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RESEARCHPAPERPETROLEUMEXPLORATIONANDDEVELOPMENTVolume44,Issue4,August2017OnlineEnglisheditionoftheChineselanguagejournalCitethisarticleas:PETROL.EXPLOR.DEVELOP.,2017,44(4):625–635.Receiveddate:30Aug.2016;Reviseddate:02Jun.2017.*Correspondingauthor.E-mail:tongkaijun714@126.comFoundationitem:SupportedbytheChinaNationalOffshoreOilCorporationScienceandTechnologyProject(CNOOC-KJ125ZDXM06LTD-02).Copyright©2017,ResearchInstituteofPetroleumExplorationandDevelopment,PetroChina.PublishedbyElsevierBV.Allrightsreserved.Sparsewellpatternandhigh-efficientdevelopmentofmetamorphicburiedhillsreservoirsinBohaiSeaarea,ChinaTONGKaijun1,*,LIBo1,DAIWeihua1,ZHENGHao2,ZHANGZhannü2,CHENGQi2,WANGJianli2,FANGNa21.CNOOCInternationalLtd.,Beijing100027,China;2.TianjinBranchofCNOOCLtd.,Tianjin300459,ChinaAbstract:InordertoachievethedevelopmentobjectiveoffewerwellswithhigherproductionofmetamorphicburiedhillreservoirsintheBohaiSeaarea,theJZ251SoilfieldatBohaiBayBasinwastakenasanexampletocarryoutelaborationofreservoirfracture,quantitativecharacterizationofwaterdisplacingoilmechanismatdual-porosityreservoir,optimizationofnewwellpatternmode,formulationofrationaldevelopmenttechnologypolicy,maintainingproductivityandcontrollingwaterrisingbasedondevelopmentexperienceofsimilaroilreservoir,thusformingthekeyhighefficiencydevelopmenttechniqueofsparsewellpatternofoffshoremetamorphicrockreservoir.BasedonthecharacteristicsoftheJZ251Sburiedhillreservoir,forwardsimulationofwaveequationforfractureanisotropywascarriedout,verifyingtheeffectivenessofnarrowazimuthseismicdatatofracturesdetectioninworkarea.Multi-parametersprestackinversionandgeostressfieldsimulationwereappliedtoforecastlocationoffracturesanddirectionoffracturesrespectively.Basedonthelarge-scale3Dphysicalmodelandnumericalsimulation,anewtop-bottominterlaced3Dinjec-tion-productionwelldeploymentmodelconcerningthehorizontalwellwaspresented.Consideringtheproductiondemand,therea-sonableoilproductionrateofJZ251Soilreservoirshallbecontrolledat3%-4%,depletiondevelopmentuntilthattheformationpressurelevelmaintainsat70%ofinitialformationpressurecanbeimplementedfortheoilfieldattheinitialstage,andthenthede-velopmentmodeofwaterfloodingtokeeppressurecanbecarriedout.Consideringthepilotproductiondataattheworkzone,dif-ferentwaterbreakthroughmodelsofthehorizontalwellweresimulatedtoformfourdiagnosischarts,whichhelptostabilizeoilandcontrolwatereffectively.Thefieldpracticeshowsthatthesetechniquesgreatlyincreasethecrudeoiloutputandimprovethewa-ter-injectiondevelopmenteffect.Keywords:BohaiSeaarea;metamorphicburiedhill;buriedreservoir;fractureforecast;welldeploymentmodel;reservoirdevelopmenttechniques;maintainingproductivityandcontrollingwaterrisingIntroductionAmetamorphicburiedhilloilfieldgroup(includingJZ251S,PL91,JZ324,BZ262andotheroilfields)withthelargestre-servescale(about2.8×108t)sofarinChinawasdiscoveredinBohaiBayduring2002to2012,anditisofgreatsignificanceformaintainingtheproductionof3000×104t[13]inBohaifieldstorealizehighefficiencydevelopmentofthiskindofspeciallithologicalreservoir.Atpresent,thedevelopmentofmetamorphicburiedhillreservoirinChinaconfrontsmanychallenges,suchasdifficultfinedepictionoffracturedreser-voirbody,complicatedmatchingrelationshipcharacterizationbetweenreservoirspaceandseepage,difficultdevelopmentwellpatternmodeselection;ifconventionaldevelopmentmodeisadopted,problemssuchaslowdrilling-encounterratioofreservoir,lowsinglewellyield,earlywaterbreak-throughandrapidwatercutincreasecouldoccur,makingitdifficulttorealizeeconomicdevelopmentofoffshoreoilfield.Inthisstudy,takingJZ251SoilfieldinBohaiBayBasinasanexample,finedescriptionofreservoirfracture,characteriza-tionofwaterdisplacingoilmechanismindual-porosityres-ervoir,optimizedselectionofnewwellpatternmode,formu-lationofrationaldevelopmenttechnologypolicy,maintainingproductivityandcontrollingwatercuttechnologyareinvesti-gatedbasedondevelopmentexperienceofsimilaroilreser-voirs[18],formingkeyhighefficiencydevelopmenttechniquesofsparsewellpatternforoffshoremetamorphicrockreser-voir.TONGKaijunetal./PetroleumExplorationandDevelopment,2017,44(4):625–6356261.Overviewofoilfield1.1.GeologicalcharacteristicsofJZ25SfieldJZ251SoilfieldatBohaiBayBasin,withtypicalweather-ingblockyburiedhillreservoir[2],isafracturedreservoirwithbothporeandfracture.Atburieddepthsof17001900m,withverticalspanof200matmost,thereservoiriscomposedoftwo-feldspargneissandplagiogneiss,withporeandfrac-tureasmajorreservoirspace.Theregionalstructuralfracturesaremostlymiddle/high-angleones(2060),trendingNE-SWmostlyandthenNWW-SEE,withfracturedensitiesof0.53.5lines/m.Theporositiesofmatrixandfracturearerespectively5.72%and1.08%;thematrixpermeabilityislessthan1×103m2,andthefracturepermeabilityisattherangeof(260900)×103μm2.Inaddition,thisreservoirisalsocharacterizedbyverticalzoning(uppersectionofsemi-wea-theringcrust,lowersectionofsemi-weatheringcrustandin-nerbasementsection),complicatedreservoirspaceandseep-agecharacteristics,strongheterogeneity,andweakbottomwaterenergy,etc.[6]Crudeoilofthisreservoirislightoilwithlowdensity,lowviscosity,andmedium/lowparaffincontent,whichisconducivetodevelopment.1.2.Difficultiesinhighefficiencydevelopment1.2.1.DetectionofeffectivefracturesWide-azimuthseismicacquisitionisadoptedforseismicsurveyofonshoreoilfieldtogainall-aroundseismicinforma-tion,andtheprestackazimuthanisotropycharacteristicofverticalwaveisusedtodetectfracture,whichishigherinreliability.Bycontrast,theacquisitionazimuthangleofseis-micdataofoffshoreoilfieldisgenerallynarrower;forthestudyarea,theseismicdataacquisitionisatthedirectionof120°,withtheincludedangleofacquisitionof20°,andexten-siononbothsidesof20°,namelythattheazimuthanglerangesfrom100°to140°;whetherseismicdataacquiredatnarrowazimuthcaneffectivelydetectfracture,thefractureatwhatazimuthcanbedetected,thesequestionsposegreatchallengestofracturepredictionofseismicdataacquiredatnarrowazimuth.1.2.2.FracturedreservoirdistributionAsthedrillingcostofoffshorewellisrelativelyhigh,thedrillingwellspacingattheoilfieldevaluationstageislarge;thespacingof7exploratorywellsinJZ251Soilfieldaveragesatabout3.5km.Sparsewellpatternwithwidespacingcan’tcontrolthedetaileddevelopingcharacteristicsofreservoirontheplane,andthedrillingresultsof7wellsinthestudyareare-confirmthatthefracturedreservoirdifferswidelyindifferentwellsandhasstrongheterogeneity.Undersparsewellpattern,lesscoreandloggingdatawouldbeavailable,makingitmoredifficulttofindoutthedistributionpatternoffracturedreservoir.1.2.3.CharacterizationofwaterdisplacingoilmechanisminfracturedreservoirsBasementburiedhillreservoirsinbasinsofEasternChinaoftenhaveundevelopednaturalaquifer,sotheyaremostlydevelopedbywaterfloodingtokeeppressure.Butthemeanstounderstandingwaterdrivingcharacteristicsinfracturedreservoirarelimited.Forfracturedreservoirinwhichnaturalfracturesdistributeinstrongrandomnessandcomplicatedpattern,limitedbythesizeofreservoirbodyandcoringcon-ditions,thereislackofresearchmeansthatcanquantitativelyreflectthewaterdrivingcharacteristicsandremainingoildis-tributionofdual-porositymedium.1.2.4.OffshoreoilfielddevelopmentwithfewerhigherproductionwellsTheoil-bearingintervalinthetargetareahaslargethick-ness,fractureofverticalzoningdistributionandmedium/highangledevelopingcharacteristics.Numericalreservoirsimula-tionshowsthatdevelopmentwithtraditionalverticalwellinjection-productionpatternhassuchproblemsaslowdrill-ing-encounterratiooffracture,lowsinglewellyield,earlywaterbreakthroughandrapidproductiondeclineafterwaterbreakthrough,etc.Inordertorealizeeconomicandefficientdevelopment,itisanecessitytomakeanoverallplanofoff-shoreplatformspace,lifeandwellslots,andcarryoutopti-mizationstudyonwelltype,wellpattern,andinjec-tion-productionmode,etc.tooptimizetheconfigurationofundergroundresourcesandengineeringfacilities.2.Keyhighefficiencydevelopmenttechniquesofsparsewellpatternforoffshoremetamorphicrockreservoirs2.1.ForecastingdevelopmentpositionandpredominantdirectionoffracturedreservoirsRegardingthedifficultyofpredictingfracturedreservoirinmetamorphicburiedhill,firstforwardmodelingsonthechar-acteristicsofoffshorenarrowazimuthseismicdataandhigh-anglefractureswerecarriedouttofindoutthevariationfeatureofmultipleseismicattributes(amplitude,energy,fre-quencyattribute,attenuationgradient)causedbyfracturewithoffsetandazimuthangle;thentheseismicattributemostsen-sitivetofracture(frequencyattribute)wasselectedtoforecastfracturedevelopmentposition.PrestackS-waveimpedanceinversionwasappliedtopredictthedevelopmentpositionofreservoir,andthefracturewasdetectedbasedonthefre-quencydifferencewithinthescopeofreservoir.Onthisbasis,thestressfieldsimulationbasedonrockphysicalpropertieswasconductedtofigureoutthedevelopmentdirectionsoffractures,realizingfinedepictionoffracturedreservoirandquantitativepredictionofhigh-qualityreservoirspatialdistribution.2.1.1.SeismicforwardmodelingbasedonfractureanisotropywaveequationMetamorphicburiedhillformationsarecharacterizedbydeepburial,uneventhickness,fracturedevelopment,strongheterogeneityanddifferentfillingproperties.FailingtoformTONGKaijunetal./PetroleumExplorationandDevelopment,2017,44(4):625–635627aneffectiveseismicreflectionleadstothenon-uniquenesswhenseekingfavorablefracturedreservoirswithpost-stackseismicreflectionfeatures.Startingfromseismicacquisitionparametersandfracturedevelopmentfeatures,thelogginginformation,rockphysicaltestdataandtheoreticalequationwereusedtoestablishgeologicmodelandrockphysicalmodeloffracturedreservoirofkeywells,thenseismicre-sponsesatdifferentoffsetsandazimuthswerecalculated,andthechangefeatureofseismicattributes(amplitude,energy,frequencyattribute,andattenuationgradient)causedbyfrac-turewithazimuthandoffsetwasanalyzedthroughforwardmodelingbasedontheanisotropywaveequation.Throughcomparison,thevariationcharacteristicoffrequencycausedbyfracturewithazimuthanglewasfiguredout.Theresearchshowsthat:(1)thereisnoseismicanisotropyresponseatzeroincidenceangle,anisotropyresponsewillappearonlywhenthereiscertainincidenceangle,andtheseismicanisotropystrengthoffracturewillrapidlyincreasewiththeincreaseofincidenceangle;(2)fracturehasstronginfluenceonseismicreflection,fractureporosityoflargerthan1%cancauseobviousanisotropyonseismicreflection,therefore,3DprestackP-wavedatacanbeusedtodetectfracture.Bysettingfixedacquisitionazimuth(100140)andchangingthefracturetrend,therelationshipbetweenfre-quencyattributeandfracturetrendunderdifferentfractureporositieswassimulatedandanalyzed.ItcanbeseenfromFig.1thatthefracturetrendandfrequencyabnormalitypre-sentarelationshipof“M”type.Fracturedetectionbasedonseismicattributedifferenceabnormalityatdifferentazimuthscansatisfyfracturedetectioninamajorityscopeofazimuth,andisonlyweakintheazimuthof100110and1020,verifyingtheeffectivenessoffracturedetectionwithseismicdataacquiredatnarrowazimuthinthisarea.2.1.2.FracturedreservoirpredictionbasedonprestackelasticparameterinversionItcanbeseenfromthecrossanalysis(Fig.2)ofP-waveandS-waveimpedance,shearmodulusandtotalporosityofburiedhillformationinJZ251Soilfieldthatthefracturedres-ervoirintheresearchregionischaracterizedbylowP-waveimpedance,lowS-waveimpedance,lowshearmodulusandhighporosity,andtheporositylimitofreservoirandnon-re-servoirisatabout3.5%.Aftercomprehensiveanalysis,S-waveimpedancelessthan7.5×109g/(m2/s)wasselectedasthestandardtodividereservoir,thencorrelationtransforma-tionbetweenprestackS-waveimpedanceandtotalporositywasappliedtogainthepredicteddatavolume[910]oftotalfractureporosity,andquantitativedepictionofreservoirwasrealizedaftercalibratedwithmeasuredparametersofdrilledwells.Thevariationoffrequencyabnormalityofnear-faroffsetwithfracturedensitywassimulatedundertheconstraintofreservoirframework,torealizequalitative-semi-quantitativedetectionoffracturedevelopmentdegreeintheresearchre-gion:(1)whenthevalueoffrequencyabnormalityislargerthan0.6,thereservoirisconcludedasreservoirrichinfrac-ture;(2)whenthevalueoffrequencyabnormalityisbetween0.2and0.6,thereservoirisconsideredasreservoirwithsomeorfewfractures;(3)whenthevalueoffrequencyabnormalityislessthan0.2,asthelayerisnon-reservoirortightlayer.2.1.3.Reservoirfracturedirectiondetectionbasedonmulti-informationfusionBasedonthefractureinformationgainedfromwelldrillingandfracturedreservoirspatialdistributionstudy[6],fracturedevelopmentintheresearchregioniscontrolledbyfaultandancientlandformjointly,andfracturesaredistributedalongfaultsandstructuralhighs,whichprovidegeologicalbasisforfracturedetectionwithstressfieldsimulation.Basedongeo-logic,drillingandloggingdate,andbyusingprestackseismicelasticparameterinversiontechnique,themechanicalmodelwasestablishedbycombiningstructuralmap,velocityofver-Fig.1.Relationshipbetweenfracturetrendandfrequencyab-normality.Fig.2.CrossplotofelasticparametersandtotalporosityofburiedhillreservoirinJZ251SOilfield.TONGKaijunetal./PetroleumExplorationandDevelopment,2017,44(4):625–635628ticalwave,speedratioofP-waveandS-waveandotherpa-rameterstocarryoutnumericalsimulationofstressfield.Theresultshowsthattheprincipaldirectionoffractureinthere-searchregionisNE-SWparalleltolargefault,andthesecon-darydirectionisNWW-SEE,obliquecrossingwithlargefault.Wellimagingloggingandproductiontestresultofreservoirhaveprovedthefracturedirectionpredictedisreliable,pro-vidingbasisforfollow-upwelldeploymentandwelltrackoptimization.2.1.4.DistributionoffracturedreservoirinmetamorphicburiedhillThegatherdataofprestacktimemigrationwasappliedtogainthefull3Dpredicteddataoffracturedreservoirofburiedhillthroughprestackelasticparameterinversion.Theresearchshowsthatthefracturedreservoirofburiedhillhastheparti-tionabilitycharacterontheplaneandzoningcharacterinver-ticaldirection[2].Basedonthecurrentknowledge,thefrac-turedreservoirofburiedhillatinJZ251Siscontrolledbylithology,faultandancientlandformjointly,anddevelopsalongthehighpointsofburiedhill.Ontheplane,fracturedreservoirisbetterdevelopedinthemiddleJZ251S-2—JZ251S-7wellblockandnortheastJZ251S-1—JZ251S-8wellblock(Fig.3a);vertically,theburiedhillreservoircanbedi-videdintouppersectionofsemi-weatheringcrust,lowersec-tionofsemi-weatheringcrustandinnerbasementsection(Fig.3b);ofwhich,theuppersectionofsemi-weatheringcrustissufferingstrongerlybychemicallyweathering,withdevel-opeddissolvedpores,partialfracturesanddissolvedporesarefilledwithclaymineralsandcalcitecement;lowersectionofsemi-weatheringcrustisthickandrichinfracture,withasso-ciateddissolutionporesalongfractures,whichisthemostfavorablereservoirbeltintheburriedhill.ItcanbeseenfromFig.3bthatthefavorablereservoirinthelowersectionofsemi-weatheringcrustoccursattheburiedhilltopinquasi-layerform.Theinnerburiedhillhasundevelopedfrac-tures,whichbreaksthetraditionalunderstandingthattheinnerArchaeanburiedhillis“homogeneousblock”,thereout,the“quasi-layer”reservoirmodelofmetamorphicburiedhillisraised,whichlaysafoundationforsubsequentstereoscopicinjection-productionwellpatternoptimizationandinnerwaterflooding.2.2.Injection-productionwellpatternmodeforstereoscopicdevelopmentofthick-beddedburiedhillreservoirMetamorphicburiedhillsinBohaiBayBasinaregenerallycharacte
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