wft introduction to ubd bookmarked
TRANSCRIPT
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 1 of 154
Introduction To
Underbalanced Drilling
Prepared by: Steve Nas Asia Pacific Regional Engineering Manager
Weatherford Underbalanced Systems 238a Tompson Road #16-01/04 Novena Square Tower A Singapore 307684 Tel +65 6511 3688 Email: [email protected]
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 2 of 154
Table of Contents INTRODUCTION ..................................................................................................................................5
OBJECTIVES..........................................................................................................................................5 PERFORMANCE DRILLING.....................................................................................................................6 MANAGED PRESSURE DRILLING...........................................................................................................7
WHAT IS UNDERBALANCED DRILLING? ....................................................................................8 UNDERBALANCED RESERVOIR DRILLING ...........................................................................................10
HISTORY OF UNDERBALANCED DRILLING.............................................................................11 WHY DRILL UNDERBALANCED?.................................................................................................12
UNDERBALANCED VERSUS OVERBALANCED ......................................................................................14 DISADVANTAGES OF UNDERBALANCED DRILLING .............................................................................15 LIMITATIONS ......................................................................................................................................15
CLASSIFICATION SYSTEM FOR UNDERBALANCED DRILLING.........................................16 HOW TO DRILL UNDERBALANCED? ..........................................................................................18
DATA COLLECTION ............................................................................................................................19 Reservoir Data ..............................................................................................................................19 Reservoir Screening Tool (RST)....................................................................................................20 SURE Phase II ..............................................................................................................................22 Reservoir Damage Assessment .....................................................................................................22 Drilling Data.................................................................................................................................24 Analog Data..................................................................................................................................24
EVALUATION ......................................................................................................................................25 Risk Assessment ............................................................................................................................26
CANDIDATE SELECTION......................................................................................................................29 UBS Well Type Matrix ..................................................................................................................30
HIGH LEVEL COST ESTIMATES ...........................................................................................................31 UBS FEASIBILITY...............................................................................................................................31
DETAILED WELL PLANNING ........................................................................................................33 CIRCULATION SYSTEM DESIGN ..........................................................................................................33
Fluid Selection ..............................................................................................................................33 Formation Damage.......................................................................................................................38 Gaseous Fluids..............................................................................................................................40 Mist Systems..................................................................................................................................42 Foam Systems................................................................................................................................43 Gasified Systems ...........................................................................................................................46 Single phase fluids ........................................................................................................................47 Gas lift systems .............................................................................................................................48 Drillpipe injection.........................................................................................................................49 Annular injection ..........................................................................................................................51 Parasite string injection................................................................................................................52 Gases for Underbalanced Drilling................................................................................................53 Air .................................................................................................................................................53 Natural Gas...................................................................................................................................53 Cryogenic Nitrogen.......................................................................................................................54 Membrane Nitrogen ......................................................................................................................57 Exhaust Gas ..................................................................................................................................58
FLOW MODELING ...............................................................................................................................59 Pressure calculations....................................................................................................................59 Flow modeling ..............................................................................................................................67
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 3 of 154
Equivalent Motor Throughput versus Gas Injection Rate.............................................................75 Hole cleaning ................................................................................................................................76 Annular Friction Pressure versus Gas Injection Rate ..................................................................78 Annular Liquid Hold Up versus Gas Injection Rate .....................................................................79 Drillstring Injection Pressure versus Gas Injection Rate .............................................................80 Drillstring Liquid Hold Up versus Gas Injection Rate .................................................................81 Reservoir Inflow............................................................................................................................82
DRILLSTRING AND DOWN HOLE TOOL DESIGN ..................................................................................83 Pressure While Drilling (PWD)....................................................................................................83 Conventional MWD Tools in Underbalanced Drilling .................................................................83 Electromagnetic Measurement While Drilling (EMWD)..............................................................83 Non Return Valves ........................................................................................................................86 Wireline Retrievable Float Valves ................................................................................................87 Down Hole Isolation Valves .........................................................................................................88 Drillstring Design .........................................................................................................................89 Drillpipe........................................................................................................................................91 Hard banding................................................................................................................................92 Drillpipe Rubbers..........................................................................................................................92 Jars ...............................................................................................................................................92 Down Hole Motors........................................................................................................................93
EQUIPMENT SELECTION......................................................................................................................94 Gas Injection Equipment...............................................................................................................94 Air compressors ............................................................................................................................94 Nitrogen Generation System .........................................................................................................95 Booster Compressors ....................................................................................................................97 Well Control Equipment................................................................................................................99 Rotating Diverters.......................................................................................................................100 Snubbing systems ........................................................................................................................105 Separation Equipment.................................................................................................................106 Horizontal separators .................................................................................................................107 Vertical Separators .....................................................................................................................108 UBD Choke manifold ..................................................................................................................109 Data acquisition..........................................................................................................................110 Flares ..........................................................................................................................................112
WELL CONTROL STRATEGY .............................................................................................................113 Well Kill Strategy........................................................................................................................113 Well Control................................................................................................................................113 Erosion........................................................................................................................................115 Corrosion Management ..............................................................................................................116 Corrosion Inhibitor Types...........................................................................................................118
PERSONNEL SELECTION....................................................................................................................119 TRAINING AND COMPETENCY...........................................................................................................120 OPERATIONAL PROCEDURES ............................................................................................................121 COMPLETING UNDERBALANCED DRILLED WELLS ...........................................................................123
Snubbing .....................................................................................................................................123 Workover of an Underbalanced Drilled Well .............................................................................127 Underbalanced Drilled Multi-Lateral Wells...............................................................................127
SUBSURFACE SERVICES ....................................................................................................................128 PROCESS FLOW DIAGRAMS ..............................................................................................................129 RIG AND LEASE LAYOUT ..................................................................................................................130 HEALTH SAFETY AND ENVIRONMENTAL PLANNING.........................................................................131
Environmental Aspects................................................................................................................131 Safety Aspects..............................................................................................................................131
DETAILED COST ESTIMATES.............................................................................................................133 UBS PROGRAM ................................................................................................................................134
UNDERBALANCED RECORDS .....................................................................................................136
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 4 of 154
REFERENCES ...................................................................................................................................138 SUPPLIERS OF UNDERBALANCED DRILLING SERVICES ..................................................153
WEATHERFORD ................................................................................................................................153 HALLIBURTON ..................................................................................................................................153 SHAFFER...........................................................................................................................................153 TESCO...............................................................................................................................................153 LEADING EDGE ADVANTAGE ..........................................................................................................153 BLADE ENERGY PARTNERS ..............................................................................................................153 SCANDPOWER...................................................................................................................................153 NEOTEC ............................................................................................................................................153
ABBREVIATIONS.............................................................................................................................154
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 5 of 154
Introduction This introduction is intended to provide an overview of current underbalanced drilling technology and is therefore by no means exhaustive. It should serve as a guide to the current UBD technology, explaining how, when and why underbalanced drilling is carried out.
Objectives The objective of this introduction is to provide an awareness of underbalanced reservoir drilling technology and the associated operations. It also provides a starting point and basic orientation for identification of hazards and corresponding risk assessment. This introduction covers a very brief introduction to managed pressure drilling and performance drilling to ensure a complete overview of the technologies associated with underbalanced drilling. Weatherford underbalanced drilling division currently offers three sub product lines under the underbalanced umbrella and these can be listed as :
• Performance Drilling (PD) This technology is intended to achieve maximum penetration rates through reducing the well bore pressure to a minimum possible value.
• Managed Pressure Drilling (MPD) With MPD the intention is to precisely manage and control the annular pressure to allow the bottom hole pressure to be within close limits to “walk the line”
• Underbalanced Drilling The intention of underbalanced drilling is to reduce formation damage, discover potential bypassed pay, and increase reserves by allowing access to these reserves thus ultimately increasing net NPV. In underbalanced reservoir drilling the well is designed to allow the reservoir to flow to surface whilst drilling.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 6 of 154
Performance Drilling This is the original air drilling technique to increase penetration rate. In performance drilling, the bottomhole pressure is as low as possible to increase drilling performance. Performance Drilling P formation >>>P bottomhole = P hydrostatic + P friction + P choke The objective of performance drilling is to reduce the drilling costs by drilling faster. This is normally achieved by using gas or air as a circulation medium. Reducing the bottom hole circulation pressure significantly increases the penetration rate.
Fig 3 Performance Drilling Definition
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 7 of 154
Managed Pressure Drilling A number of related technologies have now been established in the industry as a result of tools used originally in underbalanced drilling. One of these techniques is Managed Pressure Drilling, which has been defined as follows: Managed Pressure Drilling is an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly.” It means that the annular pressure profile is controlled in such a way that the well is balanced at all times, or in mathematical format: Managed Pressure Drilling P reservoir = P bottomhole = P hydrostatic + P friction + P choke
Fig 2 Managed Pressure Drilling Definition
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 8 of 154
What is Underbalanced Drilling? Underbalanced reservoir drilling is defined by the IADC UBO committee as: Drilling with the hydrostatic head of the drilling fluid intentionally designed to be lower than the pressure of the formations being drilled. The hydrostatic head of the fluid may naturally be less than the formation pressure or it can be induced. The induced state may be created by adding natural gas, nitrogen or air to the liquid phase of the drilling fluid. Whether the underbalanced status is induced or natural, the result may be an influx of formation fluids which must be circulated from the well and controlled at surface. This in effect means that in underbalanced reservoir drilling, the effective downhole pressure within the wellbore is always maintained to be lower than the reservoir pressure and it is intended to have reservoir inflow into the wellbore. Underbalanced Drilling P reservoir > P bottomhole = P hydrostatic + P friction + P choke
Fig 1 Underbalanced Reservoir Drilling Definitions
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 9 of 154
Fig 4 Shared Equipment for MPD, UBD and PD Technology These three techniques have equipment in common but are all applied in different circumstances. This introduction concentrates on underbalanced reservoir drilling and the associated equipment and techniques, not on the performance drilling or managed pressure drilling aspects.
RCDGas Handling
Choke ManifoldFloat Subs
MPD
PDUBD
RCD = Rotating Control Device
Equipment used for Managed Pressure DrillingUnderbalanced DrillingPerformance Drilling
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 10 of 154
Underbalanced Reservoir Drilling This introduction concentrates on underbalanced reservoir drilling and the associated techniques and equipment used in this drilling method. In underbalanced reservoir drilling, the wellbore pressure is maintained below the reservoir pressure at all times, and the resulting inflow from the reservoir is carefully controlled during the entire drilling process. Underbalanced Drilling P reservoir > P bottomhole = P hydrostatic + P friction + P choke The well is still controlled by controlling the wellbore pressure, but this pressure is maintained to be always below the reservoir pressure. Primary well control is no longer an overbalanced barrier of a column of fluid but is replaced by flow control using a combination of hydrostatic pressure, friction pressure and surface choke pressure. The BOP stack remains as the secondary well control barrier. It must be pointed out that a UBD well operates on a single barrier. The bottom hole circulation pressure is a combination of hydrostatic pressure, circulation friction losses and surface pressure applied at the choke. The hydrostatic pressure is considered a passive pressure and is a result of the fluid density and the density contribution of any drilled cuttings and a small contribution of any gas in the well. The friction Pressure is a dynamic pressure (It changes with pumps on or off) and results from circulating friction of the fluid used. The choke pressure arises from annular back pressure applied at surface. These three pressures are controlled at all times and ensure that flow control is maintained whilst drilling underbalanced. The lower hydrostatic head avoids the build-up of filter cake on the reservoir formation and avoids the invasion of whole mud and drilling solids into the formation. This helps to improve productivity of the wellbore and reduces any pressure related drilling problems.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 11 of 154
History of Underbalanced Drilling Underbalanced drilling has been around since the start of the oil exploration. All cable tool drilled wells were drilled underbalanced and most of us have all seen the pictures of blowouts and gushers as an oil reservoir was struck. Until 1895 all wells were drilled underbalanced. The introduction of rotary drilling technology in 1895 required fluid circulation, which initially was water. To enhance safety and hole cleaning, mud systems were developed in 1920 and drilling continued overbalanced. As deeper and larger reservoirs were encountered the reservoir damage issues became less of an issue. Until in the 1980’s the first underbalanced wells were drilled in the Austin Chalk. This proved to be the introduction to modern underbalanced drilling which started in the early 1990’s in Canada.
1284 First cable tool wells drilled in China 1859 - 1895 All wells drilled underbalanced. 1895 Rotary drilling with water. 1920 First mud systems used. 1928 First BOP’s used. 1932 First use of gasified fluids to drill 1955 Dusting or air drilling becomes popular. 1988 First high pressure gas well drilled underbalanced in Austin Chalk. 1993 First UBD wells drilled in Canada. 1995 First UBD wells drilled in Germany 1997 First UBD wells drilled offshore.
Since 1997, just after the third international underbalanced drilling conference was held, better co-operation between operators internationally was initiated. The first committees were developed as a result of Shell and Mobil requesting more information and co-operation to ensure that offshore wells could be drilled safely underbalanced. In 1998 the IADC took the safety lead in underbalanced drilling and the IADC UBO committee was formed in order to enhance the safety of underbalanced drilling operations. This committee developed the underbalanced classification matrix and continues today to develop safer and more efficient methods and procedures for underbalanced drilling operations. The development of better flow modeling systems and training systems together with international experiences shared between operators has helped to develop underbalanced drilling as one of the primary technologies for enhanced production from depleted fields and reservoir understanding in newly developed fields.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 12 of 154
Why Drill Underbalanced? The reasons for underbalanced reservoir drilling can be broken down into three main categories:
• Minimizing pressure related drilling problems • Reducing formation damage and enhancing productivity • Reservoir characterization while drilling
Fig 5 Reasons for Underbalanced Drilling The first reason for underbalanced drilling was often to reduce losses and to avoid pressure related drilling problems such as differentially stuck pipe and penetration rate improvements. This became known as drilling enabling UBD and is still widely used as a justification for underbalanced drilling but it is now more correctly classified as managed pressure drilling. Another reason was to improve reservoir productivity by eliminating reservoir damage caused by drilling fluids and fines and filtrate migration into the reservoir formations. Reduction of skin factor is then the main justification for UBD. More recently, operators are now considering underbalanced drilling to characterize the reservoirs whilst drilling. Productive features in the reservoir can be identified whilst drilling and well trajectories and well lengths can be optimized to increase reservoir productivity and to identify potentially productive horizons in the reservoir.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 13 of 154
The avoidance of drilling problems in the reservoir, especially once reservoir depletion starts and infill drilling of horizontal wells becomes more challenging as losses and differential sticking increases and it is a primary reason for operators to consider underbalanced drilling. Penetration rate increase in reservoir drilling has never been a significant driver for underbalanced drilling as tripping operations become slower and more complex if the reservoir is to be maintained underbalanced. With underbalanced drilling, as a result of drilling problems, it was noted that reservoir productivity increased approximately 300% (SPE paper 91559) with underbalanced drilling and completion of the wells. As more experience and better underbalanced systems are developed, the characterization of the reservoir is now becoming an increasing reason for operators to consider the practice of underbalanced drilling in the reservoir. Finding productive features such as fractures and high permeability streaks in the reservoir has resulted in reduced drilling costs and increased reservoir productivity. In certain areas of the world, new reservoir zones, previously masked by overbalanced pressures have now been discovered and developed.
Fig 6 Production Increases By Rock Type (From BP 2001)
BP Drilling and CompletionConference 2001
Production Increase by Rock Type
0%
100%
200%
300%
400%
500%
600%
700%
800%
900%
Standard
Frac'd D
olom
ite
Dolom
ite
Dolom
ite
Arg. L
imesto
ne
Sandstone
Uncon. Sandsto
ne
Carb. &
Sandsto
ne
Fract'd
Sandsto
ne
Fract'd
Carb
onate
Carb. &
Sandsto
ne
Fract'd
San
dstone
Fract'd
Dolo
mite
Fract'd
Sandsto
ne
Sandstone / S
hale
Sandsto
ne
Sandsto
ne
Average 360%Mode 200%
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 14 of 154
Underbalanced versus Overbalanced Comparing overbalanced drilling and underbalanced drilling allows us to establish the main differences between the two drilling techniques. Overbalanced Drilling Underbalanced Drilling
Fig 7 Overbalanced Versus Underbalanced Comparison Overbalanced Operations Mud fluid invasion and the hydrostatic pressure in the well bore can mask potentially productive zones. Reservoir damage, especially in horizontal wells, is often difficult or complicated to remove or clean up once production starts. The lower permeability and porosity zones may never be properly cleaned up, which can result in large sections of a well (especially horizontal wells) being unproductive. Lost circulation and differential sticking can often result in severe drilling problems and many wells in depleted reservoirs never get to their planned TD. Underbalanced Operations New productive horizons are often identified when drilling. No damage or minimum damage is done to the reservoir rocks, including the tighter sections of a well, resulting in better production. No losses or differential sticking as the fluid pressure is below the reservoir pressure.
“Hidden” zone:didn’t produce
“New” zone:produced while
drilling
Skin damage:• Not all zones
contribute
No skin damage• All zones
contribute
• Lost circulation,• Differential sticking• Problems getting to
TD
•Increased ROP and Bit life
•Well testing while drilling
•Different Issues•(ECD)
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 15 of 154
Disadvantages of Underbalanced Drilling It is of introduction reasonable to assume, aside from the positive aspects of underbalanced reservoir drilling, that there are also specific disadvantages associated with underbalanced drilling. Comparing the advantages and disadvantages of underbalanced drilling will allow operators to start initial considerations and candidate selection.
Underbalanced Reservoir drilling Advantages Disadvantages
Decreased formation damage Possible wellbore stability problems Eliminate risk of differential sticking Increased daily costs Reduce risk of loss circulation Generally higher risk with more inherent
problems Increased ROP More complex tripping operations Improved bit life Possible increased torque and drag Reservoir Characterization More complex drilling system More people required
Limitations There are not only advantages to underbalanced drilling. Before starting an underbalanced drilling operation, the limitations of the process must also be reviewed. There are a number of technical limitations as well as safety and economic limitations to underbalanced drilling. Conditions that can adversely affect any underbalanced operation:
• Wellbore stability issues. • Deep, high pressure, highly permeable wells can be problematic due to flow
control & safety issues. • Excessive formation water. • High producing zones close to the beginning of the well trajectory will
adversely affect the underbalanced conditions along the borehole. • Not following established design guidelines. • Wells that require hydrostatic fluid or pressure to kill the well during certain
drilling or completion operations. • Slim hole wells with high annulus friction pressures. • Wells that contain significant pressure or lithology variations. • Operators interfering with the UBD experts. • Increased complexity and HSE issues on H2S wells. • Handling and disposal of produced fluids. • Flaring of produced gas. • Erosion and corrosion issues and risks.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 16 of 154
Classification System for Underbalanced drilling A classification system developed by The International Association of Drilling Contractors (IADC) is helping in establishing the risks associated with underbalanced drilled wells Level 0 Performance enhancement only; no hydrocarbon containing zones Level 1 Well incapable of natural flow to surface. Well is 'inherently stable' and is
low level risk from a well control point of view Level 2 Well capable of natural flow to surface but enabling conventional well kill
methods and limited consequences in case of catastrophic equipment failure
Level 3 Geothermal & non-hydrocarbon production. Maximum shut-in pressures less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences
Level 4 Hydrocarbon production. Maximum shut-in pressures less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences
Level 5 Maximum projected surface pressures exceed UBO operating pressure rating but are below BOP stack rating. Catastrophic failure has immediate serious consequences
A matrix is referenced below to easily classify the majority of known underbalanced applications. This system combines the risk management categories defined above (Levels 0 to 5) with a sub-classifier to indicate if wells are drilled “underbalanced” or with a “low head” using underbalanced technology. In order to provide a complete method of classifying the type of technology used for one or more sections of a well, or multiple wells in a particular project, a third component of the classification system addresses the underbalanced technique used.
Classification A= Low head , B= UBD A B A B A B A B A B A BGas drilling 1 1 1 1 1 1 1 1 1 1 1 1Mist Drilling 2 2 2 2 2 2 2 2 2 2 2 2Foam drilling 3 3 3 3 3 3 3 3 3 3 3 3Gasified Liquid Drilling 4 4 4 4 4 4 4 4 4 4 4 4Liquid Drilling 5 5 5 5 5 5 5 5 5 5 5 5
4 50 1 2 3
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 17 of 154
Example of Classification System Use – a horizontal section of a well is drilled in a known geologic area using a drilling fluid lightened with nitrogen gas to achieve an underbalanced condition through the reservoir section. The maximum predicted bottomhole pressure is 3,000 psi with a potential surface shut-in pressure of 2500 psi. This well would be classified as a 4-B-4 indicating Classification Level 4 risk, and UBD drilling with a Gasified liquid. All wells classified as a level 4 or level 5 underbalanced well will require significant planning to ensure that these wells can be safely drilled underbalanced. More information can be found on the IADC website at www.iadc.org
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 18 of 154
How To Drill Underbalanced? Before mobilizing or selecting equipment, it is essential that the correct reservoir candidate is selected, the correct well and of course the correct way to drill underbalanced. One of the complexities of underbalanced drilling is ensuring that all the issues associated with drilling and flowing a well simultaneously are understood. What happened in conventional drilling is now all changed as the reservoir will be dictating what actions are taken during drilling and tripping operations. To ensure that all off the issues are addressed before starting an underbalanced drilling operation, a standard sequence of al the issues has been developed shown as a road map. This road map leads the way to a successful underbalanced drilling operation.
Fig 8 Underbalanced Drilling Road Map
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 19 of 154
Data Collection To understand if a reservoir can be drilled underbalanced, a significant amount of data needs to be collected and analyzed. The objectives and reasons for an underbalanced drilling project will need to be determined early on in a project, and these must remain in focus during the project preparations.
Reservoir Data Reservoir data collection is the first step to a successful underbalanced drilling project. Too often, underbalanced fails to
live up to its potential either because unsuitable reservoirs are drilled or inappropriate drilling technology is applied. Until now, there was no easy and reliable way to identify underbalanced prospects, to highlight the technical challenges presented by them, or to quantify the results that could be expected. Weatherford's SURE team and our new Suitable Underbalanced Reservoir Evaluation (SURE) Process change all of this. SURE simplifies the candidate selection process with the Reservoir Screening Tool (RSTTM); provides in-depth analysis with the Reservoir Damage Assessment (RDATM) software; and produces a risk-based economic model to aid decision making. The SURE collection of reservoir data will include issues such as:
• Reservoir depth • Reservoir pressure • Reservoir temperature • Lithology • Net to gross • Fracture data if it is a naturally fractured reservoir • Oil water / oil gas contacts • Permeability and porosity • What fluids are being produced? • Is there core data available for the reservoir and is there core available to
carry out damage analysis? • What production data is available from offset wells? • What reservoir models are being followed? • Why are the reservoir targets chosen?
The more information that can be gathered on a reservoir, the better the analysis that can be made to see if underbalanced drilling is indeed beneficial to the reservoir.
DATA COLLECTION RESERVOIR DRILLING ANALOGS
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 20 of 154
Reservoir Screening Tool (RST) To run the Reservoir Screening Tool (RST), basic reservoir data, the drive mechanism, presence of fractures, risk of borehole instability, reservoir heterogeneity, and minimum/most likely/maximum values for porosity, water saturation, reservoir thickness, pressure, clay content, etc are entered. RST Process Methodology In RST, a number of modules are incorporated in the software through which each candidate reservoir is run. These modules are a combination of classical formation damage theory coupled with proprietary experience of underbalanced analog reservoirs around the world. The RST employs a Monte Carlo simulation as an integral part of the software. The possible values for each uncertain reservoir variable are defined by a probability distribution. Within the simulation, the software randomly picks possible reservoir values from each of these probability distributions to calculate an underbalanced risked suitability rating. At the end of several thousand iterations, RST produces a distribution of the scores for each reservoir. Risked Suitability scores range from -100 (stick to conventional techniques) to +100 (drill UB without hesitation), with 0 as the break-even point. Some reservoirs are simply too technically challenging to be drilled underbalanced. So, RST accounts for exclusionary factors. These may include, regardless of other positive indicators, high reservoir pressure, borehole instability or an extremely low pore pressure gradient. As well as statistical distributions of a reservoir’s suitability for underbalanced drilling, the RST enables sensitivity analysis. For each reservoir studied, tornado charts are provided that help understand which input variables have the greatest correlation to the risked suitability score. The type (positive or negative), and strength of correlation indicate the degree to which the range of each input variable is influencing the range of outcome. RST Deliverables At the end of an RST screening study, you are presented with a quick look summary report containing:
• The suitability rating of each candidate reservoir in the form of statistical distributions
• A ranking of each candidate reservoir within all reservoirs examined • A comparison of the candidate reservoirs to analog reservoirs with proven
underbalanced drilling success • Discussion on factors influencing the scoring of candidates Sensitivity
analysis • Based on this information, a decision can be made to proceed with the in-
depth analysis phase of the SURE process.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 21 of 154
Suitability RST Score Recommendation Excellent 70 to 100 Drill Underbalanced
Good 40 to 69 Drill Underbalanced or proceed to SURE Phase II
Average 0 to 39 Proceed with RDA or SURE phase II study Poor -21 to 0 Eliminate candidate or proceed with RDA study
Non candidate -21 to -100 Eliminate candidate for underbalanced drilling
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 22 of 154
SURE Phase II The goal of SURE Phase II is to provide a risk-based approach to underbalanced systems payback for RST screened reservoirs. Phase II predicts and compares formation damage and productivity for overbalanced and underbalanced drilled wells. The first step of the in-depth analysis is amassing a wealth of geological, production and reservoir data, focusing on detailed reservoir definition (lithofacies, x-ray diffraction, cores, etc.) and the results of previous drilling and completion techniques. Gathering this data is a joint effort between the SURE Team and the client. After a formal QA/QC assessment, robust data is then input to Weatherford’s proprietary Reservoir Damage Assessment (RDA) software.
Reservoir Damage Assessment Developed in cooperation with Hycal Energy Research Laboratories, the Reservoir Damage Assessment software is a uniquely rigorous model that predicts radial isotropic near wellbore formation damage. Additionally RDA considers damage effects on productivity due to factors such as a real positioning and partial penetration. The software models 11 formation damage mechanisms for non-stimulated underbalanced or overbalanced wells. We created this model based on classical damage theory augmented with pseudo empirically calculated damage assessments mined from a large database of core and laboratory data contained within RDA. The calculated permeability reduction due to each damage mechanism is segregated as near and deep, along with an effective radius of damage. The reduction in permeability due to damage is converted into skin to feed a Babu & Odeh-based analytical model within RDA. Alternatively, for more complex reservoirs, the permeability reductions can be input into a numerical simulator for a more rigorous productivity evaluation. the production forecasts produced are then used as input to the next stage of the Phase II SURE process – economic modeling.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 23 of 154
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 24 of 154
Drilling Data Besides reservoir data, a large amount of drilling and well data is collected, mainly to ensure that underbalanced drilling can be executed safely and efficiently.
Some of the drilling data that would be collected is:
• Where are casing strings set and what is the casing design for the well? • What kind of completion is to be run? • What are the objectives of the well? • Directional profile of the well • Reservoir target area’s and expected reservoir penetration • Drilling parameters normally used in the reservoir • Drilling history of the field and offset wells. • Drilling problems encountered in the reservoir • Pore and fracture gradients in the well • What drilling fluids have been used to drill this reservoir? • Finally cost and time information will be required to ensure that it is cost
effective to drill underbalanced. The more that is know about the field and the reservoir, the better the solution is that ultimately is applied to the reservoir and surprises will no doubt still be encountered once underbalanced operations start.
Analog Data As a part of the entire selection process for wells and reservoirs, a review of any similar reservoirs that may have already been drilled underbalanced should be conducted. Using the production data
from these offset reservoirs may provide useful offset information for a UBD operation. Analog data from around the world is collected by UDB providers and stored and can be used to establish the best underbalanced methods to be used for a specific reservoir. Of course the SPE papers also provide an excellent source of reservoir information.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 25 of 154
Evaluation As part of the data collection the reservoir needs to be evaluated, and it needs to be established if the reservoir does indeed benefit from underbalanced drilling technology. Some reservoirs cannot be drilled underbalanced and other reservoirs will only show marginal benefits.
Of course a part of the whole reservoir selection process is the economic screening of the candidate reservoirs and wells. The business drivers behind a project must never be forgotten. If the business benefits cannot be achieved then the project must be reviewed and maybe cancelled. The improvements from an underbalanced operation must pay for the additional cost of the technology. This is often the most difficult limitation of underbalanced drilling to overcome. If the reservoir / production engineers cannot be convinced that there is a sound reason for drilling underbalanced and can see productivity improvements, the whole underbalanced project may never get further than the feasibility study. To drill a well underbalanced extra equipment and people are required and this additional cost of the well must be paid back. Once this information has been gathered and reviewed, and, from the data it is thought that underbalanced drilling is the absolute best method to recover more hydrocarbons in an economic and technically successful manner, it is time to review the next set of steps in the design process.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 26 of 154
Risk Assessment
The risk assessment forms an integral part of the underbalanced selection process and ensuring that operators are made aware of the potential risks, the risk assessment is carried out during the candidate selection process.
The IADC well Classification form an essential fist step in the overall risk assessment. The IADC classification for underbalanced wells should be assigned to every well drilled underbalanced. This gives the first indication of the potential risks.
Fig 9 Underbalanced IADC Classification
Level 5 – Maximum projected surface pressures exceed UBO operating pressure rating but are below BOP stack rating. Catastrophic failure has immediate serious consequences.
IADC Well ClassificationLevel 0 – Performance enhancement only; no hydrocarbon containing zones.
Level 1 – Well incapable of natural hydrocarbon flow to surface. Well is 'inherently stable' and is low-level risk from a well control point of view.
Level 2 – Well capable of natural hydrocarbon flow to surface but enabling conventional well kill methods and limited consequences in case of catastrophic equipment failure. (Flowing oil well)Level 3 – Geothermal & non-hydrocarbon production. Maximum shut-in pressures less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.Level 4 – Hydrocarbon production. Maximum shut-in pressures less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences. (Oil/gas well)
555555555555Liquid Drilling
444444444444Gasified Liquid Drilling
333333333333Foam Drilling
222222222222Mist Drilling
111111111111Gas Drilling
BABABABABABAA – Low Head, or B – UBD
543210Classification Level
555555555555Liquid Drilling
444444444444Gasified Liquid Drilling
333333333333Foam Drilling
222222222222Mist Drilling
111111111111Gas Drilling
BABABABABABAA – Low Head, or B – UBD
543210Classification Level
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 27 of 154
The next step in the risk assessment is the review of the reservoir and the produced fluids
The risk assessment for the reservoir reviews the kind of fluids that are expected, the gas rates and the production profile. It also reviews if any H2S is being produced and of course it looks at the depth of the reservoir and the pressure in the reservoir. A deep high-pressure sour gas reservoir would obviously have a classification with a higher risk compared to a low-pressure oil producer.
The reasons and objectives for underbalanced drilling are also very much a part of the risk assessment. A well drilled underbalanced to minimize skin damage will need to be maintained underbalanced at all times, thus adding complexity to the operation. As part of the QHSE section of the risk assessment, the equipment required and fluid systems to be used are also recorded as is the number of people on location and the experience of the rig crew.
feet
feet
psi
4 - 11 mmscf/day0 - 4 mmscf/day
Yes
> 32 mmscf/day315 - 1260 bbl/day No0 - 315 bbl/day
Oil/CondensateGas
0 - 315 bbl/day315 - 1260 bbl/day> 1260 bbl/day
11 - 32 mmscf/day
Measured Depth:
True Vertical Depth:
Reservoir Pressure:
> 1260 bbl/day
Water
Reservoir and Production
H2S expected
Oil/Condensate Production rate
Gas production rate
Water production rate
Produced fluids
1 2 3 4 Involved WFT Provided
Mist Pump
Rank the reasons for considering UB operations:
Technical/Equipment
Separation
CompressionMembrane N2
Rotating DiverterFluids
Upper zones will be open while drilling production section UnderbalanceDrill String
FloatsHorizontal/high angle wellFoam is to be used in operations
BOP
Cryogenic N2
Equipment to be involved in the operation:
Minimize reservoir skin damage
Minimize drilling problems: Diff. Sticking, Lost circulation, etc.
Performance enhancement (e.g.. ROP)
Other (specify):
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 28 of 154
The tripping method in an underbalanced drilled well is crucial. Avoiding pipe light and snubbing can reduce the risk level significantly.
Finally the experience of the operator is taken into account together with a look at how the job will be performed.
Once this is done, a risk assessment score will be allocated to the job. This provides a rapid assessment of the potential risk and provides the service provider with the required equipment and personnel for the job.
N2/Nat. Gas only DDVLow
The Drilling Contractor crews have less than 3 years of underbalanced experience
Air onlyMediumHigh
SnubbingWater based fluid
QHSE
Flow while tripping Pre engineered WFT Program/Template not available
Equipment operator SOP's not availableConventional trippingOil based fluid
Environ. Sensty Fluid system to be usedUBS Pers.
Tripping method:
Operator/Producer Experience The operator has less than 3 years of general underbalanced experience The operator has minimal underbalanced experience in this field
The drilling engineer has less than 3 years of general underbalanced experience
The drilling engineer has minimal underbalanced experience in this field
A feasibility study has not been performed on this field
Commodity based With full Applications Engineering
How would the operator prefer the job be performed?
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 29 of 154
Candidate Selection Although it is true to state that most reservoirs can probably be drilled underbalanced, the complexities will vary greatly from reservoir to reservoir. Some reservoirs cannot safely be drilled underbalanced because of pressure or geological issues mainly associated with rock stability issues.
Candidate selection for underbalanced drilling must not only focus on the benefits of underbalanced drilling but must also consider a number of additional considerations that will need to be appreciated when selecting candidates. It is important that the right reservoir is selected for an underbalanced drilling operation. The table below shows reservoir types that will and will not benefit from underbalanced drilling.
Will benefit from UBD
Will not benefit from UBD
Formations that usually suffer major formation damage during drilling or completion operations. Wells with skin factors of 5 or higher
Wells in areas of very low conventional drilling cost
Formations that exhibit differential sticking tendencies
Wells drilled in areas of extremely high ROP (that is ROP ≥ 1000 ft/day)
Formations with zones with severe losses or fluid invasion from drilling or completion operations
Extremely high permeability wells
Wells with large macroscopic fractures Ultra low permeability wells Low permeability wells Poorly consolidated formations Wells with massive heterogeneous or highly laminated formations characterized by differing permeabilities, porosities and pore throat throughput
Wells with low borehole stability
High production reservoirs with low to medium permeabilities
Wells with loosely cemented laminar boundaries
Formation with rock fluid sensitivities Wells that contain multiple zones with different pressure regimes
Formations that exhibit low ROP with overbalanced drilling
Reservoirs with interbedded shales or claystones
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 30 of 154
UBS Well Type Matrix
The next step in risk assessment is a quick look at the type of well that is to be drilled and this will give further insight in the planning and equipment requirements for a job.
Fig 10 Risk Matrix Some type of risk matrix will need to be designed to reduce operational risks and provide a system for effective hazard (QHSE) and change management by giving an indication of risk levels for the selected well. This example provides a guide to risk assessment and these should be prepared for each project during the HAZID / HAZOP reviews.
0.208 psi/ft
Productivity Enhancement
LOW RISK MODERATE RISK HIGH RISK
0.364 psi/ft
0.443 psi/ft
0.520 psi/ft
0.624 psi/ft
> 0.624 psi/ft
Geotherm
al
Drilling
Performance
Drilling
Managed
Pressure Drilling
Sour Oil W
ells
Sweet O
il Wells
Sour Gas W
ells
Sweet G
as Wells
Drilling Optimization
Underbalanced Classification Matrix
0
0
0
0
0
0
3
3
3
3
3
3
4A
4A
4A
4A
4A
4A
1
1
2
4
5
555 5
444
444
444
222
111
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 31 of 154
High Level Cost Estimates One of the first issues an operator wants to be solved is after it has been confirmed that his reservoir can be drilled underbalanced is how much budget is required.
The high level cost estimates that are created at this point of an underbalanced drilling project are normally budgetary cost estimates based on the expected equipment and expected people for the project. These budgetary costs are normally further defined later on in the project by the project manager once more of the detailed engineering has been finalized. An underbalanced drilled well can cost as much as double the cost of a conventional well depending obviously on the complexities that are anticipated during the drilling operations.
UBS Feasibility The UBS feasibility is the output and report part of the initial underbalanced drilling review. This feasibility report, reviews and describes all of the previous aspects such as reservoir and well candidate selection combined with the risk matrix. This report also provides a summary of the methodology that needs to be applied for the selected wells based on the matrix.
This feasibility report allows the operator’s senior management to review and approve the full underbalanced drilling project before starting with the detailed engineering. The feasibility report also allows the service provider to look at his equipment and personnel availability and schedule the job. Of course if the feasibility shows that underbalanced drilling is not feasible, it should also explain why UBD might not be the correct method and what alternative methods could be used instead. This point provides the operator with a stop / go point in an underbalanced drilling project.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 32 of 154
The first stop and go point in a UBD project is reached when the candidate selection has been completed and the feasibility report has been provided to the operator. There is a second stop/go point in any underbalanced drilling project and that is normally arrived at once all the detailed planning has been completed and the detailed program and procedures have been completed. Prior to starting rig modifications and starting to mobilize equipment a project can be delayed, postponed or cancelled.
Project Definition Design & Planning Execution
Candidate Selection
Technical Feasibility
Economic Feasibility
Tender UBD Services Award Contracts
Detailed Design
Preliminary Procedures
Modify Equipment
Mobilise Equipment
Carry out Rig Mods
Commision Equipment
Rig Up
Train Crews
Complete detailed HAZOP
Drill Underbalanced
Review
End of well report
Produce
Capture Lessons
Project Approval
Final Go-Ahead Approval
Costs
Cost Saving potential
UBD Project definition
Cos
ts
Project Time
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 33 of 154
Detailed Well Planning Providing that the feasibility report recommends that underbalanced drilling is profitable and operator management has provided the OK for the project to continue, the detailed well planning will start at this point.
These steps ensure that all UBS well planning covers all of the issues and will result in a complete detailed UBS drilling program.
Circulation System Design The well planning will start with the design of the circulation system. The first step in this is the design of the base fluid.
The base drilling fluid is the fluid that will be pumped down the drillpipe. This should not be confused with the annular fluid that, in an underbalanced situation, comprises of the base fluid plus any reservoir and formation fluids that enter as a result of the underbalanced state.
Fluid Selection Fluid selection for underbalanced drilling operations can be extremely complex. Key issues such as reservoir characteristics, geophysical characteristics, well fluid characteristics, well geometry, compatibility, hole cleaning, temperature stability, corrosion, drilling BHA, data transmission, surface fluid handling and separation, formation lithology, health and safety, environmental impact, fluid source availability, as well as the primary objective for drilling underbalanced all have to be taken into consideration before the final fluid selection can be made. The objective of the fluid selection system is to select the optimum drilling fluid for underbalanced drilling operations that meets all the health, safety, and environmental requirements as well as the required technical requirements.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 34 of 154
One of the most important aspects of the base fluid is the density of the fluid that is required to achieve an underbalanced condition in the wellbore whilst circulating. In overbalanced drilling, the fluid weight is selected so that it provides a minimum hydrostatic pressure of some 200 psi plus a trip margin above the reservoir pressure. In underbalanced drilling, a fluid needs to be selected that provides a suitable hydrostatic pressure below the reservoir pressure. This density value provides us with a starting point for the selection of a fluid system. This value is likely to be further refined, depending on the circulation system pressure losses and the expected reservoir inflow, with a given drawdown. To calculate this initial fluid density required, simply convert the reservoir pressure and the drawdown into an equivalent fluid density. Fluid gradients are calculated based on the following formula:
Reservoir Pressure - Surface Pressure - Drawdownfluid gradient (ppg) = 0.052 x Reservoir TVD (ft)
Where: Surface Pressure is assumed to be approximately 150 psi And the reservoir Drawdown is assumed to be 250 psi Note : These numbers can vary significantly for different reservoirs and must be determined during the reservoir evaluation and review. Once an equivalent mud weight is calculated, it is relatively simply to obtain the first indication of the fluid system that may be used for underbalanced drilling. Where:
Equivalent Fluid Weight Fluid System 0 to 2 ppg Nitrogen or gas 2 to 4 ppg Stable foam system 4 to 7 ppg Gasified fluids or Foam systems 7 to 8.5 ppg Native crude or Diesel 8.5 to 10 ppg Waterbased fluid systems 10 to 12 ppg Brine systems 12 ppg or higher Not recommended for underbalanced drilling
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 35 of 154
The fluid selection for underbalanced drilling has a density range that covers the entire spectrum from gas through to weighted fluids.
SO_00591 2/4/98
Air
Oil
Weighted Mud(Barite)
7
0.
0 150 20.0
Fig 11 Fluid Classification and selection As the density of the required fluid increases the associated reservoir pressure of the reservoir will normally be higher and the IADC well classification must be considered when selecting fluids. It must be remembered that an IADC level 5 well, which may require a high fluid density will also require significant planning to ensure that the risks associated with the higher pressures can be managed. There are basically 5 fluid systems that are recognized with underbalanced drilling that allows us to achieve drilling fluids with densities ranging from gas to weighted liquids.
• Gas systems • Mist systems • Foam • Gasified Liquids • Liquids
The base drilling fluid for underbalanced drilling operations has three basic functions, just like in overbalanced drilling.
1. Hole Cleaning Transportation of solids, liquids and gases. 2. Lubrication Lubricity of the drillstring and bit 3. Cooling Particular the cooling of the bit.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 36 of 154
The goals and functions of the base drilling fluid for underbalanced drilling can be further broken down into a number of further categories.
a) Non-Damaging. b) Not Over-Expensive. c) Good Hole Cleaning. d) Lubrication. e) Rheological Control - Viscosity and Friction. f) Ease of Separation and Measurement - Surface.
Fig 12 Fluid Selection Matrix The fluid selection matrix shown in fig 12 describes how many combinations are possible to obtain the required bottom hole pressures associated with the fluid system for underbalanced drilled wells. Fluid densities can be changed using gas injection, or even fluid injection. But the safety considerations in fluid selection always need to be maintained.
Gas
Mist
Foam
Gasified Fluids
Single phaseFluids
Brines
Weighted Fluids
Air
Nitrogen
Drill string Injection
WaterSea water
Fresh waterFormation Water
OilDiesel Oil
Native CrudeVegetable Oils
Distillates
BrinesKCL Brines
Chloride BrinesBromite Brines
Zinc BrinesFormate Brines
Mudwaterbased mud
oil based mud
Exhaust Gas
Natural Gas
Incr
easi
ng D
ensi
ty
Fluid System Base Fluid Gas GasInjectionMethod
Annular Injection
Combined string /annular Injection
Fluid Selection Matrix
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 37 of 154
Maintaining well control during underbalanced drilling is a primary concerns, and an essential part of this is keeping the surface pressures as low as possible. This is also one of the selection criteria for underbalanced drilling. For most separation systems, a maximum surface pressure will be provided by the service provider. The rotating control device has a certain maximum operating pressure which cannot be exceeded. Using a simple table, as shown below, the downhole pressure and surface pressure for a given fluid system, with a certain reservoir pressure, can be quickly assessed. With a reservoir pressure of 4115 psi in the example below the maximum surface pressures can be estimated quickly. Fluid System Density Pressures
at reservoir Surface Pressure
Gas 0.1 ppg 45 4071 Mist 0.3 ppg 135 3980 Foam dry 3.5 ppg 1584 2532 Foam wet 6.0 ppg 2715 1400 Gasified diesel 5.8 ppg 2625 1491 Gasified water 7.5 ppg 3394 721 Diesel oil 7.2 ppg 3259 857 Water 8.4 ppg 3802 314 Fluid System 9.0 ppg 4073 42
It is normal practice to limit the surface pressures to a safe operating value as defined during HAZOP or HAZID reviews.
Surface Pressures
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Gas
Mist
Foam dry
Foam wet
Gasified diesel
Gasified water
Diesel oil
Water
Mud System
Flui
d Sy
stem
Surface Pressure (psi)
Surface Pressure Too High Surface Pressure
need to be controlled
Fig 13 Surface Pressure Control During Fluid Selection Process A graph, as shown in fig 13 above, quickly show the implications on surface pressures when selecting different fluids.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 38 of 154
Formation Damage The fluid selection must also take into account any potential reservoir and formation interaction, and the potential for separation of the reservoir fluids from the base drilling fluid. Other key issues for fluid selection considerations are not only associated with formation pressures, but must also take into account the type of formation that is being drilled and the potential formation damage mechanisms. The assessment of the formation damage with a given fluid becomes an essential element for the engineering and fluid selection for an underbalanced project. Reservoir engineers and geologists, as well as the production engineers, will need to understand the damage mechanisms associated with the reservoir. Detailed studies and coreflush testing with a selected fluid may be required to assess the damage mechanisms of a certain reservoir. One of the most important aspects of the fluid selection is formation damage, especially if the objective of underbalanced drilling is to improve reservoir productivity and to minimize formation damage. Although this will have been looked at in some detail during the candidate selection, with the selection of the drilling fluid, reservoir damage once again needs to be reviewed. As stated earlier, coreflush testing with the final selected base fluid may have to be performed to ensure that reservoir damage is minimized. Four main damage mechanisms have been identified: Mechanical Damage Mechanical damage is mainly caused by the introduction of solids from the mud system, weighting agents, fluid loss agents, artificial bridging agents (LCM) or naturally occurring drilled solids and whole mud invasion. Biological Damage Biological damage results from the introduction of bacterial agents during drilling and completion processes. One of the main issues here is the introduction of bacteria that over time result in the formation of sulphates, causing a reservoir to become sour over time. Thermal Damage Thermal damage is mostly associated with air and gas drilling where, due to friction and insufficient cooling or due to downhole fires, heating of the formation occurs which in turn results in glazing of the formation. Chemical Damage Chemical damage is mainly caused by the swelling of clay in the formations as a result of fluid filtrate invasion. It can also be caused by the precipitation of waxes, solids or asphaltenes caused by a reduction in temperature or pressure associated with the drilling process.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 39 of 154
All four of these main damage mechanisms will need to be taken into account when selecting a drilling fluid for underbalanced operations. These four main categories are further broken down into a number of more detailed damage mechanisms and the chart below shows these main damage mechanisms.
Fig 14 Main Formation Damage Mechanisms.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 40 of 154
Gaseous Fluids Gaseous fluids are the gas systems. Although the gaseous fluids are normally associated with performance drilling in some reservoir applications the density requirement for the fluid may well require gas systems to achieve an underbalanced status. The use of air in hydrocarbon bearing formations is not recommended as the combination of oxygen and natural gas may cause an explosive mixture. There have been a number of reported cases where downhole fires have destroyed drillstrings and of course there is the obvious potential of the rig burning down if a gas/air mixture gets to surface and forms an explosive mix in a surface system. To avoid the use of air, nitrogen is normally used. The experience with nitrogen in well servicing operations made it a first choice for underbalanced drilling operations. Nitrogen has a number of options in as far as liquid nitrogen or generated nitrogen and this will be further discussed under the gas systems in this introduction. Natural gas for underbalanced drilling operations has been proven to be a worthy alternative in drilling operations. If a gas reservoir is being drilled underbalanced, a producing well or an export pipeline well may produce sufficient gas at the right pressure to drill. This avoids the use of nitrogen and may provide a cheap drilling system. Characteristics of gaseous drilling:
• Fast penetration rates • Longer bit life • Greater footage per bit • Good cement jobs • Better production • Requires minimal water influx • Slugging can occur • Mud rings can occur in the presence of fluid ingress • Relies on annular velocity to remove cuttings from the well
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 41 of 154
Fig 15 Air Drilling / Foam Drilling Setup On a Rig. The diagram above shows a typical rig set up for nitrogen or air drilling. A gas buster in the return line ensures that any produced liquid can be returned to the pits. The above depicted setup will also work for foam drilling by mixing nitrogen/air with a fluid and a surfactant. The basic setup for air drilling and foam drilling is not significantly different. The gas injection system is tied into the standpipe and gas is injected through the conventional standpipe manifold directly into the standpipe. The return line has a T-junction to route any fluid slugs to the shale shakers. A gas buster is used to separate any produced fluids, and these are routed to the shakers. The flare line or blooie line is normally routed into a flare pit.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 42 of 154
Mist Systems Mist Drilling is normally used when formations begin to produce small amounts of water (10 to 100 bbls per hour) during air/gas drilling operations. Gas or air volumes are increased and a mist pump skid is used to inject small quantities of water and a foaming agent solution. This solution entraps the water influx and enables the air phase to lift the cuttings and influx to surface. Mist drilling should only used in special applications since hole cleaning is even more difficult with mist drilling system when compared with air drilling. In mist drilling, the fluid added to a gas environment will disperse into fine droplets and form a mist. In general this technique is used in areas where some formation water exists which prevents the use of complete 'dry air' drilling. Characteristics of mist-drilling:
• Similar to air drilling but with addition of liquid • Relies on annular velocity to remove cuttings from the well • Reduces formation of mud rings • High volumes required (30%-40% more than dry air drilling) • Pressures generally higher than dry air drilling • Incorrect air/gas-liquid ratio leads to slugging, with attendant pressure
increase
Fig 16 Mist Exiting Blooie Line At Surface.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 43 of 154
Foam Systems Drilling with foam has some appeal due to the fact that foam has some attractive qualities and properties with respect to the very low hydrostatic densities, which can be generated with foam systems. Foam has good rheology and excellent cutting transport properties. The fact that foam has some natural inherent viscosity as well as fluid loss control properties, which may inhibit fluid losses, makes foam a very attractive drilling medium. During connections and trips, the foam remains stable and provides a more stable bottom hole pressure.
Fig 17 Foam Structure Foam consists of a continuous liquid phase, forming a cellular structure that surrounds and entraps a gas. Foams can have extremely high viscosities; in all instances, their viscosities are greater than that of both the liquid and the gas that they contain. During foam drilling, the volumes of liquid and gas injected into the well are carefully controlled. This ensures that foam forms when the liquid enters the gas stream, at the surface. The drilling fluid remains foam throughout its circulation path down the drillstring, up the annulus and out of the well. The more stable nature of foam also results in a much more continuous downhole pressure condition due to slower fluid and gas separation when the injection is stopped. Adding surfactant to a fluid and mixing the fluid system with a gas generates foam. Foam used for drilling has a texture not unlike shaving foam. It is a particularly good drilling fluid with a high carrying capacity and a low density. One of the problems
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 44 of 154
encountered with the conventional foam systems is that foam does what it says on the tin. It remains stable. The foam normally remains stable, even when it returns to the surface, and this can cause problems on a rig if the foam cannot be broken down fast enough. In earlier foam systems, the amount of defoamer had to be tested carefully so that the foam was broken down before any fluid entered the separators. In closed circulation drilling systems, stable foam could cause particular problems with carry over. The recently developed stable foam systems are simpler to break, and the liquid can also be re-foamed so that less foaming agent is required and a closed circulation system can be used. These systems, in general, rely on either a chemical method of breaking and making the foam, or the utilization of an increase and decrease of pH to make and break the foam. The foam quality at surface used for drilling is normally between 80% and 95%. The quality of foam means that the system is 80% to 95% gas, with the remaining 5 to 20% being liquid. Downhole, due to the hydrostatic pressure of the annular column, this ratio changes as the volume of gas is reduced. An average acceptable bottom-hole foam quality (FQ) is in the region of 50%-60%.
The compressible bubble structure of foam provides up to 10 times the carrying capacity of normal liquid based circulation systems. Due to the high carrying capacities of foam, annular velocities as low as 1 ft/min have proven to provide effective hole cleaning. Experience has proven that foam is able to handle over 100 bbls/hr of water influx.
Fig 18 Foam drilling (note the cuttings floating on top of the foam) Fluid densities for foam range from 0.2 to 0.8sg (1.6 ppg – 6.95 ppg). The density ranges are adjusted with the make up of the foam by adjusting the LVF (Liquid Volume Fraction) through the injection of liquid and gas by adjusting the backpressure on the well. The backpressure adjusts the downhole pressure and slows down the velocities in the annulus.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 45 of 154
Characteristics of foam drilling: • Extra fluid in the system reduces the influence of formation water • Very high carrying capacity • Reduced pump rates due to improved cuttings transport • Stable foam reduces slugging tendencies of the wellbore • The stable foam can withstand limited circulation stoppages without affecting
the cuttings removal or ECD to any significant degree • Improved surface control and more stable downhole environment • The breaking down of the foam at surface needs to be addressed at the
design stage • More increased surface equipment required
Gas volume percentage Name
99.99 – 96% Mist 96% - 55% Foam
0 – 55% Gasified Liquid
Guidelines for Foam Drilling Liquid injection volume 16 – 80 gpm Soap injection volume 0.3 to 1.0% by weight 0.05 – 0.5 gpm Gas injection volume 300 – 1000 scft/min
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 46 of 154
Gasified Systems The next system after a foam system is a gasified fluid system, which is used to control slightly higher pressures. In these systems, a liquid is gasified to reduce the density. There are a number of methods that can be used to gasify a liquid system and these methods are discussed within the injection systems section. The use of gas and liquid, as a circulation system in a well, complicates the hydraulics in the wellbore and the ratio of gas and liquid must be carefully calculated to ensure that a stable circulation system is used. If too much gas is used, slugging will occur. If not enough gas is used, the required bottom hole pressure will be exceeded and the well will become overbalanced. Characteristics of gasified-mud systems:
• Extra fluid in the system will almost eliminate the influence of formation fluid unless incompatibilities occur
• The mud properties can easily be identified prior to commencing the operation
• Generally, less gas is required • Slugging of the gas and fluid must be managed correctly • Increased surface equipment will be required to store & clean the base fluid • Velocities, especially at surface, will be lower, reducing wear & erosion both
downhole and to the surface equipment.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 47 of 154
Single phase fluids A well drilled underbalanced with a single phase fluid is sometimes referred to as “flow drilling”. This is the simplest form of underbalanced drilling and, providing that an underbalanced situation can be achieved when circulating at the required rates, a single phase fluid system should be a first consideration. Water based systems Water, especially offshore, can be a first consideration because it is cheap and easily accessible. Water can be easily separated or viscosified, and the circulation system is almost as in a conventional drilling operation. Any viscosity added to a system must be carefully considered when drilling underbalanced. Remember that fluids must be effectively degassed in the separation system. Any viscosity in the system may cause issues with degassing of the fluid. Oil Systems If reservoir conditions are such that water is deemed unsuitable, then crude oil, base oil or diesel can be considered as a drilling fluid with the understanding and acceptance that when drilling an oil-bearing reservoir, this will ultimately turn into a crude oil system since base oil or diesel cannot be separated from crude oil. A crude oil system can be chosen as long as the system exists, to ensure that the crude is sufficiently degassed before entering a closed pit system. The risks of using a crude oil system must be addressed in a HAZOP when selecting the fluid system. Other Systems The use of additives, such as glass beads, has been used in an attempt to lighten a fluid. However, since the glass beads come out over the shakers in the solids separation system or get crushed and damaged throughout the whole system, new beads need to be continuously added. The addition of glass beads is therefore, an expensive option and not very effective in lightening the fluid.
The main use for hollow glass beads in drilling is to reduce friction and torque within deviated holes. Solid glass spheres act as tiny ball bearings to reduce friction and reduce differential pressure. The glass beads are a transparent, solid soda lime glass, free of pits and excess air bubbles.
Fig 19 Glass Beads The glass beads are chemically inert and do not effect the chemical characteristics of the mud system. Glass beads are not recommended for prevention of differential sticking. General Description of the beads Super-Slide – Coarse – 12-20 mesh Super-Slide – Medium – 20-40 mesh Super-Slide – Fine – 170-325 mesh
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 48 of 154
Gas lift systems If a fluid needs to be reduced in density, the use of a gas injection into the fluid flow is an option. This offers a choice into not only the gas used but also in the way the gas is injected into the circulation system. Normally, natural gas or nitrogen is used as a lift gas, but both CO2 and O2 can also be utilized. However, gasses containing oxygen are not recommended for two main reasons: • The combination of oxygen and saline fluids with the high bottom hole
temperatures can cause severe corrosion to tubulars used in the well and drillstring.
• If during the drilling process, hydrocarbons enter the borehole (expected in an
underbalanced environment), a potentially explosive situation could arise, resulting in either downhole fires or surface equipment explosions.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 49 of 154
Drillpipe injection Drillstring injection is the first and simplest method of gas injection into the circulation system. Compressed gas is injected at the standpipe manifold where it mixes with the drilling fluid. The main advantage of drillstring injection is that no special downhole equipment is required in the well. The use of reliable non-return valves is required to prevent flow up the drillpipe. The gas rates used when drilling with drillpipe injection system are normally lower than with annular gas lift, and low bottom hole pressures can be achieved using this system. The disadvantages of this system include the need to stop pumping and the bleeding of any remaining trapped pressure in the drillstring every time a connection is made.
This results in an increase in bottom hole pressure. It may then be difficult to obtain a stable system and avoid pressure spikes at the reservoir when using drillpipe injection. The use of pulse type MWD tools is only possible up to 20% gas by volume. If higher gas volumes are used, the pulse system deployed on MWD transmission systems will no longer work. Specialist MWD tools such as electro-magnetic may have to be used if high gas injection rates are required. However, these tools do not work very well offshore or if a significant amount of evaporates are drilled. One alternative is to connect the MWD back to surface using an electric cable. This technique has previously been used very successfully with
Fig 20 Drillstring Injection coiled tubing as the drillstring. If drill pipe is to be used, wet connects can be utilized; however, the additional time consumed using this technique can be limiting. A further drawback for drillstring injection is the impregnation of the gas into any downhole rubber seals. Positive displacement motors (PDM's) are especially prone to fail when the rubber components get impregnated with the injection gas and then tripped back to surface. Once a trip is made, the rubber can explode or swell as a result of the expanding gas not being able to disperse out of the stator quick enough. This effect (explosive decompression) destroys not only the motors, but also affects
`
Reservoir
Liquid Gas
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 50 of 154
any rubber seals used downhole, resulting in a high turnover of motors. This can be very costly to the drilling operation. Special rubber compounds have been developed and the design of motors is changing to allow for this expansion. The majority of motor suppliers can now provide PDM’s specifically designed for use in this kind of downhole environment. But, if drillpipe injection is an option, the use of all metal turbines should be considered, depending on the operational demands. Care must be taken at surface when breaking out the drilling assembly in case there is any high-pressure gas trapped in the tool string.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 51 of 154
Annular injection Annular injection through a concentric casing string is most commonly used in a number of offshore projects. This method is worthwhile if a suitable casing or completion tubing scheme is installed in the well. For a new drill well, a liner should be set just above the target formation. The liner is then tied back to surface using a modified tubing hanger to suspend the tie back string. Gas is injected in the casing liner annulus to facilitate the drawdown required during the drilling operation. The tie back string is then pulled prior to installation of the final
completion. The alternative is for an older well to have a completion in place incorporating gas lift mandrel pockets. These can be set up to provide the correct bottom hole pressures during the drilling operation. The drawback with this type of operation is that the hole size and tools required are restricted by the minimum ID of the completion. However, the main advantage of using an annulus to introduce gas into the system is that gas injection can be continued during connections, thus, creating a more stable bottom hole pressure. As the gas is injected, via the annulus, only a single-phase fluid is pumped down the drillstring. This has the advantage that conventional MWD tools operate in their preferred environment, which can have a positive affect on the operational cost of a project.
Fig 21 Annular Injection (Concentric String) However, the drawbacks of this system are that a suitable casing/completion scheme must be available and that the injection point must be low enough to obtain the required underbalanced conditions. There may also be some modifications required to the wellhead for the installation of the tie back string and the gas injection system. Drilling a larger hole to accommodate the system and the well control issues associated with the annular injection system must also be considered.
`
Reservoir
Liquid
Gas
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 52 of 154
Parasite string injection
The use of a small parasite string strapped to the outside of the casing for gas injection is really only used in vertical wells. For redundancy reasons, two 1” or 2” coiled tubing strings are normally strapped to the casing string above the reservoir as the casing is run in. Gas is pumped down the parasite string and injected onto the drilling annulus. The installation of a production casing string and the running of the two parasite strings makes this a complicated operation. Wellhead modification is normally required to provide surface connections to the parasite strings. This system is not recommended for deviated wells as the parasite string is easily ripped off with the casing on the low side of the hole. However, the principles of operation and the advantages of the system remain the same as with annular injection.
Fig 22 Parasite string Injection
`
Reservoir
Liquid
Gas
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 53 of 154
Gases for Underbalanced Drilling Several options for gas exist when drilling underbalanced. These are:
• Air • Natural Gas • Liquid Nitrogen • Generated Nitrogen • Exhaust Gas
Air Although air is not the most obvious choice in a hydrocarbon environment, air and foam can still be used providing that foam stability is ensured and that defoaming does not cause explosive mixtures. It must also be stated that outside of drilling in hard rock formations and dry gas formations, drilling operations using air in combination with liquids have been fraught with significant corrosion and oxidation problems in addition to well documented instances of downhole fires and explosions.
Natural Gas If a source of high-pressure natural gas at the correct volumes is available, drilling with natural gas is a very good option. The use of air hammers with gas drilling is another option that can be used to increase ROP. This is an option used in tight gas reservoirs. A flow regulator and a pressure regulator are normally used to control the amount of gas injected during the drilling process. Natural gas is also non-toxic and non-corrosive if sweetened correctly. Natural gas has greater solubility in hydrocarbons when compared to nitrogen, which may result in the potential for greater disengagement problems and asphaltene precipitation. The gas produced from the system can sometimes be re-routed to the compression system and be re-used, thus, virtually eliminating the need to flare the gas. The most efficient use of natural gas is normally through annular injection. The use of natural gas through the drillstring is not recommended, as gas will have to be vented every time a connection needs to be made although this can be done safely. The use of natural gas injection through a coiled tubing system is also not recommended, as a pinhole in the coil could not be isolated and gas maybe released to form an explosive mixture inside the wraps of the coiled tubing reel.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 54 of 154
Cryogenic Nitrogen Nitrogen is by far the most common gas that is currently being used to lighten the circulating fluid column in underbalanced drilling operations. Nitrogen is a colorless, odorless and tasteless gas that makes up four fifths of the earths atmosphere. Nitrogen was discovered in 1772 by the Swedish druggist Carl Wilhelm Scheele and Scottish botanist Daniel Rutherford. Nitrogen is non-toxic, non-flammable and non-corrosive. It has very low solubility in water and hydrocarbons, and is compatible with virtually any fluid used in drilling operations. Nitrogen does not tend to form hydrate complexes or emulsions. Nitrogen forms a major part of our atmosphere in the fact that the atmosphere comprises of: 78.03 % Nitrogen 20.93 % Oxygen 0.93 % Argon 0.11 % Other gases Nitrogen used in well operations is normally delivered to the rig in liquid format. This type of nitrogen is also known as cryogenic nitrogen. It is produced by extraction from the air through fractional distillation. In this process the air is liquefied and the liquid is then separated though the following factors. Liquid air boils at -317°F Liquid nitrogen boils at -320°F Liquid oxygen boils at -297°F. Oxygen starts to evaporate leaving Nitrogen rich liquid. By repeating the boiling and condensing processes high purity of liquid nitrogen up to 99.98 % can be obtained. Only within recent years have materials and equipment been developed to handle very cold liquids like nitrogen on a commercial scale. The field of science that deals with the technology of handling liquids colder than -187°F is called cryogenics. All the liquids and equipment to handle these cold liquids are labeled cryogenic liquids and cryogenic equipment. Special steels and aluminum are the most widely used cryogenic construction materials.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 55 of 154
Chemical symbol………………………………….. N2 Molecular Weight……………..…………………... 28.016 Normal boiling point………………………………. - 320.45°F Critical pressure…………………………………… 492.3 psi Critical temperature……………………….……… - 232.87°F Triple point…………………………………………. - 345.9°F @ 1.82 psi 1 gallon of Liquid N2……………………………… 93.12 scft of gas Latent Heat of Evaporation………………………. 85.67 BTU/lb Specific Heat (cp) @ 77ºF……………………….. 0.4471 BTU/(lb)(ºF) Specific Heat (Cv) @ 70ºF………………………. 0.3197 BTU/(lb)(ºF) Ratio of Specific Heat……………………………. 1.401 Thermal Conductivity @ 60ºF…………………… 0.01462 BTU/sq ft hr Density of saturated vapour…………………...… 0.03635 lb/cu ft Specific gravity of vapour at 14.7 psia (air = 1).. 0.967 Density of liquid nitrogen at boiling point…….… 50.443 lb/cu ft In underbalanced operations in the field, Nitrogen is mostly extracted through the process of separation of nitrogen molecules and air molecules through a membrane system. This is also known as nitrogen generation or membrane technology. Conversion data for Nitrogen
Nitrogen is more expensive than straight air, but as discussed in other sections, the use of pure air is not recommended for underbalanced drilling. The cost of nitrogen is mainly driven by the fuel costs to generate nitrogen and by the equipment rental costs of the generation system. Cryogenic nitrogen is normally transported to locations in vacuum jacket tanks made out of stainless steel and tested to 50 psi. The volume of an offshore nitrogen tank is normally 2000 gallons. Commercial trucks will carry as much as 7000 gallons and rail cars will hold as much as 12,900 gallons of liquid nitrogen. Nitrogen tanks are provided with pressure relief valves to release nitrogen gas as the pressure builds up in the tank due to gas expansion by heat. As the pressure is released, the remaining liquid is cooled. Liquid nitrogen in storage loses gas continuously. This can be a significant issue in desert or tropical locations when Nitrogen has to be transported over long distances.
Nitrogen conversion data
lb scft of gas liquid (gal) liquid (cu ft) liquid (L)1 lb 1.000000 13.800000 0.148300 0.019820 0.5613001 scft of gas 0.072400 1.000000 0.010750 0.001436 0.0406801 gal of liquid 6.743000 93.050000 1.000000 0.133700 3.7850001 cu ft of liquid 50.450000 696.100000 7.481000 1.000000 28.3200001 Liter of liquid 2.782000 24.580000 0.264200 0.035310 1.000000
Standard conditions for nitrogen are 14.7 psia and 60 deg F
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 56 of 154
Cryogenic nitrogen in 2000-gallon transport tanks provides high quality nitrogen and utilizes equipment that is generally less expensive. Liquid nitrogen is passed through the nitrogen converter, where the fluid is pumped under pressure prior to being converted to gas. The gas is then injected into the string. Generally, the requirement is for the nitrogen converter and a work tank, with additional tanks being provided as necessary. For operations in excess of 48 hrs, the requirement for liquid nitrogen could be quite large, and this can result in logistical difficulties.
Fig 23 Cryogenic Nitrogen and N2 Converter In Use For UBD Operation The use of cryogenic nitrogen offshore is sometimes not recommended; this would depend on the application. Pumping 1500 scft/min of nitrogen for a 24hr-drilling period requires 15 tanks of 2000 gal each. Moving this on and off an offshore platform is a significant task and therefore could present some serious safety implications. If drilling is ongoing at this rate for several days, then two dedicated supply boats would be required to maintain supply. In order to move away from tank transport for large nitrogen dependant drilling operations, the use of nitrogen generators is recommended.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 57 of 154
Membrane Nitrogen In 1995 a US patent was issued for a process to use membrane gas separation technology to drill oil and gas wells with nitrogen produced onsite to replace higher cost cryogenically produced nitrogen as an alternative gas source for underbalanced drilling. The system uses banks of modules to separate nitrogen from the atmosphere. Nitrogen gas is generated by introducing compressed air into hollow membrane fibers, which preferentially separate oxygen and other rich gases from the air leaving high purity nitrogen at around 95%. The remaining 5% is normally oxygen.
The separation of nitrogen and oxygen is dependent on the concentration and quality of the individual fibers, and is directly related to inlet pressure and flow rate across the membrane; it is also inversely related to individual gas component partial pressures.
Fig 24 Membrane Technology for Nitrogen Production Theoretically, only the nitrogen will flow the entire length of the hollow tube membrane system to exit as the product stream with the oxygen rich permeate stream and water vapour being vented before reaching the exit point. One of the important issues associated with nitrogen generation is the purity of the nitrogen. Depending on the amount and pressure of nitrogen required, the purity will vary. At 95% purity, 5% oxygen will be delivered. A percentage oxygen probe device is included in modern systems to ensure shutdown of the flow of oxygen if it exceeds hazardous limits, which is likely greater than 8% oxygen content.
Fig 25 N2 Membrane In normal operation during underbalanced drilling, the oxygen content is limited to 5%, although this is not enough oxygen to cause explosion levels, it is sufficient oxygen to cause significant corrosion problems. The corrosion is made worse when salt brine systems are used at elevated temperatures. In many underbalanced operations using generated nitrogen, a corrosion program will have to be implemented to combat the effects of oxygen in the drilling system.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 58 of 154
Exhaust Gas A potentially very attractive source of gas is the waste gas stream from self-contained propane units or diesel fired rig engines themselves. However, when using diesel fired engines, the combustion process is relatively inefficient and the flue gas can contain 10 - 15% oxygen plus corrosive gases such as CO2 and NO2 which may react adversely with produced hydrocarbons, thus accelerating the corrosion process. The exhaust gas from a diesel engine is usually composed of approximately 83% nitrogen, 10% carbon dioxide, 3% oxygen, 2% carbon monoxide and 2% other gases. To date, there are no recorded cases of underbalanced drilling operations using diesel generated exhaust gas. Propane fired exhaust gas systems is the focus of the new exhaust gas system and one unit has gone through field trials in a major oilfield in the Western Canadian Basin after two years of research and development. The original diesel exhaust gas system has its limitations due to its ineffective combustion process. Propane fired engines, when well tuned, burn much cleaner and this results in much less oxygen (often less than 2%) in the effluent gas. But the availability and transportation issues associated with propane gas in remote locations have left the exhaust gas technology in the experimental stage.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 59 of 154
Flow Modeling Multiphase flow calculations are unlike any other hydraulics calculations. Multiphase flow is probably the most complex fluid engineering known in the industry. Multiphase, or compressible fluids, change considerably with pressures and temperatures, and
the large number of assumptions that are used knowingly or unknowingly in the various models are poorly understood by most drilling engineers. The use of computer models has in many cases led to drilling engineers working a multiphase flow model without truly understanding what happens inside the model. The result of this poor understanding is that programmes for modeling specific hydraulics are being widely used for modeling of two phase fluids or even for modeling of pure gas drilling operations.
Pressure calculations The pressure drop in any tubing or conduit for any fluid is a function of three components:
1) Static pressure 2) Friction pressure 3) Acceleration pressure
Conventional Single Phase Fluid Models How do the three components of static, friction and acceleration pressure work in conventional single-phase hydraulics programmes? Static Pressure In a conventional hydraulics model, the static pressure is directly related to the fluid density. Friction Pressure In a conventional hydraulics program, the friction pressure calculation is a 4-step process.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 60 of 154
Step 1 Determine fluid type. This is normally based on one of the following 6
types: Newtonian Fluid Bingham Plastic model Power Law model Herschel-Bulkley Robertson-Stiff Casson
Step 2
Determine the Reynolds number.
Step 3
Determine flow regime turbulent or laminar.
Step 4
Based on the flow regime determine the pressure loss.
Acceleration pressure
Since there is no expansion (or very little expansion) of the fluid between the bottom of a well and the surface, this factor is normally ignored in conventional hydraulics software programmes.
Surface Pressure In conventional drilling the BOP’s are open and no surface pressure is applied to the system.
As can be seen in conventional hydraulics models, the calculation of the system pressure losses is normally a very simple and straightforward process. In single-phase hydraulic models, a spreadsheet can easily be built to determine the pressure losses in a well.
Fig 26 Conventional Hydraulics Flow Modeling
ConventionalHydraulics
Determine fluid type
Newtonian
Bingham
Power law
Herschel Bulkley
Robertson Stiff
CassonCalculate Velocity
Calculate Shear Rate
Calculate ReynoldsNumber
Calculate criticalvelocity
Determine flow regime
Calculate Pressuredrop
END
Start
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 61 of 154
Multiphase hydraulics models In multiphase models the whole system is more complex as there are many more factors to take into account.
Fig 27 Multiphase Hydraulics Flow Modeling
Multiphase flow models
Input data
Calculate Specific gravity ofoil
Tubing Diameter
Reservoir Pressure
Reservoir Temperature
Gas gravity
Water Density
Oil Gravity in Deg API
Oil Flow Rate
Water Flow rate
Assumptions
Gas Viscosity Constant at 0.018cpSurface Tension of oil constant at 30 dynes/cmSurface Tension of water constant at 70 dynes/cm
Gas Liquid ratio
Calculate Mass of 1 bblsstock tank liquid
Calculate Oil FVF
Calculate Water oil Ratio
Calculate Solution Gas Oilratio
Calculate Density of LiquidPhase
Calculate Gascompressibility factor
Calculate Density of gasphase
Calculate oil Viscosity andwater viscosity
Calculate Liquid mixtureviscosity
Calculate Liquid surfacetension
Calculate Liquid viscositynumber
Calculate superficial liquidvelocity
Calculate superficial gasvelocity
Calculate gas velocitynumber
Check flow Regime
Bubble Flow Other Flow
Calculate hold up correlatingfunction
Determine secondary hold upfactor
Calculate liquid hold up
Calculate two phase reynoldsnumber
Determine Moody frictionfactor
Calculate average mixturedensity
Calculate mixture velocity
Calculate Pressure Drop
Determine flow regime
Bubble Slug Transition Mist
Calculate slip velocityor bubble rise velocity
Determine Moody frictionfactor
Calculate average mixturedensity
Calculate mixture velocity
Calculate Pressure Drop
Depending on flow pattern the following calculationsare carried out
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 62 of 154
Flow Regimes In order to predict friction factors and liquid hold up, the flow regime in the annulus must be known. In overbalanced drilling operations, only laminar or turbulent flow is considered. In underbalanced drilling, many more variations need to be considered. The flow regime varies with the inclination of the well and, again, a number of methods and correlations are known to predict flow regimes. Flow regimes are generally broken down into two main areas:
Fig 28 Flow Patterns For Vertical and Horizontal Multiphase Flow
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 63 of 154
The complexity of multiphase flow modeling calculation is best presented with the diagram in the figure below:
Fig 29 Variables in Multiphase Flow Calculations The number of variables – fluids (gas and liquid), density, viscosity, compressibility, cuttings density, cuttings shape (or roundness), fluid composition, etc and variable interaction makes multiphase flow calculations a difficult undertaking. These variables are calculated over every iteration element of the well model. It is understandable that this has to be done with a computer because of the resource and time requirement to undertake such complex calculations.
Multiphase components Drill String Injection
Oil Phase ofDrilling fluid
Water Phase ofDrilling fluid
Oil Phase ofReservoir fluidWater Phase ofReservoir fluid
Free HydrocarbonGas
DissolvedHydrocarbon Gas
Nitrogen
Air
Cuttings
Dril
lstr
ing
Bit Res
ervo
irA
nnul
us
Water Phase ofDrilling fluid
Oil Phase of Drillingfluid
Nitrogen
Air
Cuttings
Oil Phase ofReservoir fluid
Water Phase ofReservoir fluid
Free HydrocarbonGas
DissolvedHydrocarbon Gas
Annular Surfacepressure
Reservoir Pressure
Drill string injectionPressure
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 64 of 154
Current Multiphase Models These are some of the models that were or are for modeling multiphase flow in underbalanced drilling:
Company Name of Model Design Basis
Maurer Engineering Inc
Mudlite 2 Chevron foam model
Shell/Landmark
Flodrill Mechanistic (steady state) model
Nowsco
Circa Combination of various correlations.
Weatherford
AMFM (foam only) Chevron foam model
Petrobas
SIDHAM Unknown
Schlumberger
Sidekick (dynamic) OLGAS (blowout and well control) model designed for HPHT wells.
Neotec
Wellflo Tubing/Gaslift design Olgas
Wellflo Dynamics
Flow Model (dynamic) OLGAS blowout model in competition with sidekick
Signa Engineering
HUBS Mechanistic (steady state) model
Scandpower
Ubits Dynamic simulator based on Olgas model.
The most widely used multiphase model for underbalanced is the Neotec Wellflo 7 model. Although this is a static model, it has developed with the help from underbalanced drilling engineers over the past 10 years and is now considered as the most comprehensive model available in the industry. For dynamic simulations and training purposes, the Scandpower Ubits software is the most widely used. Circulation Design Calculations When designing an underbalanced circulation system, the bottom hole pressure must be maintained below the reservoir pressure, whilst, at the same time the surface separation system must have sufficient capacity to handle the flow rates and pressures expected while drilling. The separation system must be capable of handling sudden productivity increases from the well from fractures or flush zones and retain the ability to “choke” back production if well outflow is more than what can be safely handled by the surface separation equipment. Not only must the bottom hole pressure be controlled, but the surface separation system must also be able to work within the design parameters of the well and the reservoir.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 65 of 154
Using a surface separation system that requires 1000 psi to function properly, but designing the well with a maximum surface pressure of 250 psi, will likely cause a mis-match in the system can ultimately lead to down time during the drilling operation. The entire system from reservoir wellbore, drilling system and surface separation system must all work within the same parameters. Tuning an underbalanced system is something that requires experience, and it is something that needs to be fully understood to optimize an underbalanced operation. The design of an underbalanced drilling circulation system must take into account factors such as:
• Bottomhole pressure The bottom hole pressure must be less than the effective reservoir pressure under static and dynamic conditions to enable reservoir fluid inflow into the wellbore. This difference creates the driving force that drives well productivity.
• Reservoir inflow performance and control
The productivity of the reservoir whilst drilling underbalanced is a function not only of bottomhole pressure but also reservoir characteristics like permeability, porosity, and length of reservoir exposed to the wellbore, drainage radius and drawdown. Drawdown is one of the most important factors in controlling reservoir inflow since most of the other reservoir parameters are relatively fixed by the geology. Therefore, the bottomhole pressure must be controlled by either hydrostatic of the drilling fluid or via the choke to control reservoir inflow performance. This is an essential part of underbalanced well control.
• Cuttings transport and hole cleaning
Cuttings generated whilst drilling must be removed from the wellbore by the hydraulic action of the drilling fluid, just like in conventional drilling operations. For hole cleaning to be effective, the fluid annular velocity has to be at least twice the cuttings settling velocity. In underbalanced drilling, the gas phase is not taken into account for cuttings transport but it is generally assumed that a certain minimum liquid annular velocity is required for hole cleaning.
• Motor performance in multiphase flow environment
While drilling with multiphase fluids, it is important that the motor performance is not compromised by the hydraulics, that is, the flow rate through the motor should be sufficient to deliver the required performance and be within motor operating envelope. It must be recognized that gas, at bottom hole pressure and temperature, acts more like a fluid and as more gas is pumped, the down hole motor will see more flow. This is normally referred to as ELV (Equivalent Liquid Volume)
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 66 of 154
• Surface equipment capabilities and limitations
The productivity of the reservoir while drilling and the length of reservoir that is exposed in an underbalanced drilled well is the driving factor. The surface separation system must be designed to handle the expected inflow of fluids. An essential part of the underbalanced well control is the capacity of the surface separation system and the associated erosional velocities at surface resulting from the reservoir inflow. Surface equipment capacity must always be designed to handle the maximum expected production from the well whether instantaneous or steady state.
• Environmental considerations
Either due to governmental legislation and/or operators’ policies, underbalanced drilling operations may have to be carried out with reduced or zero emissions (without gas flaring). Where this is the case, the surface separation system has to be designed for total containment of the produced cuttings and reservoir fluids inflow – oil, gas and water. Gas recovery systems are currently under development, but the slugging and intermittent production causes significant challenges.
• Wellbore Stability
Exposing the wellbore to pressure drawdown imposes stresses on the surrounding formation. If the stresses exceed the strength of the formation, hole collapse could occur. It is therefore important that a thorough borehole stability study should be conducted in evaluating the feasibility of a reservoir as a candidate for underbalanced drilling. The exposure of overlying formations to underbalanced pressures whilst drilling also needs to be considered when reviewing underbalanced well designs. Casing strings should be placed to isolate potentially unstable formations.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 67 of 154
Flow modeling When designing an underbalanced drilled well, the following graphs should be presented to the operator.
• Annular Bottom Hole pressure versus Gas injection rate • Equivalent Motor Throughput versus gas Injection rate • Minimum hole cleaning velocity versus gas injection rate • Annular friction pressure versus gas injection rate • Annular liquid hold up versus gas injection rate • Drillstring injection pressure versus gas injection rate • Drillstring liquid hold up versus gas injection rate
These graphs will provide a complete operational picture for an underbalanced operation. All the required parameters can be selected from these graphs. There are a number of other issues that will need to be reviewed but these graphs will provide the operating window for underbalanced drilling and provide some insight into well behavior. The reservoir inflow is normally ignored in the initial design. First of all it must be ensured that an underbalanced status can be achieved in the reservoir even if there is no assistance from the reservoir.
Fig 30 Neotec Multiphase Flow Model
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 68 of 154
Annular Bottom Hole pressure versus Gas injection rate This graph provides the operating envelope for underbalanced drilling. The operating envelope is bounded by a number of curves. The annular bottom hole pressure graph is a combination chart of hydrostatic pressure versus gas injection rate.
As gas is injected into a fluid system, the hydrostatic pressure will drop as more and more gas enters the system. As the amount of gas in the system increases, the gas is compressed at the bottom of the well and the gas will expand as it rises to the surface of the well. So as more gas enters the system, the friction pressure in the well increases as shown below.
Fig 31 Hydrostatic Pressure Reduction With Gas Injection
As can be sees, the hydrostatic pressure drops as more gas is injected, but the friction pressure starts to increase as more gas enters the well and expands on its way back to the surface. If these two effects are combined into a single curve, then the very typical pressure versus gas rate curve, the so called “J: curve can be seen as shown overleaf:
Fig 32 Hydrostatic Pressure Reduction and Friction Pressure Increase With Gas Injection
Pres
sure
Gas injection rate
Hydrostatic Pressure
Pres
sure
Gas injection rate
Hydrostatic Pressure
Friction Pressure
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 69 of 154
Fig 33 Bottom Hole Pressure Decrease With Gas Injection The brown curve now shows the combined curve of hydrostatic pressure and friction pressure. In the first part of the curve, the rapid decline of pressure ca be seen with increasing amounts of gas. This part of the curve is known as the hydrostatically dominated part of the design curve. As the amount of gas increases, the friction pressure in the well will also increase as a result of the gas expansion. The flatter part of the pressure curve is known as the friction dominated part of the curve. As the gas injection rate increases further, the bottom hole pressure will start to increase as a result of the friction pressure. So contrary to popular belief in the oilfield, more gas is not always better.
Pres
sure
Gas injection rate
Hydrostatic Pressure
Friction Pressure
Hydrostatically Dominated Friction Dominated
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 70 of 154
Bottom Hole pressure stability When designing a circulation system that provides stable bottom hole pressures, the system should not only avoid pressure spikes but it should also avoid slugging. The operating envelope allows the drilling engineer to determine, for a particular gas injection rate, whether the flow is dominated by hydrostatic or frictional pressure loss. Any point on the performance curve with a negative slope is dominated by hydrostatic pressure losses. These points are inherently unstable, show large pressure changes with small changes in gas flow rate, and exhibit increasing bottom-hole pressure with decreasing gas flow rate. Operating on the hydrostatic dominated slope will mean that severe slugging is encountered while drilling. Points on the performance curve with a positive slope are dominated by frictional pressure loss. These points are inherently stable and exhibit increasing bottom-hole pressure with increasing gas flow rate. Note: "dominated by frictional pressure loss," does not necessarily imply that the frictional pressure loss is greater than the hydrostatic pressure loss. Instead, this means that the reduction in hydrostatic pressure loss associated with an increase in the gas injection rate is less than the increase in frictional pressure loss due to the increased gas rate. This information can be used in several ways. If a reduction in bottom hole pressure is required, a decrease in gas injection (the obvious answer to someone only familiar with single-phase flow) will lead to an increase in bottom-hole flowing pressure if the flow is hydrostatically dominated. Further, the cost of nitrogen (as the injection gas), if bulk liquid nitrogen is used, can be one of the most significant costs associated with UBD operations. One of the most common misconceptions in underbalanced drilling is that more nitrogen (i.e. gas) injection is better. This stems from observations of drilling operations that are hydrostatically dominated, where an increase in the gas injection rate can lead to significant decreases in the bottom-hole pressure. However, if the drilling operation is frictionally dominated, increasing the gas injection rate will not only increase the bottom-hole pressure, but may dramatically increase the cost associated with nitrogen used while drilling. Saponja recommended that underbalanced drilling be carried out in the friction-dominated part of the pressure curve. Operations conducted on the hydrostatic part of the curve often report that a cyclic bottom hole pressure occurs and that it is very hard to obtain a stable system. More gas is the answer here to move onto the friction-dominated part of the design curve. Thus, for a specific design case, the operating envelope can not only confirm the feasibility of underbalanced drilling, but also offers valuable insights into both the acceptable and optimal gas injection rates, and the influence of those rates on the
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 71 of 154
bottom hole flowing pressure. Operating envelopes should be developed for a range of design parameters. However, the operating envelope cannot tell the entire story. Each point on the operating envelope corresponds to a single wellbore calculation for a specific gas injection rate. For all such calculations, valuable additional information can be gathered by analyzing profiles of the in-situ liquid hold-up, actual gas and liquid velocities, pressure, and temperatures. At the moment, the only concern is the wellbore (bottom hole) pressure. The first graph shows the wellbore pressure versus gas injection. At a given flow rate, the wellbore pressure in the well is calculated for a given fluid system, well configuration, drill string and surface pressure. As this first graph is constructed, a number of other issues will also need to be considered.
Fig 34 Bottom Hole Pressure Decrease With Gas Injection For Single Flow Rate The first issue is, of course, the reservoir pressure. We need to establish if and underbalanced pressure can be achieved in the well bore. A target pressure will normally be established at some given value below the known reservoir pressure.
Pres
sure
Gas injection rate
Hydrostatically Dominated Friction Dominated
Flowrate 1
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 72 of 154
Fig 35 Bottom Hole Pressure Decrease With Gas Injection and Reservoir Pressure Window It can be seen that a pressure loss occurs in the well bore system that is able to achieve an underbalanced status below the reservoir pressure. The friction-dominated part of the design curve is below the reservoir pressure and this provides the first operating parameters for flow modeling.
Pres
sure
Gas injection rate
Hydrostatically Dominated Friction Dominated
Reservoir Pressure
Target Pressure
Flowrate 1
Flowrate 2
Fig 36 Bottom Hole Pressure Decrease With Multiple Gas Injection Rates and Reservoir Pressure Window This curve is normally created with 3 or 4 different flow rates, as will be seen later on in the graphs.
Pres
sure
Gas injection rate
Hydrostatically Dominated Friction Dominated
Reservoir Pressure
Target Pressure
Flowrate 1
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 73 of 154
Once four or five fluid rates are calculated, the remaining set of operating parameters can be added and the operating window can be defined. The next set of curves that are introduced in the graph are the minimum and the maximum flow rate through the down hole motor.
Fig 37 Operating Window for Bottom Hole Pressure This provides the minimum required volume through the down hole motor that is required to drive the bit, it also provides the maximum flow rate that the down hole motor can handle without being damaged. Note: The maximum motor flow rate may be of the maximum gas injection rate. It is not always possible to have the motor limits on the same graph.
Pres
sure
Gas injection rate
Hydrostatically Dominated Friction Dominated
Reservoir Pressure
Target Pressure
Flowrate 1
Flowrate 2
Minimum MotorFlow rate
Maximum MotorFlow rate
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 74 of 154
The last information on this curve is the minimum liquid velocity for hole cleaning. Again, it is sometimes impossible to show this on the design graph as the annular velocity maybe high enough without the gas injection.
Fig 38 Complete Operating Window for Bottom Hole Pressure
Fig 39 Real Projection of an Underbalanced Operating Window.
Pres
sure
Gas injection rate
Hydrostatically Dominated Friction Dominated
Reservoir Pressure
Target Pressure
Flowrate 1
Flowrate 2
Minimum MotorFlow rate
Maximum MotorFlow rate
Minimum HoleCleaning Flow
rate
Bottom Hole Pressure vs Gas Injection rate
0
500
1000
1500
2000
2500
3000
3500
4000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas rate (scft/min)
Bot
tom
hol
e pr
essu
re (p
si)
flowrate = 100 gpm
flowrate = 150 gpm
flowrate = 200 gpm
flowrate = 250 gpm
Reservoir Pressure
Target Pressure
Max Motor ELV
No Reservoir inflow
choke pressure = 150 psi
Well name : Test Well
Fluid System Waterbased + NitrogenBit Depth 16000 ft
Reservoir Formation Sandstone
Client : Oil Company
Operating Window
Single NPU 1500Second NPU 1500
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 75 of 154
Equivalent Motor Throughput versus Gas Injection Rate To ensure that a sufficient flow rate is provided through the down hole motor, the equivalent liquid rate through the motor is calculated. This graph is also important in ensuring that the motor is not working past its design envelope. The maximum and minimum parameters must be obtained from the motor provider for the motor that is being used.
Fig 40 Equivalent Flow Rate Through the Down Hole Motor The formula associated with this graph is displayed as follows:
gas rate (scft/min)Equivalent motor flow rate (gpm) = x 42 x Liquid rate (gpm)198.6 x bottom hole pressure (psi)
460 + Temperature (deg F)
⎧ ⎫⎪ ⎪⎪ ⎪⎨ ⎬⎛ ⎞⎪ ⎪⎜ ⎟⎪ ⎪⎝ ⎠⎩ ⎭
Bottom hole pressure and temperature, as well as liquid and gas rates through the motor, are taken into account. The bottom hole pressure will vary with the gas rate, as will the motor throughput.
Equivalent Motor Throughput
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas Injection Rate (scft/min)
Equi
vale
nt M
otor
Thr
ough
put (
gpm
)
flowrate = 100 gpmflowrate = 150 gpmflowrate = 200 gpmflowrate = 250 gpmMinimum Flow RateMaximum Flow Rate
Well name : Test Well Client : Oil Company
choke pressure = 150 psi
No Reservoir inflowFluid System Waterbased + NitrogenBit Depth 16000 ft
Reservoir Formation Sandstone
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 76 of 154
Hole cleaning Hole cleaning whilst drilling underbalanced must be closely monitored. There is a reduced fluid rheology (a very thin, non-solids suspending fluid, turbulent two-phase flow) and normally, an increased ROP. A positive result of two-phase flow will be acceleration of fluid and cuttings’ transport velocities (due to gas expansion) as the fluid moves upwards from the bit. The main areas of concern for hole cleaning are where the hole angle is from 45° to 50°, and the region immediately behind the bit. The area immediately behind the bit can become the critical hole cleaning area, as there is limited reservoir inflow here. Liquid phase velocity and hole cleaning in this area is only a function of the fluid(s) and rate(s) being pumped or injected down the drill string. Two-phase hole cleaning is largely dependent on the same criteria as for single phase. Hole cleaning efficiency and solids transport are primarily controlled by liquid phase velocities and solids concentration. Studies and field experience have shown removal of cuttings is more efficient with two-phase fluid. The addition of a gas medium generates a turbulent flow regime, which minimizes solids bed formation. Liquid velocity is the critical parameter controlling the system’s ability to transport solids. From past experience, it has been concluded that a minimum liquid phase annular velocity of 180 to 250 feet per minute is required in a wellbore with a deviation greater than 10°.
Fig 41 Hole Cleaning Velocity Continuous observation of returned drilled solids, including cuttings size and size distribution, is undertaken at the shakers to confirm hole cleaning efficiency and to determine if modifications to the circulation system are required.
Minimum Hole Cleaning Velocity
0
20
40
60
80
100
120
140
160
180
200
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas Injection rate (scft/min)
Ann
ular
Vel
ocity
Ope
n H
ole
(ft/m
in)
flowrate = 100 gpmflowrate = 150 gpmflowrate = 200 gpmflowrate = 250 gpmMinimum Hole Cleaning Velocity
Well name : Test Well Client : Oil Company
choke pressure = 150 psi
No Reservoir inflowFluid System Waterbased + NitrogenBit Depth 16000 ftReservoir Formation Sandstone
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 77 of 154
For a two-phase circulation system, rate of penetration is limited by the percentage of solids in the liquid phase. If the solids concentration is too high, overbalanced BHP spikes, hole cleaning problems and increased surface pipework erosion can result. The percentage of solids in the liquid phase has been assessed during actual underbalanced drilling operations and should not exceed 2.5% to 4.0% The formula for calculating this can be displayed as follows:
20.025 x flowrate (gpm) x 0.1337Max penetration rate (ft/hr) = x 60
Bit size (inch) x 12 4
π⎛ ⎞⎜ ⎟⎝ ⎠
The design curve associated with hole cleaning is the minimum liquid annular velocity versus gas injection rate. This number is associated with the largest annular diameter. Although much debate is still ongoing with the hole cleaning issues in multiphase flow, the liquid annular velocity is assumed to be a good parameter.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 78 of 154
Annular Friction Pressure versus Gas Injection Rate The annular friction pressure will provide an indication about the pressure losses seen in the annulus as a result of the flow rates. A high annular pressure loss is normally the result of small annular diameters. If the annular pressure loss is high, then this must be taken into account when making connections. Shutting down the gas and liquid flow rates will result in a rapid decrease of bottom hole pressure, and this will result in a high reservoir influx. This will be circulated out once drilling resumes and may cause the system to slug or be unstable until this influx is circulated out of the hole. A low annular pressure loss may result in the well going overbalanced sooner if appropriate measures are not taken.
Fig 42 Annular Friction Pressure Versus Gas Injection Rate
Friction pressure vs Gas injection rate
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas Injection rate (scft/min)
Fric
tion
Pres
sure
(psi
)
flowrate = 100 gpmflowrate = 150 gpmflowrate = 200 gpmflowrate = 250 gpm
Well name : Test Well Client : Oil Company
choke pressure = 150 psi
No Reservoir inflowFluid System Waterbased + NitrogenBit Depth 16000 ft
Reservoir Formation Sandstone
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 79 of 154
Annular Liquid Hold Up versus Gas Injection Rate The annular liquid hold up graph is created to understand what happens in the well once circulation is stopped for tripping or connections. Shutting down the circulation of gas and liquids will result in the gas and fluid separating down the hole. Knowing the average percentage of gas and liquid in the annulus allows us to calculate the top of the fluid level, the total amount of liquid in the well and the resulting bottom hole pressure.
Fig 43 Gas Percentage in the Annulus versus Gas Injection Rate The graph shows the percentage of gas by volume in the annulus versus the gas injection rate.
Annular Liquid Hold Up / Quality
0%
10%
20%
30%
40%
50%
60%
70%
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas Injection Rate (scft/min)
Liqu
id H
old
Up
(%)
flowrate = 100 gpmflowrate = 150 gpmflowrate = 200 gpmflowrate = 250 gpm
Well name : Test Well Client : Oil Company
choke pressure = 150 psi
No Reservoir inflowFluid System Waterbased + NitrogenBit Depth 16000 ft
Reservoir Formation Sandstone
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 80 of 154
Drillstring Injection Pressure versus Gas Injection Rate If drillstring gas injection is used, then the drillstring injection pressure must be estimated to ensure that sufficient pump pressure capacity is provided to inject gas and liquids into the drillstring when circulating. Although the standpipe pressure is not directly used for any of the underbalanced parameters, the injection pressure will still provide indications when drilling. If, as in the graph shown below, the injection pressure is too high for the nitrogen system and the rig pumps, the drillstring design or the MWD and motors may have to be reviewed.
Fig 44 Injection Pressure versus Gas Injection Rate
Injection Pressure vs Gas Injection
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas Injection rate (scft/min)
Inje
ctio
n Pr
essu
re (p
si)
flowrate = 100 gpmflowrate = 150 gpmflowrate = 200 gpmflowrate = 250 gpmMaximum Injection pressure (psi)
Well name : Test Well Client : Oil Company
choke pressure = 150 psi
No Reservoir inflowFluid System Waterbased + Nitrogen
Bit Depth 16000 ft
Reservoir Formation Sandstone
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 81 of 154
Drillstring Liquid Hold Up versus Gas Injection Rate As in the annular hold up graph, this graph is intended to provide an indication of where fluid levels are once circulation is stopped. In combination with the annular liquid hold up, it allows calculation of the total amount of fluid in the well.
Fig 45 Gas Percentage in Drillstring versus Gas Injection Rate
Injection Liquid Hold Up / Quality
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Gas Injection rate (scft/min)
Liqu
id H
old
up %
flowrate = 100 gpmflowrate = 150 gpmflowrate = 200 gpmflowrate = 250 gpm
Well name : Test Well Client : Oil Company
choke pressure = 150 psi
No Reservoir inflowFluid System Waterbased + NitrogenBit Depth 16000 ft
Reservoir Formation Sandstone
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 82 of 154
Reservoir Inflow In underbalanced drilling, as soon as the bit penetrates the reservoir, reservoir fluids will start to flow into the wellbore. Once this happens, the stabilized multiphase flow regime in the well prior to reservoir fluid entry must be adjusted to account for inflow without upsetting the circulating system or moving out of the underbalanced window already established. The rate of reservoir fluid inflow depends, in part, on the drawdown and reservoir rock properties (the differential pressure between circulating bottomhole pressure and reservoir pressure). There are a number of models that can be used to estimate the reservoir fluid inflow based on the rock and fluid parameters. However, the reservoir rock properties are fixed and the only variable that is adjustable is the drawdown (bottom hole pressure) to control reservoir fluid inflow. As previously defined, the inflow performance of a well represents the ability of the reservoir to produce fluids under a given condition of drawdown. The reservoir fluid inflow performance is one of the most important parameters in underbalanced operations because of its impact on well production and safety. The unexpected events of flush production (after drilling into a fracture) can have a significant impact on drilling operations. It must not be assumed that the well flow will be stable when drilling underbalanced.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 83 of 154
Drillstring and Down Hole Tool Design When looking at drillstring and down hole tool design, a few issues need to be addressed for drilling underbalanced.
Pressure While Drilling (PWD) Pressure while drilling sensors have proved invaluable in every underbalanced drilling operation to date where they have been included in the drillstring and operated without downtime. However, quite a number of these sensors have proved problematic because of the vibration problems and fast drilling rates encountered with underbalanced drilling. Adding a down hole gauge or sensor in the drillstring will definitely enhance the underbalanced drilling operation and help the team optimize the drilling process and increase the knowledge of the reservoir.
Conventional MWD Tools in Underbalanced Drilling The most common technique for transmitting MWD data uses the drilling fluid pumped down through the drill string as a transmission medium. Mud-pulse telemetry transmits data to the surface by modifying the flow of mud in the drill-pipe in such a way that there will be changes in fluid pressure at surface. It involves the sequential operation of a down hole mechanism to selectively vary or modulate the dynamic flowing pressure in the drillstring and thereby sends the real-time data gathered by the down hole sensors. This variation in the dynamic pressure is detected at the surface where it is demodulated back into the real measurements and parameters from the down-hole sensors. Signal strength at the surface depends on many factors including the mud properties, drill-string arrangement, flow rate, signal strength generated at the tool, telemetry frequency, and many others. When drillstring gas injection is selected for underbalanced drilling, these small pressure pulses have to be transmitted in a compressible fluid medium. Transmission of pressure pulses in a compressible fluid environment is difficult and experience to date indicates that mud-pulse telemetry systems are best applied to scenarios with a maximum gas percentage of 20% (by volume at the standpipe). This ratio can be extended somewhat depending on well depth, profile, liquid-phase fluid, drill-string/bottom hole assembly, pumping pressure and flow rates. But for drillstring gas injection, MWD pressure pulse technology is problematic. A solution for this is the use of electromagnetic telemetry for MWD and PWD tools.
Electromagnetic Measurement While Drilling (EMWD) The history of annular pressure measurements extends as far back as the mid 1980’s when Gearhart Industries, Inc. provided annular pressure sensors on their measurements-while-drilling (MWD) tools. Since then, Anadrill and other service companies have developed sensors for down hole annular pressure measurements while drilling.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 84 of 154
Electromagnetic telemetry transmits data to the surface by pulsing low-frequency waves through the earth. There are essentially two ways of doing this: one that induces an axially symmetric electric field around the drill-pipe, and a second that
Fig 46 Electromagnetic MWD Tools drives current directly from one part of the drill-pipe to another. The former is referred to as “Imag” and the latter as “Emag”. Imag transmission is typically used for short-hop systems, e.g. across a motor. It has an advantage in that its transmission is essentially independent of mud properties and layering within the rock formation. Signals are generated by wrapping solenoid coils around the drillpipe to create a magnetic dipole. The contrast in magnetic properties of metal versus rock is only about 100 to 1, but the dipole efficiency can be increased somewhat by adding ferrite cores to the coils. Emag transmission is typically used to send data over longer distances. Signals are generated from a voltage difference on the drill-collar, which is either induced from toroidal coils wrapped around the collar or created directly by adding an insulating “gap” to the drillpipe. This creates an electric dipole with one long end (to the surface) and one “short” end (to the bit). The metal drillpipe acts as a long focusing antenna because of the large conductivity contrast between it and the rock (10,000,000 to 1).
Transmitter Surface Antenna
Bi-directional Transmission
TransmittingAntenna Bit
Induced Currents
Transmitter Surface Antenna
Bi-directional Transmission
TransmittingAntenna Bit
Induced Currents
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 85 of 154
Emag has a disadvantage that contrasts in rock properties and in particular very high resistivity formations such as evaporates will strongly attenuate the signal. Emag signals on land can be received by measuring the voltage difference between stakes on the surface. For offshore applications, Emag signals can in theory be detected by measuring voltage differences on the seabed or the current flow returning to the riser, but in practice there remain many challenges to running Emag operations offshore. Both Imag and Emag are subject to an increasing attenuation as the frequency increases. Shales and low conductivity water sands are particularly attenuative. Pressure while drilling tools or PWD tools have enhanced the underbalanced process significantly. Certainly on the initial trips in an underbalanced well, the PWD sensor should be used. Once reservoir inflow starts, the PWD sensor will provide valuable information about the PI of the reservoir.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 86 of 154
Non Return Valves Non return valves are necessary for underbalanced drilling to prevent influx of reservoir fluids up inside the drillstring either when tripping or making connections. It must be recognized that there is pressure below non-return valves. The positions of the float valve in the drillstring depend on the tools in the BHA and the policy of the operating philosophy underpinning the safety management of the operation. The number of float valves in the BHA and the drillstring is also a matter of
company policy consistent with perceived risks and management thereof. If the drilling float valve(s) should all fail, the well may have to be circulated to kill weight fluid and a string trip undertaken to replace or repair the float valves. It is good practice to install a float valve in the top of the drillstring, when using drillstring gas injection, often referred to as the string float valve because it aids operational efficiency by reducing the time it takes to bleed off the pressure before making connections. This top valve is often a wireline retrievable float valve that can be retrieved, as access through the string is required. In general, a double float valve is installed just above the BHA and a further double float valve is installed above the bit so that there is redundant service. Two types of non-ported drill string floats that are commonly used are the flapper and plunger floats. The flapper valve uses a spring loaded flapper valve and allows passage of balls for coring or shut off tools. Wireline cannot be run through flapper valves since the flapper closes once the tool is passed and the tool cannot be pulled.
Fig 47 Well Control U-Tube
Drill Pipe Annulus
Reservoir
Bit
Floats
Bottom Hole Pressure
Standpipe Pressure
Annulus Pressure
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 87 of 154
Size 3F Model "G" Flapper Type floats
The Baker Model "G" Drill Pipe Float Valves are made of normalized, quenched, and tempered alloy steel to resist wear and erosion. The flapper valve opens fully during circulation, providing an unrestricted bore through the valve, thereby effectively prolonging the life of the valve and drill collar, as there is no diversion of fluid against the drill collar ID. When circulation stops, the flapper closes instantly.
Fig 48 Model G Flapper Valve Size 3F Model "F" Plunger Type floats
The Baker Model "F" Drill Pipe Float Valve provides a positive and instantaneous shut-off against high or low pressure, assuring continuous control of fluid flow during drilling. For normal drilling operations, the durable Model "F" is the most economical choice and is available in all sizes.
Fig 49 Model F Plunger Type Valve
Wireline Retrievable Float Valves Wireline retrievable float valves are normally run in the upper section of the drillstring. The purpose of the wireline retrievable valve is to allow the gas in the drillstring to be bled off rapidly and allow connections to be made. The valve prevents the entire drillstring gas volume having to be bled off every connection and the valve also adds another well control barrier to the upper part of the drillstring.
Fig 50 Wireline Retrievable Non-Return Valve The valve can be retrieved if wireline operations through the drillstring are required or if the valve has to be moved to a higher position in the string. The valves are positioned in a locking profile sub that is part of the drillstring.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 88 of 154
Down Hole Isolation Valves The down hole deployment or down hole isolation valves have been designed to eliminate the need for snubbing operations, or the need to kill the well in order to trip the drill string during underbalanced drilling operations.
In underbalanced drilling, there are a number of options when tripping the string. The well can be allowed to flow, the well is shut in and a snubbing unit is used to trip the pipe, or the well is killed and tripping is conducted overbalanced. Killing the well is not an option if reservoir productivity improvement is the objective for underbalanced drilling. To avoid the use of a snubbing unit, two types of down hole isolation valves have been developed. The down hole valve or deployment valve is run as an integral part of the casing program, allowing full bore passage for the drill bit when in the open position. When it becomes necessary to trip the drill string, the string is tripped out until the bit is above the valve, at which time the deployment valve is closed and the annulus above the valve bled off. Now the drill string can be tripped out of the well without the use of a snubbing unit and at conventional tripping speeds, thus reducing rig time requirements and providing improved personnel safety. The drillstring can then be tripped back into the well until the bit is just above the deployment valve, at that point
Fig 51 Position of DDV in a Well
the pressures are equalized and the valve can be opened and the drillstring run in to continue drilling operations. Deployment Valves are currently available in 7”, 9-5/8” and 10-3/4” casing sizes with differential pressure ratings up to 5000 psi. Fig 52 DDV System Flapper Valve and Actuator Sleeve
FlapperActuatorMandrel
FlapperSpring
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 89 of 154
Drillstring Design Drill string design has the same purpose as casing or tubing design. The purpose of drillstring design is to provide a drillstring assembly that will perform satisfactorily under the anticipated drilling conditions. A drill string must be designed to fulfill the following functions:
• Transmit and support axial loads • Transmit and support torsional loads • Transmit hydraulic power • Provide a well control barrier (UBD)
In order to fulfill that purpose, the drill string design must:
• Keep the maximum stress at any point in the drill string less than yield strength derated by a design factor.
• Retard fatigue as much as economically practical. • Be resistant to hydrogen sulphide if H2S is expected. • Be pressure sealing and gas tight (UBD)
The ranking and importance of each of these functions is dependent upon the well design and objectives. The drillstring for underbalanced drilling can be jointed pipe as well as coiled tubing. Hole size and reservoir penetration as well as directional trajectory will determine whether coiled tubing or jointed pipe is the optimal drillstring medium. If the hole size required is larger than 6-1/8", jointed pipe may have to be used. For hole sizes of 6-1/8" or smaller coiled tubing can be considered. The size of coiled tubing currently used for underbalanced drilling operations is between 2" & 2-7/8" OD and the sizing criteria for coiled tubing includes many factors such as hydraulics, weight and tension requirements and total weight of the coil. Occasionally the ideal coiled tubing for an operation may be excluded due to such factors as crane or transport limitations or that the life of the coil may not be feasibly economical. Generally, coiled tubing has several advantages and disadvantages over jointed pipe systems. For jointed pipe systems, drill string properties and tripping under pressure will need to be considered. The installation of a rotating head or snubbing system on a platform or rig with a fixed distance between rotary table and wellhead may cause severe challenges in rig up. Several previous operations on land rigs had to be re-designed to accommodate rotating control devices and rig assist snubbing systems.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 90 of 154
Coiled Tubing versus Jointed Pipe
Coiled Tubing Jointed Pipe No connections made during drilling Connections require gas injection shut
down causing pressure peaks Higher pressure containment Pressure of Rotating Diverters limited to
5000 psi static pressure. Stiff wireline makes MWD systems simpler in gasified fluids
MWD systems unreliable in gasified systems
No snubbing system required Pressure deployment requires snubbing unit
Maximum hole size 6” No hole size limit Hole cleaning more critical Hole cleaning can be assisted by rotation Potential for pipe collapse in high pressure wells
Special drillstring connections required for gas fields
Thru tubing drilling work possible Thru tubing work requires special rig floor tools on conventional rigs
BOP stack smaller BOP stackup requires rotating diverter system.
Lower costs Higher costs as a result of rig. Limited with drag for outreach Ability to drill long horizontal sections If hole size and well trajectory permits, coiled tubing is likely to be the simplest system to drill underbalanced wells. But this technical advantage also has to be considered against the economics of having a full-size drilling rig and a coiled tubing system installed just to drill underbalanced. Underbalanced drill string design is simplified compared with traditional drill strings. Since common problems with overbalanced drilling are avoided. Drill strings are normally slick with no jars and minimal number of stabilizers. Stabilizers create issues when tripping through rotating diverters under pressure. Any drill collars that are to be run must be slick so that well control can be maintained during trips. Spiral drill collars will leak when they are being pulled through a rotating diverter.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 91 of 154
Drillpipe Conventional drillpipe can be used in underbalanced drilling operations. The connections are more important for several reasons.
Torque and drag friction factors in underbalanced drilling are often double of what they are in overbalanced drilling operations. So more torque is required to turn the pipe and this has a direct impact on the connections as well as on the maximum reach with the given surface equipment.
Not too many connections can be legitimately classified as gas tight connections in the market place today. Although many manufacturers will recommend a number of connections they will not guarantee that the connections are gas tight. In fact, only Grant Prideco XTM (eXtreme Torque Metal seal) connection is guaranteed by its manufacturer as a gas tight connection. The XTM connections are designed with radial metal-to-metal seal, which ensures the gas-tight capability of the connection. Hydril WT (wedge) connection series can be used although field experience has shown that they are not gas tight under all conditions. However, most long double-shouldered connection (e.g. DSTJ or VAM, XT and HT) will be gas tight if they are properly doped and maintained. The following tool joints are commonly used in underbalanced drilling: Hydril WT38 or WT39 and HT38 comparable NC connection. For more information see (www.hydril.com) Grant Prideco XTM39 For more information see (www.grantprideco.com) It is also important that drill pipe being used for underbalanced drilling not be plastic coated. In a gasified fluid, the plastic coating is likely to be stripped off and plug the string. The new abrasion-resistant, liquid-applied, modified epoxy-phenolic or the ceramic particle loaded epoxy resin coated systems for drillpipe can be used for underbalanced drilling operations.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 92 of 154
Hard banding Any hard banding on the drillpipe must be reviewed carefully. Hard banding on the pipe will wear out the rotating diverter rubbers much more quickly then pipe without hard banding. If hard banding is required, then it must be as smooth as it can be. Several operators have started using Armacor™ hard banding on their drillpipe.
Drillpipe Rubbers Drillpipe protection rubbers cannot be run when drilling underbalanced. There are two reasons for this. One is that they suffer from gas impregnation when run deeper into the well and will explosively de-compress when pulling out of the hole. The second issue is running the drillpipe rubbers through the rotating diverter will cause blow-by when tripping and drilling.
Jars The use of drilling jars in underbalanced drilling with jointed pipe is not a straightforward decision. In underbalanced drilling, drilling jars can be used and they are just as effective as in overbalanced drilling. Differential sticking does not occur during underbalanced drilling. The one issue with jars that needs to be considered is tripping jars using a snubbing unit. The snubbing force required for pushing or pulling the pipe while tripping in or out of hole may be enough to set and/or trip the drilling jar on a number of occasions to an extent that compromises the operational effectiveness of the drilling jar. It may be prudent not to include drilling jar in cases where there is evidence that its inclusion may make tripping more difficult or may compromise the effectiveness of the jar. Some bottom hole assembly components can be hydraulically operated (locking/unlocking) by means of the pressure differential between drill string and annulus that occurs when circulation is stopped and started. Tools with these mechanisms are likely to be disturbed when drilling underbalanced, mainly due to the different fluid compositions in the drill string and annulus.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 93 of 154
Down Hole Motors The selection and use of down hole motors is of importance in underbalanced drilling as many UBD wells are horizontal wells or directional wells. Both positive displacement motors (PDM) and vane motors and turbines have been successfully used in underbalanced drilling operations.
The main issue with motors arises when drillstring gas injection is used and a compressible mixture drives the motor or turbine. Gas will increase the speed of the motor but will decrease the torque output from the motor. Multiphase fluids will reduce the operating window of a motor or turbine. One of the major problems with motors in compressible fluids is the ability to detect a motor stall. When
Fig 53 Turbine and PDM Differences pumping a compressible fluid during a motor stall, the pressure increase will be masked by the gas compressibility. Once the driller notices that the motor has stalled he will pick up off bottom. This often results in the gas pressure being released from the drill string and the result is that the motor will exceed its maximum flow rate and overspeed, causing damage. PDM motors are susceptible to fluids that will attack the rubber stator and cause deformation and jamming. High temperatures may also cause swelling of the stator and result in the motor jamming. Hence, both bottom hole temperature and well fluids are important in defining motor selection.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 94 of 154
Equipment Selection The equipment selection is started on the injection side of the underbalanced drilling operation and will be worked through the surface equipment via the wellhead and separation system to the
flare.
Gas Injection Equipment Gas injection equipment for underbalanced drilling comprises of various items. For air drilling operations, the same compressors and boosters are used, and for a nitrogen generation system a nitrogen generation unit is added.
Air compressors Primary air compressors used in underbalanced operations are normally after cooled direct drive, two-stage helical screw compressors. Most compressors produce a maximum air flow of 900 scft/min at 300 psi to 350 psi, with a horsepower rating of approximately 380 BHP at 1800 rpm.
The compressors are powered by a diesel engine and are skid mounted. It must be taken into account that compressors need to be de-rated in flow rate by 3% for every 1000ft of ground elevation.
Fig 54 Quincy Air Compressor
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 95 of 154
Nitrogen Generation System The nitrogen production unit is a single containerized system with compressed air into the system and nitrogen produced at the outlet. An NPU will normally produce a maximum of 1500 scft/min of nitrogen through the membrane system as described in the gas section of this manual.
Fig 55 Flow Path Through a Nitrogen Generator (NPU1500) Membrane performance is generally described as a function of the nitrogen purity, operating temperature, and operating pressure. In general, productivity increases with an increase in pressure and temperature. A Nitrogen Production Unit or NPU contains all the equipment required to properly condition the feed air supplied to the membrane modules. Typical equipment includes an air receiver, moisture separator, coalescing filter, carbon filter, and a particulate filter. Proper operation and maintenance of this filtration system will prevent oily water condensate, airborne particles, pipe scale from contaminating and/or clogging the membrane fibre openings. The membrane modules are completely encased in ASME coded cylindrical pressure vessels. Purity Assurance Most nitrogen generation units are equipped with two purity assurance valves. The product valves allow on spec nitrogen to flow into the outlet line. The product vent valve will vent off-spec nitrogen, which is too high with oxygen impurity. High and low oxygen impurity set points are entered into the processor via an electronic control system. Flow metering is provided internally in the unit through the use of an orifice meter.
Air Cooler
Air Cooler
HeaterAir
Demister
OilFilter
ParticulateFilter
CarbonBed
ParticulateFilter
Nitrogen
Oxygen
Nitrogen Generation System
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 96 of 154
A nitrogen generation system is 50% efficient. This means that if 1500 scft/min of nitrogen is required, then 3000 scft/min of air needs to be pumped into the nitrogen generation system. A full single nitrogen generation system can deliver 1500 scft/min of nitrogen and requires three or four large air compressors to deliver the required air. A booster compressor will be used on the outlet to boost the nitrogen flow up to the required standpipe injection pressure.
Fig 56 Nitrogen Generation System for 3000 scft/min of Nitrogen at 4000 psi The system as show in fig 54 has the capability of generating approximately 3000 scft/min of nitrogen at 4000 psi.
• Six 950 scft/min feed air compressors deliver 5700 scft/min of air at 350 psi. • The two Nitrogen Generators deliver 2850 scft/min of N2 at 350 psi. • The low pressure boosters raise this pressure from 350 psi to 1800 psi. • The final high pressure booster raises this pressure from 1800 psi to 4000 psi
into the standpipe. This equipment will take up significant space on a location, especially on an offshore location.
Air Compressors Low pressureBooster
Compressors
N2 Generators
Nitrogen Generation System
High pressureBooster
Compressor
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 97 of 154
Fig 57 Nitrogen Generation System for 3000 scft/min Rigged Up on Location The nitrogen volume and pressure requirements must form an essential part of the planning process for an underbalanced drilling operation. As not only equipment requirements must be known, space and diesel supply for the equipment must also be planned.
Booster Compressors Two types of boosters are normally used on an underbalanced drilling job, low pressure boosters and high pressure boosters. The low pressure boosters boost the outlet from the nitrogen generator from 165 psi to approximately 1800 psi. Low pressure Boosters The low-pressure boosters are normally composed of a two cylinder, single or two-stage, double acting, reciprocating, inter-cooled and after-cooled, 7 1/2” x 5” pressure booster. The low-pressure booster is capable of boosting with an inlet pressure of 165 PSI. The volume of nitrogen that can be boosted depends on the configuration of the booster compressor. The higher the volume, the lower the maximum pressure.
Fig 58 WB-12 Low Pressure Booster (1800psi)
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 98 of 154
The table below provides some indication of the volume and pressure relationship.
Set up Clearance Flow rate Pressure Single stage Min Clearance 3000 SCFM 650 PSIA Two-stage Min. Clearance 2150 SCFM 1400 PSIA Two-stage Max. Clearance 1550 SCFM 1850 PSIA High Pressure Boosters The high-pressure booster is normally a single cylinder, double-acting, reciprocating, after-cooled, 2.75” x 7” pressure booster. The high pressure booster needs an inlet pressure of 1400 psi and can boost up to a pressure of 4000 psia. The high-pressure
booster may be volume restricted and this will need to be confirmed with the equipment supplier.
Fig 59 WB-11 High Pressure Booster (4000 psi)
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 99 of 154
Well Control Equipment The conventional BOP stack used for drilling should not be compromised for underbalanced drilling operations. The conventional BOP stack must not be used for routine UBD operations and must not be used to control the well except in case of emergency. This ensures that the BOP remains the secondary well control system.
A rotating diverter system and flow line with ESD valves is normally installed on top of the conventional BOP to provide underbalanced well control. If required, additional rams can be added under the BOP stack to provide operational functions for underbalanced operations. The drilling kill and choke lines must be left in place to allow for conventional well kill operations to take place. It is recommended that any additional RAMS be operated through a separate KOOMEY system.
Fig 60 Typical BOP Stack Up for Underbalanced Drilling
1.00 m
Rig Floor
Skid Deck
Rotating Control Headsystem
Flow Spool
5.79 m
ESD Valve
Primary Flowline
1.24 m
Annular Preventer
Pipe Rams
Variable Rams
Secondary Flowline
Blind / Shear Rams
Working Blind Ram
Drilling spoolChoke / Kill Lines
Flow Spool
Snubbing SystemPotentially required
Typical BOP Stack Up SketchNot to scale forInformation only
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 100 of 154
Coiled Tubing Drilling
Well control using a coiled tubing drilling system is performed using a dual stripper rubber and not a rotating head. The rig up for underbalanced coiled tubing drilling will however need to take into account deployment and recovery of the coiled tubing BHA under pressure.
Fig 61 Coiled Tubing Side Door Strippers
Rotating Diverters The principle use of the rotating diverter system is to provide an effective annular seal around the drillpipe during drilling and tripping operations. The annular seal must be effective over a wide range of pressures and for a variety of equipment sizes and operational procedures. The rotating control diverter system achieves this by packing off around the drillpipe. The rotating control head system comprises of a pressure- containing housing where packer elements are supported between roller bearings and isolated by mechanical seals. There are currently two types of rotating diverters recognized: Active
The active type use external hydraulic pressure to activate the sealing mechanism, and normally active rotating control diverters increase the sealing pressure as the annular pressure increases.
Passive The passive type uses a mechanical seal with the sealing action activated by well bore pressure.
All surface BOP systems have limitations, in both the amount of pressure they can seal off and the degradation of the sealing equipment from the flow and composition of the different reservoir fluids and gases over time, regardless of the type of surface BOP control system chosen. The key to making the right choice of diverter for each particular underbalanced drilling operation is in the careful consideration and pre-planning of the possible well conditions, which are:
• The expected flow rates. • The expected pressures. • The type of pipe rotation to be conducted through the diverter system.
Retract Port
Pack Port
Upper Guides
PackingArrangement
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 101 of 154
The selection criterion for rotating diverters is mainly based on expected static and dynamic pressures.
Fig 62 Rotating Diverter Selection Chart The API does not currently recognize rotating control diverters as blowout preventers, which they were never designed for in the first place. The API does now recognize the rotating head as a diverter, but has not issued any documentation or certification on these systems although this is currently under development by the IADC/UBO committee. Currently there are four types of rotating equipment suitable for high pressure applications. These are:
• Weatherford /RTI RBOP • Shaffer PCWD • Williams 7100 • RBOP
The present rotating control diverter systems are capable of operating at 3000 psi while rotating at 200 rpm with a maximum static pressure of 5000 psi and a maximum pressure while stripping of 3000 psi. This latest generation of rotating control diverters is compatible with top drive and power swivel systems and has been found to be excellent pipe stripping tools.
3000
2000
1000
500
1500
2500
1000
2000
3000
4000
5000
500
1500
2500
3500
4500
Williams 7000
Williams 7100Sour Service
Shaffer PCWD
RBOP
Strata RFD 5000
RPM 3000Sour Service
Williams IP-1000
Static Pressure (psi)
Dyn
amic
Pre
ssur
e (p
si)
Rotating Diverter Selection Chart
Williams 8000/9000
RBOP
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 102 of 154
Passive Rotating Control Diverters The Williams diverter systems serve a wide range of surface pressures, down hole conditions, and drilling conditions. All of the Weatherford Williams systems are passive systems.
Fig 63 Various Williams Rotating Diverters
Working PressureStatic Pressure
500 psi500 psi
1000 psi1500 psi
1500 psi3000 psi
2500 psi5000 psi
Model 9000:.
IP 1000: Model 7000: Model 7100:
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 103 of 154
Active Rotating Control Diverters Weatherford supplies three active RCD systems.
Shaffer PCWD The Pressure Control While Drilling Preventer (PCWD) The pressure control while drilling system has combined the features of the spherical BOP with a hydraulic and electronic system that allows the spherical BOP to rotate while containing pressure.
The major components of the pressure control system are very similar to a conventional annular BOP. The PCWD is capable of safely shutting in on 5000 psi in the static mode and 2000 psi at 200 rpm. The unit allows stripping of tool joints while rotating and is capable of completely shutting off an empty wellbore at 50% of the rated working pressure (2500 psi).
Fig 64 Shaffer PCWD Rotating Diverters The PCWD design uses the standard 11-inch spherical annular preventer element with a piston arrangement similar to a standard annular preventer.
RPM 30007” ID
1000psi1500psi3000psi
Stripping PressureRotating PressureStatic Pressure
RBOP 2K7” ID
1000psi1500psi3000psi
RBOP 5K7” ID
2500psi3500psi5000psi
RPM 30007” ID
1000psi1500psi3000psi
RBOP 2K7” ID
1000psi1500psi3000psi
RBOP 2K7” ID
1000psi1500psi3000psi
RBOP 5K7” ID
2500psi3500psi5000psi
RBOP 5K7” ID
2500psi3500psi5000psi
RPM 30007” ID
1000psi1500psi3000psi
RPM 30007” ID
1000psi1500psi3000psi
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 104 of 154
RBOP One of the first active rotating diverter systems was known as the RBOP. This was
the first system developed to have two seals with active pressure control on the seals directly dependent on the wellbore pressure. As the main seal started to wear, the back up seal would provide the sealing mechanism until the main seal could be replaced. The cooling of the bearings was effected by an oil-cooled system.
Fig 65 Precision RBOP Rotating Diverters The following items are important when selecting a rotating diverter system:
• Design Criteria; expected flow rates, operating pressures and temperatures. • Design Standards and Specifications, Mechanical and Material. • Size of through bore. • Sealing mechanism, Active/Passive. • Proven Track records. • Test program including stripping and media of test fluids (gas or liquid). • Certification. • Experience.
Issues to considered for improving the life of the seals on a RCD are :
A Top Drive is preferred, if a Hex Kelly is used it will require to have smooth edges, Square Kelly’s can not be used with RCD’s. The drill pipe should be smooth with minimal grooves and tong marks. Inspection of the drillstring prior to underbalanced drilling is recommended. The smoother your drillstring the longer the stripper rubbers last. Identification ring grooves should be filled in or removed. Finally the BOP stack must be aligned to within 1/2” of the rotary table.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 105 of 154
Snubbing systems If tripping is to be conducted underbalanced without a down hole deployment valve, a snubbing system will have to be installed on top of the rotating control head system.
The current snubbing systems used in underbalanced drilling are called rig assist snubbing systems. These units need the rig draw works to pull and run pipe and are designed to deal only with pipe light situations. A jack with a 10ft stroke is used to push pipe into the hole or to trip pipe out of the hole. The ability to install a snubbing system below the rig floor allows the rig floor to be used in the conventional drilling way. Snubbing with an onshore rig where there is no space under the rig floor to install a snubbing unit will have to be conducted on the rig floor. In order to facilitate snubbing, so called push-pull units are installed on the rig floor.
Fig 66 Sub Rig Floor Snubbing Unit (Dolsnub 6) Push-pull machine in the down and in the up position
Fig 67 Rig Assist Push-Pull Unit (Courtesy Tesco) The stroke of this unit is 10ft and the maximum snubbing capacity of the unit is 50,000lbs of force or equivalent to snubbing 5” pipe with 6-1/4” tool joints with 1500 psi surface pressure.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 106 of 154
Separation Equipment In all underbalanced operations, the separation system that is to be used has to be tailored to the expected reservoir fluids. The separation system must be designed to handle the expected influx fluids and gasses, and it must be able to separate the drilling fluid from the return well flow in order for it to be pumped down the well once again. The surface separation system in underbalanced drilling can be readily compared with a process plant, and there are many similarities with the process industry. Fluid streams while drilling underbalanced are often described as four phase flow, as the return flow is comprised of:
1. Oil 2. Water 3. Gas 4. Solids
The challenge for the separation equipment is to effectively and efficiently separate the various phases of the return fluid stream into their individual streams whilst at the same time returning a clean fluid back to the drilling process. A number of approaches in separation technology have emerged recently:
Fig 68 Approaches to Separation The approach taken is largely dependent on the expected reservoir fluids. Normally the first approach is taken, but if erosion is expected to be a problem, the solids can be removed first.
Remove Gas Remove Solids Separate oil & Water Drilling Fluid
ReducePressure
Remove Solids Remove Gas Separate oil & Water Drilling Fluid
ReducePressure
Remove Solids Remove Gas Separate oil & Water Drilling Fluid
ReducePressure
Remove Gas Remove Solids Separate oil & Water Drilling Fluid
ReducePressure
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 107 of 154
In a lot of situations, the separator is the first process equipment that receives the return flow out of a well. Separators can be classified as:
Classification Operating Pressure Low pressure 10 to 20 psi, up to 180 to 225 psi Medium pressure 230 to 250 psi, up to 600 to 700 psi High-pressure 750 to 5000 psi
Separation of liquids and gasses is achieved by relying on the density differences between liquid, gas and solids. The rate at which gasses and solids are separated from a liquid is a function of temperature and pressure. Separators are classified as “two-phase” if they separate gas from the total liquid stream and “three phase” if they also separate the liquid stream into its crude oil and water components. In underbalanced drilling, the term “four-phase” separation is used to indicate the separation of 1)oil, 2) water, 3) gas and 4) solids. Horizontal and vertical separators can be used. Vertical separators are more effective when the returns are predominantly gas, while horizontal separators have higher and more efficient fluid handling capacities.
Horizontal separators In horizontal separators, well returns enter and are slowed by the velocity-reducing baffles.
Fig 69 Horizontal Separator Solids predominantly settle in the first compartment from where they can be removed by a solids transfer pump. Liquid passes over the partition plate into the second compartment where further solids separation takes place and the liquids begins to separate by virtue of their density difference and residence time. The liquid spills over to the third compartment where separation is completed. The water component and liquid hydrocarbon are discharged from different levels of the third compartment. The separator should be fitted with adequately sized pressure relief valves and an emergency shutdown valve, triggered on high/low liquid level, and high and/or low
Fluids In
Gas Out
Gas
Solids
FluidOil/water Mixture
Water
Oil
Oil
Oil Out
Water Out
Solids Slurry out
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 108 of 154
pressure. It should be fitted with sight glasses to indicate liquid levels and observe the solids level.
Vertical Separators
In a vertical separator the solids predominantly settle at the bottom of the vessel, from where they can be removed. The remainder of the liquids and gasses are separated by their density differences with gas at the top, oil in the middle and the water lower down on top of the solids. The water component and liquid hydrocarbon are discharged from different levels of the vessel. The advantage of vertical separators is their reduced footprint and better gas handling capabilities.
Fig 70 Vertical Separator A separator must have the following abilities:
• Remove the bulk of the liquids from the gasses. • Remove bulk of the solids from the liquid. • Separate oil from water. • Sufficient capacity to handle fluid surges of liquid from a well. • Sufficient length, or height, to allow the small droplets to settle out by gravity. • A means of reducing turbulence in the main body of the separator so that
proper settling may take place. • A mist extractor to capture entrained droplets, or those too small to settle by
gravity. • Proper backpressure and liquid-level controls.
The efficiency of a separator to remove gas from oil is dependent on physical and chemical characteristics of the crude, separator operating pressure and temperature, rate of throughput, size and configuration of the separator. The rate of throughput and liquid depth in a separator determine the “residence” or “settling” time of the liquid phase.
OilOil Out
Water
Gas Out
Mist Extractor
Vertical Separator
Fluids In
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 109 of 154
UBD Choke manifold Chokes, choke manifolds and standpipe manifolds are all important parts of any underbalanced drilling operation and play an integral role in the overall improved safety process involved in all underbalanced drilling operations. The choke manifold should be designed to handle the maximum expected volumes from the well (4-inch minimum piping) equipped with dual chokes (one hydraulic and the other manual). This redundancy allows one choke to be operating while the other is isolated and maintained. Without the proper piping and flow control at surface, the annular and injection flows integral to the system can become a hazard to the overall surface control system.
All choke manifolds involved with underbalanced operations should be designed to accommodate flow, pressure, temperature, possible erosion and corrosion from the return flow of the drilling fluids, gases and solids. The choke manifold used for underbalanced drilling will be a separate manifold from the standard drilling choke manifold. Both manifolds will remain fully independent of each other.
Fig 71 UBD Choke Manifold
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 110 of 154
Data acquisition The data acquisition used on a underbalanced system should provide as much information as possible not only to ensure that the drilling process remains within the required limits for safety and efficiency, but also to allow the maximum amount of information to be obtained from the reservoir whilst drilling. A good functioning data
system will also allow for formation analysis whilst drilling and of course increasing reservoir knowledge is one of the primary benefits of underbalanced drilling. However, the safety aspect of data acquisition should not be overlooked as well control is directly related to the pressures and flow rates seen at surface. The data acquisition system must be designed to obtain all the required data from the underbalanced drilling
Fig 72 Gas Flow Meter process. It must also provide the ability to analyze drilling and reservoir data whilst drilling underbalanced. Recording the data and providing the ability to analyze this data afterwards in combination with reservoir engineers and geologists can provide significant insight into the reservoir and the drilling process. This will also allow for optimization of the drilling process on subsequent wells. A number of standard plots are normally provided during the underbalanced process. These are: Against Time Reservoir Pressure Standpipe Pressure Bottom Hole Annulus Pressure Wellhead Pressure Liquid Rate In Nitrogen Rate In
Provides pressure and flow rate comparison.
Against Time Reservoir Pressure Bottom Hole Annulus Pressure Wellhead Pressure Liquid Rate In Liquid Rate Out Nitrogen Rate In Gas Rate Out
Provides calibration chart for flow modeling.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 111 of 154
Against Time Liquid Rate In Liquid Rate Out Nitrogen Rate In Gas Rate Out Cum. Liquid Out Cum. Gas Out
Provides reservoir inflow
Against Measured Depth Reservoir Pressure Bottom Hole Annulus Pressure Wellhead Pressure Liquid Rate Out Gas Rate Out
Provides reservoir PI data vs. measured depth
Against Vertical Depth Liquid Rate In Liquid Rate Out Nitrogen Rate In Gas Rate Out Cum. Liquid Out Cum. Gas Out
Provides reservoir PI data vs. vertical depth
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 112 of 154
Flares As hydrocarbons are produced whilst drilling underbalanced, these must be handled on the drilling location. Gas is normally flared whilst crude oil and condensate are stored and then pumped to a processing facility.
Where environmental regulations preclude flaring, gas re-compression and export injection can now be considered as a viable alternative to flaring. Flaring is either done in a flare pit or through a flare stack. The flare stack or flare pit should be equipped with an automatic ignition system and flame propagation blocks. For safety reasons a great deal of consideration should be given to the
Fig 73 Top of a Flare Stack surface equipment layout to avoid unnecessary rig crew exposure to noxious fumes, radiated heat, noise and flammable liquids. The equipment layout should also maintain adequate separation distances from the wellhead and all external sources of ignition. Separation criteria must meet applicable regulator or operator specifications. A flare stack or flare pit should always be positioned downwind of the prevailing winds in the area. Fig 74 Flaring Gas During the planning stage, a realistic maximum acceptable radiation level for personnel and equipment should be determined to allow a practical and economic flare and burning system to be installed.
To reduce the amount of possible heat radiation, heat shielding or typically water spray systems are used on offshore systems to maintain safe and operable conditions. Even with the use of water curtains as a means of preventing the spread of fire and thermal radiation, it is necessary to know the amount of thermal radiation that will be transmitted through the water curtains. For onshore systems a heat radiation survey can dictate the required height of the flare stack.
Fig 75 Clean Burner (courtesy of FG Engineering Services BV) Non visible flame flares are now also available.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 113 of 154
Well Control Strategy It is important that a distinction be made between well kill and well control in underbalanced drilling operations. In underbalanced drilling flow control is used by manipulating the bottomhole pressure and surface pressure to keep well productivity within safe and
acceptable operating limits. Well kill in this context, however, is actually displacing the well to kill weight mud and restoring overbalanced conditions. In underbalanced drilling, this is normally only done if safety of equipment or personnel is compromised when operational controls have strayed outside a pre-determined safe and acceptable operating envelope or where equipment failure requires the well to be killed to control the well. In underbalanced drilling, the well must be designed for 100% underbalanced condition. This means that the well must be able to contain a full column of reservoir fluid to surface.
Well Kill Strategy The well kill strategy adopted for underbalanced drilling is to isolate the wellbore and return the operations to conventional overbalanced operations. Well kill is done when:
• At any time where personnel safety or the installations are threatened. • During a persistent inability to maintain underbalanced flow control. • Where specialized underbalanced drilling equipment fails and the only way of
retrieving the situation is to revert to overbalanced conditions. Well killing may be necessary when:
• Drill string integrity is lost. • Non-return valves and the contingency DISV (drop in safety valve) system
fails. • The casing integrity is lost. • During a failure of the emergency/safety shutdown system. • A down hole problem exists or complex fishing operations are required.
The above lists are not necessarily exhaustive and there may be other situations that may necessitate a well kill. These should all be addressed in the HAZOP and HAZID documents completed for a UBD operation.
Well Control The inflow from the reservoir into the wellbore depends on a number of factors, such as drawdown, permeability, length of reservoir exposed to the wellbore and reservoir productivity index. During underbalanced drilling control of the reservoir is maintained by maintaining the reservoir drawdown within a predetermined limit consistent with the PI of the reservoir and the capacity of the surface separation equipment. In an underbalanced drilling operation, a Flow Control Matrix is prepared prior to the start of the underbalanced drilling phase. This is a summary of flow control actions as a function of reservoir gas inflow rate and flowing wellhead pressure.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 114 of 154
Pressures Range 1 = 50% RCD dynamic rating. Range 2, = 50% to 90% of the RCD dynamic rating. Surface Flow Rates Range 1 = 60% of the flow rate capacity or the upper erosion limit. Range 2 = 60% to 90% of the separation system flow rate capacity or the upper erosion limit (Erosional velocity is normally taken as 180 ft/min) Once a baseline trend of flow rates and pressures have been established, any change or deviation from trends in fluid returns, annular bottomhole pressure readings or standpipe pressures should be investigated with other surface data and the necessary course of action should be decided if well control procedures have to be activated. Depending on the changes observed and other information available, three possible actions are likely, and using traffic light colors makes the matrix easily understandable:
• Continue underbalanced drilling as normal green light • Per the flow control matrix, perform corresponding action • Stop drilling and shut-in well on the rig BOP
Flow rates 0 to 1250 psi 1250 to 2250 psi >2250 psi
0 to 5 MMscft/day ManagableAdjust System bottom
hole pressure Shut in on Rig BOP
5 to 10 MMscft/dayAdjust System bottom
hole pressureAdjust System bottom
hole pressure Shut in on Rig BOP
>10 MMscft/day Shut in on Rig BOP Shut in on Rig BOP Shut in on Rig BOP
Underbalanced Flow Control MatrixSurface Pressures
For Williams 7100 Rotating Head
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 115 of 154
The option to shut in the well on the rig’s BOP should only be used as a last resort where the wellhead pressure will exceed UBD surface equipment pressure rating or when choke control is incapable of restricting well productivity within the surface separation equipment’s safe operating limits. Where well productivity is higher than expected, consideration should be given firstly to reducing well productivity by decreasing the drawdown. The following well control issues should also be considered as part of the planning for any UBD project:
• Barrier philosophy • Is a snubbing system required?
Snubbing requirement determines what level of compressional load will be imposed on the wellhead during snubbing. On a platform, it is important to know if the platform can stand this additional load.
• Pressure rating of well control equipment vis-à-vis reservoir productivity and expected wellhead pressure.
• Sour service requirement – if there is a chance that sour gas might be encountered? It is wise to plan the UBD process to deal with the contingency.
• Wear inspection regime and procedures for BOP sealing elements and pipe work associated with routing of reservoir fluid and control.
• In the event of wear of the sealing element of the RBOP, how will it be replaced and what redundancy exists in the rig up to cope with the scenario?
• BOP systems operations requirement – ensure that the rams and sealing elements of the BOP are capable of gas service and can cope with the volume of gas expected during underbalanced drilling operation and for the length of time required.
• Hydrate prevention strategy. • What back up equipment are needed for well control purposes? For example,
NRV inserts and subs, replacement parts for critical well control equipment. Overall, always remember that in underbalanced drilling, FLOW CONTROL and not PRESSURE CONTROL is the way of controlling reservoir fluid inflow.
Erosion Although not directly a well control issue, the potential for erosion of surface and down hole equipment must be considered when drilling underbalanced. Erosion monitoring and prediction is essential for safe operations. The management of erosion and the use of erosion monitoring systems must be considered as part of the surface and down hole design. The flow velocity limits applied by the industry to control erosion are defined in the API recommended practice RP14. One drawback, however, of these guidelines is that the amount of solids in production operations is significantly lower than in underbalanced drilling operations. Erosion in surface pipe work must be considered as part of the design process, and in high rate gas wells this can be a significant cost for a UBD operation. In general, target 'T's should be used wherever necessary and
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 116 of 154
these should include a method of quickly replacing them for inspection and change-out purposes. Erosion engineering is required to ensure that wall thickness on piping and equipment will stay within the minimum required values to prevent leaks and consequent rupture of piping. Field experience has shown that where quantities of sand/solids are expected,
erosion problems are likely to be encountered if the flowing velocity is allowed to exceed the erosional velocity. In order to avoid potential erosion problems, the production rate of a well should be limited to ensure the flow velocity is reduced to the API RP 14E recommended maximum of approximately 150 ft/sec. Wall thickness of all equipment should be checked at least yearly, and every time equipment has returned from a location where
corrosive, abrasive and/or erosive materials have been processed.
Corrosion Management One of the aims of any UBD drilling project should be to minimize or manage corrosion. This is first defined by setting goals for drill string corrosion rates, defined in millimetres of metal lost or dissolved per year (mpy) as measured by corrosion coupons. Different companies have different levels of tolerance with respect to corrosion. If a company does not have pre-set standards, the following table may be considered as a place to start.
Temperature (° C)
Velocity (m/s)
Targeted Corrosion Rate
(mpy) < 1
< 60 1 – 5 < 10 60 – 120 5 – 20 10 – 50
> 120 > 20 ~ 50 Certain information can greatly enhance the effectiveness of a corrosion management program. Corrosion mechanism identification begins by analyzing the following items:
a. Reservoir fluid type and chemistry b. Bottom hole temperature c. Bottom hole pressure d. Acid gas (H2S or CO2) concentrations e. Electrical conductivity of fluids
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 117 of 154
f. Fluid velocity A full corrosion monitoring and chemical treatment program should be conducted on all underbalanced drilled wells. The aim for a corrosion inhibition program should be to maintain a specified corrosion rate as measured by API approved corrosion drill pipe circular ring coupons. Two types of monitoring devices can be used:
• Drill pipe circular ring coupons • Electrical resistance probes.
Drill Pipe Circular Ring Coupons
A circular ring coupon should be placed in the toolbox joint of the first joint above the drill collars. Another coupon should be placed in the top drive saver sub close to the surface. The ring coupons should then be exposed for more than 40 hours; normal exposure time is about 100 hours.
Fig 76 Corrosion Coupon Ring Electrical Corrosion Monitoring Probes If electrical corrosion probes are used, two probes should be installed in the following locations prior to UBD operations commencing:
• In the stand pipe manifold after the fluid and gas have been commingled. • In the inlet of the primary separator system.
Fig 77 Electrical Corrosion Measuring System Oxygen Oxygen is the most common corrosive agent of primary significance in a corrosion monitoring and inhibition program. In the presence of moisture, oxygen causes rusting of steel, the most common form of corrosion. Since oxygen is soluble in water, the drill stem is continually exposed to potentially severe conditions. Membrane nitrogen systems produce inert gas with oxygen concentrations ranging from 3% to 8%. Although this is not as high as the 20% oxygen found in compressed air, it will always produce unacceptable corrosion rates if not properly treated.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 118 of 154
Corrosion rates can still be controlled even at 9% oxygen by adding more chemicals. Oxygen concentration is a function of membrane retention time. Higher flow rates may dictate a higher oxygen concentration. The average oxygen content of a nitrogen system should not exceed 5% if possible. If membrane nitrogen is employed, oxygen sensors should be routinely monitored by operations personnel and concentrations listed on the morning operations report. Corrosion inhibitors should always be considered, even in oil based UBD mud systems. If the decision is made to treat with corrosion inhibitors, a corrosion engineer or technician on site can monitor the performance of the chemical inhibition program.
Corrosion Inhibitor Types Corrosion control chemicals can be divided into groups according to their function. Corrosion inhibitors that are cationic produce a film on the surface of the pipe. The film disrupts the circuit of the corrosion cell through the electrolyte. Cationic inhibitors are typically called filming amines. The second group of corrosion inhibitors is called anodic inhibitors because they pacify the cathode of the corrosion cell. These inhibitors are anionic and react with the cathodic areas of the pipe to neutralize them and control corrosion. Anodic inhibitors are the most effective in underbalanced drilling systems. Because they are anionic, they are compatible with foaming agents and their performance in the presence of dissolved oxygen is excellent. Cationic filming amine corrosion inhibitors are incompatible with foaming agents and do not perform well in the presence of dissolved oxygen. This is because monatomic oxygen can penetrate the amine film on the pipe wall. The result is severe pitting corrosion on the pipe wall.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 119 of 154
Personnel Selection
Personnel selection for an underbalanced drilling operation is normally left to the service provider. Competency of underbalanced drilling personnel is important, and the use of competent and experienced operators is essential for a successful underbalanced drilling operation.
Normally the following personnel would be brought out to a location for a large underbalanced drilling operation: Dayshift Nightshift Supervision 1 UBD Supervisor 1 UBD Supervisor Engineering 1 UBD Engineer 1 UBD Engineer Separation 1 Separation Supervisor 1 Separation Supervisor 2 Separation Operators 2 Separation Operators RCD/DDV 1 DDV / RCD Operator 1 DDV / RCD Operator Data 1 DAQ Operator 1 DAQ Operator Compression 1 Compression Supervisor 1 Compression Supervisor 1 Compression Operator 1 Compression Operator 1 Nitrogen Specialist 1 Nitrogen Specialist 1 Mechanic 1 Mechanic
Total 11 11 This implies that a total of 22 people are required for a full underbalanced drilling operation. Although, UBD engineer and UBD supervisor can sometimes be shared, reducing the number of people to 20. Sometimes specialists such as corrosion engineers or snubbing crews are also required during an underbalanced drilling operation, potentially boosting the requirements on bed space. This must be considered, especially on offshore operations.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 120 of 154
Training and Competency
Training of personnel on a UBD rig site is critical to a safe operation. In a UBD operation, training becomes even more important because of the number of interdependent services and personnel involved. Training, competency and PPE requirements for the entire crew must be assessed prior to operations commencing.
A competency system must be in place by both the operator and service provider to ensure that all personnel on the well site are competent. Competence standards must describe the standards that need to be achieved in a variety of company / contractor work roles and how competency is assessed. UBD impact Typically Standards of Competence will be required for:
• Operator site supervisor • Tool pushers • Drillers • Assistant Drillers • Derrickmen • UBD Supervisor • UBD Engineers • Equipment Supervisors for both separation and compression • Well Engineering Project Coordinator • Snubbing Engineers • Snubbing Supervisor • Gas Injection Supervisor • Surface Separation Supervisor
Regardless of whether the training takes place off-site, on-site or both, training for a UBD operation may have a substantial associated cost. The training programme can be area and well specific, and to minimize costs must be fit for purpose; but training is not optional. Due diligence requires us to ensure only trained competent personnel are allowed to work on a UBD site and personnel in the process of becoming competent are properly supervised by competent staff. Although there are variations in the approach to competency-based training throughout the world, the objective is the development of a competent workforce. This requires a system that sets standards for what competencies are required for a task or role, how competent staff will be trained to develop these standards, and how competence is assessed. In line with this objective, IADC has approved the UBO Rig Pass accreditation system and Underbalanced WellCAP Curriculum, which emphasizes flow control with different equipment and procedures from conventional drilling operations. Underbalanced WellCAP is aimed at training the well-site supervisors and the intent is to ensure that conventional well control thinking and procedures do not compromise UBD well objectives.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 121 of 154
Operational Procedures Operational procedures for underbalanced drilling operations are normally subdivided into four sets of procedures:
1. Pre and Post Operational Procedures 2. Operations Procedures 3. Equipment Procedures 4. Emergency Procedures 5. Administration Procedures
Pre-operational procedures cover all the transportation, rig up, testing and commissioning procedures. Post-operational procedures cover rig down and post-job inspection procedures. Pre and post operational procedures cover:
• Loading and Unloading Equipment • Transportation of Equipment • System Flushing • Pressure Testing • Signage • Spotting of UBD Equipment • UBD Equipment Rig Up • Post Rig Down Equipment Inspection • UBD Equipment Rig Down • Fluid System Commissioning • ESD Testing • Nitrogen Injection Line Testing
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 122 of 154
Operational Procedures will need to cover:
• Non-return Valve Pressure Testing • Non-return Valve Bleed Off • Make Up & Running BHA • Pull, Break & Lay Down BHA • Connections • Wireline Retrievable Valve Operations • Underbalanced Initiation • Drilling Underbalanced • Radio Communications • Tank Farm Management • Produced Oil Export • Solids Handling and Sampling • Tripping
Emergency Procedures need to cover:
• Bull-Head Kill Well • Equipment Failure Downstream of BOP • Failure of Rig BOP • Failure of Drilling Fluid Circulation System • Loss of Air or Rig Power Failure • Drillstring Failure • NRV Failure • Drillstring Washout • Plugged Bit or Drillstring • ESD Event • Fluid Management Event • Nitrogen Generation System Failure
Equipment Procedures need to cover:
• Start Up & Operation of Boosters • Compressor Equipment Start Up and Operations • Nitrogen Unit Start Up & Operations • RCD Element Change Out • Line Heater Operations • Flare Stack Operations • RCD Operations • Fuel Supply System
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 123 of 154
Completing Underbalanced Drilled Wells The majority of early wells drilled underbalanced could not be completed underbalanced. The majority of the early UBD wells were displaced to an overbalanced kill fluid prior to running the liner or completion. Depending on the completion fluid type, some formation
damage would take place. The damage may not have been as severe for completion brine as it might have been with drilling mud, but significant reductions in productivity of underbalanced drilled wells have been encountered after the installation of the completion. If the purpose of underbalanced drilling is for reservoir improvement, it is important that the reservoir is never exposed to overbalanced pressure with a non-reservoir fluid. If the well has been drilled underbalanced for drilling problems, and productivity is not impaired, then the well could possibly be killed and a conventional completion approach can be taken. A number of completion methods are available for underbalanced drilled wells:
• Liner and perforation • Slotted liner • Sandscreens • Barefoot
All of the above options can be deployed in underbalanced drilled wells. The use of cemented liners in an underbalanced drilled well is not recommended if the gains in reservoir productivity are to be maintained. It is generally not possible to cement a liner in an underbalanced mode, although the use of foamed cements may provide some solutions in certain circumstances. The completion requirements for a UBD well must be reviewed and analyzed as part of the feasibility study prior to commencing an underbalanced operation. Irrespective of the completion lining required for the reservoir, the installation process for a completion will have to be carefully reviewed during the planning process to ensure that underbalanced status is maintained during the completion installation. If a packer type completion is installed. The production packer and tailpipe are run and set on drill pipe with an isolation plug installed in the tailpipe. If the well is maintained underbalanced, well pressure will normally require the production packer and tailpipe to be snubbed into the well against well pressure. If a liner top completion is used in a monobore well drilled underbalanced, the use of a float collar may have to be considered to maintain well control.
Snubbing With well pressure acting upwards on the completion, the weight of the assembly will be less than the upward force. This means that a snubbing system is required to get the packer assembly in the hole. In an underbalanced system the well can be allowed to flow via the surface separation package. This is an advantage over
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 124 of 154
conventional snubbing operations as the surface pressure of a flowing well is normally lower than shut in pressure. At no time during the snubbing operations should the conventional well control BOP stack be compromised. Special snubbing BOP’s and a rotating diverter must be used in addition to the conventional drilling BOP’s. The use of a down hole isolation valve can significantly simplify the installation of a completion. There are very few mechanical methods of down hole isolation available for the running of a slotted liner. The Baker “Underbalanced Liner Bridge Plug (ULBP) System” is one of the few systems currently on the market. This system allows a retrievable plug to be set in the last casing. A retrieving tool that is attached to the bottom of the slotted liner releases this isolation plug. This retrieving tool unseats the isolation plug and then swallows the isolation plug or packer. The swallowing action of the retrieval tool ensures that the plug and retrieving tool are rigid and can be run to TD without hanging up in the open hole. Both the packer and retrieval tool are specifically designed to be released by the liner. If necessary, the well can be lubricated to kill fluid on top of the plug and displaced via the slotted liner when the drill string is sealed by the rotating diverter. The complete procedure for running of a slotted liner and the completion in an underbalanced drilled well is outlined in the following diagrams.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 125 of 154
Production Casing
Reservoir
Step 1Drilling Completed
Production Casing
Reservoir
Step 2Out of the Hole
Production Casing
Reservoir
Step 3Run Underbalanced
Liner Bridge Plug
Production Casing
Reservoir
Step 4Underbalanced Liner
Bridge Plug Set
Production Casing
Reservoir
Step 5Run Slotted Liner
Production Casing
Reservoir
Step 6Run Slotted Liner
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 126 of 154
Production Casing
Reservoir
Step 7Run Slotted Liner
Production Casing
Reservoir
Step 8Pull Running String
Production Casing
Reservoir
Step 9Run Packer
Production Casing
Reservoir
Step 10Production Packer Set
Step 11Completion
Production Casing
Reservoir
Fig 78 Completion Sequence for Underbalanced Drilled Wells
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 127 of 154
The main problem with running the completion in a live well is the installation of the SSSV control line. Once the control line is connected, the BOP will no longer seal around the pipe. Once again, the simplest method is to isolate the reservoir prior to running the completion. In the case of completion, the production packer with a plug installed in the tailpipe is snubbed into the live well and the production packer is set on drill pipe. The packer assembly would be lubricated into the well by utilizing the snubbing system or a down hole deployment valve. Once the production packer is set, the drillpipe can be used to pump completion fluid to provide an additional barrier that can be monitored if required. The completion is now run conventionally. The isolation plug in the tailpipe will be retrieved during the well commissioning. Before pulling this plug, the fluid should have been displaced out of the completion string. This can be achieved with coiled tubing or with a sliding sleeve. Once the completion has been installed, the well is ready for production. No clean up or stimulation is required in the case of underbalanced drilled wells.
Workover of an Underbalanced Drilled Well The workover procedure is a reversal of the completion running, i.e. a suspension plug is installed in the production packer tailpipe and the well is lubricated to kill fluid. After retrieving the completion, the packer picking assembly is run to the packer depth and the well is returned to an underbalanced condition prior to retrieving the packer. This ensures that formation damaging kill fluid does not come into contact with the reservoir at any time. Once a well is drilled and completed underbalanced for reservoir improvement purposes, the underbalanced status should be maintained for the life of the well. This includes all subsequent workovers and well operations.
Underbalanced Drilled Multi-Lateral Wells The setting of the production packer with a mechanical plug allows the lower leg in a multilateral well to be isolated and remain underbalanced whilst the second leg is drilled. After running the liner in the second leg, the completion can be run and a second packer can be installed and stabbed into the lower packer. If leg isolation is required, a flow sleeve can be installed at the junction to allow selected stimulation or production as required. Re-entry into both legs is also possible utilizing a selective system. However, more detail as to the exact requirements from a multilateral system will need to be reviewed. Drilling a multilateral well underbalanced with the main bore producing can be done, but the drawdown on the reservoir will be small. A further setback will be that cleaning up of the lateral is difficult if the main bore is a good producer. Getting sufficient flow through the lateral to lift fluids and solids can be a challenge. Flow modeling in a multilateral well can also be a challenge. Careful analysis of the lateral and the mainbore will have to be conducted prior to embarking on underbalanced drilling, especially as highly productive reservoirs can prove difficult to control if a small lateral requires a significant drawdown.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 128 of 154
Subsurface Services This is where the evaluation requirements of a well are reviewed with the operator and includes issues such as logging, coring and seismic surveys that may have to be collected whilst drilling underbalanced. Most logging and coring, as well as other data
requirements, can normally be obtained providing that the requirements and operational procedures are identified early on in an underbalanced project. Cuttings Regardless of the drilling fluid being used in the underbalanced operation, cuttings coming to the surface can be indexed to the formation at depth and its geological character evaluated. Gas Gas logging systems can detect the volume of gas in the circulating drilling fluid and detect the C1-C5 components in the gas stream. The need to obtain cuttings and gas specimens in the circulating drilling fluid must be identified early in the well planning process to ensure the appropriate surface equipment and technicians are available at the location. Gas samples can be taken from the separation or flare system, but appropriate safety systems must be installed. Electric Logging Any electric logging of underbalanced drilled wells can be designed just like conventional logging programs. Issues that need to be considered are the well control aspects of a logging operation. Wireline logging can be conducted using a wireline lubricator. Pipe conveyed logging is more complex as the pipe and annulus needs to be controlled. The use of a side entry sub and a Rotating Control Diverter will not work. If pipe conveyed logging needs to be done, memory tools should be considered. Due to the general lack of a conductive fluid in underbalanced drilling operations, logging should be limited to Induction Electric, Gamma Ray, Neutron and Caliper type logs. Coring Coring can be carried out in underbalanced drilling operations, although special tools and techniques must be used to ensure that the following issues are addressed:
• Dropping of balls to seal the corebarrel. • Recovery of the corebarrel, potentially under pressure. • Installation of non-return valves above the core barrel.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 129 of 154
Process Flow Diagrams A number of drawings are normally associated with an underbalanced drilled well. These drawings can be listed as:
1) UBD BOP Stack Up 2) Compression and Separation Equipment Piping and Instrumentation Diagram 3) Compression Equipment Piping and Instrumentation Diagram 4) Separation Equipment Piping and Instrumentation Diagram 5) Oil Export Pump Piping and Instrumentation Diagram 6) Valve Identification Piping and Instrumentation Diagram 7) Hazardous Areas (noise and fire and explosion) 8) Fire protection, location of breathing apparatus, gas sensors and alarm
positions 9) Escape Routes and ESD Stations 10) ESD Loop 11) Services Supply to UBS systems 12) Equipment spotting sequence 13) General rig and site layout 14) Process Flow Diagram 15) Primary search route 16) Secondary search routes (For Sour wells) 17) Grounding 18) Lighting and Emergency Lights
Drawings should be numbered and should also be assigned a date and sequence number, as a large number of drawings will be reviewed several times during the preparation of an underbalanced drilling project. A formal approval for drawings must be implemented and all drawings should be audited against actual prior to operations commencing. Symbols and legends must be clearly marked. The use of colors should be avoided so that diagrams and drawings can be copied on site if required.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 130 of 154
Rig and Lease Layout An example of a rig / lease layout complete with zonal area’s is shown below
On most lease layout diagrams, all of the equipment and access and escape routes are also normally shown.
Fig 79 Location Layout Drawing
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 131 of 154
Health Safety and Environmental Planning During the execution of drilling and testing operations, many of the activities have the potential for negative impact on the health and safety of workers, on the environment and on the equipment or installation being used. The potential for HSE problems increases
whenever a new operation that is different from the normal activity is introduced. Such is the case of reduced head or underbalanced operations on a drilling site. These operations are significantly different from the conventional drilling approach. To ensure a safe and efficient operation, the supervisors and crews executing these operations have to be familiar with the process, the equipment and the procedures. Therefore, in setting up an Underbalanced Drilling (UBD) project, it is critical that HSE issues are considered from the very early phases of the project planning cycle. HSE Management Systems A Company’s HSE management system describes the way in which it will manage its stated HSE objectives. As in any management system, this is accomplished by focusing on key activities. This ensures that:
• Critical activities are effectively controlled. • Procedures and documentation are in place. • Performance is measured and reported. • Areas for improvement are identified.
A detailed and approved document for HSE management of underbalanced drilling operations is available from the IADC UBO website.
Environmental Aspects The underbalanced drilling system is a fully enclosed system. When combined with a cuttings injection system and an enclosed mud pit system, a sour reservoir can be safely drilled using an underbalanced drilling system. The pressures and flow rates are kept as low as possible. It is not the intention to drill a reservoir and produce it to its maximum capacity. A well test can be carried out during underbalanced drilling to provide some productivity information. The hydrocarbons produced during the UBD process can be routed to the platform process plant, exported or flared. There is work currently being undertaken to reduce flaring and recover the hydrocarbons for export. In a prolific well, a significant amount of gas can be flared during the drilling process. Recovering this gas provides an environmental benefit and an economic benefit. Oil and condensate recovered are normally exported via stock tank into the process train.
Safety Aspects Besides the full HAZOP, a significant amount of crew training is required for underbalanced drilling. A drilling crew has been instructed during its entire career that if a well kicks it must be shut in and killed. During underbalanced drilling, the single item to be avoided is to kill the well. This may undo all the benefits of underbalanced
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 132 of 154
drilling. Working on a live well is not a normal operation for a drilling crew and good training is required to ensure that accidents are avoided. The underbalanced drilling process is more complex when compared to conventional drilling operations. Gas injection, surface separation, and snubbing maybe required on a well. If the hydrocarbons produced are then pumped into the process train, it is clear that drilling is no longer a stand-alone operation. The reservoir is the driving force in the UBD process. The driller must understand the process and all the interaction required between the reservoir, the liquid pump rate, the gas injection and the separation process system to safely drill the well. When tripping operations start, the well must remain under control. Snubbing pipe in and out of the hole is not a routine operation, and a specialized snubbing crew is normally brought on to snub the pipe in and out of the hole. The extra equipment also brings a number of extra crew to the rig. So besides a more complex operation, a number of service hands are on the rigs that now need to start working with the drilling crew. Yet the drilling crew will move back to conventional drilling once the well is completed. The drilling crew will need to be trained in this change of operating. If a number of wells are to be drilled underbalanced in a field, it may be an option to consider batch drilling of the reservoir sections. This saves mobilization and it also sets a routine with the drilling crew. It must be stated that few accidents occur during underbalanced drilling; this is mainly believed to the high emphasis on safety during live well operations.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 133 of 154
Detailed Cost Estimates The detailed cost estimates provide all the rates for equipment and personnel that will be used on the project. These detailed cost estimates are normally provided as a part of the commercial contract between the UBD service provider and the Operator.
Many contracts have these detailed rates included in the contract, and the monthly invoices for a project can be rapidly checked against these rates. One of the issues is often re-dress charges as a part of the operation and disputes often arise as a result of interpretation of the contract and refurbishment costs. Detailed cost estimates can normally be provided once all the engineering issues have been reviewed and the complete range of additional services and requirements is known.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 134 of 154
UBS Program An underbalanced drilling program can form part of the general drilling program or it can be presented as a separate drilling program. As a minimum requirement, a UBS program should have the following chapters:
Introduction Introducing the objectives of underbalanced drilling, the risk classification of the well and the reasons for the well operations. Well Information Basic well information such as location, reservoir targets and well trajectory as well as a short reservoir description complete with reservoir pressures and depths. An overview of the expected well condition and installed casing and tubulars used prior to underbalanced drilling is useful. Well trajectory such as length, build rates, deviation and hole size is normally provided at this point. Expected reservoir Properties
Reservoir Name Well Name/ No
Depth TVDSS Gross thickness (ft) Net/Gross Average Porosity Net Pay Thickness Average SW Formation Type Permeability Porosity Influx Top Depth (ft) Influx Bottom Depth (ft) Reservoir Fluid GOR (scf/stb) Reservoir PI (bbls/d/psi/ft) Reservoir Pressure Temperature Gas Specific Gravity Oil API
Section Objectives A short review of why the well is being drilled underbalanced and what the objectives of the operation are. The TD criteria for a well should also be included if the well program calls for the maximum production or maximum depth/length of the well.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 135 of 154
Operational Program Provides a step-by-step process of the underbalanced drilling operation. Drilling Parameters Provides a listing of the expected bits to be used and also provides details of the expected drilling parameters. Drillstring Design and Composition This section lists the detailed drillstring design for the underbalanced operation and what equipment is run where in the string and why. Also the location and type of the non-return valves should be listed here. If a down hole motor is to be used, list the manufacturer’s details including the maximum flow rates and pressures expected through the motor. Well Trajectory and Directional Issues This section lists the directional requirements and any directional drilling issues that may be encountered during the underbalanced drilling operation. UBD Hydraulics All of the flow modeling charts and underbalanced drilling parameters should be listed here including the fluids and gases that will be used. Also, the expected bottom hole pressure, reservoir pressure, and velocities expected in the well should be listed here. Well Control The expected well control matrix must be a part of the underbalanced drilling program. Timings List the expected rate of penetration and associated timings. Equipment Details of the underbalanced drilling equipment and, in the case of a coiled tubing operation, the details of the coiled tubing and the associated underbalanced drilling equipment are normally a useful addition to a drilling program. This detailed listing often can save considerable time during HAZOP / HAZID reviews and meetings. Once the detailed program has been written, considerations can be made to put all this practice into an actual operation.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 136 of 154
Underbalanced Records Underbalanced drilling in Europe started with the technology from Canada in 1995. Wells were initially drilled onshore, but migrated offshore with the first well drilled underbalanced offshore by Shell in Lowestoft in June 1997.
Year Country Operator Details 1995 Germany BEB Ulsen RWE-DEA Breitbrunn gas storage Australia WAPET 1996 Denmark Maersk Coiled Tubing Netherlands NAM Coiled Tubing UK Pentex Oil field onshore using coiled tubing. 1997 UK Shell First Offshore Well Mexico Pemex Offshore well GOM Indonesia Mobil Arun gas field (depleted) Spain SESA Algeria Sonarco Oman PDO Argentina YPF 1998 UK Shell Offshore Barque & Clipper UK Edinburgh Oil
& Gas Hatfield Moors Field
Netherlands NAM K17 offshore UBD trails. Indonesia Kufpec Oseil Field Italy Agip/SPI Sicily 1999 UK Shell Galleon & Barque Sharjah BP Amoco Horizontal oil wells Brazil Petrobras Onshore Brazil Estreito Field Indonesia YPF Maxus First offshore well in Krisna Field Algeria Sonarco Rhourde El Baquel Field 2000 UK Shell Southern North Sea Brazil Petrobras First lowhead well from a floater Sharjah BP Amoco Horizontal oil wells Oman PDO Coiled tubing underbalanced in the
Yibal field UK Talisman First UBD well from floater in North
Sea using coiled Tubing Lithuania Minijos Nafta New wells in Oil field 2001 Australia Santos Onshore in Cooper Basin Lithuania Minijos Nafta New wells in Oil field UK Talisman Second CTD well from floater Indonesia Exxon Mobil Arun gas field (depleted)
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 137 of 154
Year Country Operator Details
2001 Oman Occidental Onshore in Safah Field Colombia BP Five wells in Cusiana/Cupiagua Sarawak Shell First wells in Miri drilled UBD China Daqing First UBD wells in Daqing oilfield 2002 UK Talisman Further wells drilled on Buchan UK Shell Further wells drilled in Southern
North Sea Colombia BP Drilling underbalanced with coil in
the Cusiana/Cupiagua fields Lithuania Minijos Nafta Continue previous UBD program Indonesia Pertamina Onshore 1600 ft Horizontal lateral Oman PDO Continued operations in Nimir Field
in Oman a total of 8 wells were drilled in 2002
Jordan NPC Jordan New well in Eastern Jordan through gas bearing dubeibed Sandstone. One new well and two sidetracks were drilled.
2003 Syria Al Furat Drilled two new wells underbalanced through the Shiranish Sandstone
Brazil Petrobras Drilled several UBD wells in Carmopolis field in the Alagoas basin
Venezuela PDVSA Drilled 1200 ft horizontal section lowe head in Eastern Venezuela.
Venezuela PDVSA Underbalanced jointed pipe drilling on Lagomar block in Lake Maracaibo
Venezuela PDVSA Onshore operation in Barinas field using nitrified oil based mud system
Since 2003, the number of underbalanced drilled wells has increased continuously and more and more operators are using underbalanced technology to access reserves and to increase productivity and decrease drilling problems.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 138 of 154
References 14734 Westermark, R.V., “Drilling With a Parasite Aerating String in the Disturbed
Belt, Gallatin County, Montana “, SPE paper 14734, presented at the 1986 IADC/SPE Drilling Conference held in Dallas, TX, February 10-12 1986
37066 Saponja, J,”Challenges With Jointed-Pipe Underbalanced Operations” Paper SPE 37066 first presented at the 1996 SPE International Conference on Horizontal Well Technology held in Calgary, 18–20 November.
37138 B.D. Brant, T.F. Brent, R.F Bietz, “Formation Damage and Horizontal Wells - A Productivity Killer?” SPE paper 37138 presented at the 1996 SPE International Conference on Horizontal Well Technology held in Calgary, Canada, 18-20 November.
39303 Bijleveld, A.F, Koper, M, Saponja, J. “Development and Application of an underbalanced drilling simulator”, SPE 39303, paper presented at the SPE/IADC Drilling Conference held in Dallas, Texas 3-6 March 1998.
39924 D.L Purvis, SPE, and D,D. Smith, SPE, BJ Services, ”Underbalanced Drilling in the Williston Basin”, SPE 39924, paper presented at the SPE Rocky Mountain low permeability reservoir symposium held in Denver Colorado 5-8 April 1998.
46042 Graham, R.A. “Planning for underbalanced drilling using coiled tubing”, SPE 46042, paper presented at the SPE/Icota Coiled tubing round table held in Houston, Texas 15,16 April, 1998.
46039 Chitty, G, H. “Corrosion Issues with underbalanced drilling in H2S reservoirs” SPE 46039, paper presented at the SPE/Icota Coiled tubing round table held in Houston, Texas 15,16 April, 1998.
48982 Saintpere S, Hertzhaft, B, “Stability and flowing properties of aqueous foams for underbalanced drilling”, SPE paper 48982, paper presented at the SPE annual technical conference and exhibition held in New Orleans, Louisiana, 27-30 September 1998.
51500 Smith SP, Gregory G.A, Munro, N, “Application of multiphase flow methods to underbalanced horizontal drilling”, SPE paper 51500, paper presented at SPE international conference on horizontal well technology, held in Calgary, Alberta, Canada, 1-4 November 1998.
52826 Nas.S, “Underbalanced drilling in a depleted gas field onshore UK with coiled tubing and stable foam” SPE paper 52826, presented at the SPE/IADC drilling conference held in Amsterdam, Netherlands 9-11 March, 1999.
52827 Robichaux, D. “Successful Use of the Hydraulic Workover Unit Method for Underbalanced Drilling” SPE paper 52827, presented at the SPE/IADC drilling conference held in Amsterdam, Netherlands 9-11 March, 1999.
52829 Lage, A, Nakagawa, E, Time, R, Vefring, E, Rommetveit,R, “Full-scale Experimental Study for Improved Understanding of Transient Phenomena in Underbalanced Drilling Operations”, SPE paper 52829, presented at the SPE/IADC drilling conference held in Amsterdam, Netherlands 9-11 March, 1999.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 139 of 154
52832
Alvaro Felippe Negrão, SPE, IADC, Halliburton Energy Services; Nilo Azevedo Duarte Planning an Effective Aerated Drilling Operation in Hard Formation Based on Cost Analysis., “ SPE Paper 52832” presented at the 1999 SPE/IADC Drilling Conference held in Amsterdam, Holland, 9-11 March 1999.
52833 Gedge, B, “Underbalanced Drilling gains acceptance in Europe and the International Arena.”, SPE paper 52833, presented at the SPE/IADC drilling conference held in Amsterdam, Netherlands 9-11 March, 1999.
52889 Bennion, D.B. Thomas, F.B.. Bietz, R.F and Bennion, D.W, “Underbalanced Drilling: Praises and Perils”, SPE paper 52889, presented at the 1996 SPE Permian Basin Oil and Gas Recovery Conference held in Midland, Texas, 27–29 March 1996.
54483 Luft H.B, Wilde G, “Industry Guidelines for Underbalanced Coiled Tubing Drilling of Critical Sour Wells”, SPE paper 54483, presented at the SPE/ICoTA Coiled Tubing Roundtable held in Houston, Texas, 25–26 May 1999.
54717 Cor P.J.W. van Kruijsdijk, and Richard J.W. Cox, “Testing While Underbalanced Drilling: Horizontal Well Permeability Profiles” SPE paper 54717, presented at the 1999 SPE European Formation Damage Conference held in The Hague, The Netherlands, 31 May–1 June 1999.
55036 R.J. Cox, Jeff Li, and G.S. Lupick. “Horizontal Underbalanced Drilling of Gas Wells with Coiled Tubing” SPE paper 55036 presented at the SPE/IADC Drilling Conference held in Amsterdam, 4–6 March 1997.
55606 D R. Giffin, W. C. Lyons, “Case Histories of Design and Implementation of Underbalanced Wells”, SPE paper 55606 presented at the 1999 SPE Rocky Mountain Regional Meeting held in Gillette, Wyoming, 15-18 May 1999.
56633 S. Saintpere, B. Herzhaft, A. Toure, S. Jollet, “Rheological Properties of Aqueous Foams for Underbalanced Drilling”, SPE paper 56633 presented at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.
56865 A.F. Negra, A.C.V.M. Lage, and J.C. Cunha, “An Overview of Air/Gas/Foam Drilling in Brazil “, SPE paper 56865 presented at the 1997 SPE/IADC Drilling Conference held in Amsterdam, 4-6 March.
56684 L. Larsen, F. Nilsen, “Inflow Predictions and Testing While Underbalanced Drilling”, SPE paper 56684 presented at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.
56877 N.P. Tetley, V. Hazzard, and T. Neciri, “Application of Diamond-Enhanced Insert Bits in Underbalanced Drilling” SPE paper 56877 presented at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.
56920 R. Rommetveit, O. Sævareid, A. Lage, A. Guarneri, C. Georges, E. Nakagawa, and A. Bijleveld, “Dynamic Underbalanced Drilling Effects are Predicted by Design Model.”SPE paper 56920 presented at the 1999 Offshore Europe Conference held in Aberdeen, Scotland, 7–9 September 1999.
57569 R. Mathes, L.J. Jack, “Successful Drilling of an Underbalanced, Dual-Lateral Horizontal Well in the Sajaa Field, Sharjah, UAE”, SPE paper 57569 presented at the 1999 SPE/IADC Middle East Drilling Technology Conference held in Abu Dhabi, UAE, 8–10 November 1999.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 140 of 154
57571 J van Venrooy, N van Beelen; T Hoekstra; A Fleck, G Bell, A Weihe,”
Underbalanced Drilling With Coiled Tubing in Oman” SPE paper 57571 presented at the 1999 SPE/IADC Middle East Drilling Technology Conference held in Abu Dhabi, UAE, 8–10 November 1999.
58739 H. Santos, J. Queiroz, “How Effective is Underbalanced Drilling at Preventing Formation Damage?” SPE paper 58739 presented at the 2000 SPE International Symposium on Formation Damage Control held in Lafayette, Louisiana, 23–24 February 2000.
58800 S. Luo, Y. Meng, H. Tang, and Y. Zhou, “A New Drill-In Fluid Used for Successful Underbalanced Drilling”, SPE paper 58800 presented at the 2000 SPE International Symposium on Formation Damage Control held in Lafayette, Louisiana, 23–24 February 2000.
58972 V. Silva Jr. S Shayegi, E.Y. Nakagawa, “System for the Hydraulics Analysis of Underbalanced Drilling Projects in Offshore and Onshore Scenarios”, SPE paper 58972 presented at the 2000 SPE International Petroleum Conference and Exhibition in Mexico held in Villahermosa, Mexico, 1–3 February 2000.
59054 D. Velázquez Cruz, H. Rodríguez-Hernández, I. Cortes-Monroy, D. Azpeitia-Hernández, J. Blanco-Galan, “Underbalanced Drilling Analysis of Naturally Fractured Mexican Fields through 2D Multiphase Flow”, SPE paper 59054 presented at the 2000 SPE International Petroleum Conference and Exhibition in Mexico held in Villahermosa, Mexico, 1–3 February 2000.
59161 S. K. Tinkham, D. E. Meek, T. W. Staal, “Wired BHA Applications in Underbalanced Coiled Tubing Drilling” SPE paper 59161 presented at the 2000 IADC/SPE Drilling Conference held in New Orleans, Louisiana, 23–25 February 2000.
59166 D R. Giffin, W. C. Lyons, ”Case Histories of Design and Implementation of Underbalanced Wells” SPE paper 59166 presented at the 2000 IADC/SPE Drilling Conference held in New Orleans, Louisiana, 23–25 February 2000.
59260 S. Luo, R. Hong, Y. Meng, L. Zhang, Y. Li, C. Qin, “Underbalanced Drilling in High-Loss Formation Achieved Great Success – A Field Case Study”, SPE paper 59260 presented at the 2000 IADC/SPE Drilling Conference held in New Orleans, Louisiana, 23–25 February 2000.
59261 S. Luo,Y. Li, Y. Meng, L. Zhang, “A New Drilling Fluid for Formation Damage Control Used in Underbalanced Drilling” ”, SPE paper 59261 presented at the 2000 IADC/SPE Drilling Conference held in New Orleans, Louisiana, 23–25 February 2000.
59743 J. L. Hunt, and S. Rester, “Reservoir Characterization During Underbalanced Drilling: A New Model”, SPE paper 59743 presented at the 2000 SPE/CERI Gas Technology Symposium held in Calgary, Alberta Canada, 3–5 April 2000.
60708 D. A. A. Thatcher, G. A. Szutiak, M.M Lemay, “ Integration of coiled tubing underbalanced drilling service to improve efficiency and value”, SPE paper 60708 presented at the 2000 SPE/ Icota Coiled Tubing Roundtable held in Houston Texas 5-6 April 2000.
62203 S. Robinson, V. Hazzard, M. Leary, C. Carmack, “Redeveloping the Rhourde el Baguel field with underbalanced drilling operations” SPE paper 62203
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 141 of 154
62742 Q.T. Doan, M. Oguztoreli, Y. Masuda, T. Yonezawa, A. Kobayashi, A. Kamp,
“Modelling of Transient Cuttings Transport in Underbalanced Drilling”, SPE paper 62742 presented at the 2000 IADC/SPE Asia Pacific Drilling Technology held in Kuala Lumpur, Malaysia, 11–13 September 2000.
62743 A.C.V.M. Lage, K.K. Fjelde, R.W. Time, “Research Underbalanced Drilling Dynamics: Two-Phase Flow Modeling and Experiments”, SPE paper 62743 presented at the 2000 IADC/SPE Asia Pacific Drilling Technology held in Kuala Lumpur, Malaysia, 11–13 September 2000.
62896 C.P. Labat, D.J. Benoit, and P.R. Vining, “Underbalanced Drilling at its Limits Brings Life to Old Field”, SPE paper 62896 presented at the 2000 SPE Annual Technical Conference and Exhibition held in Dallas, Texas, 1–4 October 2000.
62898 B. Herzhaft, A. Toure, F. Bruni, S. Saintpere, “Aqueous Foams for Underbalanced Drilling: The Question of Solids”, SPE paper 62898 presented at the 2000 SPE Annual Technical Conference and Exhibition held in Dallas, Texas, 1–4 October 2000.
64379 J. L. Falcao and C. F. Fonseca, “Underbalanced Horizontal Drilling: A Field Study of Wellbore Stability in Brazil”, SPE paper 64379 presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition held in Brisbane, Australia, 16–18 October 2000.
64382 A.L.Martins, A.M.F.Lourenço, C.H.M. de Sá, “Foam Properties Requirements for Proper Hole Cleaning While Drilling Horizontal Wells in Underbalanced Conditions”, SPE paper 64382 presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition held in Brisbane, Australia, 16–18 October 2000.
64999 Y. Rojas, S. Kakadjian, A. Aponte, R. Márquez, and G. Sánchez, “Stability and Rheological Behavior of Aqueous Foams for Underbalanced Drilling”, SPE paper 64999 presented at the 2001 SPE International Symposium on Oilfield Chemistry held in Houston, Texas, 13–16 February 2001.
65512 J.G Parra, C. Celis, S. De gennaro, “ Wellbore Stability Simulations for Underbalanced drilling operations in highly depleted reservoirs”, SPE paper 65512 presented at the 2000 SPE petroleum Society of CIM international conference on Horizontal well Technology held in Calgary, Alberta Canada 5-8 Nov 2000.
67688 S. Jansen, P. Brett, J. Kohnert, and R. Catchpole, “Safety Critical Learnings in Underbalanced Well Operations”, SPE paper 67688 presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 27 February–1 March 2001.
67689 D. Park, P. R. Brand, B. Allyson, and G. Sodersano, “Planning and Implementation of the Repsol-YPF-MAXUS Krisna Underbalanced Drilling Project”, SPE paper 67689 presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 27 February–1 March 2001.
67690 W. Kneissl, “Reservoir Characterization Whilst Underbalanced Drilling”, SPE paper 67690 presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 27 February–1 March 2001.
67693 R. J. Lorentzen, K. K. Fjelde, J. Frøyen, A. C. V. M. Lage, G. Nævdal, and E. H. Vefring, “Underbalanced Drilling: Real Time Data Interpretation and Decision Support”, SPE paper 67693 presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 27 February–1 March 2001.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 142 of 154
67829 S. Nas, A. Laird, “Designing Underbalanced Thru Tubing Drilling Operations”,
SPE paper 67829 presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 27 February–1 March 2001.
68491 D. M. Hannegan, “Underbalanced Operations Continue Offshore Movement”, SPE paper 68491 presented at the SPE/ICoTA Coiled Tubing Roundtable held in Houston, Texas, 7–8 March 2001.
68495 F. Jun, G. Changliang, S. Taihe, L. Huixing, and Y. Zhongshen, ,”A Comprehensive Model and Computer Simulation for Underbalanced Drilling in Oil and Gas Wells” SPE paper 68495 presented at the SPE/ICoTA Coiled Tubing Roundtable held in Houston, Texas, 7–8 March 2001.
69448 S. Nas, A. Laird, “Designing Underbalanced Thru Tubing Drilling Operations” SPE paper 69448 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Buenos Aires, Argentina, 25–28 March 2001.
69449 R. Rommetveit, A. C. V. M. Lage, “Designing Underbalanced and Lightweight Drilling Operations; Recent Technology Developments and Field Applications”, SPE paper 69449 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Buenos Aires, Argentina, 25–28 March 2001.
69490 J. C. Cunha, F. Severo Rosa, H. Santos, “Horizontal underbalanced drilling in northeast Bazil: a field Case history”, SPE paper 69490 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Buenos Aires, Argentina, 25–28 March 2001.
69496 F.J. Romero, D. Pi and A. Cinquegrani. “Underbalanced EMWD-AP At La Concepción Block, Maracaibo Basin, Venezuela”, SPE paper 69496 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Buenos Aires, Argentina, 25–28 March 2001.
71384 R. J. Lorentzen, K. K. Fjelde, J. Frøyen, A. C. V. M. Lage, G. Nævdal, and E. H. Vefring, “Underbalanced and Low-head Drilling Operations: Real Time Interpretation of Measured Data and Operational Support”, SPE paper 71384 presented at the 2001 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, 30 September–3 October 2001.
72153 R. Bullock, J. Karigan, T. Wiemers and S. McMillan, “ New generation underbalanced drilling 4-phase surface separation technique improves operational safety, efficiency and data management capabilities”, SPE paper 72153 presented at the SPE Asia Pacific Improved Oil Recovery Conference held in Kuala Lumpur, Malaysia 8-9 October 2001.
72300 M. C. Stuczynski PE, “Recovery Of Lost Reserves Through Application Of Underbalanced Drilling Techniques In The Safah Field”, SPE paper 72300 presented at the IADC/SPE Middle East Drilling Technology held in Bahrain, 22–24 October 2001.
72328 B. Guo, “ Use of Spreadsheet and analytical Models to Simulate Solid, Water, oil and gas flow in underbalanced drilling”, SPE paper 72328 presented at the IADC/SPE Middle East Drilling Technology held in Bahrain, 22–24 October 2001.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 143 of 154
72373 J. V. McCallister, K. L. Haddad, R. Keenan, “Underbalanced Coiled-Tubing
Drilling in a Thin Gas Storage Reservoir: A Case Study”, SPE paper 72373 presented at the SPE Eastern Regional Meeting held in Canton, Ohio, 17–19 October 2001.
74333 A.L. Martins, A.M.F. Lourenço, and C.H.M. de Sá, “Foam Property Requirements for Proper Hole Cleaning While Drilling Horizontal Wells in Underbalanced Conditions”, SPE paper 74333 revised for publication from paper SPE 64382, first presented at the 2000 SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Australia,16–18 October 2000.
74426 C. Perez-Tellez, J.R. Smith and J.K Edwards, “ A new comprehensive Mechanistic Model for underbalanced drilling Improves wellbore pressure predictions”, SPE paper 74426 presented at the SPE International Petroleum Conference and Exhibition in Mexico held in Villahermosa Mexico, 10-12 February 2002.
74445 R. D. Murphy, P. B. Thompson, “A Drilling Contractor’s View of Underbalanced Drilling”, SPE paper 74445 presented at the IADC/SPE Drilling Conference held in Dallas, Texas, 26–28 February 2002.
74446 G. Pia, T. Fuller, T Haselton, R. Kirvelis, “Underbalanced / Undervalued?, Direct qualitative comparison proves the technique!”, SPE paper 74446, presented at the IADC/SPE Drilling Conference held in Dallas, Texas, 26–28 February 2002.
74447 C.D. Hawkes, S.P. Smith P.J. McLellan “Coupled Modeling of Borehole Instability and Multiphase Flow for Underbalanced Drilling”, SPE paper 74447, presented at the IADC/SPE Drilling Conference held in Dallas, Texas, 26–28 February 2002.
74448 D. Hannegan and R. Divine, “Underbalanced Drilling–Perceptions and Realities of Today’s Technology in Offshore Applications” SPE paper 74448, presented at the IADC/SPE Drilling Conference held in Dallas, Texas, 26–28 February 2002.
74461 Weisbeck, D., Blackwell, G, Park, D, and Cheatham, C., “Case History of First Use of Extended-Range EM MWD in Offshore, Underbalanced Drilling”, SPE paper 74461, presented at the IADC/SPE Drilling Conference held in Dallas, Texas, 26–28 February 2002.
74841 R.G Fraser, J. Ravensbergen, “ Improving the performance of coiled tubing underbalanced horizontal drilling operations”, SPE paper 74841 presented at the SPE/Icota Coiled Tubing Conference and Exhibition held in Houston Texas, USA, 9-10 April 2002.
74846 J. Hibbeler, L. Duque, L Castro, A. Gonzalez and J. Romero, “ Underbalanced Coiled Tubing leads to improved productivity in slotted liner completions”, SPE paper 74846 presented at the SPE/Icota Coiled Tubing Conference and Exhibition held in Houston Texas, USA, 9-10 April 2002.
77237 B. Guo, A. Ghalambor, “ An Innovation in designing underbalanced drilling flow rates: A gas-Liquid rate window (GLRW) Approach”, SPE paper 77237 presented at the IADC/SPE Asia Pacific Technology Conference held in Jakarta Indonesia 9-11 September 2002.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 144 of 154
77240 S.Herbal, R. Grant, B. Grayson, D. Hosie and B. Cuthberson, “ Downhole
Deployment Valve Addresses Problems Associated with Tripping Drill Pipe During Underbalanced Drilling Operations”, SPE paper 77240 presented at the IADC/SPE Asia Pacific Technology Conference held in Jakarta Indonesia 9-11 September 2002.
77241 J. Aasen, E. Skaugen, “ Pipe Buckling at surface in underbalanced drilling”, SPE paper 77241 presented at the IADC/SPE Asia Pacific Technology Conference held in Jakarta Indonesia 9-11 September 2002.
77352 J.W. Colbert, and G. Medley, “Light Annular MudCap Drilling - A Well Control Technique for Naturally Fractured Formations”, SPE paper 77352, presented at the SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, 29 September-2 October 2002.
77529 G. Pia, T. Fuller, T Haselton, R. Kirvelis, “Underbalanced Production Steering Delivers Record Productivity”, SPE paper 77529, presented at the SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, 29 September – 2 October 2002.
77530 E. H. Vefring, G. Nygaard, K. K. Fjelde, J. Frøyen, R.J. Lorentzen, A. Merlo and G. Nævdal, “Reservoir Characterization during Underbalanced Drilling: method, accuracy and necessary data”, SPE paper 77530 presented at the SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, 29 September – 2 October 2002.
78978 Y. Wang, B. Lu, “ Fully coupled chemico-geomechanics model and applications to wellbore stability in Shale formation in an underbalanced field conditions” SPE paper 78978 presented at the SPE International Thermal Operations and Heavy Oil Symposium and international horizontal well technology conference held in Calgary, Alberta, Canada 4-7 November 2002.
79792 R. Cuthberson, A Green, J. A. G. Dewar and B. Truelove, “ Completion of an underbalanced well using expandable sand screen for Sand Control”, SPE paper 79792 presented at the SPE/IADC Drilling conference held in Amsterdam, The Netherlands 19-21 February 2003.
79852 A. C. V. M. Lage, G. Sotomayor, A. Vargas, P. da Silva, H. L. Lira and P. Silva Filho,” The first underbalanced multilateral well branches drilled in Brazil, a field case history”, SPE paper 79852 presented at the SPE/IADC Drilling conference held in Amsterdam, The Netherlands 19-21 February 2003.
79853 P.A. Francis, I.A. Davidson, S Harti, W. Geldof, M.S. Culen, T. Jenkins, “Low Risk/High reward strategy drives underbalanced drilling implementation in PDO”, SPE paper 79853 presented at the SPE/IADC Drilling conference held in Amsterdam, The Netherlands 19-21 February 2003.
79854 D. Hannegan, G. Wanzer, “ Well Control Considerations – Offshore Applications of underbalanced drilling Technology”, SPE paper 79854 presented at the SPE/IADC Drilling conference held in Amsterdam, The Netherlands 19-21 February 2003.
79857 D. Lee, F. Brandao, G.Sotomayor, H. Lucena, and P. Silva Filho, “A New Look for an Old Field - Multilateral, Underbalanced, Semi-Short Radius Drilling Case Study: Installation of a Seven Leg Multilateral Well”, SPE paper 79857 presented at the SPE/IADC Drilling conference held in Amsterdam, The Netherlands 19-21 February 2003.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 145 of 154
80207 S. Kakadjian, B. Herzhaft, L. Neau, “HP/HT rheology of Aqueous
Compressible Fluids for Underbalanced Drilling Using A Recirculating Rheometer”, SPE paper 80207 presented at the SPE International Symposium on Oilfield Chemistry held in Houston, Texas, U.S.A., 5-7 February 2003.
81069 K. Schmigel, L. MacPherson, “Snubbing Provides Options for Broader Application of Underbalanced Drilling Lessons”, SPE paper 81069 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Port-of-Spain, Trinidad, West Indies, 27-30 April 2003.
81620 A.C.V.M. Lage, G. Sotomayor, A. Vargas, R. Rodrigano, P.R.C. da Silva, H.L. Lira and P. Silva Filho, “Planning, Executing and Analyzing the Productive Life of the First Six Branches Multilateral Well Drilled Underbalanced in Brazil”, SPE paper 81620 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81621 H. Xiong and D. Shan, “Reservoir Criteria for Selecting Underbalanced Drilling Candidates” SPE paper 81621 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81622 T. Harting, J. Gent, and T. Anderson, “Drilling Near Balance and Completing Open Hole to Minimize Formation Damage in a Sour Gas Reservoir”, SPE paper 81622 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81623 J. Ramalho, R. Medeiros , P.A. Francis, I.A. Davidson, “The Nimr Story: Reservoir Exploitation Using UBD Techniques”, SPE paper 81623 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81625 I.C. Gil, S. Shayegi, “Comparison of Wellbore Hydraulics Models to Maximize Control of BHP and Minimize Risk of Formation Damage”, SPE paper 81625 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81626 R. Cade, R. Kirvelis, and J. Jennings, “Does Underbalanced Drilling Really Add Reserves?”, SPE paper 81626 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81627 R. Divine, “Planning is Critical for Underbalance Applications with Under-experienced Operators”, SPE paper 81627 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81628 P. Brett, D. Weisbeck, R. Graham, SPE, “Northland Energy Services (UK) Innovative Technology Advances Use of Electromagnetic MWD Offshore in Southern North Sea”, SPE paper 81628 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 146 of 154
81629 M.S. Culen, S. Harthi, and H. Hashimi, “A Direct Comparison Between
Conventional and Underbalanced Drilling Techniques in the Saih Rawl Field, Oman” SPE paper 81629 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81630 S. Saeed “Underbalanced Data Acquisition: A Real-Time Paradigm”, SPE paper 81630 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81631 C.G. Mykytiw, I.A. Davidson, P.J. Frink, “Design and Operational Considerations to Maintain Underbalanced Conditions with Concentric Casing Injection”, SPE paper 81631 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81632 T.Devaul, A. Coy, “Underbalanced Horizontal Drilling Yields Significant Productivity Gains in the Hugoton Field”, SPE paper 81632 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81633 R. Malkowski, ”The Challenge Of Well Control In Under Balance Drilling And The Role Of Training In Meeting It”, SPE paper 81633 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81634 E.H. Vefring, G. Nygaard, R.J. Lorentzen, G. Nævdal and K.K. Fjelde, , “Reservoir Characterization during UBD: Methodology and Active Tests”, SPE paper 81634 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81636 K.K. Fjelde, R. Rommetveit, A. Merlo, and A.C.V.M Lage, “Improvements in Dynamic Modeling of Underbalanced Drilling”, SPE paper 81636 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81638 J.L. Hunt, S. Rester, “Multilayer Reservoir Model Enables More Complete Reservoir Characterization During Underbalanced Drilling”, SPE paper 81638 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81639 P.V. Suryanarayana, S. Rahman, R. Natarajan, R. Reiley, “Development of a probabilistic model to estimate productivity improvement due to underbalanced drilling”, SPE paper 81639 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81640 B. Guo, K. Sun, and A. Ghalambor, “A Closed Form Hydraulics Equation for Predicting Bottom-Hole Pressure in UBD with Foam” SPE paper 81640 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
81644 A.A. Garrouch and H.M.S. Labbabidi, “Using Fuzzy Logic for UBD Candidate Selection” SPE paper 81644 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 147 of 154
81645 G.W. Nance, “Little Known Lubrication Method: Great Tool for UB Work” SPE
paper 81645 presented at the 2002 IADC/SPE Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 25-26 March 2003.
84841 R.G Fraser, J. Ravensbergen, “ Improving the performance of coiled tubing underbalanced horizontal drilling operations”, SPE paper 74841 presented at the SPE/Icota Coiled Tubing Conference and Exhibition held in Houston Texas, USA, 9-10 April 2002.
85061 Q.T. Doan, M. Oguztoreli, Y. Masuda, T. Yonezawa, A. Kobayashi, S. Naganawa, and A. Kamp, “Modeling of Transient Cuttings Transport in Underbalanced Drilling (UBD)”, SPE paper 85061 presented at the 2000 IADC/SPE Asia Pacific Drilling Technology Conference, Kuala Lumpur, 11-13 September.
85319 M. Sarssam, R. Peterson, M. Ward, D. Elliott, and S. McMillan, “Underbalanced Drilling For Production Enhancement in the Rasau Oil Field, Brunei” SPE paper 85319 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
86465 D. Biswas, and P.V. Suryanarayana, “Estimating Drilling-Induced Formation Damage Using Reservoir Simulation to Screen Underbalanced Drilling Candidates”, SPE paper 86465 presented at the SPE International Symposium and Exhibition on Formation Damage Control, held in Lafayette, Louisiana, U.S.A., 18-20 February 2004.
86558 Y. Ding, B. Herzhaft and G. Renard, “Near-Wellbore Formation Damage Effects On Well Performance - A Comparison Between Underbalanced And Overbalanced Drilling”, SPE paper 86558 presented at the SPE International Symposium and Exhibition on Formation Damage Control, held in Lafayette, Louisiana, U.S.A., 18-20 February 2004.
87986 A. Coy, D. Hall, and M. Vezza, “A Safe Approach to Underbalanced Drilling in H2S Producing Fields”, SPE paper 87986, presented at the IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition held in Kuala Lumpur, Malaysia, 13-15 September 2004.
88698 H. Qutob “Underbalanced Drilling; Remedy for Formation Damage, Lost Circulation, & Other Related Conventional Drilling Problems”, SPE paper 88698, presented at the 11th Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, U.A.E., 10-13 October 2004.
89324 Y. Li and E. Kuru, “Prediction of Critical Foam Velocity for Effective Cuttings Transport in Horizontal Wells” SPE paper 89324 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
90185 H. Pinkstone, A. Timms, S. McMillan, R. Doll, and H. de Vries, “ Underbalanced Drilling of Fractured Carbonates in Northern Thailand Overcomes Conventional Drilling Problems Leading to a Major Gas Discovery”, SPE paper 90185 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 148 of 154
90836 T.W. Cavender and H.L. Restarick, “Well-Completion Techniques and
Methodologies for Maintaining Underbalanced Conditions Throughout Initial and Subsequent Well Interventions” SPE paper 90836 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91220 R.A. Graham and M.S. Culen, “Methodology for Manipulation of Wellhead Pressure Control for the Purpose of Recovering Gas to Process in Underbalanced Drilling Applications”, SPE paper 91220 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91239 J.E. Olsen, E. Vollen and T. Tønnessen, “Challenges in Implementing UBO Technology”, SPE paper 91239 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91242 Ø. Arild, T. Nilsen, and M. Sandøy, “Risk-Based Decision Support for Planning of an Underbalanced Drilling Operation”, SPE paper 91242 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91243 R. Rommetveit, K.K. Fjelde, J. Frøyen, K.S. Bjørkevoll, G. Boyce, and J.E. Olsen, “Use of Dynamic Modeling in Preparations for the Gullfaks C-5A Well” SPE paper 91243 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91356 B. Guo and A. Ghalambor, “Pressure Stability Analysis for Aerated Mud Drilling Using an Analytical Hydraulics Model” SPE paper 91356 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91519 D.D. Moore, A. Bencheikh, J.R. Chopty, “Drilling Underbalanced in Hassi Messaoud”, SPE paper 91519 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91544 S. Salimi , M. Golan, and K.I. Andersen, “Enhancement Well Productivity—Investigating the Feasibility of UBD for Minimizing Formation Damage in Naturally Fractured Carbonate Reservoirs”, SPE papers 91544 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91556 A. Coy, D Hall and M. Vezza, “A Safe Approach to Underbalanced Drilling in an H2S Producing Field Leads to Operational Success and Productivity Improvement”, SPE paper 91556 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91566 G. Medley, and C.R. Stone, “MudCap Drilling When? Techniques for Determining When to Switch From Conventional to Underbalanced Drilling” SPE paper 91566 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 149 of 154
91558 T. Friedel and H.-D. Voigt, “Numerical Simulation of the Gas Inflow During
Underbalanced Drilling (UBD) and Investigation of the Impact of UBD on Longtime Well Productivity”, SPE paper 91558 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91559 I. Davidson, R. Medeiros, D. Reitsma, “Changing the Value Equation for Underbalanced Drilling”, SPE paper 91559 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91581 J. Knight, R. Pickles, B. Smith, and M. Reynolds,” HSE Training, Implementation, and Production Results for a Long-Term Underbalanced Coiled-Tubing Multilateral Drilling Project”, SPE paper 91581 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91583 J.A. Cantu, J. May and J. Shelton, “Using Rotating Control Devices Safely in Today’s Managed Pressure and Underbalanced Drilling Operations”, SPE paper 91583 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91593 D. Kimery and M. McCaffrey, “Underbalanced Drilling in Canada: Tracking the Long-Term Performance of Underbalanced Drilling Projects in Canada” SPE paper 91593 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91598 C. Mykytiw, P.V. Suryanarayana, and P.R. Brand, “Practical Use of a Multiphase Flow Simulator for Underbalanced Drilling Applications Design—The Tricks of the Trade”, SPE paper 91598 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91607 D. Kimery and T. van der Werken, “Damage Interpretation of Properly and Improperly Drilled Underbalanced Horizontals in the Fractured Jean Marie Reservoir Using Novel Modeling and Methodology”, SPE paper 91607 presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition held in Houston, Texas, U.S.A., 11-12 October 2004.
91610
E. Kuru, O.M. Okunsebor, Y. Li, University of Alberta, “Hydraulic Optimization of Foam Drilling For Maximum Drilling Rate”, SPE paper 91610, presented at the SPE/IADC Underbalanced Technology Conference and Exhibition, 11-12 October 2004, Houston, Texas
91725 Calvin Holt, Weatherford International, “Proving UBD's Value in Brownfields and Beyond”, SPE paper 91725, presented at the SPE/IADC Drilling Conference, 23-25 February 2005, Amsterdam, Netherlands
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 150 of 154
92484 L. Zhou, R.M. Ahmed, S.Z. Miska, N.E. Takach, M. Yu, University of Tulsa; A.
Saasen, Statoil ASA, “Hydraulics of Drilling with Aerated Muds under Simulated Borehole Conditions” , SPE paper 92484, presented at the SPE/IADC Drilling Conference, 23-25 February 2005, Amsterdam, Netherlands
92513 Randal Pruitt, Charlie Leslie, BP; Bruce Smith, WUU; Olivier Desplain, Tom Kavanagh, Schlumberger; Tony Woolham, Halliburton Energy Services; Allistar Law Baker, Hughes Inteq; Nick Christou, Weatherford GSI; Daniel Borling, BP, “Underbalanced Coiled Tubing Drilling Update on a Successful Campaign”, SPE paper 92513, presented at the SPE/IADC Drilling Conference, 23-25 February 2005, Amsterdam, Netherlands
93346 H. Qutob and H. Ferreira, Weatherford Intl. Inc., “The SURE way to underbalanced Drilling”, SPE paper 93346, presented at the SPE Middle East Oil and Gas Show and Conference, Mar 12 - 15, 2005, Kingdom of Bahrain
93695
D. Murphy, Petroleum Development Oman; I. Davidson, Shell UBD Global Implementation Team; Kennedy, Blade Energy Partners; R. Busaidi and J. Wind, Petroleum Development Oman; C. Mykytiw, Shell UBD Global Implementation Team; and L. Arsenault, Precision Drilling UBD, “Applications of Underbalanced Drilling Reservoir Characterization for Water Shut Off in a Fractured Carbonate Reservoir - A Project Overview”, SPE paper 93695 presented at the Middle East Oil and Gas Show and Conference, Mar 12 - 15, 2005, Kingdom of Bahrain
93784 A. Timms, Amerada Hess, and K. Muir, and C. Wuest, Weatherford UBS, “Downhole Deployment Valve - Case History”, SPE paper 93784 presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, 5-7 April 2005, Jakarta, Indonesia
93974 T. Friedel*, G. Mtchedlishvili, H.-D. Voigt, and F. Häfner, Freiberg U. of Mining and Technolog, “Simulation of Inflow Whilst Underbalanced Drilling (UBD) With Automatic Identification of Formation Parameters and Assessment of Uncertainty”, SPE paper 93974 presented at the SPE Europec/EAGE Annual Conference, 13-16 June 2005, Madrid, Spain
94164 M.E. Ozbayoglu and C. Omurlu, Middle East Technical U. “Flow-Rate Optimization of Aerated Fluids for Underbalanced Coiled-Tubing Applications”, SPE paper 94164 presented at the SPE/ICoTA Coiled Tubing Conference and Exhibition, 12-13 April 2005, The Woodlands, Texas
94169 J. Weber and D. Stilson, SPE, Kinder Morgan Inc., and D. McClatchie, S. Denton, and L. King, SPE, BJ Services Co., “Improving the Efficiency of Gas-Storage-Well Completions Using Underbalanced Drilling With Coiled Tubing”, SPE paper 94169 presented at the SPE/ICoTA Coiled Tubing Conference and Exhibition, 12-13 April 2005, The Woodlands, Texas
94469 Y. Meng, SPE, Southwest Petroleum Inst.; L. Wan, Tubular Goods Research Center; X. Chen and G. Chen, Great Wall Drilling Co.; L. Yang, Tubular Goods Research Center; and J. Wang, Xi'An ShiYou U., “Discussion of Foam Corrosion Inhibition in Air Foam Drilling”, SPE paper 94469 presented at the SPE International Symposium on Oilfield Corrosion, 13 May 2005, Aberdeen, United Kingdom
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 151 of 154
94763 M.S. Culen and D.R. Killip, Precision Drilling Services & Co, “Forensic
Reservoir Characterisation Enabled with Underbalanced Drilling”, SPE paper 94763 presented at the SPE European Formation Damage Conference, 25-27 May, Sheveningen, The Netherlands
95861 P.V. Suryanarayana, and Z. Wu, Blade Energy Partners; J. Ramalho, Shell Intl. E&P; and R. Himes, Stim Lab Division of Core Laboratories, “Dynamic Modeling of Invasion Damage and Its Impact on Production in Horizontal Wells” SPE paper 95861 presented at the SPE Annual Technical Conference and Exhibition, 9-12 October 2005, Dallas, Texas
96282 B. Webster and M. Pitman, Baker Oil Tools, and R. Pruitt, BP, “Worlds First Coiled Tubing Under-balanced Casing Exit Using Nitrogen Gas as the Milling Fluid”, SPE paper 96282 presented at the Offshore Europe conference, 6-9 September 2005, Aberdeen, United Kingdom
96646 D. Reitsma, E. van Riet, Shell International Exploration & Production B.V., “Utilizing an Automated Annular Pressure Control System for Managed Pressure Drilling in Mature Offshore Oilfields”, SPE paper 96646 presented at the Offshore Europe Conference, 6-9 September 2005, Aberdeen, United Kingdom.
96992 A.P. Gupta, A. Gupta, J. Langlinais, Louisiana State U., “Feasibility of Supercritical Carbon Dioxide as a Drilling Fluid for Deep Underbalanced Drilling Operation” SPE paper 96992 presented at the SPE Annual Technical Conference and Exhibition, 9-12 October 2005, Dallas, Texas.
97025 H. Santos and P. Reid, Impact Solutions Group; J. Jones, Drilling Systems; and J. McCaskill, Power Chokes, “Developing the Micro-Flux Control Method—Part 1: System Development, Field Test Preparation, and Results”, SPE paper 97025 presented at the SPE/IADC Middle East Drilling Technology Conference and Exhibition, 12-14 September 2005, Dubai, United Arab Emirates.
97028 J.E. Gravdal, R.J. Lorentzen, K.K. Fjelde, and E.H. Vefring, RF-Rogaland Research, “Title Tuning of Computer Model Parameters in Managed-Pressure Drilling Applications Using an Unscented Kalman Filter Technique” SPE paper 97028 presented at the SPE Annual Technical Conference and Exhibition, 9-12 October 2005, Dallas, Texas
97317 S.R. Shadizadeh, Petroleum U. of Technology, M. Zaferanieh, Petroiran Development Co. , “The Feasibility Study of Using Underbalanced Drilling in Iranian Oil Fields”, SPE paper 97317 presented at the SPE/IADC Middle East Drilling Technology Conference and Exhibition, 12-14 September 2005, Dubai, United Arab Emirates
97372 G. Nygaard, K.-K. Fjelde, G. Nævdal, R.J. Lorentzen, and E.H. Vefring, RF - Rogaland Research, “Evaluation of Drillstring and Casing Instrumentation Needed for Reservoir Characterization During Drilling Operations”, SPE paper 97372 presented at the SPE/IADC Middle East Drilling Technology Conference and Exhibition, 12-14 September 2005, Dubai, United Arab Emirates.
98083 B. Guo, and A. Ghalambor, U. of Louisiana at Lafayette, “A Guideline to Optimizing Pressure Differential in Underbalanced Drilling for Reducing Formation Damage”, SPE paper 98083 presented at the International Symposium and Exhibition on Formation Damage Control, 15-17 February 2006, Lafayette, Louisiana U.S.A.
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 152 of 154
98787 J. Saponja, Weatherford Canada Partnership; A. Adeleye, Anadarko Corp.
Canada; and B. Hucik, Canadian Natural Resources Ltd. “Managed-Pressure Drilling (MPD) Field Trials Demonstrate Technology Value” SPE paper 98787 presented at the IADC/SPE Drilling Conference, 21-23 February 2006, Miami, Florida, USA
98926 L. Zhou, Scandpower Petroleum Technology Inc., “Hole Cleaning During UBD in Horizontal and Inclined Wellbore”, SPE paper 98926 presented at the IADC/SPE Drilling Conference, 21-23 February 2006, Miami, Florida, USA.
99075 K.S. Bjørkevoll, and R. Rommetveit, SINTEF Petroleum Research, and A. Rønneberg and B. Larsen, Statoil, “Successful Field Use of Advanced Dynamic Models”, SPE paper 99075 presented at the IADC/SPE Drilling Conference, 21-23 February 2006, Miami, Florida, USA
99113 A.M.F. Lourenço, A.L. Martins, P.H. Andrade Jr., and E. Y. Nakagawa, Petrobras, “Investigating Solids-Carrying Capacity for an Optimized Hydraulics Program in Aerated Polymer-Based-Fluid Drilling”, SPE paper 99113 presented at the IADC/SPE Drilling Conference, 21-23 February 2006, Miami, Florida, USA
99116 R. Soto, J. Malavé, M. Medina, and C. Díaz, PDVSA, “Managed Pressure Drilling (MPD): Planning a Solution for San Joaquin Field, Venezuela”, SPE paper 99116 presented at the IADC/SPE Drilling Conference, 21-23 February 2006, Miami, Florida, USA
99165 M. Azeemuddin, and D. Maya, Baker Atlas; E.A. Guzman, PDVSA; and S.H. Ong, Baker Atlas, “Underbalanced Drilling Borehole Stability Evaluation and Implementation in Depleted Reservoirs, San Joaquin Field, Eastern Venezuela” SPE paper 99165 presented at the IADC/SPE Drilling Conference, 21-23 February 2006, Miami, Florida, USA
Gas research Institute. Underbalanced Drilling Manual. Published by Gas research Institute Chicago Illinois.GRI-97/0236 Proceedings from 1st International Underbalanced drilling Conference & exhibition held in The Hague, Holland 1995 Proceedings from 3rd International Underbalanced drilling Conference & exhibition held in The Hague, Holland 1997 Maurer Engineering Inc. Underbalanced Drilling and Completion Manual. DEA 101 phase 1. October 1996 Proceedings from the North Sea Underbalanced Operations Forum held in Aberdeen 1996 Proceedings from the first IADC Underbalanced drilling Conference & exhibition held in The Hague, Holland 1998 Proceedings from the 2000 IADC Underbalanced drilling Conference & exhibition held in Houston Texas Proceedings from the IADC Underbalanced drilling Conference & exhibition held in Aberdeen 2001
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 153 of 154
Suppliers of Underbalanced Drilling Services
Weatherford Weatherford has the comprehensive solutions for safe and effective underbalanced drilling to meet growing production demands worldwide. Extensive R&D and strategic acquisitions of leading UBS technologies from Tesco, Dailey, Alpine Oil Services and ECD Northwest and recently Precission Energy Services have launched Weatherford UBS as the main player in the global arena for underbalanced drilling solutions in offshore and deepwater environments. http://www.weatherford.com
Halliburton Halliburton provides underbalanced solutions focused on enhancing reservoir performance with concern for safety and the environment remaining a top priority. Halliburton provide their own UBD separation systems and reservoir engineering. http://www.halliburton.com
Shaffer Shaffer supply rotating control head systems to the underbalanced drilling market
Tesco Tesco corporation supplies rig floor mounted snubbing systems to the underbalanced industry. http://www.tescocorp.com
LEAding Edge Advantage Leading Edge Advantage provides independent engineering and project management mainly focused on underbalanced coil operations. http://www.lealtd.com
Blade Energy Partners Blade provides independent engineering and project management to the underbalanced drilling industry. They also included advanced UBD training and UBD well control training as one of their products. http://www.blade-energy.com
Scandpower They also have developed a dynamic UBD simulator for training and wellsite purposes in conjunction with Scandpower. www.scandpowerpt.com
Neotec WELLFLO 7 has also become the industry standard software for flow modeling of underbalanced drilling (UBD) operations worldwide. http://www.neotec.com/
APR-WUBS-WFT-001 Revision 001 Issue Date:
January 2006 Introduction to underbalanced drilling
Ref APR-WUBS-WFT-001 Page 154 of 154
Abbreviations BHA Bottom Hole Assembly BHP Bottom Hole Pressure BOE Barrel of Oil Equivalent BOP Blow out Preventer ECD Equivalent Circulating Density EMWD Electromagnetic Measurement While Drilling ERD Extended Reach Drilling ESD Emergency Shutdown GPM Gallons Per Minute HAZOP Hazard Analysis Operations HPHT High Pressure High Temperature HSE Health Safety and Environment IADC International Association of Drilling Contractors MMscft/day Million standard cubic foot per day MWD Measurement While Drilling NDT Non Destructive Testing PCWD Pressure Control While Drilling PDM Positive Displacement Motor PSI Pounds per Square Inch RCD Rotating Control Diverter RBOP Rotating Blowout Preventer ROP Rate of penetration TD Total Depth TVD Total Vertical Depth UBD Underbalanced Drilling