ContentslistsavailableatScienceDirectJournalofPetroleumScienceandEngineeringjournalhomepage:www.elsevier.com/locate/petrolEvaluationofheterogeneityimpactonhydraulicfracturingperformanceHadiParvizia,SinaRezaei-Gomaria,⁎,FarhadNabhania,AndreaTurnerbaSchoolofScienceandEngineering,TeessideUniversity,MiddlesbroughTS13BA,UKbEONE&P,129WiltonRoad,LondonSW1V1JZ,UKARTICLEINFOKeywords:Hydraulicfracturing:Heterogeneousreservoir:Welltestanalysis:Netpressure:TightsandABSTRACTHydraulicfracturingoperationintightreservoirsincreasestheconnectivityofthewelltomorereservoirlayersandfurtherregions,thusboostingtheproduction.Heterogeneityinfluencesthehydraulicfracturingperformance;thisisobservedwhencomparingtheperformanceofdifferentfraccedwells.Thosethatfaroutperformotherfraccedwellsaregenerallyconnectedtomorepermeablerockornaturalfractures.Modellinghydraulicfracturingnetpressureprovideshydraulicfracturedimensionsandconnectivityperfracturejob.Moreover,welltestinterpretationcanimplytheactivenumberofhydraulicfracturesandanaverageestimationoftheirdimensionsandconnectivityaftercleaningupandflowingthewell.Thereisatechnicalgapintheintegrationofwelltestdatawithfraccingoperationaldatafordiagnosingandevaluatingthehydraulicfractureperformance.Thispaperintroducesanovelapproachtolinkthehydraulicfracturingmodellingwithwelltestinterpretation.ThismethodquantifiesheterogeneityimpactonhydraulicfractureperformancethroughintroducinganewparameterdefinedasHeterogeneityImpactFactor(HIF).ThecalculatedHIFforthefraccedwellsvariesbetween74%(indicatingthatthewellfaroutperformedtheexpectedhydraulicfractureperformance)to−65%(dramaticallyunderperformedwell).Theoutcomeoftheproposedtechniquewasvalidatedbygeologicalobservationsandwassubsequentlyappliedtothedynamicsimulationmodel.Thepressurepredictionofthemodelwascomparedwiththethree-weekannualshut-down;thebuild-upresponseanditsderivativedisplayanexcellentmatchwhichprovidesevidencefortherobustnessofthedynamicmodelandtheeffectivenessoftheproposedtechnique.1.IntroductionSinceitsintroductioninthelate1940s,hydraulicfracturinghasbeenwidelyusedinNorthAmericatoachievehigherrecoveryfromlowpermeabilityreservoirsand/ortobypasstheformationdamagearoundthewellbore(Economidesetal.,2002).Inaddition,successfulapplica-tionsofthistechniquehavebeenreportedinotherlocationsincludingNorthSea(Vosetal.,2009),SouthAmerica(AntociandAnaya,2001),Asia(Shaouletal.,2007)andMiddleEast(Al-ZarouniandGhedan,2012)and(Mirzaei-Paiaman,2013).Generally,manystepsofanalysisareperformedpriortoanyhydraulicfracturingjobtoensureitseffectiveness.But,incomparison,hydraulicfracturinginheteroge-neousreservoirsrequiresmuchmoreanalysisforanoptimumdesignandoperation.Thisismainlyduetothefactthatinheterogeneousreservoirs,rockpropertiesvarydramaticallyandcanaffectthehydraulicfractureperformance.Toovercomethetechnicalandopera-tionalchallengesassociatedwithhydraulicfracturinginsuchreser-voirs,multi-disciplinaryapproachesarerequiredtogainimprovedinsightintothehydraulicfracturingperformance.Thisaimcanbefulfilledbydevelopingmethodstocapturetheimpactsofreservoirheterogeneity,mostdesirablyinaquantitativemanner,inawaythattheresultscanbeeasilytranslatedintoreservoirdynamicmodellingsystems.SouthernNorthSea(SNS)reservoirsareamongthemosthetero-geneousreservoirsinwhichhydraulicfracturinghasbeenconsideredforimplementation.TheSNSreservoirsarecharacterisedbytheirmorepermeablelayersandnaturalfracturesasthetwopossibleelementsofheterogeneity(Parvizietal.,2015a)and(Parvizietal.,2015b).Thesedistinctionsmakethefraccingdesignsmorecomplicatedandsignifytheimportanceoftakinganintegratedapproachtogetthemostoutoftheavailabledata.However,integrationofdifferentdatasourcesisoftennotstraightforwardandrequiresinnovativetechniques.Thispaperintroducesanewtechniquetodiagnosethehydraulicfractureperformancebyintegratingwelltestanalysisandcollectingdataateachhydraulicfracturingstage.Uponidentificationofatechnicalgapindataintegration,thispaperproposesaninnovativetechniqueforquantifyingtheimpactofheterogeneityonhydraulicfractureperfor-mance.Resultsobservedafterapplyingthistechniquetorealfielddatahttp://dx.doi.org/10.1016/j.petrol.2017.05.001Received20February2017;Receivedinrevisedform10April2017;Accepted2May2017⁎Correspondingauthor.E-mailaddress:s.rezaei-gomari@tees.ac.uk(S.Rezaei-Gomari).JournalofPetroleumScienceandEngineering154(2017)344–353Availableonline03May20170920-4105/©2017ElsevierB.V.Allrightsreserved.MARKproviderobustnesstothemethod.Therearedifferentapproachestoevaluatethewellperformanceofahydraulicallyfracturedwell.Eachapproachhasitsownadvantagesandrequiresadifferentlevelofdetailsformodelling.Accordingly,thepredictionreliabilitydependsonthemethodologystrengthincaptur-ingmoreofthecontributingproductionmechanismsandtheunder-lyingphysics.Themostcommonmodellingapproachesforincorporatingtheeffectsofhydraulicfracturingincludenegativewellskinfactor(Schulte,1986),course-gridtransmissibilitymultiplier(El-AhmadyandWattenbarger,2004;Iwereetal.,2004),andlocalgridrefinement(LGR)transmissibilitymodification(Bennettetal.,1986;Hegre,1996).TheLGRmethodoffersmoremodellingflexibilitysince3Dpropertieswithhigherresolutioncanbemodelledtohelpincorporatethereservoirheterogeneity.Ideally,thefracturecell(i.e.thecellwhichhoststheinducedfracture)shouldhavesimilarwidthtotheinducedfracturewhichcanbe,forexample,intherangeof0.03–0.51in.(basedonthedatafrom24hydraulicfracturejobsperformedinaSouthernNorthSeafield;seeAppendixA).Usingsuchsmallcellsizesviolatesoneoftheassumptionsofwellmodellinginfinitedifferencesimulators(Peacemanradiusformula)andaddserrortothewellperformancecalculations.Itisalsoextremelyslowandgeneratesconvergenceproblemsinnumericalreservoirsimulations(Hegre,1996).Onesolutionistoconsiderthickerfracturecellsandupscalethehydraulicfractureconductivitytothefracturecells.Inthepresenceofnaturalfracturesintightformations,thephysicsandmodellingbecomemorecomplicatedandchallenging.Duetodifficultiesofdesigningandperformingexperimentalworkonfracturenetworkpropagationinthelaboratorysettingsandthedifferenceoflaboratoryandreservoirscales,numericalmodellinghasbecomeanessentialtoolinhydraulicfracturestudies,asitfacilitatesincorpora-tionofmanydetailsandconditionsinmodellingandpredictionoffracturenetworkgeometries(Zhangetal.,2015).Someauthorshaveattemptedtosimulatehydraulicfracturinginnaturallyfracturedreservoirsconsideringthecomplexitiesinvolved.FracturemodellingapproachesbasedontheBoundaryElementSystem(BES)wereappliedbysomeresearchers(Sousaetal.,1993;Zhangetal.,2007;SessettyandGhassemi,2012).ZhaoandYoung(2009)developedadynamic3DDistinctElementModel(DEM)basedontri-axialfractur-inglaboratoryexperimentstosimulatefluidinjectionintoareservoirwithnaturalfractures.Benetal.(2012)usedDiscontinuousDeformationAnalysis(DDA)tosimulatehydraulicfracturing.HuangandGhassemi(2012)usedtheVirtualMultidimensionalInternalBonds(VMIB)evolutionfunctionfornumericalsimulationof3Dfracturepropagationatmicroscale.Usingthismethod,theysuccess-fullyrepresentedthefeaturesoftensileandcompressivefracturepropagationandsuggestedthat3Dsimulationoffracturepropagationhelpsunderstandinganddesigningmultiplehydraulicfractures.Zhangetal.(2016)usedthelatticecellversionofthediscretizedvirtualinternalbondmethodtomodelthereservoirrockfornumericalsimulationofthefracturedevelopmentbehaviourincomplexuncon-ventionalreservoirs.HamidiandMortazavi(2014)simulatedthehydraulicfractureinitiationandpropagationthroughintactrockusing3DDistinctElementCode(3DEC)andintroducingafictitiousjointtechniquetofacilitateimportingthefractureinitiationcapabilityintheDEMapproach.Zhangetal.(2015)havegivenafullaccountofhydraulicfracturingsimulationapproachesandconcludedthatDisplacementDiscontinuityModels(DDM)canbestsimulatethecomplexfracturenetworks.Consideringthefactthatmostofthenumericalfracturemodellingapproachesaremainlysuitableforhardrocksduetoassumingplanarfracturegeometryandlinearplasticfracturemechanics,Wang(2015)usedExtendedFiniteElementMethod(XFEM)togetherwithCohesiveZoneMethod(CZM)andMohr-Coulombtheoryofplasticitytoinvestigatetheinitiationanddevelopmentofnon-planarfracturesinbrittleandductilerocks.Toaddressthesameissuesandinvestigatenon-planarhydraulicfracturesby3Dsimulation,Sobhaniaraghetal.(2016)alsocombinedtheCohesivesegmentswithPhantomNodeMethodandcalleditCPNM.Nadimietal.(2016)presentedanewmeshfree3DsimulationmodelbasedonPeridynamic(PD)methodforinvestigationofhydraulicfracturedevelopmentandgeometryincomplexandheterogenousformations;themethodalsoconsiderstheinteractionoftheinducedfractureswiththenaturalfractures.Despitetheirbasicnature,couplingofthesemodelswithacommercialsimulatorforinvestigatingtheinteractionofinducedfractureswithnaturalfracturesisdifficultandcurrentlynotfullypractical.Therefore,onperformanceevaluationofhydraulicfractures,amethodologyisrequiredthatcan:1.Serveasdiagnostictooltoidentifytheheterogeneityintermsofnaturalfracturesand/orhighpermeabilitystreaks;2.Supportthetunedinitialguessforconnectivitycalculationofupscaledfracturecellstoreducetheassociateduncertainty;3.Linkthefindingstogeologicalfeatures.Thesefeatureshavenotbeenquantitativelyintegratedinthemethodologiesproposedbytheinvestigatorssofar.Inthiswork,theauthorssuggestthatsuchatechnicalgapcanbefilledthroughintegrationofwelltestresultswithfracturingoperationaldataanalysisfordiagnosingandevaluatinghydraulicfracturingperformance.Tolinkthehydraulicfracturingmodellingwithwelltestinterpretation,inthispaper,anewmethodologyisproposedtoquantifythehetero-geneityimpactonhydraulicfractureperformanceintermsofanewparameterdefinedasHeterogeneityImpactFactor(HIF).Thispara-meterrepresentsaquantifiedvaluefortheexpectedperformanceofhydraulicfracturingoneachwellconsideringthecontributionofheterogeneity.HIFcreatesabasisforcomparingthewellsofthesamefieldwitheachotherandalsocanexhibitthedegreeofheterogeneityindifferentfields.NomenclatureCfDDimensionlessFractureconductivityDDADiscontinuousDeformationAnalysisDEMDistinctElementModel(DEM)FCFiniteConductivityFCFractureconductivityHIFHeterogeneityImpactFactorkPermeabilityKf.wConnectivityofhydraulicfractureLGRLocalgridrefinementLWDlogging-while-drillingMDMeasureddepthMMSFDMillionstandardcubicfeetNPMNetpressurematchPKNPerkins-Kern-NordgrentheoryPLTProductionloggingtoolPnCriticalnetpressurePTAPressuretransientanalysisSSkinSCfSurfaceConductivityforawellwithonehydraulicfractureSCSurfaceConductivityforawellwithmultiplehydraulicfractureswfFracturewidthWTAWelltestanalysisxfFracturehalf-lengthH.Parvizietal.JournalofPetroleumScienceandEngineering154(2017)344–353345QuantificationofheterogeneityimpactasavalueisimportantasitcanbeusedforpredictionofwellproductionbyintegratingthetoolsofproductionsimulationwithHIF.ThewayforwardistoworkonanewmethodologyofintegratingHIFwithDeclineCurveAnalysis.TheresultsoftheapplicationoftheproposedtechniqueinoneoftheSNSreservoirswereinverygoodagreementwithgeologicalanddrillingobservations.TheHIFanalysiswasthenincorporatedintothedynamicsimulationmodelandpressurepredictionsofthemodelwerecomparedwiththethree-weekannualshut-down.Thebuild-upresponseanditsderivativedisplayedanexcellentmatchwhichprovidesevidenceofsuccessfulapplicationoftheproposedtechnique.Inthefollowingsections,first,theworkflowintroducedbytheauthorsinapreviousstudy(Parvizietal.,2015b)toanalysethehydraulicfractureperformanceisdiscussed.Inthatstudy,aninte-gratedmulti-disciplinaryapproachwasproposedfordeployingthedataandinformationavailableallthewayfromseismicinterpretationtoreservoirdynamicmodellingtoevaluatetheperformanceofthehydraulicfracturing.Itshouldbenotedthatthecurrentworkis,infact,acontinuationofthepreviousstudyandthenewlyproposedHIFanalysisisbuiltuponthefoundationofthehydraulicfractureperformanceanalysisworkflow.Forthisreason,anin-depthdiscussionontheapplicationofthisworkflowforthereservoirunderthestudyhasbeenpresentedaswell.Then,anewratio(WTA/NPM)forevaluatingtheperformanceoffraccedwellsaswellasanewparameter(HIF)toshowtheimpactofreservoirheterogeneityhavebeendefinedandappliedtotherealfielddata.Itisworthmentioningthatthemethodologyinthisstudyis,indeed,focusedoncombiningtheresultsofwelltestanalysis(wheretheproduction-pressureismatched)withtheresultsofnet-pressurematch(wherepressuredepletionischar-acterizedandmatchedusingspecificparameters).Oncethesetwomatchesareobtained,sincewelltestconsidersalargerradiusofinvestigationandnet-pressureconsidersasmallerradiusofinvestiga-tion,therelationbetweenthemcanbeusedtoconcludeazero-dimensionpropertyofthereservoirheterogeneitywhichwehavequantifiedanddefinedasHIF.Finally,theresultsoftheworkarecomparedwithgeologicalevidencesandvalidatedbymatchingthepressurepredictionsoftheresultingdynamicreservoirmodelwiththerealwelltestdata.2.Methodology:hydraulicfractureperformanceanalysisworkflowInthissection,thedetailsofapplicationofthecomprehensiveworkflowofhydraulicfractureperformanceanalysisintroducedbytheauthorsinapreviouspublication(Parvizietal.,2015b)ispresented.Inthiswork,actualfielddatawasacquiredfromanoilandgasoperatorwiththeviewtoevaluatethehydraulicfractureperformance.ThisfieldissituatedintheSouthernNorthSeagasbasin.Itis10kmlong,1.5kmwidewithanestimatedreservoirthicknessof270ft.ThereservoirrockisofRotliegendage,mainlysandstonewithlayersofsiltstoneandminorshaledeposits,accordingtologandcoredata.Thisproducinghorizonisoverlainbya400-feetshaleformationwhichconstrainsthepropagationofthefractures.Thereservoirformationisalsounderlainbyaverytightsandstonewithunsuccessfulattemptsofproductionwhichrenderitunexpectedtohavenoticeablecontributiontoproductionoffluids.Basedoncoredata,thereservoirporosityrangesfrom5%to20%andtheaveragereservoirpermeabilityislessthan1mD.Slightlyhigherpermeabilitiesareobservedwherethereservoirformationisencounteredinwellsatlowerdepthswithlessillitisation.Theinitialwelltest,doneontheexplorationwellinthe1980's,hadindicatedagasflowrateof4MMSCFD;thelowratewasattributedtothesignificantlyillitisedformation.Anappraisalwellwasdrilled16yearslaterandflowed10MMSCFD.Aphaseddevelopmentplanwaspreparedwiththreeofthefiveinitiallyplannedhorizontalwells(A,B,andC)beingdrilledandfraccedeachwithfivestages.Thetworemainingwellsweredrilledafterthreeyearsofproduction.Thesewellswerealsohorizontaleachwithfivestagefraczones(DandE),similartophase-1wells.Performanceevaluationofthesemulti-fraccedhorizontalwellsiscrucialforforecastingandevaluatingfurtherdevelopmentopportu-nities.Tosimulatethehydraulicfractureinthisstudy,pseudo3Dhydraulicfracturemodellingwasperformedusingacommercialsimulatorforfracturedesignandanalysisincomplexsituations.However,itshouldbenotedthat,inthispaper,thefocusisonthecombineduseofhydraulicfracturemodellingresultsandpressuredeclineanalysisresultsregardlessofthespecificmethodologies/soft-wareusedforobtainingsuchresults.Inotherwords,inanyothersimilarstudy,oncethefracturemodellingisperformedusinganyapproachchosenbytheengineer/researcher,theresultscanbeintegratedwiththeresultsofwelltestanalysis,whichcould,inturn,beaccomplishedusinganymethodselected.Suchintegrationoftheresultsisthengovernedbytheworkflowpresentedhere.Acompletepictureofthehydraulicfracturingmodellingworkflowrequiresanintegratedmulti-disciplinaryapproachtobeapplied.Thissystemat
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