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SPE-20707-PA

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SPE 20707 PA
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OptimalFractureStimulationofaModerate-PermeabilityReservoir~KuparukRiverUnit,AlaskaC.M.Pearson,SPE,A.J.Bond,SPE,M.E.Eck,SPE,andK.W.Lynch,SPE,ArcoAlaska,Inc.Summary.Sixty-fivepercentofthereservesoftheKuparukRiverfield,thesecond-largestproducingoilfieldintheU.S.,iscontainedina20-to80-md-permeabilitysandstone.Thispaperprovidesdetailsofstimulationdesignadvancesmadeoverthepast3yearsinthisformation.Thedesignstepsforoptimizingfracturetreatmentsinamoderate-permeabilityformationrequireprimaryemphasisonfractureconductivityratherthanontreatmentsizeorfracturelength.Thisphilosophywasusedforthe140newwellsdocumentedinthispaper.Treatmentsizewasgraduallyincreasedonceacommensurateincreaseinfractureconductivitywasobtained.Applyingthenewdesigntotherefracturingof88producingwellsinthefieldresultedinanincremental40,000BOPD,asignificantportionofthefield's300,000BOPD.IntroductionDuringthepast6years,morethan550wellshavebeenfracturestimulatedintheKuparukRiverUnit.Thelargenumberoftreatmentshasprovidedtheopportunityforsignificantadvancesinthetechnicalandoperationalaspectsofhydraulicallyfracturingamoderate-permeabilityformation.Theinitialsuccessofthisprogramwasdocumentedpreviously.1Thispaperoutlinesthecontinuedtreatmentoptimizationandthesubsequentrefracturetreatments.Asaresultofthesuccessfulapplicationofhydraulicfracturing,theeconomicdevelopmentlimitoftheKuparukRiverUnithasbeenextendedsignificantly.TheKuparukRiverUnit,locatedintheAlaskanarctic,coversabout115,000acres.Theinitialdevelopmentison160-acrewellspacingwithsome80-acreinfilliocations.TheKuparukreservoirisasandstonewhoseprimaryproducingmechanismissolutiongasdrive.Themajorityofthefieldisundersecondaryrecovery,receivingpressuresupportthroughacombinationofwaterfloodandwater-alternating-immiscible-gasinjection.Anenhancedrecoverypilot,miscibleenrichedgasalternatingwithwaterinjection,isunderway.ProductionoccursfromtwohorizonswithintheKuparuksandstone.Fig.1isatypicallogoftheKuparukinterval.Thedevelopment'sprimarytargetwastheupperzone,knownastheCSand.Itconsistsofverycoarsetoveryfine-grainedsiderite-andquartzcementedsandstone.Netpayrangesupto80ft,withanaveragepermeabilityof150md.Fig.2,afieldmapidentifYingtheindividualdrillsites,showsthearealextentoftheCSandintheKuparukRiverUnit.Sincefieldstartupinlate1981,theprolificrateoftheCSandzonehascontributedthemajorityofthefield'sproduction.Thelowerproducingzone,theASand,ispresentthroughouttheunit.Althoughtheaveragethicknessistypicallylessthan30ft,withpermeabilityrangingfrom20to80md,theASandcontains65%ofthetotalreservesintheKuparukfield.Itisafinetoveryfine-grainedsandstoneinterbeddedwithshaleandcementedwithquartzandvaryingamountsofankerite.TheBSand,madeupofsands,siltstones,andshales,rangesingrossthicknessfromoto150ft.Thishigh-shale-contentzoneprovidesabarrierthatisimpermeabletoflowbetweenthetwoproducingzones.ThisbarrierbenefitstheoilrecoveryatKuparukbyallowingthetwozonesofdistinctlydifferentproducingcharacteristicstobewaterfloodedseparately.ItalsoprovidesthereservoirbarriertoisolateandtreattheASandeffectivelybyhydraulicfracturing.Kuparukwellswithdeparturesupto9,000ftaredrilledfromcentrallylocatedgravelpadstominimizetheenvironmentalimpactinthearctictundra.ThemajorityofthewellsaredrilledatananglethroughtheKuparuktominimizedrillingcosts.Untilrecently,noattemptwasmadetoalignthewellborewiththefractureorientation,andthetypicalholeangleacrosstheformationis40°.Fig.3isatypical16-welldrillsitedevelopmentillustratingthecentralpadlocationandthewelldepartures.InterpretationoffractureCopyright1992SocietyofPetroleumEngineersSPEProductionEngineering,August1992treatingpressuresandheightgrowthiscomplicatedbythedeviatedwellboresthatrequirespecialperforatingandfracturingstrategies.Twoprimaryoptionsareusedtopreventcommunicationinthewellboreduringthefracturetreatment,dependingontheCSanddevelopment.Inthecoreofthefield,wheretheCSandiswelldeveloped,selectivesinglecompletionsareinstalledtoallowapackertoisolatetheCSand.ForthewellswithminimalCSanddevelopment,theASandisstimulatedgenerallybeforetheCSandisperforated.Fig.4showscompletionschematicsofthesetwocompletionpractices.Thesecompletionsallowzonalisolationtobemaintainedduringfracturetreatments,assistinmanagingwaterfloodoperations,andimprovereserverecovery.Themoderate-permeabilityASandhaslowinitialrates.Unstimulated,itwouldbeuneconomicalinthehigh-costarcticenvironment.Prefractureflowefficienciesaverage55%(flowefficiencyistheratioofthewell'sactualPItoitsPIiftheskinequalszero).2MatrixstimulationtreatmentsareunsuccessfulbecauseofthehighlylaminatednatureoftheASand,whichpreventseffectivecommunicationbetweentheperforationsandallthesandintervals.Fracturetreatmentsareusedtoovercomethenear-wellboredamagecausedbydrillingandcompletionoperationsandtoprovidehighflow-capacityfracturestomaximizewithdrawals.ThehydraulicfractureprogramallowsthesuccessfuldevelopmentofthereservoirandsignificantlyexpandstheeconomicacreageoftheKuparukRiverUnit.Inthismoderate-permeabilityformation,thedesignchallengescenteraroundacombinationofmaximizingfracturewidth,proppantconductivity,andfracturelengthwhileminimizingheightgrowthandgeldamage.Theremotelocationandharshenvironmentrequirestrongconsiderationoftheoperationalaspectsofthetreatmentstomaximizeoverallsuccess.Initialfracturetreatmentsfocusedprimarilyonthenear-wellboredamage.Duringthedesignevolution,avarietyoffluids,proppants,andpumpingscheduleswereusedtobalancetheoperationalandtheoreticaldesignconsiderations.1Thestandarddesignusedduring1984-86(Table1)wasabout11,000Ibmof20/40-meshsand,200bblofgelleddiesel,andfluid-lossadditivesofsilicaflourandlOO-meshsand.Gelleddieselhadtheaddedbenefitsofminimizingformationdamageandeliminatingthefreezingrisksassociatedwithgelledwater.Themajorityofthesetreatmentswereperformedinitiallyinthesouthwesternportionofthefieldandinolderwellsthatwererecompletedwithselectivesinglecompletiondesigns.Thistreatmentdesignovercamedamagecausedbydrillingandcompletionfluids,minimizedthepotentialforcommunicatingwiththeCSand,andreducedscreenoutfrequency.Typicalpostfractureproductionrateswere755BOPDwithaflowefficiencyof153%.Newwellsdrilledduringthepast4yearsprovidedtheopportunitytooptimizetheKuparukfracturingprogram.ThecoreofthestimulationactivitywasthenewdevelopmentareainthenorthernportionofthefieldwithminorCSanddevelopment.BecausetheriskoffracturingintothehighlyproductiveCSandhadbeenelim-259MD,FTTVD.580068505850690059006950700059507050600071006050715072006100120-GRNM,APINGRNM,APINKRU2W-06220.1RILO,OHM-MBulkDenSity,GM/CCIPERFS_NETPAYFig.1-LogintervaloftheKuparukformation.inated.thetreatmentsizeandproppantpermeabilitieswereincreasedtodeterminetheadvantagesoflargerfractures.TheincreaseinjobsizebeganonDrillsite31inlate1986andDrillsite3Kinearly1987,wheretheaveragejobsizeswere20,000and35,000Ibmof20/40-meshsand.Nosignificantincreaseinproductivitywasrealized.Calibrationtreatments,minifractures,wereperformedoneachofthenewdevelopmentdrillsitestodeterminethefluidefficiencyofthedifferentfracturingfluidsandthenecessaryrequirementsforfluid-lossadditives.Thisresultedintheeliminationofl00-meshsandfromtreatmentspumpedonDrillsite3Qin1987andareductionintheuseofsilicaflouronDrillsite3Minearly1988.Thesechangesresultedinsignificantimprovementinthefractureconductivityandpostfractureproductivity.Furtheradvancesin1988occurredwiththeuseofceramicproppantsonDrillsites3Hand30.Largertreatmentsalsowereusedtoprovidefurtherstimulationbenefits.Themostrecentadvancewasmadein1989withthechangetoawater-basedgeltoimprovefracturewidthandtheproppant-carryingcapacityofthefluid.Drillsite2Kdesignspumpedin1990usedupto200,000Ibmof16120-and12/18-meshceramicproppant.ContinualoptimizationoffracturetreatmentsoccurredintheevolutionoffracturingattheKuparuckRiverfieldduringthelast6years.Theresultsoftheseadvancesprovidesignificantoil-ratebenefits.Overthelast4years,anadditional140newwellshavebeenstimulated.Productivitymorethandoubledasaresultofthedesignchanges.Thechangesindesignhaveimprovedpostfractureflowefficiencyfromanaverageof153%tomorethan300%.Inaddition,asofJune1990,88wellshavebeenrefracturedtotakeadvantageofthetechnicalandoperationaladvances.Thecurrentincrementalratecausedbytheserefracturesisabout40,000BOPD,asignificantportionofthefield's300,OOO-BOPDcurrenttotalproduction.Ongoingeffortstoimprovethedesignincludeoptimizingthefracturelwellboreintersectioninadeviatedwellthroughorientedperforating,pumpinglargertreatments,andimprovingonsitequality-controlpractices.Moderate-PermeabilityTreatmentDeSignDeterminingthepotentialrateimpactofahydraulicfracturetreatmentisoneofthemostdifficultproblemstoquantify.Numerousstudiesandmodelingeffortshavebeenundertakensincethelate1950'stopredictthesebenefits.3-5Thesestudiesshowedthattheproductivityofahydraulicallyfracturedwelliscontrolledbythe260o1981-1986DEVELOPMENTI2Z11986-1988DEVELOPMENTITO1988-1989DEVELOPMENTB1989-1990DEVELOPMENTDUNDEVELOPEDFig.2-Kuparukfieldmap.lengthandconductivityofthecreatedfracture.Themlljorityofthesemodelsindicatethat,forafixedfractureconductivity,thereisamaximumeffectivefracturelength.4OneofthewidelyrecognizedexamplesofthisworkispresentedbyMcGuireandSikora5(Fig.5).Inthisfigure,theproductivityincreaseofafracturedwelloveranundamagedwellisplottedvs.therelativefractureconductivityforfracturelengths.Ifafracturewithaconductivityof1,250mdftisplacedina30-mdformationdraining160acres,Fig.5predictsapostfractureflowefficiencyofabout140%,regardlessofthefracturelength.ThisismarkedasPointAinFig.5andrepresentsthedesignexpectationofinitialtreatmentspumpedinthefield.Ifthesamefracturestimulationisachievedwithahigherpermeabilityproppant,yieldingaconductivityof9,OOOmd-ft,Fig.5predictsapostfractureflowefficiencyfrom230%to270%forafracturelength10%to30%ofthedrainageradius.Thesepoints(PointsBandB')representtreatmentspumpedwithhighconcentrationsofceramicproppant.Asimilarbutlessdramaticeffectcanbedemonstratedbyalteringtheproppedwidthoftheresultantfracture.Theproductivityofahydraulicallyfractured,moderatepermeabilitywelliscontrolledprimarilybyfractureconductivity.Fracturelengthcanbecomeanimportantfactoriftheconductivitycontrastbetweenthecreatedfractureandtheformationislargeenough.Inthiscase,itmaybeeconomicaltopumpalargevolumeofhigh-permeabilityproppanttomaximizeproduction.Thefollowingdesigncriteriashouldbeconsideredduringtheinitialdesignphaseandtheongoingoptimizationofafracturingprograminamoderate-permeabilityreservoir.ProppantSelection.Proppant-packpermeabilityisoneofthemostimportantfactorsgoverningthefinalfractureconductivity.Itcanvarybymorethananorderofmagnitude,dependingontheproppanttypeandsizedistribution.6Fig.6showsproppantpermeabilitycurvesforvariousproppanttypesandsizedistributions.7,8Itispossibletochangetheproppantpermeabilitysignificantlybyonlyslightlychangingthesizedistribution,sphericity,orcrushresistanceoftheproppant.Achangefromsandtoaresin-coatedorceramicproppantcanyieldlargeincreasesinconductivity.Itisimportanttoselecttheproppantforitsperformanceatthetypicalproducingconditionsoverthelifeofthewell.Recentstudies6,7haveshownthatsignificantdecreasesinconductivityareexperiencedafterlong-termexposuretoclosurestress,temperature,andreservoirfluids.Theseverityofthesedecreasesvariesfordifferenttypesandsizesofproppants.Formoderate-permeabilityfor-SPEProductionEngineering,August1992•Producer..InjectorFig.3-Drillsite2Wspidermap.mations,proppantevaluationshouldbemadeintermsofcostpermillidarcyandnotcostperpound.FractureWidth.Maximizingthecreatedfracturewidthwillincreasefractureconductivity.Largercreatedwidthsallowalargermeshproppanttobeplaced.Dependingonthestrengthoftheproppantandtheproducingconditionsofthewell,thelargermeshsizesmaysignificantlyincreasethepermeability.Largercreatedwidthsalsoallowahigherconcentrationofproppanttobeplaced,resultinginagreaterproppedwidthwithacorrespondingincreaseinconductivity,andalargeramountofproppantperunitlength.Foragivenslurryconcentration,alargerwidthwillgiveaproportionatelylargerproppantconcentrationonapound-per-square-footbasis.Theonevariablethedesigncontrolsthatcansignificantlyalterfracturewidthistheviscosityofthetreatingfluid.9Viscositiescanbevariedbyordersofmagnitude,dependingonthetypeoffracturinggelused.Toavoidproblemswithunnecessarygelresidueandtominimizeheight-growthconcerns,thegelviscosityshouldbelimitedtotheamountrequiredtotransportandplacethemaximumsandconcentration.Gelqualitycontroliscrucialbecauseproblemsoftenresultinreducedviscosityandless-than-expectedfracturewidth.TreatmentSize.Treatmentsizeusuallyislimitedbyoneoftwodesignissues:pumpingrateandfractureheightgrowth.AlargerSingleCompletionSelectiveSingleCompletionI>CIBlastJointsCSandASandFig.4-Kuparukcompletionschematics.treatmentgenerallywillresultinanincreasedlengthandsomecombinationofincreasedwidthand/orheight.Heightgrowthiscontrolledprimarilybyin-situstressesandshouldbeaddressedthroughcomputermodeling.10Pseudo-3Dandfully3Dfracturesimulatorsshouldbeusedtopredictthefinalgeometryofacreatedfracture.Themaximumtreatmentsizeforagivensetofwellcompletionandreservoirconditionscanbedeterminedwithafracturesimulator,andtheproductivitybenefitsoftreatmentsizesuptothismaximumcanbeevaluatedwithbothanalyticalandnumericaltechniques.Fluid-LossAdditives.Largevolumesoffluid-lossadditivesaregenerallyperceivedasbeingnecessarytoplaceatreatmentsuccessfullyinmoderate-permeabilityformations.Thehighleakoffratesneedtobecontrolledandoftenexcessfluid-lossadditiveisviewedas"cheapinsurance"toaidinpumpingthetreatment.Withveryfewexceptions,fluid-lossadditiveswillbedamagingtothefinalproppant-packconductivity.Totallyinertfluid-lossadditives,l00-meshsandorsilicaflour,shouldbeavoidedbecausetheytendtoplugtheproppantpackirreversibly.11,12Althoughsomeinertfluid-lossadditivesmayberequired,theyshouldberunonlyin~heminimumconcentrationneededtoplacethetreatment.Ifpossible,thefluid-lossadditiveshouldbeaproductthatdegradesanddissipatesundernormalproducingconditions.FieldEvaluationofFluld·LossAdditivesInitialeffortstoimprovefractureconductivityfocusedonthefluidlossadditivesusedtopumpthetreatments.AseriesofminifractureswasrunonDrillsite3Ktodeterminetherequiredamountoffluid-lossadditivesandspecificallytoevaluatetherelativerolesTABLE1-GELLED·DIESELTREATMENTDESIGN(1986)PumpCleanProppantProppantProppantRateVolumeMeshConcentrationMassStageDescription(bbllmin)~Size(Ibm/gal)(Ibm)1Prepad,gelleddiesel20652Pad,gelleddiesel20361000.58003Pad,gelleddiesel20604Gelleddiesel201620/4032,0005Gelleddiesel202720/4089,100Total:Fluid(excludingflush),bbl204Proppant(excluding100-meshsand),Ibm11,100Allfluidsaregelleddieselwith40·lbm/1,000·galsilicaflour.SPEProductionEngineering,August1992261,.12~10~~"':d~~~=Averageformationpermeability,md.(bMedongroathlckneu)=Fractureiengthfromwellbore.Ft.r.=Drainageradius,ft.A=Wellspacing.Acres.WkF=Crackconductivity.mel.-In.wkF'wJo=Proppedwidthoffracture,In.=Permeabilityofproppjngmatertal,m-o3K•5%100Mesh(/)o3Q-1to4%100meshi4000'663Q•No100mesh11X06)(0!20)()()(~
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