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SPE-2854-PA

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SPE 2854 PA
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.FracturingGradientsinWaterfloodsofTfi..,~a.menhili~~,p~~~iall~DepletedLuw‘1VAl.lLvuuLAlZonesMartinFelsenthal,SPE-AIME,ContinentalOilCo.HowardH.Ferrell,SPE-AIME,ContinentalOilCo.IntroductionFormationfracturing,or“pressureparting”asitissometimescalled,canoccuratsurprisinglylowpres-suregradientsduringwaterinjection.Forconditionsconducivetoverticalfracturingthesefracturinggradi-entsexpressedasbottom-holeinjectionpressurevsdepthmaybeaslowas0.50psi/ft.Thisismuchlowerthanthegradientsusuallyobservedinthesamefieldswhenwellsarehydraulicallyfracturedduringinitialcompletions.Thereasonfort.hkdfierenceisthatthefracturinggradientisdependentonforma-tionpressure.whiletheformationpressureishigh,thefracturinggradientobsavedincompletionworkwillalsobehigh,butasthepressuredeclinesduringprimaryproduction,thefracturinggtilentlikewisedeclines.FieldObservationsF,ractwinggra&msof0.57psi/ftwerereportedbyEatonforapressure-depletedWestTexasfieldwhereoldproducingwellshadbeenconvertedtowaterin-L.--t=field,~~=di~ntsequ~edjectionwetis.’hih,=.2,.,W~:6~to0.70psi/ftduringhydraulicfracturingofnewwells.Eatonalsorepom-dfracturinggrdentsequalto0.748,0.864,and0.993psi/ftaftersuccessive6-monthperiodsinaCaliforniainjectionwell,withthechangesbeingattributedtogradualincreasesinformationpressurearoundthewellbore.HeckalsomadereferencetosimilarobservationsintheBrad-ford,Pa.,field.zThese,andsimilarliteraturerefer-ences,illustratethephenomenoninaqualitativeman-ner,butnotenoughdataaregivenforquantitativecorrelations.Tofillthislackofcorrelativedata,weobservedfracturegradientsandalsoobtainedcor-respondingshut-informationpressures;thelattervalueswedeterminedfrompressurefall-offtests,usingthemethodofHazebroeketaL3OnesuchsetofdatawasobtainedinFieldA,a6,500-ftdeepMississippianlimestone.reefreservoir.Inthisreservoirtheaveragefracturinggradientwasreportedas0.78psi/ftatadiscoverypressureofabout2,675psig.Duringprimaryproductiontheformationpressuredeciinedtovahtiesraaigingfrom780to1,700psiinthewellsofinterest(thelargerangeofvtduesbeingtheconsequenceofpermea-bilitybarriersthroughoutthereservoir).Approxi-matelyamonthafteroldproducminthisreservoirwereconvertedtoinjectionwells,fractwegradientsandshut-informationpressureswereobservedinfive.m.1---c.~+h.#+wells.ThiswasrepeateaarmanottIGrJLIsoUUi=w.waterinjection.ItmaybeseenfromthedatainFig.1thatduringtheperiodofobservationtheshut-informationpressureintheinjectionweiisincreased,ontheaverage,750psi.Thiswasaccem.patiedbyadefiniteriseinthefracturinggradients,asevidencedbythefactthatfracturegradientsaveraged0.55psi/ftinthefirstmonthand0.65psi/ftinthesixthmonthofinjection.TheoryofFormationFracturing...-.—GA.@-tm+nhasbeenThetneoreucalbasisfolfullUauwLL~-.-...=-------,Fracturingduringwaterjloodingcanbediagnosedfrompressurefall-offandstep-rateinjectivitytestdata.IntectonicallyrelaxedzonesthesewelltestdataalsoyieldPoissonratio.InformationgivenheresuggeststhatPoissonratiosrangingjrom0.23to0.33arenormalforlow-permeabilityrocks,andthathigherratiosmayindicatefracturingthroughshalycavrock.JUNE,1971727.Fig.I—Increaseinfracturinggradientswithincreaseinshut-informationpressures.POISSONRATlO.~ezFig.2—RatioofmaximumIatemldeformationtomaxi-mumlongitudinaldeformationobservedduringcompressionloading.IDFRAC6RADIENT0.5BHPpsim’n.IASOOLOMIIE3.S00a75csAmvloNEWOo1I-ALLu+o0.5S.LFORM.PRESS.,&OEPTHFT.Fig.3-Field-observedfraetunnggradientsvsshut-informationpressure:depthratiosforthreaIowpermeabilityreservoirs.Poissonmtiocunfeswerederivedfromformat-ionfracturingtheory.WA&A4..“,..........-.:.-Akwu+a~n-i~,tHuhkti~~~~s........Martin,eWhalen,Tandothers.s’9HubbertandWfiexplainedthatintectordcallyrelaxedzonescharacter-~izedbynormalfaultingtheleastpticipalstressisinahorizontaldimction.sThesezonesaregenerallythoughttoexistatdepthsofmorethan3,000ftbutfrequentlyoccuraisoatsidowerdepths.Fracturegradients(pf/Ointhesezonesarealwayslessthantheoverburdenpressuregradient(S/D)of1.0psiperfootofdepth.Theresultingproducedfracturesareverticalandgenerallyhavedirectionalorientations.Martin’sworkshowsthatinmostC,*Stheproducedfracturewillparallelthenormalfaultsinthearea.aThefracturingpressuregradientisafunctionoftheoverburdenpressure(S),theshut-informationpressurc(p.)andthePoissonratio(v)asshowninEq.1:ThePoissonratio,V,isillustratedinFig.2.Itdescribesthemaximumrockdeformability(justbe-foretherockstartstodisintegrateundertestloading).Thisratiomaybeashighas0.45forhighlyelasticrockand,underlaboratorytestcondltions,10fallsinthe0.20to0.30rangeforbrittledenserockssuchaslimestonesanddolomites.CorrelationofFracturingPressurewithFormationPressureandPoissonRatioThegraphinFig.3showsthatthePoissonratiowasusedasacorrelationvariable.Thedatapointsahownareactualfieldobsenfations,andthecorrelatingcurvesarebasedonEq.1,usinganoverburdenpres-suregradientof1.0psi/ft.Fig.3contains,inadditiontodatafromthepre-viouslymentionedFieldA,datafroma3,900-ftdeepPermiandolomite(F]eldB)andan8,600-ft-deepPermiansandstonereservoir(FieldC).Thelatterreservoircontainedanextremelyhard,fine-grainedsiltysandstone(porosity=10percentpermeabilitytoair=0.05to1.5red).ItappearsfromFig.3thatthefracturinggradients1.+.wmrmnhlvwenW:thforthetimereservotmCerre..-A.-.W--”-Jtheformation-pressum:depthratio,aswouldbeex-pectedfromthefracturingtheory.Notethattheuseofpressure:depthratiosinthecorrelationgraphmakesforconvenientcomparisonofdatafromfieldsofvaryingdepth.NotealsothatmostofthedatapointsliebetweenindicatedPoissonratiosof0.25and0.35.Onedatapointliesbetween0.20and0.25andfourfallbetween0.35and0.40.ItisspeculatedthatthepointsinFig.3withanunusuallyhighPoissonratiomaybeindicativeofafractureextensionintoamoreelasticrockmaterialthanwaspresentwithintheoriginalreservoir.Fig.4,whichillustratesthispointinmore&tail,showsthattheindicatedPoissonratiochangeddrasticallyinsuccessivetestsforWells20,74,and94.Thismayindicatethatthefracturebrokethroughtheconfinesofthereservoir.Supportingthisconceptarethere-sultsofcalculations(madebythemethoddescribed72SJOURNALOFPETROLEUMTECHNOLOGY.byClark”)thatinFieldAindicatedgreaterfracturelengthsinsuccessivetests.Initialfracturelengthswereinthe50-to100-ftrange,whereasthelaterindicatedfracturelengthsexceeded660f~whichishalfthedistancetothenearestproducingwell.HowToRecognizeandAvoidExcessivelyLargeInducedFracturesWerealizethatamoderatedegreeofinjectionwellfracturingisoftennecessaryanddesirableintightreservoirsinordertocompletewaterfkmdkginareasonabletime.Caremustbetaken,however,toavoidinjectionpressuresthatwouldleadtoexces-sivelylongfracturesthatcouldbreakoutofthereser-voir.Iffractureheightorlengthisallowedtogrowwithoutanycontrolduringinjection,theresultisUke!ytoheseriouswaterfloodinetliciencies,includhsglargelossesofwaterfloodoilandinjectionfluid.Thesetypesofoperationsaregenerallycharacterizedbypersistentlylopsidedfluid-intofluid-outratiosof2:1,5:1,oreven10:1.Ifitisdecidedtooperateatorabovepartingpres-sure,aknowledgeoftherocktypeaboveandbelowthepayzonecanhelpinestimatingthepressuregradientthatcansafelybeapplied.Hardlimestonecaprockmaypartatpressureslowerthantheforma-tionpressureandresultinexcessivedamagingfrac-tures,butplastic-likeshalemayhelpcontainthefracturewithintheformation.Afterinjectionrateshavestabilized.pressurefall-A+t+cch~llldbem&-kandrCSU!tS.shou!dbeana-“,1.-aa0.,-----...----lyzedbypublishedmethods’.”forshut-informationpressure(p.)andfracturelength(X~).Furthermore,theapparentPoissonratio(]’)shouldbeevaluatedeitherwiththeaidofF@3orwiththeaidofEq.1,whichmayberestatedasfollows:v=(P/D–p,/D)/(S/D+P/D–2pe/D),............(2)wherePisnowthedown-holeinjectionpressurepriortopressurefaii-ofitesting.Ifthesecalculationsindicateeitheranexcessivefracturelength(say.severalhundredfeet)oraPoissonratiothatappearstobemuchhigherthanisnormalFRAC.GRADIENTBHPpsiOm‘F.0.9rPOISSONRATIOG.4‘,:.Y,I1,Io0.10,20.s0.4Q50.60,7Fig.4-Analysisoffracturinggradientsobsewedduringdifferentstagesofwaterinjection.Numbersoncurvesrefertowellnumbers.JUNE.1971fortheresesvoirrockofinterest,theinjectionpressureshouldbereduced.HowToOptimizeFracturingDuringWaterfloodAfterinjectionhasstabilizedatareducedpressure,anewpressurefall-offtestshouldbeconductedandresultsshouldbeanalyzedagain.Thisshouldbere-peateduntilboththecalculatedfracturelengthsandPoissonratioappearnormal.Inmostcases,moderatefractures(e.g.,200ftorless)areacceptable;butinanyparticularcasereservoirfeatures,suchaspaythickness,thenatureoftheoverburdenorunder-burdenandthenearnessofwaterorgaszonesalsomustbeconsidered.Poissonratiovaluesmaybediagnosedbycomparisonwithdataforsimilarreser-voirs,asdisplayedinFigs.3and4.Forthereservoirsshowninthesefigures,Poissonratiosof0.23to0.33appeartobenormal;otherratios,however,mayindi-cateafracturethrougheithermoreelasticrockorharder,morebrittlerocklyingaboveorbelowthereservoirofinterest.ApparentPoissonratioscalculatedfromfieldob-servationsduringdrillingandinitialhydraulicfrac-turingwouldbemosthelpfulinpredictingfracturesforwateriloods.Wethereforeproposethatafileoffield-observedPoissonratiosbeestablished.Experienceinseveralwater!loodshasshownthatattentionshouldalsobegiventheproducingwells.These,too,mayneedtobefracturedorotherwisestimulatedtoovercomethelowmatrixpermeabilityaroundtheproducingwellbore.Fielddataindicatethatthiscansignificantlyimprovewaterfloodoilproduction.step-xac--R-%lfi@tivityDataasanAidto.ArMl!ysisThedatapointsshownforWell20inFig.4areinpartbasedontheresultsofthestep-rateinjectivitytests’””illustratedinFig.5forthesamewell.Thisfi3JR~how~Lfiat~cmiona!lyadoublebreakappearsinthesecurves.Bothbreaksareassociatedwithdis-tinctivelydifferentPoissonratios,asillustratedinFig.4.ThesecondbreakisbelievedtoindicateaFig.5--Breaksinsteprateinjectivitycurves,indicatingformationfracturing.Numbersondatapointsrefertosuccessivetimes.729probablefracturethroughtheoverburdenorunder-burden.Thestep-rateinjectivitytestsillustratedinFig.5wereconductedasfollows.Firstinjectionwagmain-tainedatastabilizedrateineachwellcorrespondingtoPoint1oneachcutve.Thena10-daypressurefall-offtestwasconducted.Next,injectionwasre-sumedatalowratecorrespondingtoPoint2.Injec-tionpressureswereallowedtostabilizefor24hours,afterwhichtheinjectionratewasincreasedslightly.Thiswasrepeatedseveraltimestoyieldtherem=htgdatapoints.ResultingdatawereplottedasshowninFig.5.Theabscissaofthisgraphgivesthedfierencebetweenthebottom-holepressureattheendofeach24-hourperiodandtheshut-informationpressure.ThepointorpointsatwhichratesincreasedabruptlybeyondtheamountexpectedfromtheincreaseinApwereconsideredtoindicateformationfractunng.OtherDiagnosticTechniquesFordiagnosingsuspectedlossofinjectedwateraboveorbelowtheformationofinterestthetemperaturesurveyisuseful.Stillothertechniquesz4-z3areavail-ablefordiagnosingdirectionalorientationsoffrac-tures.Knowledgeofsuchdirectionalityisofcriticalimportancetothesuccessofanywaterflood,especiallyWnt.-+toodcinlfiwm~rrm=ahilitvm=cervoirc2*-25.....”...--=”....“..r-....--”--...-”-..-..”.ConclusionsInjectionpressuresthatcauseformationfracturingduringwaterfloodingdependinpartontheshut-informationpressure.Thus,fracturingpressuresinagivenreservoirdeclineastheshut-informationpres-suredeclines.Formationfracturingcanbedetectedbypressurefall-offtestsfollowedbystep-rateinjec-tivitytests.Thesetestsyieldformationproperties,fracturelength,anddeformationpropertiesexpressedasPoissonratio.Injectionpressuresatornearthefracturingpres-surerequireclosesurveillancebecauseexcessivein-jectionpressurecanleadtoprematurewaterbreak-throughandpoorsweepefficiency.Ifeitherthefrac-turelengthorthePoissonratioappearsabnormal,areductionininjectionpressuresiscalledfor.DatashowninthisreportsuggestthatPoissonratiosrangingfrom0.23to0.33arenormalforlow-perme-abilitylimestone,dolomites,andsandstones.References1.Eaton,B.A.:“FractureGradientPredictionandItsApplicationinOilfieldOperations”,J.Pet.Tech.(Oct.,1969)1353-1360.2.Heck,E.T.:“FracturesandJoints”,Prod.Monthly(Feb.,1955)20.3.Hazebroek,P.,Rainbow,H.andMatthews,C.S.:“PressureFall-OffinWaterInjectionWells”,Trans.,AIME(1958)213,250-260.4.Harrison,E.,Kieschnick,W.F.andMcGuire,W.J.:.“TheMechanicsofFractureInductionandExtension”,Trans.,AIME(1954)201,252-263.5.Hubbert,M.K“.andWillis,D.G.:“Mechanicsof~#aulicFracturing”,Trans.,AIME(1957)210,153-6.Martin,F.G.:“MechanicsandControlinHydraulicFracturing”,Pet.Eng.(Dec.,1967)62-66,69-72.7.Whalen,H.E.:“UnderstandingandUsingFracPres-~8>inWellPlanning”,Pet.Eng.(Sept.,1968)75-76,..8.Blanchet,P.H.:“DevelopmentofFractureAnalysisasExplorationMethod”,Buii.,AAPG(Aug.,1957)41,N,-.Q1’7AQ-17<0‘.”.“,,r-r”-,,/7.9.Dunlap,I.R.:“FactorsControllingtheOrientationandDirectionofHydraulicFractures”,J.Inst.ojPet.(Sept.,1963)49,No.477,282-288.10.Wuerker,R.G.:“AnnotatedTablesofStrengthandE!agicp~~nertiesQfROCkS”,DriUinc.AIMETransac-—-------e,tionsRepri~~-&iesNo.6,23-45.11.Clark,K.K.:“TransientPressureTestingofFracturedInjectionWells”,J.Pet.Tech.(June,1968)639+43.12.Yuster,S.T.andCalhoun,J.C.,Jr.:“PressurePart-ing”,Prod.Monthly(Feb.,1945)9,No.4,16-26.13.Spencer,O.F.:“SecondaryRecoveryofOit”,Pennsyl-vaniaStateCollege(1949)169-172.14.Goolsby,J.L.:“Here’stheRelationofGeologytoFluidInjectioninthePermianCarbonateReservoirs,WestTexas”,OilandGas1.(July31,1967)188-190.15.Donohuc,D.A.T.:“CanAerialPhotographyImproveOilRecovery?”,OilandGasJ.(July10,1967)201-203.16.Overbey,W.K.,Jr.,andRough,R.L.:“SurfaceJointPatternsPredictWellBoreFractureOrientation”,Oil;uProd.Monthly.OLO..x*n(JUnC,1700)10-17.17.Vieira,L.P.:“FracturedReservoirs”,Prac.,SeventhWorldPet.Cong.,MexicoCity(1967)219-227.18.Elkins,L.F.andSkov,A.M.:‘“DeterminationofFractureOrientationfromPressureInterference”,Trans.,AIME(1960)219,301-304.19.Kunkel,G.L.andBagley,J.W.:“ControlledWater-ftooding,MeansQueenReservoir”.J.Pet.Tech.(Dec.,1965)1385-1390.20.McKinley,R.M.,Vela,S.andCarlton,L.A.:“AFieldApplicationofPulseTestingforDetailedReservoirDescription”,J.Pet.Tech.(March,1968)313-321.21.Fraser,C.D.andPettitt,B.E.:“ResultsofaFieldTesttoDeterminetheTypeofCementationofaHy-draulicallyInducedFormationFracture”,J.PerTech.(May,1962)463-466.22.Anderson,T.O.andStahl,E.J.:“AStudyofInducedFracturingUsinganInstrumentalApproach”,J.Pet.Tech.(Feb.,1967)261-267.23.Zemanek,J.,Caldwell,R.L.,Glenn,E.E.,Holcomb,S.V.,Norton,L.J.andStraus,A.J.D.:“TheBoreholeTeleviewer—ANewLoggingConceptforFractureLocationandOtherTypesofBoreholeInspection”,J.Pet.Tech.(June,1969)762-774.24.Carries,P.S.:“EffectofNaturalFracturesorDirec-tionalPermeabilityinWaterftooding”,SPEpaper1423presented_at_Seve~thBiennia!-SecondaryRecoverySym-posium,WichitaFalts,Tex.,May~-3,i~66.25.Donohue,D.A.T.,Hansford,J.T.andBurton,R.A.:“TheEffectofInducedVerticallyOrientedFracturesonFive-SpotSweepEfficiency”,Sot.Pet.Eng.J.(Sept.,1968)260-268.J_PTOriginalmanuscriptreceivadinSocietyofPatroleumEngineeraofficeJan.8,1970.RevisedmanuscriptreceivedMarch24,1971.Paper(SPE2854)waspresentedatSPESymposiumonPracticalAspectsofImprovadRecoveryTechniques,baldinFortWorth,Tax.,March8-10,1970.@Copyright1971AmericanInstituteofMining,Metallurgical,andPetraleumEngineers,Inc.730JOURNALOFPETROLEUMTECHNOLOGY
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