underbalanced drilling technology-candidate

7/29/2019 Underbalanced Drilling Technology-Candidate

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UNDERBALANCED DRILLING TECHNOLOGY -CANDmATE SELECTION FOR OPTIMAL APPLICATION

D. Brant Bennion, Hycal Energy Research Laboratories Ltd.

What Is Underbalanced Drilling?bstractBy rigid definition. an "underbalanced" condition is

generated any time the effective circulating downholepressure of a drilling. completion. stimulation or workoverfluid (the pressure exerted by the hydrostatic weight of thefluid column, plus whatever pump pressure is applied to thefluid system to circulate or inject it, and the associatedfrictional pressure drop) is greater than the effective porepressure in the formation adjacent to the sand face. Mostformations, unless abnormally Pressured.are naturally placedin an overbalanced state when water based fluids of normaldensity are used in typical oilfield operations. In someabnormally high pressured onnations (and in somenormallypressured formations), a naturally underbalanced conditioncan be generated using conventional oil- or water-baseddrilling fluids. This condition, if it occurs during a drillingoperation. is normally termed "flow drilling" and has beenused successfully for many years in fonnations such as theAustin O1alk in Texas.

Underbalanced drilling (UBD) is used increasinglyworldwide as an alternative technique to conventionaloverbalanced drilling to reduce invasive near wel/borefonnation damage problems in oil and gas producingfonnations. Properly designed and executed underbalanceddrilling operations can eliminate or significantly reducefonnation damage concerns with respect to such phenomenaas mud or drill solids invasion, lost circulation, fluidentrainment and trapping effects, and potential adversereaction of invaded drilling or completion fluids with thereservoir matrix or in-situ reservoir fluids.

Underbalanced drilling and completion operations are notnecessarily a panacea for all formation damage problemsand. if an underbalanced drilling operation is poorlydesigned and executed, there is the potential for greaterdamage than if a properly implemented overbalancedoperation had been used.


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Most of the current discussion related to underbalanceddrilling, and that which this article focuses on, is a conditionwhere the fonnation pressure is sufficiently low that aneffective underbalance pressure cannot be achieveddownhole without the entrainment of some type of oon-condensable gas with the circulating drilling fluid to lowerthe effective fluid density to the point where anunderbalanced condition is obtained. This is often referred toas the "artificial" generation of the underbalance condition.

becauseof periodic overbalanced pressure pulses dueto:

Advantages of Underbalanced DrillingSome of the advantagesof underbalanced drilling include

- Pipeconnections- Bit trips- ConventionalMWD signal tansmissions- Localizeddepletioneffects- Frictional flow effects- Hydrostaticcolumn effects- Multiple different pressured ones- Variablepressuren a commonzone- Poorknowledgeof original reservoirpressure- Operationalor supply problems

Spontaneous ountercurrentmbibition effectsGravity drainageeffects n high penneability zones.evenunderconstantunderbalancedlow conditionsCondensate ropout/gasiberation effectsGlazing or mashing (near weUbore mechanicaldamage)Increased ropensity or corrosionproblems f air oroxygen content reduced air is used as the non-condensablegas medium used to generate theunderbalancedondition

The underbalanced condition must be maintainedcontinuously;however,even more significant damagemayhave occurred than in the conventional overbalancedsituationdue o the ow viscosity fluid, high solids contentand ack of a protective ilter cakepresentduring the UBDoperation f the underbalance ondition is compromised.11ris s schematicallyllustrated n Figure 1.

potential eduction n invasive onnation damageminimize the potential or lost circulationminimize the potential or differential stickingeliminate need for costly mud systemsand costlydisposalof exotic mudsimprovedROPon drilling, reducingdrilling costsandincreased it lifemitigation of extensive ndexpensive ompletionandstimulationoperationsrapid indication of productive zonesof the reservoirduring drillingpotential economic benefit from flush productionduring drillingpotential o flow test while drilling

Disadvantagesof Underbalanced DrillingMany studies have investigatedsome of the potential

disadvantages f the UBD process,particularly if poorlydesigned nd executed Thesewould includesuchconcerns

What Are The Issues o Be AddressedUnderbalanced rilling is not a solution o poor reservoir

quality. The process does not inherently manufacturepenneability. It can.however,yield significant ncreasesnproduction f the problem with conventionaloverba1ancedcompletionshas been a high degreeof fom1ationdamagewhich cannot be overcome by conventional fluid oroperational design practices (ie - severe ost circulation.extremepermeabilityzones,significant pressure epletion,major rock-fluid sensitivity issues with reactive clays,emulsions, etc.), or if technical problems (differentialsticking) have occurred.Many operatorsattempt o applyUBD technology in extremely poor quality fom1ations,expectinghigh production rates when, even f the well is

Wellbore stability and consolidation oncernsSafetyand well control concernsn high pressure rsour environmentsIncreased osts or drilling (andcomplexcompletionsif requireddue o the essential atureof completingthe wells in a live condition)Inability to use conventionalMWD technology orthrough string njection techniquesFailure to maintain a continuously underbalancedcondition resulting in significant invasive damage

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drilled and completed n a completelyundamagedashion.productivity s uneconomicdue o permeability imitations.The application of UBD technology n horizontal wells toattempt to intersect macro or micro ractures in a lowpermeability matrix, or to obviate gas or water coningproblems n vertical wells, would be potentialexceptionsothis rationale.

The majority of these through string injection UBDprojects have been conducted using water-based luids (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 fairassessmentas t is often considered an expensive alternativeto conventional overbalanced drilling which is expected tosolve a vast array of impaired productivity problems and is,therefore, applied in situations of marginal reservoir qualitywhere we know that a conventional overbalanced approachhas not been successful. This, perhaps, s one of the reasonswhy we see a relatively high economic failure rate amongUBD projects as the field of applications have, in general,been biased towards more marginal quality reservoirs inmany 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 - 6500bbl/day) range. Detailed calculations and monitoring arerequired to ensure hat 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 pipesticking problems.

The main criteria n decidingwhetheror not to implementUBD technology n a given reservoirsituationare:

Real Time Data AcquisitionReal time data recording and gathering systems,bothdownhole and at the surface, are essential o a properlyexecutedUBD ope~tion. This allows for the monitoring ofthe perfom1ance f the entire operation to maintain anoptimum underbalanced ondition.

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3 Many early UBD operations did not have the ability toconduct 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 hat 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 theproject?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 ProcessesFigure 2 provides a schematic llustration of a typical UBD

process 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 hisis 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 Useof 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 lossesaround the BRA are high enoughto create a restricted flow zone. Jointed pipe operations willlikely continue to be used n 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 or an effective underbalanceddrilling 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 n thearea of underbalanced drilling.

What Are the Optimum Operating Conditions?The question is often asked as to what type of

underbalance pressure gradient is required for an effectiveUBD operation. Typically, circulating pressuressufficient 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 jointedpipe systems.

Any UBD operation results in localized depletion effectsoccurring in the near wellbore region as llustrated in Figure3. Periodic slight increases n circulating pressure, even f 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 OCCUJTenCCSuring 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 controlsystemsused 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 5-100/0over the length of the horizontal sectionare recommended to counteract this effect. Such reductions

What is the Best Operational System o Use?This question must be answered from a technical, safety

and 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 pressurelosseswill also mimic this effect if efforts are maintained tokeep a consistent pressure at the bit as the drilling operationproceeds.

Formations Exhibiting Extreme Rock-Fluid SensitivityConsiderable formation damage can be caused by the

adverse 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, n 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 or UBD ProcessSelectionMany factors must be considered by the integrated team to

ascertain f 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. extremepermeabilityconsolidated/unconsolidated ntercrystalline formationsand zonesof extreme ressuredepletion)

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 causedby contact withincompabole invading foreign hydrocarbon based fluids orthe formation of scales and precipitations causedby 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 overbalancepressuresof7 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 fustplace.

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 ead to expensive and sometimes erminalproblems associated with stuck.pipe.

Formations Exhibiting Subirreducible Oil or WaterSaturationsThe mechanism of aqueous and hydrocarbon phasetrapping has been discussedn detail in the literature'I.'1Pennanent ntrapment f wateror hydrocarbons n the nearwellbore region can result in significant reductions n theproductivity of the formation due to adverse relative

permeability effects. Underbalanced rilling, if the wrongbase fluid is used, may aggravate his problem due tospontaneouscountercurrent imbibition effects', but ifappropriatedesignedUBD technology can be an efficientmeansof mitigating potential problemswith adverse luidretentionand rappingeffects.Use of a non-wetting luid as

Some luid lossesmay occur when drilling horizontal wellsin systems exbt"biting extreme macroscopic permeabilityfeatures due to gravity induced drainage.

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surface njection pressures re required hen a large volumeof pressuredluid in presentat surface n the uninjectedcrstring which may alsopresenta potential safetyhazard.the base luid 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 o 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 pressuremeasurementsare essential o ensure he successof 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 QualityHighly laminated fonnations or more massive sandstone

or carbonate fonnations which exhibit a wide variation inreservoir permeability and porosity (ie - a large variation inpore 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.FornrationsExhibiting 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 orpurposes 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 PermeabilityAlthough deep, high pressure, high penneability zones

represent 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 n 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

Reservoirswhich may exhibit zonesof multiple different6


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or because t appears o 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 necessarydatato 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 andproduction potential.

pressures r a significant areal variation of pressure n agiven targetzone may be difficult candidatesor 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 hose described above it is likely thatwell designed conventional overbalanced operations can bedesigned and implemented yielding comparable or superiorresults to a more expensive and risky underbalancedoperation.

AcknowledgementsThe author expresses ppreciation o Vivian Whiting forher assistance n the preparation of the manuscript and

figures and to the management f Hycal Energy ResearchLaboratories or the funding of this project andpermissionto present he paper.

ConclusionsUnderbalanced rilling is a very complex processwhichshouldnot be designed nd mplemented n a gut feel basis,

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underbalanced drilling technology-candidate

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