PElROLEUM SOCffiTY OF CIM PAPER NO. PTS 98-58

D.B. Bennion. F.B. Thomas. A.K.M. Jarnaluddin. T.MaHycal Energy Research Laboratories Ltd.

increase rates of penetration, reduce invasive fonnationdamage and reduce significant problems with drillingdue to lost circulation and differential sticking. Manysuccessful in1plementation stories of underbalanceddrilling are evident in the literaturel.2; however,underbalanced drilling is not a panacea for all fonnationdamage problems in that inappropriately designedunderbalanced drilling jobs may actually result in moreformation damage than if a weU-designed andcontemplated overbalanced job had been used in thesame situation in some circumstances.

Abstract

UnderbaJanced drilling (UBD) is becomingincreasingly used as a technique to reduce significantinvasive formation damage in vertical and horizontalwells to improve production rates of oil and gas. andenhance injectivity in gas andwater injection situations.UBD may be a technically demanding procedure toexecute in certain reservoirs and much of the benefitwith respect to mitigation offormation damage may becompromised if the underbalanced drilling operation isnot screened; designed and conducted in an appropriatefashion. This paper reviews commonformation dam.agemeclKmisms which may occur in reservoirs and how, incertain situations, these types of damage may bereduced or eliminated through the appropriate use ofunderbalanced drilling technology. Various situationsin which underbaJanced drilling technology may resultin potential significant formation damage are alsodiscussed

This paper reviews some of the current technology inuse at the present time in underbalanced drilling andillustrates some ofdte points which operators should beaware of before embarking on an underbalanced drilling

operation.

What is Underbalanced Drilling (UBD)?

A rigorous definition ofUBD is the situation in whichthe exerted circulating pressure of the drilling fluid incontact with the formation is less than the effective porepressure in the adjacent section of the matrix. Thedesirable course of action is to have this occur along the

Introduction

Underbalanced drilling is utilized worldwide for thedrilling of horizontally and vertically oriented wells to

entire exposed section of the net productive pay of thereservoir under consideration, resulting in a net inflowof oil, water or gas (which may be contained in thematrix) into the wellbore. These produced fluids arethen retw11ed to the surface along with the circulatingdrilling fluid. A number of nomenclatures for adescription ofUBD ex.ist in die literature. They can bedefined as follows:

In this situation. the circulating fluid density is furtherreduced to generate the underbalanced pressurecondition by the inclusion of some type of non-condensible gas (generally nitrogen) in the circulatingdrilling fluid to reduce overall equivalent circulatingdensity. This results in dte net inflow of fluids from theformation. The maintenance of these artificially inducedunderbalanced conditions is obviously moreproblematic than the preceding drilling operationdiscussed (i.e. flow drilling) due to the necessitY of thecontinuous injection of gas, resulting in theestablishment of a quasi steady state equilibrium whichmay be disturbed during drilling operations. Thesephenomena are discussed in greater detail in this paper.

Overbalanced Drilling. A situation in which theequivalent circulating density of the drilling mud issufficient that, at bottomhole conditions, the drillingfluid pressure is greater than the formation pressure,resulting in an effectively killed state of the well (whereno inflow of fonnation of fluids occurs) during aconventional drilling operation. This was the mostcommon technique utilized to drill wells in the past, andis still the dominant technology utilized in manyreservoir situations today.

Gas Drilling. Gas and air drilling have been utilized formany years as a technique to reduce drilling problemsand avoid significant lost circulation into highlyunderpressured or sensitive formations. Pure gasdrilling consists of a situation where pure air or nitrogenis utilized at extremely high flow rates, generally incombination with PDC bits. to generate small drillcuttings which are transported in highly turbulent flowback to dIe surface. Gas drilling may suffer fromproblems in certain situations due to poor hole cleaningability and near wellbore mechanical damage known asglazing or mashing (discussed in greater detail later onin this paper).

Low Head Drilling. Low head drilling refers to asituation where an overbalanced pressure condition,similar to that descn"bed above, is maintained with theuse of lower density oil-based fluids or possibly aeratedor gasified fluids to reduce the effective overbalancepressure exerted on the formation. This is done toreduce formation damage and the potential for severeinvasive losses due to drilling problems, such as lostcirculation, stuck pipe and differential sticking. Lowhead drilling is still classified as a form of overbalanced

drilling.Mist Drilling. Mist drilling is a variant of pure gasdrilling operations where a small amount ofhydrocarbon or water-based fluid is entrained in thecirculating drilling fluid to reduce frictional and heatproblems and reduce problems with downhole MWDassemblies. Mist drilling typically uses 5 - 30liters/minute of base fluid in combination with 40 - 80cmlminute of non-condensible gas, in comparison toconventional artificial UBD gasified fluid operations,discussed previously, which may use liquid ratesranging from 300-800 liters/mill at equivalent gascirculation rates.

Flow Drilling. Flow drilling refers to a situation wherethe fonnation of interest has sufficient pressure that apure liquid based fluid can be utilized and an effectiveunderbalanced pressure condition will be generated atthe bottomhole level. This may be accomplished insome slightly underpressured reservoirs through the useof low density oil-based fluids or invert drilling fluids.In some cases, in normally pressured or overpressuredreservoir situations, water-based fluids may also beutilized successfully for flow drilling operations. Flowdrilling operations are commonly utilized in some areasof the Austin Chalk.

How is Underbalanced Drilling Conducted?

Equipment utilized to drill underbalanced (withrespect to bottomhole and surface control equipment)has been extensively discussed in the literature3.4.Figure I provides a basic schematic illustration of atypical flow loop which is utilized to conduct anunderbalanced drilling operation. The vast majority ofunderbalanced drilling operations conducted to datehave utilized conventional jointed pipe technology,

Underbalanced Drilling - Artificially Induced This

type of underbalanced drilling is commonly utilized inthe Western Canadian Basin and also in oilier locationsin the world where sufficient reservoir pressuredepletion ex.ists that an effective Wlderbalanced pressurecondition cannot be naturally generated downhole usingeither oil or water-based fluids of normal density ranges.

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although an increased use in coiled tubing in recentyears has been noted, particularly in the westernCanadian industry.

underbalanced drilling is often thought of as apreventative technique are generally defined as follows.

Fines Migration. Fines migration refers to the naturalmotion of pre-existing particulates contained within theformation. In general, fmes will tend to migrate fairlyexclusively with motion of the wetting phase, be this oilor water depending on the given reservoir situation.Significant loss of oil- or water-based drilling fluid. ifthis drilling fluid represents the wetting phase in ahighly overbalanced pressure situation, may result in thephysical migration of fines or, subsequent to this, theattempt to clean up the wellbore under high drawdownconditions to remove significant volumes of invadedloss filtrate may also result in particulate mobilization,blocking and plugging. The overall motivation foroverbalanced drilling in such situations would be toreduce or eliminate fluid losses to the formation toreduce significant concerns with fines migration duringboth drilling and subsequent cleanup operations. Theonly potential risk associated with fines mobilizationproblems during underbalanced drilling is if a sufficientunderbalanced pressure condition exists, and themobilized phases from the formation (either oil orwater) represent the wetting phase, in which case,sufficiently high fluid production rates from thereservoir during a UBD operation may result in thepremature initiation of fines migration. The case may bemade that flow rates of this magnitude would normallyoccur during the normal production operations of thewell subsequent to the drilling operations.

For many Canadian operations, closed surfacesystems, in which a four phase separator is used toseparate gases, liquids and solids, combined with arotating control head and onsite flare stack and tankagefor produced fluids are commonly utilized. In otherareas of the world, non-closed surface control systemswith open separators, sometimes called "gas busters",are utilized to conduct various types ofUBD operations.

What are the Motivations for Conducting anUnderbalanced Drilling Job?

A variety of possible reasons exist for a givenoperating company to conduct an underbalanced drill jngjob. A number of papers are present in the literaturewhich discuss this technology as well as presentingscreening criteria for the appropriate selection of thebest reservoir candidates for underbalanced drillini .

The primary motivations most often quoted forconducting underbalanced drilling operations include:

. Reduction in invasive fonnation damage.

. Increase in effective rates of penetration and overalldrilling rates and drilling time/cost reduction.

. Reduction in significant lost circulation problems.

. Reduce drilling problems in highly penneable orheterogeneous reservoirs containing fractures orvugular porosity. External Solid\" Entrainment. During conventional

overbalanced drilling operations, a net overbalancedpressure differential exists which has a tendency topotentially drive fluids and associated solids into thefonnation. Conventional drilling fluids contain a widevariety of potential suspended solids material such as aweighting agents, (barite, hematite, calcium carbonate,etc.), fluid loss agents (organic clays, bentonite, etc.) ora variety of granular and solid loss circulation materials(sized carbonates, sized salts, cellulosic fibers, oilsoluble resins, gas sublatable crystals) and other moreinteresting LCM materials, such as chicken feathers,walnut hulls, bamboo fibers, cardboard, golf balls, etc.In a normal hydrostatic overbalanced condition, there isan impetus for these fluids and associated solids to bedisplaced into the fonnation. In most relativelyhomogeneous mabix situations, the physical depth ofsolids invasion tends to be rather shallow ( I - 2 cm into

the formation). The result is that this mechanism ofdamage becomes relatively inconsequential in situations

Formation Damage and Underbalanced Drilling

In many situations, the prime motivation forconducting an underbalanced drilling operation may beassociated with severe fonnation damage that may havebeen observed in horizontal or vertical wells drilled inthe area. It must be emphasized that underbalanceddrilling is not a solution for poor reservoir quality. It isnot a stimulation technique and it does not manufacturepermeability. It only allows one to maximize the useand potential of the already existing permeability.Fonnation damage is a complex phenomenon havingvarious root causes and mechanisms, depending onparticular reservoir situations and lithology and ondrilling, completion and production practices utilized ina given reservoir situation. Many detailed discussionson formation damage are present in the literature6.7.' butthe primary mechanisms of formation damage for which

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where cased, cemented and perforated completions arecontemplated, as the damage radius will typically notextend beyond the radius of a normal perforationcharge. However, the vast majority of underbaIanceddrilling operations are completed in an openhole mode,as are most horizontal wells. For this reason. thisparticular damage mechanism is one of significantimportance and one of the prime motivations for theapplication of UBD technology in some situations.

materials such as kaolinite, kaolin, halite, etc. whichmay be partially solubilized or softened by the contactof water-based fluids. The use ofunderbalanced drillingattempts to minimize the contact of the formation withtbesepotentially solubilizing fluids, resulting in minimalinvasion and release of fines and on gauge hote

applications.

Formation of Stable Emulsions. In certain situations,invading drilling fluids may combine with insitu fluidsto form highly viscous stable water in oil emulsions(Figure 4). These emulsions may have high apparentviscosity, and result in the formation of what is knownas an emulsion block in the near wellbore region whichmay significantly impair oil or gas production rates inthe affected zone. The use of underbalanced drillingavoids the inclusion or entrainment of these potentiallyemulsifying fluids in the near wellbore region, althoughsignificant emulsion issues may exist between producedfluids and circulating drilling fluids (which will need tobe addressed in the design of the basic underbalanceddrilling operation used in a given situation).

Phase Trapping. Phase b'apping refers to the pennanententrainment of an increased trapped water orhydrocarbon saturation in a porous media causingadverse and deleterious relative penneability effects(Figure 2). The blockage of the entrained fluid causesa reduction in the effective relative permeability to oilor gas resulting in a roDe of potentially significantdamage surrounding the wellbore region. The primemotivation of underbalanced drilling in such a situationis to prevent the significant loss of potentially damagingand trapping water or oil based filtrates into theformation. thereby minimizing and mitigating thepotential severity of damage associated with phasetrapping effects. Scales. lncompauole waters may result in the formation

of carbonate or sulphate based scales which may behighly damaging to injectivity or productivity in the nearwellbore region. The use of appropriately designedunderbalanced drilling minimizes the potential for fluidcontact (in the fonnation) and the associated

penneability impairment.

Reactive Clays. Many sandstone fonnations containreactive clay minerals such as smectite ormontmorilinite, or potentially detlocculatible clays suchas kaolinite. These clays can be destabilized by contactwidt fresh or low salinity brines or, in some cases,associated widt rapid changes towards a caustic pHsituation. The motivation of underbalanced drilling is,of course, to minimize dIe potential losses of potentiallydamaging water-based filtrates to dIe formation whichmay result in dIe creation of a wne of significantlyimpaired penneability and productivity in dIe nearwellbore region.

Wettability Alterations. Many drilling fluids contain avariety of polar surfactants and additives which are usedfor corrosion, imbibition. torque reduction. emulsioncontrol, etc. Some of these components may have apropensity to be adsorbed on both carbonate andsandstone surfaces and cause a wettability alteration ortransition which may significantly alter the water-oilrelative penneability characteristics in the near wellboreregion. In situations where a mobile water saturation isapparent, this may increase effective water-oilproducing ratios of a given well and adversely affectwell economics (Figure 5). The use of properunderbalanced drilling technology prevents the contactand entrainment of the near wellbore matrix volumewith potential wettability altering fluids and reduces thepotential for damage.

Polymer Ad\'orplion. Many drilling fluids containrheology enhancing agents and polymers to improvefluid viscosity and reduce fluid losses in anoverbalanced situation to the formation. Thesepolymers may have a physical tendency to be adsorbedon the face of the formation and, particularly in lowpermeability rock (Figure 3), may result in significantpermeability impairment (localized in general to the nearwellbore region). The use of underbalanced drillingoperations eliminates both the inclusion of thesepotentially absorptive materials and the loss of thesefiltrates to the formation.

Bacterial Damage. Water-based fluids may contain, ifimproperly subjected to biological control, viablepopulations of aerobic and anaerobic bacteria whichmay be entrained in the near wellbore region andMineral Dissolution. Certain fomlations contain soluble

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subsequently resuh in bacterial growth and propagation.This may result in die creation of polysaccharide richbacterial biofilrns, possibly reducing productivity orinjectivity, downhole and surface corrosion issues and,if the bacteria are sulphate-reducing in nature, thepotential reduction of elemental sulphate (that may bepresent in insitu formations or injected fluids) into toxichydrogen sulphide gas. The appropriate use of UBDtechnology prevents die long-tenD losses of potentialwater-based fluids which may contain viable bacteriacolonies into die formation. However, in mostsituations, if water-based fluids are contemplated forany UBD operation, the appropriate bacterial andbiological control is recommended as a low costinsurance should, for some, reason, the underbalancedcondition be compromised and fluids be imbibed ordisplaced into the formation.

fractured fonnations wiili high vertical penneability,ilie effect ofilie damage is relatively insignificant and,even in situations of high damage, relativelyproductive horizontal wells may be obtained.However, in situations wiili adverse vertical tohorizontal penneability ratios d1at are oftenencountered in many carbonate and clastic formationapplications, ilie damage extends to the greatest extentin ilie streamline of greatest permeability, greatlycompromising ilie productive nature of the horizontalwell. Even moderate amounts of near wellboreformation damage may therefore become radtersignificant in very adverse vertical to horizontalpermeability ratio situations.

Potential Issues Associated With the Use ofUnderbalanced Drilling

Underbalanced Drilling and Horizontal Wells A number of potential problems can be associatedwith the implementation of under balanced drilling withthe primary motivation to reduce invasive fonnationdamage. The single largest issue, in most situationswhere underbalanced drilling yielded poor results, havebeen cases where there is clearly documented evidencethat it has been difficult or impossible to maintain aneffectively underbalanced condition 100% of the timeduring the drilling and completion operation. Much ofthe benefit of the UBD operation may be compromised,and it is possible to be in an even worse situation (if theUBD pressure condition is periodically compromised)than if a conventional drilling operation had been used

(Figures 6-9).

Horizontal wells have long been recognized as moresevere candidates for formation damage than theirvertical well cotmterparts. These phenomena have beendiscussed in detail in the literature9. A number ofreasons exist why formation damage is known to be amore significant problem in horizontal wells than invertical wells. A brief synopsis of these damagemechanisms include:

Figure 6 illustrates a poorly designed and executedconventional overbalanced drilling operation where highamounts of both filtrate and potentially damaging solidsmay be invading mabix, fractures or interconnectedvugs in the near wellbore region, resulting in a zone ofsignificant near wellbore damage.

Figure 7 represents the better designed and conceivedoverbalanced drilling operation where appropriaterheology and bridging/LCM materials have been utilizedto create a stable and external filter cake which acts asan impenneable barrier to prevent significant loss ofdamaging solids to the matrix, fractures or wgs in thenear wellbore region. With an appropriately designedoverbalanced drilling fluid, the objective is that thisstable filter cake may be easily removed by directmechanical backflow of the formation or, due to itslocalized condition directly adjacent to the weUboreformation interface, subsequently removed by some type

. Longer duration of time required to drill horizontal

wells resulting in potential deeper invasion ofdamaging filtrates and solids.

. The majority of horizontal wells are completed in an

openhole fashion resulting in relatively shallowinvasive fonnation damage. In a typically cased.cemented and perforated completion, this would notbe considered significant enough to result insignificant near wellbore penneability impairment ina horizontal completion.

. Localized drawdown effects resulting in partial

cleanup of only a relatively small portion of thehorizontal section.

. Due to the large exposed inflow area of horizontal

wells, the vast majority of stimulation treabnentstends to be fairly superficial in nature, in comparisonto highly penetrating stimulation treatments which canbe conducted relatively economically on equivalentvertical damaged wells.

. Anisotropic flow effects - In situations where thevertical permeability is not equal to the horizontalpermeability, damage is preferentially created out inthe streamline of greatest permeability. In vertically

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of a localized chemical or mechanical stimulationtreatment which may be used during the completionphase of the well.

reduce some of these problems). This being die case,the nitrogen injection must be stopped at each pipeconnection resulting in possible oscillation of dieeffective bottomhole pressure and periodic loading ofthe fluid system with slugs of fluid which may result inpotential overbalanced pulses (Figure 10).

Figure 8 provides a schematic illustration of theoptimally designed and executed UBD operation. Sincethe fonnabon pressure in this situation is greater thanthe net circulating fluid pressure, there is a net inflow offluids from the fonnation. This eliminates the potentialinvasion of damaging filtrates or solids materials intothe matrix, fractures or vugs in the near wellbore region.However, it must also be emphasized that there is alsoan absence of potentially sealing and protective filtercake. This, therefore, may be disadvantageous when, asseen in Figure 9, if the underbaIanced pressure conditionis compromised, very rapid invasion of whole drillingfluids into the fractures, vugs or near wellbore matrixmay physically occur even if the underl>alanced pressurecondition is compromised even for a relatively shortperiod of time. To further exacerbate this phenomena,typical underbalanced drilling operations rely primarilyon turbulent flow to clean the hole and gravitysegregation to disengage gas and solids from thecirculating drilling fluids at surface. This being thecase, in general, very low viscosity drilling fluids(commonly produced water or low viscosityhydrocarbons) tend to be utilized as the base drillingfluids for most artificial UBD operations. If this is thecase, and the underbalanced pressure condition iscompromised, in addition to having no protective filtercake, very low viscosity fluid with high API fluid losscharacteristics which typically contains a relatively highconcentration of very fine powered solids (due to thepoor hole cleaning capabilities of many UBDoperations) is contained in the annulus. This fluid maybe displaced rapidly into the formation, resulting in aconsiderable degree of invasive formation damage in analmost instantaneous fashion, negating much or all ofthe benefit associated with respect to the use ofUBD toreduce fonnation damage in a given wellbore situation.

Conventional Mud Pulsed MWD Operatiom. Forhorizontal applications, it is often necessary to ttansmitgeo-steering infonnation back to surface toappropriately guide the trajectory of the horizontal wellto ensure that drilling operations proceed in the zone ofrnaximwn quality in the pay interval desired. The mosteconomic classical technology for use is theincompressible drilling fluid and a downhole pulse unitto ttansmit survey data back to surface via thetransmission of incompressible pressure waves throughthe fluid contained in the central portion of the drillstring. The use of artificial UBD technology, wheregases are often injected in the drill string, negates theuse of conventional mud pulsed MWD technology dueto the presence of a highly compressib1e gas phase in thedrilling fluid. In some situations, conventional mudpulsed telemetry has been uti1ized in UBD operationsby circulating the hole to a pure incompressible fluidprior to transmitting a survey back to surface on aperiodic basis. This, of course, results in die applicationof a full hydrostatic head of fluid to the hole, resultingin periodic overbalanced pulses of pressure beingapplied to the formation, which may negate much of theresults of the underbalanced drilling operation withrespect to reducing fonnation damage.

Kill Jobs/Bit Trips. Due to mechanical problems andthe necessity of bit trips in may situations, wells whichhave been drilled underbalanced have beenhydrostatically killed to facilitate these operations. Thisresults in the physical loss of the underbalancedpressure condition with the adverse effects mentioned

previously.

How is the Underbalanced Pressure ConditionCompromised Resulting in Aggravated FormationDamage Effects?

Localized Depletion Effects. The phenomena oflocalized depletion is schematically illustrated as Figure11. It can be seen, particularly for a horizontal wellapplication, that an extended period of time may berequired to drill the entire length of the horizontalsection. Portions of the formation intersected early onin the life of the underbalanced drilling operation willtherefore be exposed to an extended flow situation.This will result in the formation face directly adjacent tothe wellbore being subjected to localized drawdowneffects and, after a period of flowing time, the pressure

Pipe Connectiom. Most UBD operations are currentlyconducted using conventional jointed pipe technology.When the underbalanced condition must be artificiallygenerated through the use of injection of a non-condensible gas. this gas is most often injected directlydown the standpipe (although some variance which willbe discussed later may also be utilized which tends to

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in this region may approximate the average circulatingpressure of the drilling fluid. At the bit location, anunderbalanced pressure is still thought to be maintained(in comparison to the virgin formation pressure inpreviously drilled portions of the well) due to localizeddepletion effects. This may result in a zone of superpressure directly adjacent to the partially depleted zoneof the wellbore resulting in the potential influx and lossof fluids with the associated fonnation damage effectsmentioned previously.

Poor Hole Cleaning. Underbalanced drilling operationsrely on highly turbulent flow to transport cuttings backto the surface and maintain a clean hole. This mayresult in the fonnation of mud rings or other situationsin certain cases which may result in high backpressureeffects and potential overbalance pressure issues.

Countercurrent Imbibition Issues

In certain situations, UBD is contemplated for use inreservoirs which contain or low subirreducible watersaturations. Adverse capillary pressure effects in suchsituations, if water is used as die base fluid fordle UBDoperation, can result in die spontaneous countercurrentimbibition of die water based fluid into die nearwellbore region resulting in die establishment of asignificant and potentially damaging phase b'ap. Thisphenomena has been discussed in detail in dieliterature1o.I' (Figure 14).

Multiple or Variable Pressured Zones. Underbalanceddrilling becomes a challenging operation in situationswhere significant variation in pressure in the expectedtarget interval( s) is expected. This may occur in verticalor deviated applications or multiple sand lenses whichmay be in varying conditions of depletion. For theoptimum preservation of maximum formationproductivity, the underbalanced condition mustobviously be tailored to account for the zone of lowestpotential pressure. However, potential crosstlowbetween highly productive overpressured and lowerpressured zones may result in making the effectivemaintenance of an underbalanced pressure condition inall portions of the well an impossible situation.

Gravity and Macroporous Drainage Effects

In certain situations when UBD is used in an attemptto reduce massive lost circulation in highly permeablefonnations (such as reservoirs containing large openfractures or large interconnected vugs), the superficialflow velocity of fluids entering a horizontal well maynot be sufficient to counteract gravity drainage effects,which may result in significant losses of these fluids tothe penetrated macroporous systems. Although if thesemacroporous features are arranged in a vertical directionthe associated damage may not be significant, theprimary issue associated may be significant lostcirculation which may result in drilling problems (Figure

15).

Frictional Flow Effects. Considerable frictionalbackpressure may be exerted by the high ratedisplacement of multi phase gas and fluid in a typicalunderl>alanced drilling operation. This may result in thefonnation of a downhole pressure profile similar to thatillustrated in Figure 12. It can be seen d1at cessation offlow could result in a dropout of frictional backpressureeffects and thus result in the drop in bottomholepressure, initiating some of the pressure swingsassociated with pipe connections as previouslymentioned. In addition, it can also be illUSb'ated byexamination of Figure 12, that, for a given wellboregeometry, fluid rate and fluid rheology regime, there isa certain maximum well depth and honzontallength thatcan be achieved in which it is still possible to maintainan effectively underbalanced pressure condition,regardless of the amount of gas which may be entrainedin the circulating drilling fluid. In certain situations,frictional backpressure effects may become sosignificant that an underbalanced drilling operationbecomes what is known as "frictionally pressuredominated". Increasing gas injection rates (rather thanreducing the bottomhole pressure, as would classicallybe expected) may actually cause the bottomho Ie pressureto effectively increase in this situation (Figure 13).

Glazing and Mashing

In pure gas drill operations, a significant amount ofheat may be generated. The combination of heat,connate water and die fine crystalline cuttings generatedfrom many UBD operations may result in die fonnationof a ceramic coating like glaze in die near wellborematrix which may significantly impair well productivity .Physical depth of invasion of this type of damage tendsto be very shallow in most situations, and may nornlallybe penetrated by a conventional perforation job.Therefore, die most significant potential scenario fordamage is associated with openhole completions,particularly in lower permeability homogeneous clasticfonnations, where the glaze is quartzose in nature andcannot be readily removed by an acid wash. The

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inclusion of a small amount of liquid (i.e. mist drilling)generally tends to mitigate this problem for mostapplications. Mashing is caused by sliding drill pipe orpoor drill string centralization, and refers to the workingof small drill cuttings and fmes into die formation facedirectly adjacent to the wellbore, causing a phenomenasimilar in extent and severity to glazing.

the refinement of these techniques with actual real timedetennination of the effects with surface operations andthe ability to maintain in a much closer fashion arelatively uniform effective bottomhole pressurecondition.

Parasite or Concentric Strings. Another variation forjointed pipe drilling technology is the use of amicro annular or concentric string or a cemented parasitestring to continuously inject nitrogen in the verticalannular section of ilie wellbore. This allows a 100%compressive fluid stream to be injected in ilie centralportion of ilie drill string and allows for conventionalMWD operations and limits problems with connections,while maintaining a relatively uniform bottomholepressure condition for an UBD operation. Typicalparasite and concentric string configurations for UBDare illustrated in Figures 16 and 17. These techniquestypically tend to be more utilized for new wellapplications and are often not utilized due to expenseand greater gas consumption, but have been successfullyapplied in a number ofUBD applications in the WesternCanadian Basin and elsewhere.

Techniques for More Continuous Maintenance of anEffective Underbalanced Pressure Condition

Increased Use ofCoi/ed Tubing (C1). The advantagesof coiled tubing are obvious with respect to die morecontinuous maintenance of an underbalanced pressurecondition. Since no pipe connections are required, amore uniform underbalanced pressure condition may begenerated and maintained. The use of an internalwireline associated widi cr also eliminates some of dieproblems mentioned widi conventional mud pulsedMWD techniques. A detailed discussion of die use ofCT for underbalanced drilling has been contained in anumber of literature articlesl2.13 and this technology isoccupying an increasing portion of die westernCanadian underbalanced drilling market. The majordisadvantages of CT are associated with restrictedhorizontal outreach. depdilimitations, potential safetyconcerns associated widi critical sour wells widilargevolumes of CT present on die surface, and frictionalflow and hole size limitations which result in relativelyslim hole applications in most situations.

Foamed Fluids. Foamed fluids appear to have naturalapplication for UBD operations due to their loweffective density, good rheology and hole cleaningeffectiveness. In addition, because most stable foamsystems retain apparent viscosity and rheology evenwhen circulation is ceased, cuttings, transportation andsignificant invasion of these fluids into the formation,even if the underbalance pressure condition iscompromised, tends to be reduced. In the past,significant problems associated with foam-based drillingfluids have been associated with the stability of the foamand the ability to disengage gas and cuttinp at surfaceand significant handling problems. The development ofa new generation of pH regenerable foams which havehigh resistance to oil contamination has lead to anincrease in the potential application of these fluids forUBD technology. It is expected that foamed drillingfluids will occupy an increasing fraction of the UBDmarket in the years to come. The high apparentviscosity rheology of foam systems may be a limitationwith respect to high frictional backpressure effectswhich may, in some cases, make it difficult to maintainan effective underbalance pressure condition in somehighly pressure depleted formations.

Use of Electromagnetic Telemetry Techniques forMWD. Some of the major limitations of UBD withrespect to mud pulsed MWD have been overcomethrough the use of electromagnetic telemetry technologywhich uses an electromagnetic pulse to transmit MWDdata back through the formation to the surface. EMTtechnology has been successfully utilized in many UBDapplications but may have problems in very highlyresistive fonnations when using oil-based fluids or atdepths below approximately 2500 meters.

Rapid Connections. Jointed pipe underbalanced drillingstill occupies approximately 80 - 85% of the UBD

market. Recent refinements in techniques widt respectto the appropriate use of float shoes in die drill stringand very rapid connection times and specific circulationtechniques (which are utilized during connections) hasgreatly improved the stability of underbalance pressureoperations during conventional jointed pipe operationsand artificially generated UBD operations. The use ofreal time bottomhole pressure measurement has allowed

Glass Spheres. The idea of using gas filled silicaspheroids as a means of reducing the density of fluidsis not a new one, and has been utilized for a number of

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years to generate low weight cements for fieldapplications. The idea of entraining glass spheroids forUBD operations has been postulated and field testedwid1 varying degrees of success. The objective is thatthe glass spheres will reduce the equivalent circulatingdensity of a drilling fluid while still maintaining aconventional, incompressible phase which can be usedfor normal conventional MWD operations. The sizingof the spheroids may also be adjusted to specify certainbridging requirements. Major problems to date havebeen crushing and commutative losses of the spheroids,and severe erosive properties of the fluid in circulatingdrilling operations. Ongoing research still continues inth is area.

References

DOANE, R.D., BENNION, D.B., EILERS, E.R.,"Successful Drilling of an UnderbalancedHorizontal Well in the Rigel Halfway Pool -Laboratory Screening and Field Results", Paperpresented at the International Conference onHorizontal Well Technology, Calgary, Alberta,Canada, Nov. 18-20, 1996.

2. CHURCHER. P.L., YURKlW, F.J., BIETZ,R.F., BENNION, O.B. "Designing and FieldTesting of UnderbaJanced Drilling Fluids toLimit Fonnation Damage: Examples from theWesterose Field. Canada".

Pure Oils. In some cases, synthetic or low density oil-based drilling fluids have been utilized to generate anunderbalanced pressure condition in slightly pressuredepleted or normally pressured fonnations. Theserepresent die simplest applications ofUBD technologyand, in many normally pressured fonnations, producedcrude oil may be die prime underbalanced drillingcandidate fluid of choice due to availability and ease of

operation.

3 LUNAN, B., "Surface Control Systems forUnderbalanced Drilling", JCPT, Sept. 1995.

4. SAPONJA, J. et at. "Considerations forU nderbalanced Drilling with Jointed Pipe", Paperpresented at the First InternationalUnderbalanced Drilling Conference andExhibition, The Hague, Netherlands, Oct. 2-4,1995.

Conclusionss. BENNION, D.B., LUNAN, B., SAPONJA, J.

"Underbalanced Drilling and CompletionOperations to Minimize Fonnation Damage:Reservoir Screening Criteria for OptimumApplication", Paper presented at die 4~ AnnualTechnical Meeting of the Petroleum Society ofCIM, Calgary, Alberta. June 10-12, 1996.

This paper has described a variety of ways in whichinvasive formation damage mechanisms can be reducedthrough die appropriate use of underbalanced drillingtechnology. It has also been illustrated thatinappropriate application of die technology may resultin damage more severe dian if appropriate conventionaloverbalanced technology had been utilized in die samesituation. Reservoir evaluation must be carefullyundertaken for every underbalanced drilling applicationto ensure that the best technology and design parametersare utilized to ensure the maximum degree of successwill be achieved for die extra cost generally associatedwith the technology.

6. ENG, J.H., BENNION, D.B., STRONG, J.B.,"Velocity Profiles in Perforated Completions",JCPT, Oct. 1993, Volume 32, No.8.

7. BEATrY, T., BENNION, D.B., HEBNER, B.,mSCOCK, R., "Minimizing Fonnation Damagein Horizontal Wells: Laboratory and Field CaseStudies", Paper presented at the CIM 1993Annual Technical Conference, Calgary, Alberta,May 9-12,1993.

Acknowledgments

The authors wish to express their appreciation toHycal Energy Research Laboratories Ltd. forpermission to publish this paper, and to Vivian Whitingin the preparation of the manuscript and associated

figures.

8. BENNION, D.B., THOMAS, F .B., BENNION,D. W., BIETZ, R.F., "Fonnation Damage Controland Research in Horizontal Wells", Paperpresented at die Sll1lntemational Conference onHorizontal Well Technology, Houston, TX, Nov.

9-11,1993.

0

9. BENNION, O.B., THOMAS, F .B., BIETZ, R.F.,JAMALUDOIN, A.K.M., "Recent InvestigationsInto Formation Damage and RemediationTechnology for Horizontal Well Applications",Paper presented at the 9111 InternationalConference on Horizontal Well technology andApplications, Houston, TX, Aug. 25-27, 1997.

10. BENNION, D.B., THOMAS, F .B.,"Underbalanced Drilling of Horizontal Wells:Does It Really Eliminate Formation Damage?",Paper presented at the SPE inti. Symposium onFonnation Damage Control, Lafayette, LA, Feb.7-10, 1994.

11 BENNION, D.B., THOMAS, F .B., BIETz, R.F.,BENNION, D. W., "Underbalanced Drilling,Praises and Perils: Lab and Field Experience",Paper presented at the Sda Annual Conference onHorizontal Well Technology, Calgary, AB, Nov.21, 1995.

12 BENNION, D.B., rnOMAS, F .B., BIETZ, R.F °,JAMALUDDIN, A.K.M., "Coiled Tubing - The

Future of Underbalanced Drilling?", Paperpresented at d1e Sib International Conference onCoiled Tubing Technologies, Dallas, TX, Jan. 8-10,1997.

13, BENNION, D.B., "UBD wid1 CT has Pluses,Min "

uses,

10

~

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