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SPE-14085-MS

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SPE 14085 MS
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SPESocietyofPetroleumEngineersSPE14085AnOverviewofRecentAdvancesinHydraulicFracturingTechnologybyR.W.VeatchJr.andZ.A.Moschovidis,AmocoProductionCo.SPEMembersCopyright1986,SocietyofPetroleumEngineersThispaperwaspresentedattheSPE19861nternationalMeetingonPetroleumEngineeringheldinBeijing,ChinaMarch17-20,1986.Thematerialissubjecttocorrectionbytheauthor.Permissiontocopyisrestrictedtoanabstractofnotmorethan300words.WriteSPE,P.O.Box833836,Richardson,Texas75083-3836.Telex:730989SPEDAL.ABSTRACTTherehavebeensignificantadvancesintheapplicationanddevelopmentofhydraulicfracturingtechnologyinthepastseveralyears.Thispaperpresentsanoverviewofsomeoftheseadvancestoprovidethereaderwithaperspectiveofthecurrentfracturingstateoftheart.Thediscussionaddresseseconomicdesignconsiderations;fracturingmaterialbehavior(proppingagents,fractureconductivity,fluidloss,fluidrheologyandproppanttransport);fieldacquiredfracturedesign,diagnosticandanalysistechnology(in-situstressesandstressprofiling,downholefracturingpressureandpressuredeclineanalysis,real-timeon-sitemonitoringandcontrol,andfracturemapping);andthree-dimensionalfracturepropagationsimulation.Acomprehensivebibliographyisprovidedasaresourceforin-depthperusalofeachareabytheinterestedreader.INTRODUCTIONIn1982attheInternationalMeetingonPetroleumEngineeringinBeijing,China,Veatch1presentedanoverviewofthestatusofhydraulicfracturingtreatmentanddesigntechnology.Manyofthefacetsofthispaperwereupdatedinasubsequentpaperin1983.2Duringthepast2-3years,fracturingtechnologyanditsapplicationthroughouttheindustryhavemadesignificantprogress.Thispaperfocusesonmanyoftherecentadvancementswhichhavedevelopedsincethepreviouspaperswerepublished.Itattemptstoprovidethereaderwithaperspectiveofthecurrentstateoftheartoffracturing.Thediscussionsurveysthemanyaspectsoffracturing,touchingonlybrieflyoneach.Itisoutsidethescopeofthepapertopresentanin-depthcoverageofallthedetailsofthetechnology.WhatisgivenReferencesandillustrationsatendofpaper.421isarathergeneraloverviewoftherecentadvancementsintechnologyandtheapplicationbytheindustry.Forthesakeofcontinuity,manyofthereferencescontainedinthepreviouspapersareincludedalongwithreferencesofrecentwork.Thisprovidestheinterestedreaderarathercomprehensiveresourcetoamorein-depthexplorationofthetechnologyoffracturing.Theworkpresentedhereprimarilycovershydraulicfracturingtreatmentapplicationsanddesign.Thereisminimalreferencetothereservoirperformanceanalysistechnologyassociatedwithfracturing.Thediscussionemphasizes(1)economicdesignconsiderations;(2)fracturingmaterialbehaviorincludingproppingagentsandfractureconductivity,fluidloss,fluidrheology,proppanttransport,andnewdataonfoamedfracturingfluids;(3)fieldacquireddataforfracturedesignincludingin-situstressdataandprofiling,diagnosticdatafromdownholefracturingpressuresandfrompressuredecline,real-timeon-sitemonitoringandcontrolcapabilities,andfracturemappingtechnology;and(4)three-dimensionalfracturepropagationsimulationmodels.FRACTURINGECONOMICOPTIMIZATIONTheconceptofoptimizingfracturingtreatmentdesigns,generallyspeaking,hasthreebasicsteps:Thefirst(upperleftportionofFig.1)istoevaluatetheincreasedincomewhichmightbeexpectedfromoilorgasproducingperformanceresultingfromvariousfracturelengthsandconductivities;thesecond(lowerleftportionofFig.1)istodeterminethecostsrequiredtoachievethevariouslengthsandconductivities;andthethird(rightsideofFig.1)istoevaluatethenetrevenue(i.e.,incomeminuscosts)versusfracturelengthtodeterminethetreatmentdesignthatyieldsthemaximumnetrevenue,i.e.,theoptimumdesign.Thespecificproceduresusedfordeterminingtheoptimum2ANOVERVIEWOFRECENTADVANCESINHYDRAULICFRACTURINGTECHNOLOGY14085fracturingtreatmentdesignforagivenformationmaynotalwaysconformpreciselytotheseconceptualsteps.Buttheywillalwaysinvolvesometypeofbalancebetweentreatmentcostsandrevenuesgeneratedfromtheproductionresponseassociatedwithatreatment.Ithasbeengenerallyrecognizedthatthefracturelengthrequirementsdependgreatlyonreservoirpermeabilityandfractureconductivity,suchasisshownbyElkins3inFig.2.Here,weseethatextremelylowpermeabilityformations(k=0.0001md)mayrequirehalf-lengthsaslongas3500-4500ft(seeshadedarea).However,lengthandconductivitymaynotbetheonlyparameterswhichaffectfracturingdesignoptimization.Thisissometimesnotobviousinparametricfracturingstudieswheretheprimaryfocusisonformationpermeability,fracturepenetrationandconductivityrequ1rements.Insomecases,otherfactors(e.g.,netpay,fractureheight,etc.)canbecomeimportantconsiderationsinfracturingeconomics.Theirincrementaleffectscanbeverysignificant.Forexample,considertheeffectofnetpayonfracturepenetrationrequirementstooptimizethenetpresentworthofatreatment(i.e.,thepresentworthofthehydrocarbonproductionforthefracturedformationminusthepresentworthofthehydrocarbonproductionfortheunfracturedformationminustreatmentcosts).Theresultsofanexamplecase,asshowninFig.3,depictthepercentincreaseinnetpresentworth(i.e.,netpresentworthforthefracturedcaseexpressedasapercentoftheunfracturedcasepresentworth)versusfracturepenetrationfornetpayswhichrangefrom2to100ft(0.6to30.6m)ina5-mdformation.Here,fractureconductivityis6000md-ft(1829md-m)andthewellsareon160acres/wellspacing.Figure3showsthattheoptimumfracturepenetration(i.e.,thepenetrationatwhichthemaximumnetpresentworthincreaseoccurs)getslongerasnetpayincreases.Theresultsforthiscaseandtwootherformationpermeabilitylevels(1and10md)aresummarizedinFig.4whichshowstheoptimumfracturepenetrationplottedversusnetpay.Hereweseeoptimumfracturelengthswhichrangefrom200to1320ftforthe5-and10-mdformations,andanalmostconstantoptimumlengthforthe1-mdformation.Thisshowsthatoptimumlengthscanvarywidelyforagivenpermeabilityandfractureconductivity,dependingonthenetpaymagnitude.Addressingfractureheightfromaneconomicstandpointreinforcestheneedforhavingreliableheightdatawhendesigningtreatments.Inadditiontotheobviousincreaseincosts,fractureheightcanhaveasignificantimpactonoptimumeconomicpenetration,whichinturncouldaffectwellspacingrequirements.Asanexample,caseswererunfora1mdformationwith10ft(3.0m)ofnetpay,a2000md-ftfracture,and160acres/wellspacing.Fractureheightsfrom180to720ft(55to219m)wereinvestigated.TheresultingoptimumfracturelengthsandtreatmentvolumerequirementsareshowninFig.5.Theoptimumvalueswerethosewhichyieldedthemaximumnetpresentworthforeachgivenheight.Ascanbeseen,theoptimumtreatmentvolumerequirementsdidnotchangedramaticallyoverthewiderangeoffractureheights,buttheoptimumlengthsdid.Ataheightof180ft,theoptimum422fracturepenetrationapproachesthedrainageboundary(i.e.,1320ft);atheightsontheorderof600-700ft(183-213m),theoptimumlengthswere300-400ft(91-122m).Thissuggeststhatonemayneedtoinvestigatetheeconomicsforcloserwellspacingforsuchsituations.Warembourg,etal.,4presentedaneconomicstudyonthreeexamples,andaddressedseveralotherimportantfactorsthatshouldbeconsideredforoptimizingtreatmentdesigns.Theseincluded:(1)thedurationoftheproductionforecastfromwhichnetpresentworthiscalculated,(2)thenetdiscountedproductionrevenue,and(3)theamountofinvestmentrequiredtoachievethedesignoption.Otherfactors,suchashydrocarbonprice,interest(discount)factors,technologylevelandriskasdiscussedbyRosenberg,etal.,5andBrashear,etal.,6havealsobeenshowntoplayacriticalroleineconomicoptimization.PROPPINGAGENTSANDFRACTURECONDUCTIVITYTherehasbeenconsiderableprogressinthedevelopmentofintermediatestrengthproppingagents(sometimescalledintermediatedensityproppants,IDP)tosupplementsinteredbauxiteforhighin-situstressapplications.Varioussupplementalindustrytestsonawidespectrumofproppingagents(sands,intermediates,sinteredbauxites,resin-coatedsands,etc.)andrecentinvestigationsonfractureconductivityhaveconsiderablyextendedpreviouslypublishedwork.7-15StudiesbyPhillifsandAnderson,16LarsenandSmith,17Becq,etal.,8andNorman,etal.,19plusrecentcomprehensivedatasetspublishedbythestimulationservicecompaniesandproppingagentmanufacturers,20-25provideanextensiveresourceforfractureconductivitylaboratorytestresults.PhillipsandAndersondemonstrateamethodtomodifythetraditionalconductivityversusclosurestressdatatoincludethecostforvarioustypesofproppantsasisshownbythecurvesinFig.6.Theserepresentthecost/unitfracturearea/unitofconductivity($/ft2/Darcy-ft)overawiderangeofclosurestresses.Theyaccountforaproppantpackdamagefactorofapproximately20-25%.Theauthorssuggestedprescribingdamagefactorvaluestoadjustlaboratorytestdatatorepresentamorerealisticestimateofin-situfieldperformance.Graphssuchasthesecanbeconstructedforagivenfracturingfluidfromcurrentproppantpriceschedules,laboratoryconductivitytestdata,andestimatesofproppantpackdamagefactorsforagivenfluid.Whenconstructingthesegraphs,oneiscautionedtouseproppantperformancedatatestedbyconsistentproceduresfromonetestinglaboratorybecausedatafromdifferentsourcesmaynotbeaccuratelycompared.Therehasbeensomeworkconductedonproppantpackdamageandplugging.Kim,etal.,26conductedproppantpackdamagetestson20/40meshsandfordifferentfracturefluidsoverawiderangeofclosurestressesandatdifferenttemperatures.Theresultsshowedthatfractureconductivitiescouldbereducedby40-60percentjustfrompluggingbythegelresidue.Cheung27reportedthatvariousconcen-14085RALPHW.VEATCH,JR.ANDZISSISA.MOSCHOVIDIStrationsofHCl/HFacidsolutionscandissolveasignificantamountofproppantandthiswouldreduceconductivity.UnpublishedworksponsoredbyNorten-AlcoCompanysuggeststhathighlysiliceousproppantsmaydegradeseverelyinbrinesathightemperatures.AlmondandBland28reportedonvariouswaysthatbreaktemperatureandbreakermechanism(i.e.,oxidizers,enzymes,etc.)playanimportantrolein20/40meshsandproppantpackflowimpairmentfromguar,derivatizedguar,andcellulosebasedfluids.Tomitigatewaterblockingproblems,PhillipsandWilson29showedthatusingasolventinthepadfluidswithasurfactantintherestofthefluidreduceswaterblockinginthefractureandsignificantlyenhancesfracturingfluidrecoveryandproduction.Acomprehensivefractureconductivity/reservoirperformancestudybyBritt30addressesoptimizationoffractureconductivityforanoilreservoirunderbothprimaryandsecondarydepletion.Itshowedcpeeconomicbenefitsofhighconductivityshortfracturesformoderatelypermeable(i.e.,1-10md)formations.TheresultsdepictedinFigs.7and8showtheimpactofdifferentfracturelengthsandconductivitiesonincrementalpresentworth.Bydevelopingcurvessuchasthese,onecandeterminetheappropriatefractureconductivity/lengthrelationshiprequiredtomaximizeeconomicreturnsforagivenreservoir.StudiesbyElbe131andMontgomeryandSteanson32addressmethodsforusingreservoirperformancetypecurvesandcomputerizedsimulatorstodeterminetheappropriatefractureconductivitydesignrequirementsforvariousreservoirpermeabilitylevels.ElbelsupportspreviousfindingsbyBennett,eta1.33thatavaryingconductivityinthefracturefromthewellboretothetipcansignificantlyaffectproductionrates.Studiessuchasthesecanbeveryimportantwhendeterminingproppantplacementandschedulingprogramsforatreatmentdesigntoassurethattheappropriatedistrib~tionofconductivityinthefractureisachieved.FLUIDLOSSMuchoftherecentinterestinfluidlosshasfocusedon(1)dynamicfluidlossbehavior,(2)insitumeasurements,and(3)fluidlosscontrolfornaturallyfracturedformations.Recentlaboratoryandfieldstudieshaveextendedthefindingsofpreviousinvestigators34-41forbothstaticanddynamicfluidlossbehavior.Recentdynamicfluidloss,studiesbyGulbis,42HarrisandPenny43Penny,etal.,44Roodhart45andZigrye,etal.,~~indicatedthatdynamicfluidlosstestscanyielddifferentresultsthanstatictestsdoandthatshearrateandshearhistorycanaffectthetestssignificantly.Figure9showsatypicalflowsystemwiththefluidlosscells,rheologyloopandheatingcapabilitiesusedbymostoftheinvestigators.Figure10showsthedifferentfluidlossbehaviorsobservedbyGulbisfordifferentshearrates,shearhistoriesandtemperatures.Thesetestswererunonthesamefluid,i.e.,aHydroxypropylGuar(HPG)fluid,crosslinkedwithatitaniumcompound.Thetestconditionsshowninthelegend423ofFig.10aregiveninTables1and2.Theresultsdemonstratethesignificanteffectthatdynamicscanhaveonfluidlossbehaviorforfluidsflowinginafractu~e.ThestudiesbyRoodhartandbyHarrisandPennyalsoshowedthatfluidloss(i.e.,bothspurtlossandfluidlosscoefficient)behaviorisaffectedbyfluidflowdynamics.Someotherinterestingobservationshavebeenreported.Roodhart'stestsdemonstratedasignificanteffectofpressuredifferentialonwallbuildingfluidlosscoefficient,C•ThisisshowninFig.11forbothacrosslinked~PGfluidwitha5%dieselandahydroxyethylcellulose(HEC)basedfl~idwithsilicaflour.HereweseeasignificantincreaseinCwathigherpressuredifferentials.HarrisandPennyobservedaneffectofincreasedviscosityintheflowingfluidduetodehydrationfromfluidloss.ThisphenomenonisshowninFig.12,bythecontinuallyincreasingviscosityforatestinaradialflowcell(Fig.9)wherefluidlossisoccurring.Itsuggeststhatthegelisthickeningbecauseoffluidloss.Theothercurveshowsthatviscositydecreasesfromshearandtemperaturedegradationwhenfluidleakoffispreventedbyreplacingthecorewithanimpermeableblank.Observationssuchastheseemphasizetheneedforabetterunderstandingofinsitufluidlossbehavioranditseffectsonrheologyandproppanttransport.Thisisparticularlyimportantbecauseofthesignificantrolethatfluidlossplays,beingoneofthemoredominantparameterscontrollingthefracturingprocess.Gulbis'workindicatedthatatshearratesbelow80sec-1,dynamicandstaticfluidlossbehaviorwassimilar.ObservationsbyPenny,etal.,showedcorrespondingresultsatshearratesbelow40sec-1•However,athighshe~y4rates,theysuggestedthatfluidlossfollowsa172trendratherthanthecommonlyobservedtforstatictests.W~rkbythepreviouslymentionedinvestigators4246allsupportedearlierstudieswhichshowedthatahydrocarbonphase(e.g.,5%diesel)couldsignificantlyreducefluidloss,especiallyifmixedwithsilicaflour(orotherfinemeshparticulates)andasurfactant.ThiswasespeciallyeffectiveinfracturedcoresasisshownbytheexampleinFig.13.Gulbisreportedthattheeffectsofthehydrocarbonphase/silicaflouradditivesonreducingfluidlosswerelesspronouncedindynamicteststhaninthestaticones.FluidLossFromFieldData:SeveralinvestigatorshavesupplementedtheliteraturewithmethodstoinferfluidlossfromfielddatasinceNolte37introducedthepressuredeclinemethodin1979.Nierode47proposedadifferentapproachfordeterminingfluidlossusingmeasurementsofincreasinginstantaneousshut-inpressure(ISIP)dataduringthetreatment.Thisworkisbasedontherelationship.34ANOVERVIEWOFRECENTADVANCESINHYDRAULICFRACTURINGTECHNOLOGY14085whereFG(t)istheISIPfracturegradient(ISIP/depth?attimet(psi/ft);t1is.thetime.offirstshut-tnpressuremeasurement,(mlns);t1sthetimeoflatershut-inpressuremeasurement,?mins);AandBareempiricalfitconstants;andCisthefluidlosscoefficient(ft/~min).NierodeproposedthevaluesA=0.19043andB=0.46767foraKristianovich-Geertsma-deKlerk,KGD(sometimescalledKristianovich-Zeltov,Kz48)shapedfracture;andA=0.20233andB=0.47850foraPerkins-Kern-Nordgren,49'50PKN,shapedfracture.ThesevaluesservedasthebasisfordevelopingthecurvesshowninFig.14forusingISIPincreaseandpumpingtimesincethefirstshut-intoestimatefluidlosscoefficient.Cooper,etal.,51presentedtheresultsofacomprehensivefieldstudycomparingthemethodsofNolteandNierodewiththeoreticalexpressionsofSmith52andofWilliams,etal.41Thethe~reticalexpressionsemploythethreetypesoflinearflowleakoffmechanisms:(1)(2)(3)fluidviscosityan
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