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RESEARCHPAPERPETROLEUMEXPLORATIONANDDEVELOPMENTVolume40,Issue1,February2013OnlineEnglisheditionoftheChineselanguagejournalCitethisarticleas:PETROL.EXPLOR.DEVELOP.,2013,40(1):84–95.Receiveddate:23Apr.2012;Reviseddate:23Aug.2012.*Correspondingauthor.E-mail:311@petrochina.com.cnFoundationitem:SupportedbytheNaturalScienceandTechnologyMajorProject(2011ZX05015).Copyright©2013,ResearchInstituteofPetroleumExplorationandDevelopment,PetroChina.PublishedbyElsevierBV.Allrightsreserved.Welltypeandpatternoptimizationtechnologyforlargescaletightsandgas,Suligegasfield,NWChinaHEDongbo*,JIAAilin,JIGuang,WEIYunsheng,TANGHaifaPetroChinaResearchInstituteofPetroleumExploration&Development,Beijing100083,ChinaAbstract:SuligegasfieldisatypicaltightsandgasfieldinChina.Welltypeandpatternoptimizationisthekeytechnologytoimprovesinglewellestimatedreservesandrecoveryfactorandtoachieveeffectivefielddevelopment.Inviewofthelargearea,lowabundanceandhighheterogeneityofSuligegasfield,aseriesoftechniqueshavebeendevelopedincludinghierarchicaldescriptionforthereservoirarchitectureoflargecompositesandbodiesandwellspacingoptimization,wellpatternoptimization,designandoptimizationforhori-zontaltrajectoryanddeliverabilityevaluationfordifferenttypesofgaswells.Thesetechnologiesprovidemostimportanttechnicalsup-portsfortheincreasesofclassIandIIwellsproportionto75%–80%withrecoveryfactorenhancedbymorethan35%andfortheindus-trialapplicationofhorizontaldrilling.Tofurtherimproveindividualwellproductionandrecoveryfactor,attemptsandpilottestsinvari-ouswelltypesincludingsidetrackingofdeficientwells,multilateralhorizontalwells,anddirectionalwells,andhorizontalwellpatternandcombinedwellpatternofvariouswelltypesshouldbecarriedoutthroughoutthedevelopment.Keywords:Suligegasfield;tightsandgasfield;welltype;wellpattern;hierarchicaldescription;horizontaldrilling;singlewellcon-trolledreserves;recoveryfactorIntroductionAsthelargestgasfieldinChina,Suligegasfieldisatypi-caltightsandgasfieldtoo.Becauseofitslowpermeability,singlewellproductivityinSuligegasfieldisverylow.Singlewellcontrolledreservesarealsolowduetostrongreservoirheterogeneityandsmalldrainageareanearthewellbore.Thus,todevelopthisgasfieldefficientlyandeconomically,fractur-ingandunconventionaltechniquesmustbeemployedtoin-creasesinglewellproductivityandcontrolledreserves.Atthesametime,wellpatternshouldbeoptimizedtoimprovetherecoveryofthegasinitiallyinplace(GIIP).Throughafewdecadesofdevelopment,twosetsoftechni-calseriesfortightgashavealreadytakenshapeintheUS.Oneseriesinvolvesverticalwellsindensewellpatternsandmultilayerfracturing,whichhasbeenprovensuitableforthicklayersandmultilayerreservoirs;maximumwelldensityisupto10wells/km2[1]andover20stagescanbefracturedinasinglewell.Thesecondtechnicalseriesistheapplicationofhorizontalwellsintightgasdevelopmentastechnicalbreak-throughhasbeenmadeonmulti-stagefracturing.Pilotdevel-opmenthasbeenconductedinSuligegasfieldinthepastdecades.Accordingtoitsgeologicalcharacteristicsoflargearea,lowabundanceandstrongheterogeneity,inlinewithseparatelayerfracturinginverticalwellsandmultistagedfracturinginhorizontalwells,welltypeshavebeendiversifiedfromverticalwellstoclusterwellsandhorizontalwells;wellpatternhasalsobeenbranchedoutfromdenseverticalwellpatterntoarealwellpatternwithclusterwellsandhorizontalwellpatterninlocalfavorableblocks,andspecialtechnicalserieshavealsobeenestablishedforSuligegasfield,improv-ingthedevelopmenttechnologyfortightgastoahigherlevel.1GeologicalcharacteristicsofSuligegasfieldanditsinfluenceonfielddevelopmentplanning1.1BasicgeologicalcharacteristicsofSuligegasfieldLocatedmainlyatWushenqicounty,Ordoscity,Suligegasfieldhasanexplorationareaofabout40000km2andthepaysarePermianHe8MemberandShan1Memberwithaburieddepthof3000to3600m.ThestructuralunitofSuligegasfieldbelongstoYishaanSlope,whichhasthefollowinggeo-logicalcharacteristics.(1)Firstly,itisatypicaltightsandgasfield.Accordingtotheinternationalgeneralstandards[2−4],tightsandgasreser-voirsmeettwobasicconditions.Oneisthatthesandstonematrixin-situpermeabilityislowerthan0.1×10−3μm2(ex-cludingfracturepermeability).Thesecondisthatthetightsandgaswellscommonlyhavenonaturaldeliverabilityoreconomicproductionwithoutstimulationtreatmentsordevi-HEDongboetal./PetroleumExplorationandDevelopment,2013,40(1):84–95−85−ateddrilling.Theuniversalstandardsfortightsandgasrevealthattheclassificationcriteriabasicallyfocusonthedevelop-mentconditionsandtheeconomicconditions.Netpayporos-ityinSuligegasfieldismainly3%−12%andtheairperme-abilityis0.01×10−3–1.00×10−3μm2.Accordingtotheresultsoflaboratoryexperiments,50%ofrocksampleshaveairper-meabilitylowerthan0.1×10−3μm2and85%havematrixin-situpermeabilitylowerthan0.1×10−3μm2[5].Fortheserocksampleswithdifferentporestructures,thein-situper-meabilityof0.1×10−3μm2correspondstoairpermeabilityof0.5×10−3−1×10−3μm2.Thus,therockpermeabilityofSuligegasfieldfulfillsthefirstcriterionoftightgas.BothverticalwellsandhorizontalwellsinSuligegasfieldhavenodeliv-erabilityoreconomicproductionwithoutfracturing.Therefore,thereservoirsinSuligegasfieldshouldbeclassifiedintotightsandgas.(2)Largeinarea,lowinabundanceanduneveninplanarenrichment.Duringitsdepositionalstage,anumberoflargebraidedchannelsedimentarysystemsweredevelopedinSu-ligearea,creatingwidespreadbraidedchannelsandsoftensofthousandsofsquarekilometers.Buriedasdeepasover4000m,thechannelsandsbecametightthroughstrongdiagenesisprocessandwereupliftedtothegentleslopes.Ac-cordingtoavailabledrillingdata,thesand,notwater-bearingandnoobviousreservoirboundary,containsgasoveranareaoftensofthousandsofsquarekilometers,exhibitingfeaturesof“continuousoilandgasaccumulations”[6].ThesandsinthepayformationinSuligegasfieldisabout30−50mthickandthegasbearingsandsareabout10mthick.Theabundanceiscommonly50MCM/km2to200MCM/km2.Enrichmentareaswithabundanceover100MCM/km2aremainlydistributedinstackedbraidedchannelareas.(3)Strongheterogeneityanddual-texturearealsothechar-acteristicsofreservoirsinSuligegasfield.Accordingtopetrophysicalproperties,thereservoirscanbeclassifiedintomajorgasbearingsandsandmatrixreservoirs(Fig.1).Themajorgasbearingsandsareofcoarsesandstonelithofacies,withtheporosityof5%−12%,airpermeabilityof0.1×10−3μm2–1.0×10−3μm2,andgassaturationof55%−65%.Gasac-cumulationincoarsesandsisthemaincontributortooriginalgasinplace(OGIP)andwellproduction.Whereas,matrixreservoirsmadeofmiddle-finegrainedsandstonesare3%−5%inporosity,lessthan0.1×10−3μm2inairpermeabilityand30%−40%ingassaturation.Usuallygasaccumulatedinmatrixreservoirsarenottakenintoaccountintheestimatedprovedreserves.Channelbars,themajorgasbearingsands[7],areinsolatedincontinuousmatrixreservoirs,mainlybraidedchannelfillingsandstones,duetothecontrolofsedimentaryfacies.Accordingtostatisticsondrillingdata,channelbarsandsaresmallinscale,2−5mthickandseveralhundredmeterswide.Usually2-3channelbarsandscanbeencoun-teredbyaverticalwell.Thechannelbarsand(s)isabout1/3oftotalbraidedriversandsinnumberandthickness.Althoughgasbearingsandsareofscattereddistribution,theywillcoveralmostthewholeareaofthegasfieldiftheyareprojectedontooneplane,indicatingthewholeareaofSuligegasfieldsaregasbearing.(4)Lowsinglewellcontrolledreservesandlowproductionrate.Limitedbytightreservoirandstrongheterogeneity,thesinglewellcontrolledreservesandproductionratearebothlow.Accordingtoresultsfromproductiondeclineanalysis,cumulativeproductionmethodandtransientdeliverabilityanalysis,theestimatedreservescontrolledbyasingleverticalgaswellinSuligegasfieldare10MCMto35MCM.Theabsoluteopenflowrateismainly30000−300000m3/d.Un-dertherequirementofmaintainingstablepeakproductionforthreeyears,theaveragerationalproductionrateforgaswellsFig.1Cross-sectionprofileshowingthereservoirdual-texturecharacteristicsinthepilotinfilldevelopmentblockinSuligegasfieldHEDongboetal./PetroleumExplorationandDevelopment,2013,40(1):84–95−86−inSuligegasfieldwouldbe10000m3/d.Atthemiddletolatestagesofgaswelllifecycle,aproductionrateofafewthou-sandcubicmetersinvolumecouldbemaintained.1.2InfluencesofreservoircharacteristicsonwellplacementReservoirheterogeneityisaprimaryfactortobeconsideredwhenplacingwells.EnrichmentareasscreeningandwelltypeselectiononthebasisofsandstackingpatternarethetwomainwaystoimprovesinglewellEURandproductionrate.Wellpatternoptimizationaccordingtosandscale,geometryanddistributionfrequencyisthemosteffectivemeanstoim-proveproducingdegreeandrecoveryofreserves.AlthoughgasbearingsandsarewidelydistributedinSuligegasfield,thereserveabundanceislowandthedistributionisuneven.Foreffectivedevelopment,improvementofsinglewellcontrolledreservesistheprimarytargetwhenplacingwells.Wellplacementandwelltrajectorydesignareopti-mizedtoenhancepenetrationratioofgasbearingsands,whichtogetherwithstimulationwillmakewellreachthehighestcontrolledreservesandhighestprofit.Pilotproductionrevealsthatonlyverticalwellswithgasbearingsandsover6mthickproduceatarateabovetheeconomiclimit.Thusgasenrichmentareascreeningshouldbeconductedfirstly.Welltypeselectiontheniscarriedoutfortheseareas.Verticalwellsorclusteredverticalwellsaretheprimarychoiceformulti-layeredreservoirswiththinsinglelayers.Whereasforblockswheregasbearingsandsareconcentratedandlaterallystablewithfewbarriersorinterbedlayers,horizontalwellsormul-ti-lateralhorizontalwellsshouldbethefirstchoicetoimprovereservescontrolledbysinglewellandeconomicprofit.Forlowabundanceblockswithgasbearingsandsthicknesslessthan6m,horizontaldrillingisrecommendedinareaswherethegasbearingsandsarestabletoimprovethereservescon-trolledbysinglewell.Wellpatternoptimizationshouldbeconductedonthebasisofwellplacementoptimizationandwelltypeselectiontofurtherimprovereservescontrolledbysinglewell.Suligegasfieldhasanobviousdual-texturecharacteristic,withsmallscaleandisolategasbearingsands,therefore,infilldrillingisnecessarytoimproveproducingdegreeofreserves.Pilotde-velopmentshowsthattherearebarelystepoutwellsinSuligegasfield,becauseofthewidedistributionandlongitudinalstackingofgasbearingsands.Scaleofgasbearingsandsra-therthandistributionfrequencyisthecontrollingfactorinwellspacingoptimization.Awidewellspacingwillleavesomegasbearingsandsnotpenetrated,whileaclosespacingwillcausesomegasbearingsandspenetratedbytwowellsormore,loweringreservescontrolledbysinglewellandeco-nomicprofit.Economiclimitwellspacingmustbeworkedout.Furthermore,forfracturingoperation,thescaleofgasbearingsandsshouldbetakenintoaccount.Fracturelengthshouldbeinaccordwithtosandscale.Patterngeometryde-pendsmainlyonthesandshapeandwidthtolengthratio.Asquare,rectangularanddiamondshapedwellpatternarethemostcommonlyused.Attheearlystageofitsdevelopment,researchandapplica-tionofwellplacementoptimizationforverticalwellsorclus-teredverticalwellswerecarriedoutinSuligegasfield.Verti-caldrillingalongwithseparatelayerfracturinghasrealizedeffectivedevelopment.Withtheadventofmulti-stagefrac-turing,horizontaldrillinggrowsrapidlyandiswidelyapplied,playinganimportantroleinimprovingdevelopmentandeconomicprofitofSuligegasfield.2WelltypesandwellpatternoptimizationinSuligegasfield2.1HierarchicalarchitecturedescriptionoflargecompositesandsandwelllocationoptimizationTooptimizethedevelopmentwellsdeployment,theresolu-tionofgasreservoirdescriptionneedstobeenhanced.Itisdifficulttoaccuratelyidentifyandquantitativelypredictthemaingas-bearingsandsbecauseoftheirsmallscale,disperseddistributionanddeepburialdepth,bymeansofgeophysicsinSuligegasfield.Therefore,theprogressivereservoirdescrip-tioncombininggeology,geophysicswithwelldata(increasingwiththenumberofdrilledwells)shouldbeutilizedtoim-provetheresolutiongraduallyfromregiontolocalarea,blocktointerwellareaandqualitativetoquantitativecharacteriza-tion.Inaccordancewithgeologicalcharacteristicsofreser-voirsinSuligegasfield,thehierarchicaldescriptionofreser-voirarchitectureoflargecompositesandshasbeendeveloped,predictingthedistributionofenrichmentareas,favorablesandsuperpositionzonesandinerwellreservoirsinsequenceandprovidinggeologicalmodelsfortheplacementofevaluationwells,pilotwellsandinfillwells.2.1.1HierarchicalarchitectureofcompositesandsForalargecomplicatedoilandgasfield,theknowledgeofdepositionalcharacteristics,reservoirdistributionandsandscaleisakeyprerequisitefortheconceptualdevelopmentdesign,enrichmentareascreening,wellpatterndesignandwelllocationdetermination.Thedepositionalbodycouldbedividedintodifferentgeneticunitsaccordingtoitsgrowthprocess.Takingfluvialfaciesasanexample,itcouldbedi-videdintolaminae,bedseries,singlesand,singlechannel,channelcomposite,fluvialsystemandbasinfillingcompositeandsoon[8−9].Inthepracticalapplication,suchdivisionschemeshouldbeadjustedcorrespondinglyaccordingtothegeologicalcharacteristicsofspecificareaandresearchre-quirements.Fromlargetosmallscale,largebraidedrivercompositesandsinSuligegasfieldaredividedintofourgradesofarchi-tectures:braidedriversystem,mainchannelsuperpositionzone,singlechannel,channelbar(Table1,Fig.2).Braidedriversystemtakesmemberasthestudyunit.TherearetwomembersinthepayformationinSuligegasfield,theeighthmemberof“Shihezi”formationH8,(whichisfurtherdividedHEDongboetal./PetroleumExplorationandDevelopment,2013,40(1):84–95−87−Talbe1FourgradearchitecturesofcompositesandsinSuligegasfieldArchitectureScaleArchitectureStratumUnitThicknessWidthLengthGeometryIdentificationMethodObjectiveGrade1(Braidedriversystem)Formation-MemberTensofmetersTenkilo-metersHundredsofkilometersWidebeltDistributionofsandsandmud,seismicfaciesPredictingenrichmentareas,placingevaluationwellsGrade2(Mainchannelsuperpositionzone)MemberOvertenmetersKilometersTensofkilometersBeltCore,loggingfacies,seismicfaciesPredictinghigh-energychan-nelsuperpositionzones,plac-ingpilotwellsGrade3(Singlechannel)LayerMetersHundredsofmetersKilometersBeltCore,loggingfaciesPredictingsinglesands,placinginfillwellsGrade4(Channelbar)LayerMetersHundredsofmetersHundreds-thousandsofmetersIrregularellipsoidCore,loggingfacies,welltestingPredictingsinglesands,placinginfillwellsFig.2SketchmapofthehierarchicalarchitectureoflargecompositesandsinSuligegasfieldintotheupperandthelowerH8),andthefirstmemberof“Shanxi”formationS1.Braidedriversystems,ofwidebelt-shape,areusuallytensofmetersthick,overtenkilome-terswideanduptoahundredkilometerslong,showingmac-roscopicstratumstructureof“mud-in-sand”.Onthebasisofthedifferencesinsandsuperpositionpattern,thebraidedriversystemisdividedintomainchannelsuperpositionzonesandmarginalzones.Superpositionzone,withnet-grossratioofmorethan70%,istherelativeenrichmentareaofgas-bearingsands.Thesesandsinincisedcompositelensshapeonthesection,beltshapeontheplane,areovertenmeterstotensofmetersthick,severalkilometerswide,andtensofkilometerslong.Marginalzones,withnet-grossratiobetween30%and70%,aredistributedonbothsidesofsuperpositionzone.Takingthinlayerasstudyunit,superpositionandmarginalzonecouldbefurtherdividedintosinglechannelsandchannelbars,whicharethethirdandthefourtharchitecturerespec-tively.Tensofmetersthick,thousandsofmeterswideandtensofthousandsofmeterslong,channelbars,ofirregularellipsoidshape,arethebasicunitofmaingas-bearingsand.Thebraidedriversystemcontrolsthegas-bearingrange,themainchannelsuperpositionzonecontrolsthedis
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