Formation Damage and Horiz Wells

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2 FORMA lON DAMAGE AND HOIUZONTAL WELLS - A PRODucnVITY ~? SPE37138Phase Trapping. The loss of both water or oil baseddrillingmud filtrate to the fonnation in the near wellbore region due toleakoff during overbalanceddrilling operations, or due tospontaneousmbibition in somesituationsduring underbalanceddrilling operations4, an result in pennanent entrapmentof aportion or all of the invading fluid resulting n adverse elativepermeabilityeffectswhich can reduceoil or gaspermeability nthe near wellbore region'.

forDlations end o be the most probablecandidatesor this typeof damage.Glazingwjll not generallyocclude argepenneabilityfeatures,such as fractures or vugs, and the glaze is usuallyreadily removable n carbonate ased onnationswith a ight acidwash due to its highly soluble nature.Factors Which Will Tend to Increase he Severity of NearWellbore Damage

Chemical IncompatibUity of Invading Fluids with the In-situRock Mat'~ Many fonIlations contain potentially reactivematerial n-situ in the matrix, including reactiveswelling clayssuch as smectite or mixed layer clays, or deflocculatablematerials such as kaolinite or other loosely attached fines.Expansionor motion of thesematerialswithin the pore system,which may be inducedby the invasionof non-equilibriumwaterbased mud filtrates into the near wellbore region, can causeconsiderableeductions n penIleabili~.7.'.9.

The overriding factor which will increase he severity of nearweUboredamagewill be the extent of incursion of fluids andsolids nto the reservoirand how thesematerialswill react withthe formation once hey come nto contactwith the rock matrix.Factors which will tend to increase the fluid/solid lossperformance of a drilling mud in a horizontal drilling applicationmay include:Overbalance PresslUe. The greater the density of die hydrostaticfluid column and resulting downhole pressure generated incomparison to the net effective reservoir pore pressure, thegreater the tendency for losses of both fluids and mud solids tothe formation. Highly weighted mud systems (due to eitherdeliberate high concentrations of weighting agents for wellcontrol or poor surface solids control resulting in a undesirablebuild-up of a high concentration of dense natural silicate orcarbonate based formation drill solids in the fluid system), highbackpressures or drilling operations in significantly pressuredepleted formations (particularly in deep zones) may allcontribute to high overbalance pressures. Overbalance pressuresin excess of about 7000 kPa (1000 psi) are generally consideredto be severe and may cause serious losses to the formation,particulary in zones of high reservoir quality.

Fluid-Fluid Incompatibility Effects Between Invading Fluidsand In-Situ Fluids. Oil or water based mud filtrates invadinginto the near wellbore region during overbalanced drillingprocessescan react adversely with in-situ hydrocarbons or waterspresent in the matrix with detrimental results which may reducepermeability. Problems would include the fonnation of insolubleprecipitates or scales between incompatible waters, de-asphaltingof the in-situ crude or hydrocarbon based drilling fluid caused byblending of incompatible oils, or the fonnation of highly viscousstable water in oil emulsions due to turbulent blending of invadedfiltrates with either in-situ water or oil.

Near Wellbore Wettabllity Alteration and Surface AdsorptionEffects. Many drilling fluid additives used for mud rheology,stability, emulsion control, corrosion inhibition, torque reductionor lubricity contain polar surfactants or compounds which can bepreferentially adsorbed on the surface of the rock. The physicaladsorption of these compounds can cause reductions inpenneability by the physical occlusion of the pore system, in thecase of high molecular weight long chain polymers, particularlyin low penneability porous media where the small pore throatsmay be easily bridged by long chain polymer molecules. Polarcompound adsorption may alter the wetting characteristics of thematrix in the near wellbore region, generally in most cases o apreferentially more oil-wet state. This causes a potentiallysignificant increase in water phase relative penneability in thisregion, which may adversely elevate producing water oil ratio forthe well if the completion is in a zone where a mobile water

High Solids ContenL A high concentration of artificial ornaturalsolids n the mud system,which are nappropriately izedto form a low permeability ilter cake,can either nvade nto therock matrix (if the solids are too small, that is, less thanapproximately300/0 f the median pore throat aperture),or mayscreen off on the formation face forming porous, highpermeability, thick filter cakeswhich may result in 10ng-tefDlfiltrate seepage nd stuck pipe problems if the solids are toolarge. For an openhole completion scenario,an appropriate izedistribution of particulate matter in the mud is essential oeStablish sealing, ow pefDleability filter cake rapidly on theface of the formation. This will minimize solids invasion todirectly at the wellbore-formation interface where it can


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D.B. BENNION, F.B. mOMAS, R.F.8IE'JZPE37131solids in clear fluid systems often results in near wellboremechanicaldamageas large volumes of the base luid. alongwith the often inappropriately sized naturally generated inesfrom the drilling process, re carriedoff into the forDlation. Theuse of appropriate viscosifiers/polymers can assist in thereduction of uncontrolled luid losses o the formation in somecases, nd should be evaluated or eachspecific situation underconsideration.

the wellbore during the drilling process. Although shallow insome cases, he penneability of this affected zone may beextremely ow, creatinga very high zone of what is referred oas "skin" damageabout the wellbore. Thus, even relativelyshallow nvasivedamage,which may be insignificant in a casedand perforatedcompletion, can be very substantial n an openhole scenario. Other reasons ontributing to increased everityof damage n horizontal versusvertical weUscould include:

Poor Base FluId CompatIbilIty. Even n the best designedoverbalanced rilling operation, and often in many so called"underbalanced" rilling operations, omeunavoidable ossesofmud filtrate to the fonnation occur. Shallow nvasion may notbe significant or cased/perforatedompletions,but may be quiteproblematic or open hole situations. This being the case, t isusually prudent to design the base mud filtrate with fullcompatibility with the formation matrix in mind. This wouldinclude anticipating problems with reactiveclays, in-situ fluids(emulsionpotential and precipitationability) and phase rapping(possibility of including 1FT reducingagents uchas surfactantsor alcohols o lessen he impact of phase rapping f fluid lossesdo occur).

GreaterDepth of Invasion. Drilling times for horizontal wellsare usually greater than conventional vertical wells. Fluidexposure ime at the heel of the well may be significant if poormud rheology s present n an overbalanced ondition, or if themud filter cake s continuouslydisturbedby a poorly centralizeddrill string or multiple tripping operations, nvasion depth ofdamagingmud filtrate and solids into the near wellbore regionmay be substantiallygreater han n a conventionalvertical wellapplication.Selective CleanupIDanulge. The large exposed ength of ahorizontalwell often results n zonesof highly variable eservoirquality being peneb"ated. High penneability streaks maypreferentially clean up upon drawdown resulting in minimaldrawdownpressure eing applied o more heavily damaged ndinvaded portions of the well, making it difficult to obtain aneffectual cleanup. Production logs on horizontal wells oftenindicate he majority of the produced luid is being sourced romonly a very small sectionof the total length of the well.

Presenceof Zones of Extreme Pemaeability. Fluid lossesandpotential damagewill generally be more significant in zonesofhigh penneability, such as high penn intercrystalline streaks,fractures or interconnectedvugular porosity which may bepenetrated y the horizontal well. Conversely, f invasiondepthis not too significant, hesezonesmay be the most forgiving andeasy to clean up in some respectsdue to more favourablecapillary pressure elations and larger pore sizes. Difficulty of Stimulation. Damaged ertical wells may often beeffectively stimulatedeconomicallyusing a variety of penetrativetechniquessuch as hydraulic or acid fracturing, acid or othertypesof chemicalsqueezes, eat reatments,etc. These ypes ofprocessesrenot readily economicallyapplied o horizontalwellsdue to cost and technical considerations associated withattempting o stimulate a section hundredsof meters n length(insteadof only a few meters n length as often is the case n avertical well). Therefore, most horizontal well stimulationtreatments end to be relatively non-invasive n nature, such asacid washes,and may only be effective in penetratingshallownear wellbore damage.

Why Is Damage More of a Concern In Horizontal VersusVertical Wells?Thereare a numberof reasonswhy horizontal wells appear o bemore susceptible o formation damage han their vertical wellcounterparts. One of the major reasons s related to thecompletionpracticesused or most horizontal wells. The fact isthat the majority of horizontal wells are completed n either adirect open hole fashion. or with some type of slotted orprepackediner, which, as far as produced luids are concerned,is equivalent o an openhole completion. This is in comparisonto vertical wells wheremost of the wells arecased, emented ndperforated. One can thus see hat a degreeof relatively smallinvasive ormation damage, everalcentimetresn depthabout avertical wellbore may be insignificant, as a normal perforation

Anisotropic Flow- The flow patterns nto a horizontal well arecompletely different than a vertical well, this is schematicallyillustrated as Figure 1. It can be seen hat a vertical well in auniform strataof crossbeddedplaneswhich it penetratesn anorthogonal ashion will drain the reservoir n a uniform planar


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4 FORMAl1ON DAMAGE AND HORIZONTAL WELLS - A PRODucnvnY Kn.LER? SPE37138horizontalpermeabilitywill be in a cylindrical pattern,with thedepth of invasion in an unimpeded luid loss situation beinggoverned by the variable permeability of the strata underconsideration. In a horizontal well, due to the frequentanisotropy of horizontal versus vertical penneability in manyreservoirsystems,he nvasionpatternwill be elliptical in nature,with the direction of the primary axis of the invasion ellipsoidbeing oriented n the direction of highestpermeability.

(4)

where Lx-- Length of horizontal section (m)Distance o horizontal no flow boundary m)Flow Into Horizontal and Vertical Wellbores To incorporatehe effect of skin damagen a horizontalwell, theskin factor, as describedn Equation3, can be substituted or thewell radius as given in Equation 4. To account or the common

situation of asymmetrichorizontal and vertical penneabilities,which exist in many formations, he terms h and Rw inEquation4 can be further modified as follows:

Uniform flow into a vertical well of constantand non directionalhorizontal permeability can be describedby the Equation1o:Q = 21T kb{Po- P..)

~ [1n Re/R.)] (I)lJOC ~ (5)

whereQkhp.

--..

R; = 0.5R. (1 v'IJ1J (6)p.P-ReR.

where:..- ~ = Horizontal pel1Jleability m2)kv - Vertical pel1Jleability m~low rate of reservoir luid (m3/s)Averagehorizontal penneability (m2)Pay zone height (m)Reservoir pressure at effective drainageradius (Pa)Wellbore pressure pa)Viscosity (Pa"s)Effective drainage adius (m)Wellbore radius (m) The resulting final forDlulation for inflow production rate to ahorizontal well with disparate horizontal versus verticalperDleabilitycan be expressed s follows:The effect of near wellbore damage can be included inEquation I dtrough the use of a resistance actor commonly

called he "skin" factor. This conceptwas ntroduced n 1953byvan Everdingen":(7).~~] (2)AP..

Table 1 providesa list of the test parameters hich were used orthe comparativecalculations. All results from the calculationsare representedon the basis of "nonnalized flow", Thenonnalized flow represents he actual flow rate for the givenhorizontal or vertical well situation at the effective "skin"condition, divided by the flow rate at the sameconditions n anundamaged ero skin condition. Therefore, all curves at zeroskin havean effectivevalue of one. The purposeof this fonn ofanalysis s not to illustrate the increase n absoluteproduction

Thus, for a constant flow to the wellbore, the skin, induced by acombination of the invasive damage mechanisms which havebeen discussed previously, adds a constant pressure drop to thetotal drawdown (or, in other terms, a portion of the availabledrawdown is dissipated in overcoming the fluid resistance o flowthrough the zone of impaired permeability created about thewellbore). A wellbore with normal radius of Rw therefore,witha skin effect present, behaves as if the well were a "clean" wellwith a reduced wellbore radius given by:


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SPE37138 DB. BENNION, F.B. mOMAS, R.F. BIETZComparison of Skin Damage in Horizontal VersusVertical Wells in an Isotropic Permeability Condition fonnation damage s radically increased s formationkJk. ratiobecomesmore and more adverse. A large number of horizontalwells aredrilled in formationsexhibiting kjkh ratios of less han0.1, so the impact of even a relatively small amount of near

wellbore skin on ultimate well productivity is apparent. It isinteresting o note at very adverse Jk.ratios he horizontalwellproductivity ratio is actually affectedmore significantly than hevertical well value, a possibleexplanationas to why someverybadly damagedhorizontal wells actually perform more poorlythan offsetting vertical well counterparts.

Table 2 provides the results of the calculations on horizontal andvertical well geometries using identical reservoir parameters (asdetailed in Table 1). This data is based on isotropic (equal)horizontal and directional permeabilities. Figure 3 illustrates thehorizontal and vertical well normalized productivity in a low skinfactor (S = 0-10) regime, and Figure 4 at the range of skinfactors up to 500. The data illustrates that preferentially thehorizontal well, due to greater length and reservoir exposure (inthis geometry) suffers less relative productivity reduction than theequivalent vertical well. However, it should be noted at extremeskin factors (which may occur in a badly damaged overbalancedopen hole completion) that the horizontal well productivity isreduced to only 13.98/0 f the original value (in comparison to thevertical well whose productivity is reduced to 1.49% of the initialvalue).

Table 4 and Figures 7 and 8 also illustrate dte effect of theopposite situation, high vertical permeability in comparison ohorizontalpermeability. This situationmight occur n a reservoirwhere the horizontal well is orthogonally intersectingverticalfracture planes as commonly occurs with horizontal wellapplications n areas uch as he Austin Chalk in Texas. In thiscase, he reversesituation holds true that, the higher the ~ratio becomes, he less sensitive o skin damage he productionrate. At a kv to ~ of 1000,which would not be uncommon ora tight matrix highly fractured ormation, it can be seen hat theproduction ate s virtually insensitive o skin, even at very highskin values. This is somewhatof an oversimplification, as weare assuming n these calculations hat the entire wellbore iscontributing to flow, rather than a few isolated fractures.Therefore,massive luid losses o a vertical fracture systemmaystill result n productivity reductions. Well results n areas uchas the Austin Chalk, where highly damaging drilling practicessuch as mud cap drilling are routinely utilized and highlyproductivehorizontal wells are still obtained,suggest, owever,that the general trend predicted here is correct for verticallyfractured ormations.

Comparison of Horizontal to Vertical Well PerformanceIn Zones of Isotropic Permeability But Variable PayTable 3 summarizes he results of the calculationsconductedusing vertical and horizontal well geometries or pay zonesthicknesses f 2, 10 and 50 meters espectively n an isotropicpenneability situation. The data has been plotted for low skinfactors as Figure 5 and for high skin and damagevalues asFigure 6. Since he data s presented n a nonnalizedbasis heprofiles for the vertical well are identical for all three paysituations as he flow rate ncreases a simple inear multiple ofpay zone hickness n this situation). It can be seen hat on anonnalizedbasishorizontal well openhole perfonnance ecomesmore sensitive o near wellbore formation damage ffectsas netpay ncreaseseven hough on an non-normalized asis otal flowrate will likely increase). This can be explainedby the greatercontribution to flow from the over/underlying sectionsof thefonnation in a thick pay zone. Boundary effects causedbydamagen sucha situation result n preferential eductions n theability of the well to effectively access he entire drainagevolume and energy of the overlying formation zones.

UnderbalancedDrillingThe previous analysis has illustrated that near wellbore skindamagen a horizontalwell can significantly reduceproductivityto the point where, n somesituations, he wells are uneconomic.Much of this damage s associatedwith invasion of fluids andsolids during the conventional overbalanceddrilling process.Underbalanced rilling has beenused n recentyearsas a meansto attempt o reduce nvasive ormation damage nd mprove heproductivity of wells in high damage/high luid loss pronescenarios. Successwith underbalanced rilling operationshasbeenmixed, primarily due o misapplicationof the technology nmany situations and a failure to maintain a continuouslyunderbalancedonditionat all times during he drilling operation.

Comparison of Horizontal to Vertical Well Performancein Reservoir Zones of Anisotropic PermeabilityTable4 summarizes Ie resultsof thesecalculationsanddIe datahavebeenplotted for low skin factorsas Figure 7 and high skinsas Figure 8. Thesesituationsmimic the more common eal life


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FORMAllON DAMAGE AND HORIZONTAL WB.LS . A PRODUCDVnY KIUER.? SPE37138drilling operation. A detaileddiscussionof problemsassociatedwith many underbalanceddrilling operations and suggestedscreeningcriteria for the proper design of an underbalanceddrilling program s contained n the literature4.12.13.

from the face of the fonnation, re-initiate flow and trackfonnation cleanupas a function of drawdownpressure p to themaximum expected drawdown pressure evel which can berealistically applied n the field to obtain a realistic evaluationofb'ue luid perfonnance.Fluid Design Criteria to Optimize a Drilling Program toMinimize Near Wellbore Damage In HorizontalCompletionsFor heterogenous ore systems, ariationsof the technologycanbe appliedusingnaturallyor syntheticaUyracturedandshimmedcores Figure 14) or new full diameter adial coreflood ormationdamage echnology which precisely mimics the radialleakoffpattern seen n the reservoir (Figure 15).Technology hat has proven viable and reliable for successfulvertical well applications n a given field often doesnot providesimilar results or horizontal wells. This frequently esults n theneed o re-design he drilling program rom a grass oots basis o

obtain a successfulhorizontal well. The use of the maximumamount of reservoir data, special core analysis echniques oscreenand evaluatevarious fluid systemsand practices,and anexperienced esign eamare all integral componentsn obtainingthe greatest hanceof successor a horizontal well application.A good understanding f the following reservoir parameterssrequired:

Figure 16 illustratesa modified coreflood apparatuso evaluateunderbalanceddrilling. These experiments are defined inadditionaldetail in the literature.. The objective of these ests sto detennine f problemssuchas spontaneousmbibition may beapparent uring a true underbalanced rilling operation,and alsothe degree of damage and invasion to be expected f theunderbalance ressure ondition is lost and low viscosity fluidand solids abruptly invade nto the fonnation. In this fashion,the amount of damage can be compared to a conventionaloverbalanced luid system in the same situation, and a riskanalysisconductedo ascertain f the extra expense nd potentialproblemsassociated ith an underbalanceddrillingoperationarejustified.ConclusionsFonnation damage n horizontal wells can be a significantimpediment o economicproductionof oil or gas. Nearwellborefonnation damage mechanisms,which can occur during thedrilling process, entreabout luid and solids osses o the matrixand fracture/vug system adjacent to the wellbore duringoverbalanced perationsas well as possiblemechanicaldamagein somesituations. Shallow damages more significant in openhole horizontal wells due to the need to be able to producethrough the zone of impaired permeability during ultimateproduction, n comparison o a cased ompletion where shallowinvasivedamage s normally penetrated y a typical perforationcharge. Flow calculations ndicate hat the severity of damagenhorizontalwells is significantly increased s he ratio of verticalto horizontalpermeabilitydegrades nd also o a lesserextentasfonnation thickness ncreases.Underbalanced rilling may be apartial solution o many nvasive onnation damageproblems nopen hole horizontal wells, but only if properly executedand fa continuousunderbalance ressure ondition is maintained

current pressurevariations n lithologypermeabilityandporosity distribution n the argetzoneand the presenceof macroporous eatures such asfracturesor vugscomposition of the matrix and presence f potentiallyreactive clays (such as swelling smectitic clays), ormobile or deflocculatable lays (such as kaolinite)initial fluid saturations,wettability of the matrix andrelative permeability characteristics to obtain anindication of potential severityof problemswith phasetrapping and retentionpore throat size distribution and fracture aperturesize(if present) o quantify size distribution of particulatesrequired o createa stable non-invasive ilter cake onthe face of the formation to reducedamageeffectschemicalcompatibility betweenmud filtrate and n-situformation fluids (emulsion, scale and precipitationpotential)

Specialcore analysis estsare often conductedon representativesamplesof reservoir core to verify the perfonnanceof a givenfluid systemonce preliminary designhas beenconducted. r tocompare he perfonnanceof severalpotential fluid systems oselect the least damaging alternative for use in the field.Figure 9 providesa schematicllustration of the regionattemptedto be simulated n the nearwellbore regime using a specialcore


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SPE37138 D.B. BefNlON, F.B. nIOMAs, R.F. BIE'IZ 713. Bennion, D.B., et ai, "ForDlation and Fluid Criteria for theScreeningof UnderbalancedDrilling Processes", resentedat the Annual Conference f the PetroleumSociety of CIM,Calgary, Canada. une, 1996.

AcknowledgementsThe authorsexpress ppreciation o Vivian Whiting and MaggieIrwin for their assistancen the preparation f the manuscript ndthe figures. and to Hycal Energy Research aboratories or thefunding of this work and permission o present he data.References

Porter, K.E., "An Overview of Formation Damage",JPT, pp780-786, August, 1986.

1

2. Bennion, D.B., F. Brent Thomas, Bennion, D. W., Bietz,R.F., "Fluid Design to Minimize Invasive Damage inHorizontal Wells", Paper HWC 94-71 presentedat theCanadianSPFlCIM/CANMET InternationalConference nRecentAdvances n Horizontal Well Applications, Calgary,Canada.Mar. 20-23, 1994.

3. Eng, J.E., et ai, "Velocity Profiles in PerforatedCompletions",CPT,August.1993.Bennion, D.B., et aI, "Underbalanced rilling of HorizontalWells, Does t Really Eliminate FormationDamage?",JCPT,November, 1995.

4.

.s, Bennion, D.B., et ai, "Aqueous and Hydrocarbon PhaseTrapping in Porous Media, Diagnosis, Prevention andTreatment", presentedat the 1995 CIM Annual TechnicalConference,Banff, Alberta, Canada.6. Bennion,D.B., et ai, "Injection WaterQuality, A Key Factorto SuccessfulWaterflooding", PaperCIM-AOSTRA 94-60,presentedat the 1994 Annual Technical Meeting of thePetroleumSociety of CIM.

Fogler. S. et aI. "Rock-Clay Interactions". PT. 1989.8. Scheunnan,R., et ai, "Injection Water Salinity, FormationPre-Treatment and Well Operations Fluid Selection

Guidelines",JPT, July, 1990,836.Bazin. B. et al. "Control of ForDlationDamage y ModellingRock-Water nteractions",PaperSPE27363.presented t theSPESymposiumon ForDlationDamage. afayette.LA, Feb.7-10. 1994.

9.


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TABLE 2: Normalized Production Rate vs Skin Vertical andHorizontal Wells -Vertical and Horizontal Permeabilties EqualSkiD Facto. Q-Norm(Vertical Well) Q-Norm(HorizontalWell:

0 I.(XM)0.&840.7920.6030.4320.2750.1320.0710.0370.015

1.0000.9880.9760.9420.8900.8020.6190.4480.288/).140

2S

1020SO

100200SOO

TABLE 3: Normalized Production Rate vs Skin Vertical and Horizontal Wells, Kv=KhEffect of Pay ThicknessQ-Norm

Vertical Wellh-2mQ-Norm

lIorizontl. Wellh-2mSkill ,,'.ctor Q-norm

Vertical WelLh = 10 mQ-norm

Horizontal Wellb = 10 mQ-norm

Verde.1 Wellb=50mQ-norm

Horizontal Wellb=SOml.cm0.9970.9940.9840.9690.9400.8630.7590.612n,187

1.(xx)0.8M0.7920.6030.432O.27S0.1320.0710.0371).01S

1.(xx)0.9850.9700.9290.8680.7660.5670.3960.2471).116

I.CXM>0.8840.7920.6030.4320.27~0.1320.0710.037Q.OI~

I. (XX)0.9190.8500.6940.5320.3620.1850.1020.0540.022

II 1.~0.8840.7920.6030.43'-0.27S0.1320.0710.037nms

:1~

102050

1001.00~OO

TABLE 4: Normalized Production Rate Vs Skin Vertical and Horizontal WellsVariable kJk.Skin Vertical Horizontal WeD Horizontal Well Horizontal Well Horizontal Wen Horizontal Well


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Formation Damage and Horiz Wells

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