transient hydraulics and multiphase kick tolerance...

ii

TRANSIENT HYDRAULICS AND MULTIPHASE KICK TOLERANCE STUDY

TO IMPROVE DESIGN OF NARROW MARGIN WELL

GOO JIA JUN

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Master of Petroleum Engineering

Faculty of Chemical and Energy Engineering

Universiti Teknologi Malaysia

JANUARY 2018

iv

To my beloved mother and father

v

ACKNOWLEDGEMENT

In preparing this thesis, I was in contact with many people, researchers,

academicians, and practitioners. They have contributed towards my understanding and

thoughts. I wish to express my sincere appreciation to my main thesis supervisor,

Associate Professor Issham Ismail, for his endless encouragement, guidance, critics

and friendship.

I am also very thankful to Schlumberger Wellog, Vestigo Petroleum,

PETRONAS Carigali and Shell Malaysia for providing the data required and software

in my research study. Librarians at UTM also deserve special thanks for their

assistance in supplying the relevant literatures.

My fellow postgraduate classmates should also be recognized for their support.

My sincere appreciation also extends to all my colleagues and others who have

provided assistance at various occasions. Their views and tips are useful indeed.

Unfortunately, it is not possible to list all of them in this limited space. I am grateful

to all my family members.

vi

ABSTRACT

Hydraulic and well control studies are the essential parts of well construction

planning, especially for drilling of complex and challenging wells with narrow drilling

margins. However, the complete applications of dynamic hydraulic analysis and

multiphase kick tolerance studies in well design are scanty, which result in ineffective

mud pressure management and extra cost spent on unnecessary casing strings, due to

excessive emphasis on previous practices (steady-state model) with liberal sprinkling

of safety factors. This research project was set out clearly to improve the well design

for narrow margin field, in terms of hydraulics and well control. A deductive

quantitative method constitutes major part of the research methodology, in which

simulation of real case studies and interpretation were conducted. The dynamic

hydraulics simulated equivalent circulating density (ECD) was compared with steady-

state results in terms of accuracy and extensiveness in providing a good well design.

In addition, the single bubble kick tolerance results which are commonly used by the

industry in spreadsheet format were compared with the multiphase model results.

Sensitivity studies were performed to understand the effect of each of the operational

or well design parameters towards primary and secondary well control. As compared

to steady-state hydraulics, transient model covers important parameters like pressure

and temperature dependent fluid properties, thermophysical properties, detailed

geometry description and operational effects, thus it is more representative to the

operational ECD. Meanwhile, multiphase kick model is proven to be more effective

for the evaluation of kick tolerance as it is able to provide the information of pressure

development during a well control operation, from initial influx and shut-in until influx

is circulated out of the well at the surface. This includes all phase transitions including

dissolving of a gas kick in oil based mud and breakout of free gas when the gas

contaminated mud reaches the bubble point at shallower depth in the well. The flow

model is much more accurate and reliable than the over-conservative traditional single

bubble theory.

vii

ABSTRAK

Kajian kawalan telaga dan hidraulik ialah komponen yang penting dalam

perancangan untuk membina telaga terutama bagi penggerudian yang rumit dan mencabar

dengan margin tekanan yang sempit. Namun begitu, tahap kesedaran yang rendah tentang

kepentingan penganalisisan hidraulik dinamik dan toleransi tendangan multifasa dalam

kerja mereka bentuk telaga telah menjejaskan keberkesanan pengurusan tekanan lumpur.

Akibatnya, kos tambahan diperlukan untuk pemasangan rentetan selongsong berikutan

penekanan terhadap model keadaan mantap yang sentiasa memberikan margin

keselamatan secara berlebihan. Kajian ini bertujuan untuk memperbaik reka bentuk telaga

yang menghadapi margin tekanan yang sempit, dari segi kawalan telaga dan hidraulik.

Kajian secara kuantitatif and deduktif ini melibatkan penyelakuan kes telaga sebenar dan

pentafsirannya. Ketumpatan peredaran setara (ECD) yang diperoleh daripada penyelakuan

hidraulik dinamik telah dibandingkan ketepatan dan rangkumannya dengan keputusan

daripada model keadaan mantap bagi menghasilkan reka bentuk telaga yang baik. Di

samping itu, keputusan toleransi tendangan gelembung tunggal yang menjadi amalan

industri turut dibandingkan dengan hasil daripada model multifasa. Kajian kepekaan

dilaksanakan untuk memahami kesan setiap parameter operasi dan reka bentuk telaga

terhadap kawalan telaga utama dan sekunder. Model hidraulik dinamik yang mencakupi

parameter penting, misalnya sifat bendalir yang peka terhadap suhu dan tekanan, sifat

termo-fizikal, huraian geometrik telaga, dan kesan operasi, didapati mampu memberikan

keputusan operasi ECD yang lebih praktikal berbanding model keadaan mantap. Selain

itu, model tendangan multifasa adalah lebih jitu dalam penilaian toleransi tendangan

kerana model terbabit mampu memberi maklumat tentang perubahan tekanan ketika

operasi kawalan tekanan, bermula daripada berlakunya tendangan gas dan kurungan,

hingga ke bendalir tendangan dikeluarkan di permukaan. Maklumat turut mencakupi

perubahan semua fasa termasuk keterlarutan gas tendangan di dalam lumpur dasar minyak

dan pelepasan gas bebas bila lumpur yang dicemari gas mencapai takat gelembungnya

pada kedalaman yang dangkal. Model aliran ini ternyata lebih tepat dan boleh dipercayai

berbanding teori gelembung tunggal yang konservatif.

viii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGCMENTS

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xiii

LIST OF EQUATIONS xvii

LIST OF ABBREVIATIONS xviii

LIST OF APPENDICES xix

1 INTRODUCTION 1

1.1 Background 1

1.2 Problem Statement 6

1.3 Hypothesis 9

1.4 Objectives 9

1.5 Research Scope 10

1.6 Significance of Study 12

1.7 Chapter Summary 12

2 LITERATURE REVIEW

2.1 Well Planning Cycle 13

2.2 Wellbore Stability Analysis 15

2.3 Wellbore Instability in Narrow Margin Drilling 20

2.4 Steady State Hydraulics and Dynamic Hydraulics 21

ix

2.5 Parameters Effecting Variation of ECD 24

2.5.1 Mud Pressure, Volume, Temperature (PVT)

Dependencies

24

2.5.2 Cutting Loadings 27

2.5.3 Mud Rheology 28

2.5.3.1 Bingham Plastic Model 29

2.5.3.2 Power Law Model (Ostwald–de

Waele Relationship)

29

2.5.3.3 Robertson-Stiff / Herschel Buckley

Model

30

2.5.4 Well Thermodynamics 30

2.5.5 Annular Flow Path 32

2.6 Single Bubble Theory 33

2.7 Application Software 37

2.7.1 WellPlan Hydraulics 38

2.7.2 Drillbench Dynamic Hydraulics 38

2.7.3 Sysdrill Hydraulics 39

2.7.4 WellPlan Well Control 40

2.7.5 Drillbench Well Control 40

2.7.6 Sysdrill Well Control 41


3 RESEARCH METHODOLOGY

3.1 Introduction 43

3.2 Research Design 43

3.2.1 Research Approach 44

3.2.2 Research Strategy 45

3.3 Detailed Research Flow Chart 46

3.4 Methodology 47

3.4.1 Data Request, Gathering and Validation 47

3.4.2 Hydraulics Simulation Model Building and

Sensitivity Study

50

x

Operators Well Control Standards and Single

Bubble Kick Tolerance Spreadsheet Review

52

Well Control Simulation Model Building and

Sensitivity Study

53

3.5 Well Data Input 56

3.5.1 Well 1 Data (Pressure Ramp Scenario) 56

3.5.2 Well 2 Data (Deep water, high temperature

wild cat drilling scenario)

60


4 RESULTS AND DISCUSSIONS

4.1 Comparison between Steady-state and Dynamic

Hydraulics Model

65

4.2 ECD Variations 66

4.2.1 Mud Pumping Rate 66

4.2.2 Wellbore Temperature 67

Surface Torque 68

Mud Weight 69

Rate of Penetration 70

Annular Flow Area 71

4.3 Dynamic Hydraulic Results 72

4.4 Hydraulic Results vs. Real Time Operational Data 75

4.5 Well Control Simulation Results 78

4.6 Model Applications, Benefits and Limitation 85


5 CONCLUSIONS AND RECOMMENDATIONS 87

REFERENCES 92

APPENDICES 99

GANTT CHART 101

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

1.1 Data input and parameters for research 11

2.1 Common well design modules 14

2.2 Mud weight window limits 16

2.3 The wells drilled in the US Gulf of Mexico deepwater and

shallow water, wellbore instability accounted for 34% to

56% of nonproductive time.

20

2.4 List of oil PVT correlation established between 1947 and

1998

24

2.5 List of water PVT correlation established between 1944

and 1987

25

2.6 Affecting parameters on temperature distribution profile of

a well

31

2.7 Common drilling hydraulics software 37

2.8 Common well control software 37

3.1 Data requirement 47

3.2 Data Quality Assessment Checklist (DQA) elements 49

3.3 Drilling parameters for sensitivity study 51

3.4 Example of kick tolerance requirement for different hole

section

52

3.5 Example of kick tolerance spreadsheet assumption and

formulas

53

3.6 Well control parameters for sensitivity study 56

3.7 Geothermal gradient 56

3.8 Well trajectory 56

3.9 Wellbore stability 57

xii

3.10 Casing profile 58

3.11 12-1/4in section drillstring 58

3.12 Drilling mud rheology 59

3.13 Geothermal gradient 60

3.14 Well trajectory 60

3.15 Wellbore stability 61

3.16 Casing profile 62

3.17 12-1/4in section drillstring 63

3.18 Drilling mud rheology 63

4.1 Dynamic and steady state hydraulic model comparison 66

xiii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Normal drilling margin (a) and narrow drilling margin

<1ppg (b)

1

1.2 Increasing water depth and under-compaction narrow the

drilling margin

2

1.3 Equivalent static density (a) and equivalent circulating

density (b)

3

1.4 Well design plot 4

1.5 Steady-state hydraulics results 7

1.6 The single bubble gas kick expands when circulated out of

well and create pressure upsurge

8

1.7 Research scope 10

2.1 Well planning workflow, starting from pre-drill well

design (blue) and while-drilling drilling monitoring (green)

13

2.2 ESD and ECD must be within the weight window (green)

to ensure wellbore stability is achieve. Four limits: pore

pressure, breakout, mud loss and breakdown are

established as operating envelops.

15

2.3 Compressional failure 16

2.4 Schmidt-pole graph which indicates the azimuth of 90° and

270° has minimum breakout pressure at depth 6353 ft.

(recommended direction)

17

2.5 Leak off test pressure profile 18

2.6 Schmidt-pole graph which indicates the azimuth of 90° and

270° has maximum breakout pressure at depth 6353 ft.

(recommended direction)

18

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2.7 High mud weight causes tensile failure 19

2.8 (a) Example of mud weight window produced by

geomechanics expert before well design. (b) Mud weight

window vs azimuth chart is shown to investigate the best

azimuth for well directional drilling by courtesy of

Schlumberger.

19

2.9 Governing equations for hydraulic analysis 22

2.10 Steady-state hydraulic computation workflow 22

2.11 Dynamic hydraulic computation workflow 23

2.12 Parameters effecting variation of ECD 24

2.13 PVT relationship for a drilling mud by courtesy of MI

Swaco

25

2.14 Drilling cuttings increases ECD 27

2.15 Drilling fluid rheology models 28

2.16 (a) Shearing of liquid due to resistive force due to viscosity

(b) Shear rate

28

2.17 Heat transfer schematics across well center to/from

formation

31

2.18 Increase of ECD due to (a) rough flow surface (b)

drillstring tool joints

32

2.19 Example hydraulic analysis results by courtesy of

Schlumberger

33

2.20 Single bubble and multiphase flow regimes 36

2.21 Screenshot of WellPlan steady-state ECD results 38

2.22 Screenshot of Drillbench ECD results 39

2.23 Screenshot of Sysdrill ECD results 39

2.24 Screenshot of WellPlan Well Control simulation 40

2.25 Screenshot of Drillbench Well Control simulation 41

2.26 Screenshot of Sysdrill Well Control simulation 41

3.1 Deductive Research Approach 44

3.2 The case study approach 45

3.3 Research flow chart 46

3.4 Drilling operational hydraulics modeling 50

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3.5 Drillbench Dynamic Hydraulics module version 2016.1.1 50

3.6 WellPlan version R5000.1.14 51

3.7 Interactive mode input of Drilbench Dynamic Hydraulics 52

3.8 Example of single bubble kick tolerance spreadsheet 52

3.9 Drillbench Dynamic Well Control module version

2016.1.1

54

3.10 Surface equipment delay input in Drillbench Dynamic

Well Control

54

3.11 Drillbench Dynamic Well Control operational control

parameters

54

3.12 Well Control operation sequence modeling 55

3.13 (a) Well schematics (b) well geothermal profile 58

3.14 (a) Well schematics (b) well geothermal profile 62

4.1 Steady-state and dynamic simulation scope 65

4.2 ECD vs. pump rate 66

4.3 ECD vs. mud inlet temperature 67

4.4 ECD vs. surface torque or RPM 68

4.5 ECD vs. mud weight used 69

4.6 ECD vs. ROP 70

4.7 ECD vs. drillpipe diameter 71

4.8 The trend of ECD at bit while drilling 72

4.9 The trend of bottomhole temperature while drilling 73

4.10 The trend of ECD at shoe while drilling 74

4.11 Steady state vs dynamic hydraulics results 75

4.12 Actual standpipe pressure (blue dotted line) vs dynamic

hydraulics results (black line)

76

4.13 Actual downhole temperature while drilling (blue dotted

line) vs dynamic hydraulics results (black line)

76

4.14 Actual ECD while drilling (blue dotted line) vs dynamic

hydraulics results (black line)

77

4.15 Single gas bubble and multiphase influx model 79

4.16 While single bubble kick tolerance modeling by using

spreadsheet neglect the dynamic effect and will provide

80

xvi

over-conservative results. It uses simplified and idealistic

model, Boyle’s Law to calculate the gas expansion which

is the model will never happen and make the drillable

depth to be shallower and more casings are needed.

4.17 Choke pressure profile generated by transient model, the

backpressure by choke is adjusted to maintain constant

bottomhole pressure (overbalance by 100psi)

80

4.18 Downhole pressure in time manner, the well pressure

overbalances pore pressure (denoted with dotted line) by

100psi during killing operation

81

4.19 Multiphase kick tolerance will provide realistic results and

is aimed to maximize the safe drilling and reduced the

number of casing

81

4.20 Surface pit gain profile. 50 barrels (required kick tolerance

value) of influx observed at surface before shut in. Pit gain

increase when free gas expands and reaches surface. Pit

gain back to zero when well killing is completed.

82

4.21 Gas flow rate out during killing operation. The top of gas

first reaches annular surface at 233rd minute. Maximum

gas rate out to be checked with degasser capability to

ensure it can divert the gas safely out of the well.

82

4.22 Standpipe pressure of around 350psi during shut in

signifies that the downhole pressure is underbalanced

350psi. Pump pressure of 350psi + 100 overbalance +

frictional mud pressure lost is needed during for well

killing

83

4.23 During killing operation, most of the influx gas (methane

in this case) is dissolved in the oil based mud. It is

circulated up until the top of dissolved gas reaches 400m

MD, the free gas starts to pop out at certain rate and

continues to travel out of borehole. Pit gain will increase

significantly when free gas reaches surface.

84

5.1 Flow simulation for blowout control from relief well 90

xvii

LIST OF EQUATIONS

EQUATION NO. TITLE PAGE

2.1 Volume of cutting 27

2.2 Bingham plastic rheology model 29

2.3 Power Law rheology model 29

2.4 Robertson-Stiff rheology model 30

2.5 Pressure at bit depth 34

2.6 Pressure of mud column in the open hole 34

2.7 Wellbore pressure at the assumed casing setting depth 34

2.8 Height of the gas bubble 34

2.9 Kick tolerance volume 34

4.1 ECD calculation 69

xviii

LIST OF ABBREVIATIONS

AMSL - Average Main Sea Level

BHA - Bottom Hole Assembly

BHP - Bottom Hole Pressure

BHT - Bottom Hole Temperature

BOP - Blowout Preventer

DP - Drill Pipe

ECD

ESD

-

-

Equivalent Circulating Density

Equivalent Static Density

FBHP - Flowing Bottom Hole Pressure

gpm - Gallons per minute

GUI - Graphic User Interface

ID - Inner Diameter

IWCF - International Well Control Forum

KT - Kick Tolerance

MD - Measured Depth

OBM - Oil Based Mud

OD - Outer Diameter

PI - Productivity Index

ppg - Pounds per Gallon

RKB - Rotary Kelly Bushing

SDM - Synthetic Based Mud

TD - Total Depth

TFA - Total Flow Area

TOC - Top of Cement

TVD - Total Vertical Depth

WBM - Water Based Mud

xix

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Non-disclosure Terms 98

1

CHAPTER 1

INTRODUCTION

1.1 Background

Well construction activities are executed within a pressure window bounded

on the lower side by reservoir pressure (is also known as pore pressure) and on the

upper side by the fracture pressure of the formation in an openhole (Figure 1.1). As the

drilling continuously expands into harsher and high complexity terrains, the pressure

window can be very narrow as less than 1 ppg, requiring the operations to be performed

by walking the tight rope between these limits.

(a) (b)

Figure 1.1 Normal drilling margin (a) and narrow drilling margin <1ppg (b)

2

Thus, conventional drilling methods typically become strained when

attempting to drill wells with narrow operational windows (Li et al., 2012). For

instance, non-productive time to drilling operational problems, such as kicks, loss

circulation, and stuck pipe events are commonly mount and result in hefty cost

(Noniface and Marcus, 2014).

Several subsurface settings can contribute to narrow margin, such as increasing

of water depths and widely varying formation pressures (Figure 1.2). Particularly in

deepwater environment, the high water depth alters the subsurface pressure profiles by

decreasing the overburden stress on the formation, which eventually translates to a

reduced fracture gradient in the vicinity of the wellbore. Meanwhile the pore pressure

gradient is typically determined by hydrostatic head of overlying fluid, therefore it is

not affected by the water depth (Rocha et al. 2004).

Apart from that, the presence of abnormal pore pressure pockets (where the

pore pressure is greater than hydrostatic pressure) further reduces drilling margin.

These pockets are originated from tectonic movements, salt dome effects, under-

compaction, as well as chemical and thermal actions (Freire et al., 2010). The ensuing

challenges share a commonality relating to safe navigation between the drilling

margins.

Figure 1.2 Increasing water depth and under-compaction narrow the drilling margin

3

The hydrostatic pressure created by the drilling fluid column is commonly

expressed as equivalent static weight (ESD), by relating the pressure at a specified

depth to its corresponding fluid density. During circulation conditions, the mud creates

an equivalent circulating density (ECD) which is higher than ESD. Referring to IADC

Lexicon, ECD, in the unit of ppg, is defined as the summation of fluid’s hydrostatic

pressure, cuttings, and annular friction pressure loss divided by vertical depth and by

0.052 (API RP59, 2012). ESD and ECD (Figure 1.3) must be kept between pore

pressure and fracture pressure gradients throughout the drilling process. Once the

wellbore pressure falls below the pore pressure, an influx of wellbore fluids can lead

to a kick or even an underground blowout. Similarly, excessive hydrostatic pressures

must also be avoided, as it can initiate and propagate fractures, which will cause loss

of circulating fluid into the formation (Stave, 2014).

(a) (b)

Figure 1.3 Equivalent static density (a) and equivalent circulating density (b)

4

Figure 1.4 illustrates the drilling window of a hypothetical well, the pressure is

represented by the step profile created when a successively heavier mud is used

(vertical line) and the wellbore is drilled to a maximum depth in which mud pressure

approaches pore pressure (horizontal line) which a casing is set.

In addition to ECD and ESD computation, the well design has to take

consideration of kick effect in which every section must be able to withstand the well

killing pressure without exceeding the fracture pressure. The maximum volume of gas

kick in barrels, known as kick tolerance is usually set by operator as a well design

standard which measure the integrity of the openhole to successfully shut in during

kick event and circulate the kick out of the hole without breaking the weakest

anticipated formation straight in wellbore (usually at previous casing shoe) (Redmann,

1991).

Figure 1.4 Well design plot

5

For instance, the section of 8.5 in hole requires 25 barrels kick tolerance as per

well design standard of the operator. Based on the bottom-up casing seat design

method, by using 11.1 ppg mud, the previous casing shoe can be set at 5000 ft.,

however, due to kick tolerance compliance, additional casing is set at 8000 ft. before

drilling to total depth (TD) at 12000 ft..

Generally, the challenges of the narrow margin drilling can be divided into two

main categories:

1. Drilling mud pressure variation

The series of standard drilling operations comprise of start and stop pumping

intervals as drill pipe connections are made during tripping and drilling operations.

When circulation stops, static conditions apply in the wellbore, and bottom-hole

pressure (BHP) is determined solely by the mud column hydrostatic pressure.

Restarting the pump will bring the well back into a dynamic state, thereby increasing

downhole pressure and re-establishing ECD. Such variances in bottomhole condition

can present added challenges, especially when seeking to maintain wellbore pressure

within a window.

Moreover, mud pressure changes based on well depth, thus there is a high delay

time of changing wellbore pressure when required as mud must first be mixed on the

surface and circulated downhole. In addition of relying on the MWD data, pressure

simulator is used to model and predict the downhole pressures, in order to give a quick

look ahead to gauge the hydraulics condition and precise control of ECD.

6

2. Casing string usage

In order to reach the target depth in narrow margin condition, the excessive use

of casing string is required. This is necessary to prevent mud losses as mud density is

incrementally elevated to contain wellbore pressure. Furthermore, the compliance to

kick tolerance worsens the case as additional casings are necessary to ensure well

integrity during well control events (Leblanc and Lewis, 1968). Further to heightened

expenses and drilling time, extra casing string can affect the production performance

as the flow area becomes smaller.

1.2 Problem Statement

As mentioned in the earlier section, the narrowing of the operating margin

available within the drilling window increases the technical challenges associated with

drilling operations. Strategic planning always yields huge improvement potential and

provides the highest impact on well performance without compromising the safety

aspect. First and foremost, in terms of drilling hydraulics, the need of accurate

computation of ESD and ECD is important in this case as it would optimize the well

design and determine the safe drilling and casing depths.

The steady-state hydraulics model is widely used by the well engineers during

planning phase which the magnitude of ECD is constant with time throughout the

entire domain analyzed at certain bit depth. Figure 1.5 shows the results of steady-state

hydraulics study which the ECD and ESD values are usually computed when the

drilling bit reaches TD. These results do not change in time, in which symbolize that

the bit reaches TD with certain rate of penetration (ROP) and circulation rate, drilling

operation is performed continuously until the ECD reaches static condition. This

condition is impossible as during drilling operation, the ECD has been observed to be

sensitive to cuttings and temperature changes, and also it does not explain how the

ECD varies starting with the operations of drilling out cement, making-up connection,

continuing drilling until TD, and hole cleaning. Multiple runs of computation have to

be performed in order to understand the whole well construction hydraulics effect.

7

Therefore, steady state hydraulics does not represent the operation condition

and most of the time the steady state model does not take into account parameter such

as fluid thermal effect and temperature profiles, cuttings (slip) effect, mud gel strength

effect, pressure-volume-temperature (PVT) relationship of the mud, etc. (Rommetveit

and Bjorkevoll, 1997). A good well design with narrow margin requires an in-depth

understanding of the transient hydraulics during all phases of the operation. The

drilling process is usually highly dynamic and complicated to model, thus, transient

modeling of drilling hydraulics has traditionally been neglected, although the model is

able to replicate a real drilling operation and provide a more accuracy prediction than

steady-state models.

Figure 1.5 Steady-state hydraulics results

Admittedly, kick tolerance is a key element when establishing a well

design. Apart from the mud weight design, the number of hole sections also relies on

the kick tolerance results computed using simple spreadsheet, which certain volume of

kick must be bearable by each sections (Wessel and Tarr, 1991). NORSOK (2004)

states that the recommended kick tolerance for each section size and the operators have

their own standard values which governs their well integrity, as a measure to prevent

underground blowout. However, neither the American Petroleum Institute (API) nor

International Association of Drilling Contractors (IADC) provides the method for

computing kick tolerance.

8

Generally, the kick tolerance is very well defined in open literature, the kick

models consider single gas bubbles (Figure 1.6) with simplifications regardless the

effect of frictional pressure loss during killing operation, equipment handling delay,

gas migration rate and gas solubility in oil based fluid (Mosti et al., 2017). The

assumptions are made too conservatively to ensure strict compliance for safety and

simplicity (Denney, 2012). The single gas bubble model does not characterize the

actual downhole behavior during a kick scenario, and the condition of single bubble is

impossible to happen (Santos et al. 2011). Conservative designs may increase costs or

prohibit drilling due to safety concerns, it does not mean the narrow margin well is

impossible to drill. A comprehensive optimization kick tolerance study is achievable

by multiphase kick modeling without compromising the safety and drilling standards.

Figure 1.6 The single bubble gas kick expands when circulated out of well and create

pressure upsurge: (a) A well control event starts with a gas influx, with pressure

gradient of 0.1 psi/ft. at the bottom of the well, (b) Gas bubble expands while traveling

up, (c) The casing shoe depth experiences the highest pressure when the top of gas

reaches the casing shoe depth, (d) The overall annular pressure profile during well

killing is greater than mud hydrostatic pressure, larger influx yields higher pressure.

(a) (b)

(c) (d)

9

1.3 Hypothesis

The detailed modeling of hydraulics in transient mode and multiphase kick

tolerance can assist in understanding better the degree of drillability of the narrow

margin hole which subsequently can improve the well planning. Hypothetically, an

accurate dynamic hydraulics model combines transient effects with a detailed

specification of fluid properties and geometry allows engineers to view the ECD in

time and develop, in order to promote effective pressure control and efficiently reach

the well’s objectives. ECD rises with the increase of mud pumping rate, mud weight

used, fluid viscosity, circulation time, and rate of penetration. Contrariwise, it declines

with the increase of temperature, rotational speed of the drill string, annular flow area,

and drilling depth.

During gas migration, the influx interacts with the mud and dissolves in the oil

phase of the mud until it reaches the bubble point condition at shallow depth. At bubble

point, it pops out from the oil and starts to expand. In the water-based mud, the influx

does not dissolve, instead it expands once it enters the well, with certain liquid hold up

(Umar et al., 2014). The single-bubble models cannot represent these dynamic

processes and often provide unrealistic results with higher casing shoe pressure.

1.4 Objectives

The objectives of the research cover the hydraulics and well control study

specifically for narrow margin wells:

1. To improve current steady-state drilling hydraulics study to transient model,

which can be a precise representation of operational equivalent circulating

density and offer strong potential to significantly impact the effectiveness of

well design and operations. It delivers quantum value to oil operators in terms

knowledge gained to achieve the operational excellence and reduce non-

productive time. It informs better operational planning and provide more

10

effective means of dealing with the narrow margin challenges in exploration

projects.

2. To model accurate multiphase kick tolerance which can improve the

effectiveness of the single bubble kick tolerance concept. The research

outcome can eliminate the need of unnecessary budget as the well is deemed

undrillable with too many casings or on the other hand an unreliable value that

could put in jeopardy the achievement of the drilling and safety objectives.

1.5 Research Scope

The research scope (Figure 1.7) focuses on the well design using the simulation

of realistic operation, covering the hydraulics changes in operational parameters over

time (circulating, static, drilling, and tripping) and multiphase kick tolerance analysis.

Primary well control: dynamic hydraulics

1. Comparing the steady state hydraulics and dynamics hydraulics results

2. Investigating the transient variation in ESD and ECD and identifying the

operational parameters that affect ECD (Table 1.1)

3. Optimizing drilling parameters which can result in the best hydraulics

condition

4. Verifying the dynamic hydraulics results with real time drilling data

Secondary well control: multiphase kick tolerance

1. Comparing the single bubble spreadsheet method from several oil

operators with multiphase kick tolerance simulation

2. Recognizing the improvement gaps of spreadsheet method

3. Performing multiple simulations for sensitivity assessments (Table 1.1)

Figure 1.7 Research scope

Finalized mud weight

11

Table 1.1 Data input and parameters for research

Simulation cases 1. Shallow water slanted well with pore pressure

ramp (overpressured reservoir)

2. Deep water vertical well with narrow margin

drilling window

Sections 12.25 in. and 8.5 in.

Parameters for ECD study 1. Mud temperature effect

2. Mud viscosity

3. Rate of Penetration / Cutting loadings

4. Drill string rotational speed and torque

5. Annular flow area

6. Mud weight

Kick tolerance sensitivity

study parameters

1. Water based or oil based mud

2. Kick intensity

3. Swabbed kick and drill kick scenario

Exclusions and

assumptions

1. The centrifuge effect of drill string rotational on

cuttings is negligible.

2. Normal drilling scenarios are simulated, managed

pressure drilling and dual gradient drilling

conditions are not studied.

3. Kick tolerance study with influx of pure methane

is presumed to happen at TD of each hole section,

at constant rate of 3 bbl/min.

4. Surge, swab effect and cementing pressure are not

studied.

5. Driller’s method is used for well control modeling.

6. Glasso oil PVT correlation model and Dodson-

Standing water PVT correlation model are used to

model the mud density.

7. Mud rheology model of Roberton-Stiff is used to

represent the correlation of shear rate and shear

stress of the mud.

12

1.6 Significance of Study

The narrow margin well condition creates the need to look beyond the

traditional way of well planning (steady state modeling) to increase operational

margin. Modeling the wells dynamically is essential and strategically important in

order to replicate the real drilling operations and provide accuracy which is not possible

with steady-state models. The transient analysis increases the drillers’ confidence in

accessing to this challenging field safely that would otherwise have been near

impossible to drill without understanding of the pressure control. This rigorous

approach in wellbore pressure management has led the engineers to decision making,

cost saving, and reaching maximum drillable depth safely.

Dynamic modeling in hydraulics and well control of a narrow margin deep

water hole delivers accurate predictions of wellbore pressure and temperature, linking

of safe drilling practices with drilling efficiency. The casing seat and maximum

drillable depths are optimized based on multiphase kick tolerance. The comprehensive

study of drilling hydraulics and kick tolerance thus offers strong potential to positively

impact well planning and operational practices for the operators.

1.7 Chapter Summary

The scope of studies covers primary and secondary well control of narrow

margin well which can be set as the best practice for operators to increase the

confidence in exploring or drilling the tight-margin well safely. The subject

background, objectives, research scope and significance of study were outlined to

realize the proposed research work and to understand the need of author to explore

more in this field of study.

92

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