UNDERBALANCED DRILLING TECHNOLOGY -CANDmATE SELECTION FOR OPTIMAL APPLICATION

D. Brant Bennion, Hycal Energy Research Laboratories Ltd.

What Is Underbalanced Drilling?Abstract

By rigid definition. an "underbalanced" condition is

generated any time the effective circulating downhole

pressure of a drilling. completion. stimulation or workover

fluid (the pressure exerted by the hydrostatic weight of thefluid column, plus whatever pump pressure is applied to the

fluid system to circulate or inject it, and the associated

frictional pressure drop) is greater than the effective pore

pressure in the formation adjacent to the sand face. Most

formations, unless abnormally Pressured. are naturally placedin an overbalanced state when water based fluids of normal

density are used in typical oilfield operations. In some

abnormally high pressured fonnations (and in some normally

pressured formations), a naturally underbalanced condition

can be generated using conventional oil- or water-based

drilling fluids. This condition, if it occurs during a drilling

operation. is normally termed "flow drilling" and has beenused successfully for many years in fonnations such as the

Austin O1alk in Texas.

Underbalanced drilling (UBD) is used increasingly

worldwide as an alternative technique to conventional

overbalanced drilling to reduce invasive near wel/bore

fonnation damage problems in oil and gas producingfonnations. Properly designed and executed underbalanced

drilling operations can eliminate or significantly reduce

fonnation damage concerns with respect to such phenomena

as mud or drill solids invasion, lost circulation, fluidentrainment and trapping effects, and potential adversereaction of invaded drilling or completion fluids with the

reservoir matrix or in-situ reservoir fluids.

Underbalanced drilling and completion operations are not

necessarily a panacea for all formation damage problemsand. if an underbalanced drilling operation is poorly

designed and executed, there is the potential for greater

damage than if a properly implemented overbalanced

operation had been used.

Most of the current discussion related to underbalanced

drilling, and that which this article focuses on, is a condition

where the fonnation pressure is sufficiently low that an

effective underbalance pressure cannot be achieved

downhole without the entrainment of some type of oon-

condensable gas with the circulating drilling fluid to lower

the effective fluid density to the point where an

underbalanced condition is obtained. This is often referred to

as the "artificial" generation of the underbalance condition.

because of periodic overbalanced pressure pulses due

to:

Advantages of Underbalanced Drilling

Some of the advantages of underbalanced drilling include

- Pipe connections- Bit trips- Conventional MWD signal ttansmissions- Localized depletion effects- Frictional flow effects- Hydrostatic column effects- Multiple different pressured zones- Variable pressure in a common zone- Poor knowledge of original reservoir pressure- Operational or supply problems

Spontaneous Countercurrent imbibition effectsGravity drainage effects in high penneability zones.even under constant underbalanced flow conditionsCondensate dropout/gas liberation effectsGlazing or mashing (near weUbore mechanical

damage)Increased propensity for corrosion problems if air oroxygen content reduced air is used as the non-condensable gas medium used to generate theunderbalanced condition

The underbalanced condition must be maintainedcontinuously; however, even more significant damage mayhave occurred than in the conventional overbalancedsituation due to the low viscosity fluid, high solids contentand lack of a protective filter cake present during the UBDoperation if the underbalance condition is compromised.11ris is schematically illustrated in Figure 1.

potential reduction in invasive fonnation damageminimize the potential for lost circulationminimize the potential for differential stickingeliminate need for costly mud systems and costlydisposal of exotic mudsimproved ROP on drilling, reducing drilling costs andincreased bit lifemitigation of extensive and expensive completion andstimulation operationsrapid indication of productive zones of the reservoir

during drillingpotential economic benefit from flush production

during drillingpotential to flow test while drilling

Disadvantages of Underbalanced Drilling

Many studies have investigated some of the potentialdisadvantages of the UBD process, particularly if poorlydesigned and executed These would include such concerns

What Are The Issues to Be Addressed

Underbalanced drilling is not a solution to poor reservoirquality. The process does not inherently manufacturepenneability. It can. however, yield significant increases inproduction if the problem with conventional overba1ancedcompletions has been a high degree of fom1ation damagewhich cannot be overcome by conventional fluid oroperational design practices (ie - severe lost circulation.

extreme permeability zones, significant pressure depletion,major rock-fluid sensitivity issues with reactive clays,emulsions, etc.), or if technical problems (differentialsticking) have occurred. Many operators attempt to applyUBD technology in extremely poor quality fom1ations,expecting high production rates when, even if the well is

Wellbore stability and consolidation concernsSafety and well control concerns in high pressure orsour environmentsIncreased costs for drilling (and complex completionsif required due to the essential nature of completingthe wells in a live condition)Inability to use conventional MWD technology for

through string injection techniquesFailure to maintain a continuously underbalancedcondition resulting in significant invasive damage

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drilled and completed in a completely undamaged fashion.productivity is uneconomic due to permeability limitations.The application of UBD technology in horizontal wells toattempt to intersect macro or micro fractures in a lowpermeability matrix, or to obviate gas or water coningproblems in vertical wells, would be potential exceptions tothis rationale.

The majority of these through string injection UBDprojects have been conducted using water-based fluids (oftenproduced water) and nitrogen. The average gas injectionrates have typical1y been in the range of 30-40 mJ /min (1.5 -

2.0 MMscf/day), although this is highly dependent on thereservoir pressure and the amount of reservoir gas which canbe produced to assist in the UBD operation once theproducing zone has been penetrated. In UBD operationsexecuted in gas reservoirs, once penetration of the zoneoccurs, the UBD operation may be partially self-supportingwith the produced gas, and very little supplemental gas maybe required with sufficient fluid injection required only foradequate downhole motor and drilling operations.

Often, underbalanced drilling technology is not given a fair

assessment as it is often considered an expensive alternative

to conventional overbalanced drilling which is expected to

solve a vast array of impaired productivity problems and is,

therefore, applied in situations of marginal reservoir quality

where we know that a conventional overbalanced approach

has not been successful. This, perhaps, is one of the reasons

why we see a relatively high economic failure rate among

UBD projects as the field of applications have, in general,

been biased towards more marginal quality reservoirs in

many situations.

Liquid circulation rates vary widely depending on thebottomhole pressure required and the amount of fluid beingproduced from the fom1ation. The liquid circulation rate is adominant factor in the control of the effective bottomholepressure. Typical liquid flow rates in many recent UBDoperations have been in the 0.3-0.8 m3 /minute (2400 - 6500

bbl/day) range. Detailed calculations and monitoring arerequired to ensure that significant annular slug flow does notoccur, which can cause large bottomhole pressure swings,but that sufficient turbulence is maintain to ensure adequatehole cleaning and cuttings transport to minimize pipe

sticking problems.

The main criteria in deciding whether or not to implementUBD technology in a given reservoir situation are:

Real Time Data Acquisition

Real time data recording and gathering systems, bothdownhole and at the surface, are essential to a properlyexecuted UBD ope~tion. This allows for the monitoring ofthe perfom1ance of the entire operation to maintain anoptimum underbalanced condition.

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3Many early UBD operations did not have the ability to

conduct real time downhole pressure measurements,resulting in many supposedly underbalanced wells beingdrilled in a completely or periodically overbalanced state.Later improvements used memory pressure gauges whichallowed acquisition of downhole pressure proflles, but onlyafter the fact and damage had been done. when it wasascertained that a continuously underbalanced condition hadnot been maintained. The more recent development of EMdownhole tools. capable of transmitting downhole pressureand survey data directly back to surface via anelectromagnetic pulse or cr applications with an internalWireline for direct downhole data transmission. have greatlyenhanced the technology for UBD by allowing real timeacquisition of pressure and location data while drilling.

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Considering past experience, can this well be drilledand completed using overbalanced technology?(Note: Not necessarily the same overbalancedtechnology that bas been used in the past which gavemarginal or disappointing results).Based on an engineering evaluation of reservoirparameters, do we feel that UBD will provide asignificant technical or economic advantage to the

project?What is the risk of failure/problems associated withthe UBD operation?Does the expected increase in value justify theincreased risk relative to the conventionaloverbalanced drilling/completion practices?

Overview of Current UBD Processes

Figure 2 provides a schematic illustration of a typical UBDprocess using a closed surface control system. The vastmajority of UBD operations executed to date have utilizedjointed drill pipe and the direct injection of the base drillingfluid and some type of noncondensible gas directly down thecentre of the drill string. This is not necessarily because thisis the best technology to use, but because it is the leastexpensive and easiest to execute with the number of rotaryrigs and conventional drilling equipment available.

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The Use of Coiled Tubing parasite string approach. The presence of the weight of a fullhydrostatic column of fluid in the drill sbing may result in azone of localized overbalance pressure and flushing at the bitand around the BRA, if pressure drop in the motor assemblyis low, and frictional losses around the BRA are high enoughto create a restricted flow zone. Jointed pipe operations willlikely continue to be used in the majority of systems for thenext few years until cr costs and availability, plusimprovements in cr technology, allow it to assume a moredominant position in the UBD marketplace.

In many aspects coiled tubing is a system which is customdesigned for an effective underbalanced drilling process. Theadvantages of coiled tubing are obvious in that the singleunjointed pipe facilitates the more ready operation of theUBD process in a continuously underbalanced mode. Theuse of an internal wireline facilitates accurate real timeMWD measurements which may be significantly lessproblematic than EMT tools and surface pressure operatingconstraints with coiled tubing are much greater (10,000 psi)in comparison to the limited pressure range of 1500 psicurrently offered by the majority of rotating control headsused in jointed pipe UBD operations. Coiled tubingtechnology has now proven to be a viable and practicalmethod for the effective drilling of horizontal and verticalwells, especially in re-entry work out of smaller casing sizes,deepenings and sidetracks where one wants to just barelyenter the zone to get flow back to surface plus the re-enteringof existing horizontal wells to drill more extensions and legsunderbalanced. As the technology continues to improve thereis definitely going to be a niche market for CT services in thearea of underbalanced drilling.

What Are the Optimum Operating Conditions?

The question is often asked as to what type ofunderbalance pressure gradient is required for an effectiveUBD operation. Typically, circulating pressures sufficient toyield a 10-30% drawdown are often considered normal, butthis is highly reservoir specific. In high pressure and highpermeability formations this may yield uncontrollable flowrates at surface and possibly contribute to premature water orgas coning or hydrocaJbon dropout in rich gas systems.Current surface control equipment exists to handle up to2.8xlQ6m3/day (100 MMscf/day) of gas flow and up to 4000m3/day (25,000 bbVday) of produced fluids. but detailedevaluation of the flow control and coning characteristics ofthe target formation must be carefully evaluated prior toimplementing the UBD operation and designing the effectiverequired UBD drawdown gradient.

The disadvantages of coiled tubing are the time andcommitment involved to use a new technology, availabilityof equipment, concerns over accurate depth control andsafety concerns in high pressure wells with large volumes ofpressurized gases at surface in the exposed non-injected crsb"ing. Also of concern are depth control and reachlimitation for horizontal drilling applications. The majorcunent challenge for a cr operation is obtaining theexperienced personnel and mobilizing the equipment on siteto make the technology a cost effective serious contenderwith the more readily available and heavily favoured jointed

pipe systems.

Any UBD operation results in localized depletion effectsoccurring in the near wellbore region as illustrated in Figure3. Periodic slight increases in circulating pressure, even if theoperator feels that an underbalanced condition is still beingeffectively maintained at the bit, can result in potentialoverbalanced fluid invasion into a previously drilled sectionof the horizontal wellbore. These oscillations in BHP arenatural OCCUJTenCCS during pipe connections, slug flow,system upsets and as the base system mud weight increasesdue to a higher solids load as the mud and fluid system isused for a greater period of time (due to the generallymediocre solids separation capability of most surface controlsystems used in typical UBD operations). This phenomenonmay be partially counteracted by gradually attempting toincrease the effective underbalance pressure as the well isdrilled to maintain every section of the formation. regardlessof the time at which it was drilled, in a continuous state ofgradually increasing drawdown which will ensure that acontinuous underbalance condition is uniformly maintainedthroughout the well. Increases in effective drawdowngradient of 5-100/0 over the length of the horizontal sectionare recommended to counteract this effect. Such reductions

What is the Best Operational System to Use?

This question must be answered from a technical, safetyand economjc point of view. Technically, the system whichwill yield the best bottomhole pressure control with acontinuously underbalanced condition is optimum with therequired necessity of continuous real time bottomholelocation and pressure measurements. This heavily favourscoiled tubing (particularly for small diameter holes and re-entries), but cost, availability and required horizontal reachmay eliminate cr as a serious contender. Alternate parasitestring or annular injection techniques may also beconsidered. These techniques are often costly and, from anabandonment perspective, may be more problematic for the

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in pressure may not always be possible due to frictionalbackpressure effects and fomJation inflow which may occurduring the drilling operation. Natural frictional pressurelosses will also mimic this effect if efforts are maintained tokeep a consistent pressure at the bit as the drilling operation

proceeds.

Formations Exhibiting Extreme Rock-Fluid Sensitivity

Considerable formation damage can be caused by theadverse reaction of incompatible water based filtrates within-situ clays or other reactive materials. Many formationscontain hydratable clays such as smectite or mixed layerreactive clays. These clays will expand on contact with 000-inhibited water based systems and can significantly affectproductivity and, in some cases, near weUbore consolidation.Some formations may also contain deflocculatible clays andfines or velocity migratable materials (ie - kaolinite clay,

detrital rocks fragments, pyrobitumen, anhydrite, etc). Manyof these types of problems can be addressed through theappropriate use of overbalanced technology withhydrocarboo based or inhibitive water based fluids.

Screening for UBD Process Selection

Many factors must be considered by the integrated team toascertain if a particular reservoir is, in fact, a prime candidatefor an underbalanced drilling operation. The followingsection provides some examples of some cotlUDon reservoirtypes which are and are not generally suitable forconsideration as UBD candidates.

Potential Prime Reservoir Applications for UBD Formations Exhibiting Significant Fluid-Fluid Sensitivity

Reservoirs with Significant Loss Circulation or FluidInvasion Potential (fractures, vugs. extreme permeabilityconsolidated/unconsolidated intercrystalline formationsand zones of extreme pressure depletion)

The invasion of incompatible drilling fluid filtrates into theformation can result in potential incompatibility betweenthese invading fluids and the insitu formation brine or oil.Reactions could include the formation of extremely viscousoil in water emulsions which may become entrapped in thenear wellbore region and cause reductions in penneability,deasphalting of insitu reservoir crude caused by contact withincompabole invading foreign hydrocarbon based fluids orthe formation of scales and precipitations caused by reactionwith water based filtrates and in-situ formation brines.

This would include zones of exb"eme intercrystallinepermeability (1000 roD and greater), large macroscopic openfractures, heterogenous carbonates with massiveinterconnected high permeability vugular porosity, zones ofextreme pressure depletion (resulting in extreme overbalancepressures of7 MPa or greater) or the worst case scenario ofa combination of one of these types of high permeabilityfeatures in a significantly pressure depleted situation.

Appropriate geochemical testing and compab"bility testingcan generally eliminate this problem for most conventionaloverbalanced operations. Conversely, in some situationswhere the potential for damage is exb"eme. UBD may becontemplated as a mechanism to avoid the introduction ofthe potentially reactive material into the formation in the fust

place.

These reservoir candidates are prime applications forunderbalanced drilling due to the difficulty, particularly inthe fractured and heterogenous carbonate situations, ofdesigning effective overbalanced fluid systems which willgenerate uniform and stable filter cakes which will preventsignificant invasion of the near wellbore region bypotentially damaging mud filtrate and solids, yet still bereadily removable to allow unrestricted flow from theformation. When filter cakes do form in situations such asthey often tend to result in problems with differentialsticking which can lead to expensive and sometimes terminalproblems associated with stuck. pipe.

Formations Exhibiting Subirreducible Oil or WaterSaturations

The mechanism of aqueous and hydrocarbon phasetrapping has been discussed in detail in the literature'I.'1.Pennanent entrapment of water or hydrocarbons in the nearwellbore region can result in significant reductions in theproductivity of the formation due to adverse relativepermeability effects. Underbalanced drilling, if the wrongbase fluid is used, may aggravate this problem due tospontaneous countercurrent imbibition effects', but ifappropriate designed UBD technology can be an efficientmeans of mitigating potential problems with adverse fluidretention and trapping effects. Use of a non-wetting fluid as

Some fluid losses may occur when drilling horizontal wellsin systems exbt"biting extreme macroscopic permeabilityfeatures due to gravity induced drainage.

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surface injection pressures are required then a large volumeof pressured fluid in present at surface in the uninjected crstring which may also present a potential safety hazard.

the base fluid for and UBD operation will generally negatethe potential for spontaneous imbibition and reduce thepotential for phase trapping as long as a continuouslyunderbalanced condition is maintained and we avoid thedirect displacement and entrapment of the base drilling fluidinto the fonnation.

Situations Where a Continuouslikely to be Compromised

Inderbalance Condition is

This is one of the single greatest flags to an underbalanceddrilling operation. Much of the benefit ofUBD is lost and theoperator may actually be in a greater damage scenario ifunderbalanced conditions are not maintained continuouslyduring both the drilling and completion operation. There islittle advantage to drilling in a underbalanced mode (unlessincreased ROP is the only motivation for UBD), and thenusing normal overbalanced practices to complete the well. Itshould be emphasized that real time downhole pressuremeasurements are essential to ensure the success of any UBDoperation. finding out after the fact that we were notunderbalanced is little consolation for a failed job. The useofEMT technology for conventional jointed pipe drilling hasgreatly improved the ability to monitor downhole pressuresin these operations (without periodically pulsing theformation with overbalance pressure for conventional MWDoperations or using costly parasite or concentric stringapproaches). The reliability ofEMT technology still limits ituse in deeper formations (greater than 2500 m), but currentimprovements, such a mid string repeater stations, promiseto extend the technology to potentially deeper applications.

Fonnations of Highly Variable Quality

Highly laminated fonnations or more massive sandstoneor carbonate fonnations which exhibit a wide variation inreservoir permeability and porosity (ie - a large variation in

pore throat size distribution) represent major challenges withrespect to designing effective overbalanced fluid systemswhich will effectively bridge and protect the wide range ofpore features that exist in these situations. In general,overbalanced systems are designed in these scenarios toattempt to protect the better quality portions of the matrix, asthis is expected to be the zone from which the majority ofproduction occurs. This can result in significant damage toportions of potentially productive fonnation in some cases.The use ofUBD technology in some of these situations mayresult in more unifoml production from the target interval.

Fornrations Exhibiting Law ROP

The preceding motivations for UBD primarily centredabout formation damage, drilling execution and productionconcerns. For some hard rock formations significantlygreater ROP's can be obtained with UBD due to greaterweight on bit. This may significantly reduce drilling timesand associated costs. In a limited number of cases theprimary motivation for UBD in some operations has been forpurposes of ROP. rather than some of the previouslymentioned factors.

Coiled tubing drilling represents what many feel is thefuture in UBD due to our ability to maintain a relativelycontinuously underbalanced condition and MWD using aless problematic internal wireline approach. Current crtechnology is limited with respect to depth and horizontaloutreach capability for extended reach well applications.Pressure pulses during connections with a conventionaljointed pipe can be minimized by using double pipe stands,rapid connections and appropriate circulation practices priorto breaking for connections to minimize the degree ofdegradation of underbalance pressure that occurs during orafter the connection is made. Top drive rigs offer theadvantage of drilling with triple pipe stands which furtherreduces the number of connections required.

Major Reservoir CriteriaUnderbalanced Drilling

Which Contraindicate

Combinations of Extreme Pressure and Permeability

Although deep, high pressure, high penneability zonesrepresent perhaps one of the best potential applications forUBD from a formation damage perspective, safety andcontrol issues with respect to well control at the surface maybecome problematic when downhole pressures in excess of30 MPa are contemplated (particularly in gas reservoirapplications) when using conventional rotary drillingequipment and rotating diverter heads. The use of coiledtubing drilling in such a situation is preferable, as surfacepressure ratings are much higher, but conversely if high

Other factors would include any type of reservoir whereany type of hydrostatic kill would be required for specializedcompletions, bit trips, etc.

Reserroir Pressure Constraints

Reservoirs which may exhibit zones of multiple different

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or because it appears to be the b"endy approach to a difficultproblem. When properly designed and executed UBDprovides a whole new approach to complex reservoirmanagement problems and may facilitate the economiccompletion and exploitation of reserves unobtainable by anyother type of currently available technology. Detailed studyand design of the UBD process by a multifaceted reservoirteam is required along with acquisition of the necessary datato ensure that the operation is viable for the reservoir underconsideration. If UBD is considered, the process must bedesigned, implemented and monitored correctly. Failure tocarefully plan and design may result in the improperapplication of UBD technology (in a potentially viablesituation), resulting in significant losses of capital and

production potential.

pressures or a significant areal variation of pressure in agiven target zone may be difficult candidates for UBD.

Normally Pressured /ntercrystalline Formations of LessThan 500 mD Permeability and Limited Rock-Fluid andFluid-Fluid Sensitivity

The case can be made that most fonnations may benefitfrom a perfectly designed and executed UBD operations,which is likely true. The unfortunate fact is that UBDoperations tend to be considerably more expensive thanconventional overbalanced drilling and by their nature maybe fraught with risks and problems. For normal "plain Jane"type formations such as those described above it is likely thatwell designed conventional overbalanced operations can bedesigned and implemented yielding comparable or superiorresults to a more expensive and risky underbalanced

operation.Acknowledgements

The author expresses appreciation to Vivian Whiting forher assistance in the preparation of the manuscript andfigures and to the management of Hycal Energy ResearchLaboratories for the funding of this project and permissionto present the paper.

Conclusions

Underbalanced drilling is a very complex process whichshould not be designed and implemented on a gut feel basis,

.,