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RESEARCHPAPERPETROLEUMEXPLORATIONANDDEVELOPMENTVolume39,Issue4,August2012OnlineEnglisheditionoftheChineselanguagejournalCitethisarticleas:PETROL.EXPLOR.DEVELOP.,2012,39(4):491–497.Receiveddate:07Feb.2012;Reviseddate:20May2012.*Correspondingauthor.E-mail:311@petrochina.com.cnFoundationitem:SupportedbytheNaturalScienceandTechnologyMajorProject(2011ZX05015).Copyright©2012,ResearchInstituteofPetroleumExplorationandDevelopment,PetroChina.PublishedbyElsevierBV.Allrightsreserved.WellspacingoptimizationforSuligetightsandgasfield,NWChinaHEDongbo*,WANGLijuan,JIGuang,WEIYunsheng,JIAChengyePetroChinaResearchInstituteofPetroleumExploration&Development,Beijing100083,ChinaAbstract:ReasonablewellspacinghasbeenstudiedandassessedtoincreasethereservesandimprovetherecoveryfactoroftheSuligegasfield,wherethereservescontrolledbysinglewellaresmall.Anevaluationmethodologyforoptimizingwellspacinginatightsand-stonegasfieldwasestablishedfromfiveaspectsincludinggeologicmodel,drainageradius,interferencetesting,numericalsimulationandeconomicbenefits.Thewellspacinghasbeenoptimizedusingthismethod.Throughgeologicalmodeling,therationalinter-welldis-tanceisestimatedtobelessthan600mandrationalwellarraydistanceshouldbelessthan1000m.Resultsofdrainageareasestimationrevealthattherationalinter-welldistanceshouldberangedfrom400mto600m.Analysisofinterferencewelltestshowsthatthelowlimitforinter-welldistanceis400mandthefavorablewellarraydistanceis600m.Numericalsimulationandeconomicevaluationshowsthataninter-welldistanceof500mandwellarraydistanceof700marefavorable.Itisconcludedfromtheabovefiveaspectsthat,incurrenteconomicandtechnologicalconditions,thewellpatternwithinter-welldistanceat500mandwellarraydistanceat700misrationalintheSuligegasfield,anditcanimprovetherecoveryfactorto45%.Keywords:tightsandgasfield;wellspacingoptimization;sandbodyscale;drainagearea;economicevaluation;recoveryfactorIntroductionTightsandgasisoneofthemaintypesofunconventionalnaturalgas.Itreferstonaturalgastrappedintightsandstonewithpermeabilitylessthan0.1u103m2underoverburdenpressureandreleasedafterstimulations[1].Productionoftightgashasalreadybeenthetopofunconventionalgasandreachedover160BCMperyearinnorthAmerica[2].Asrep-resentativeoftightsandgasinChina,theSuligegasfieldintheOrdosBasinhasbeendevelopedandachievedrapidpro-gressesinrecentyears.Itsannualproductionisexpectedtobe20BCMin2014[3,4],thelargestnaturalgasfieldinChina.Tightsandgasreservoirsgenerallyhavenoobviousboun-dariesandarewidelydistributedacrossthousandsoreventensofthousandsofsquarekilometers.Lowpermeability,poorlateralconnectivityandcontinuityofthiskindofreservoirsresultinsmalldrainageareasandlowcontrolledreservesofsinglewell.Widespacingwhichiscommonindevelopmentofconventionalgasisnotproperfortightsandgasanddensewellspacingisrequiredtoimprovethepro-ducingdegreeofinitialgasinplace(GIIP)andtherecoveryratio.Besidesbecauseoflowproductionrateandrapidde-clinerateofsinglewell,tightgasfieldsrelyonproductionreplacementbetweengaswellstomaintainproductionplateau.Thatisthatintightsandgasdevelopmentinordertoreachmuchhighproductionscaleandkeepstableproductionforaperiodoftime,thenumberofgaswellsshallbeverylarge.Thereforeresearchonrationalwellspacingatearlystageoffielddevelopmentisquitenecessarywhichcanavoiddifficul-tiesinwellpatternadjustmentatmaturestageandimprovetheprojectbenefits.Inthispaper,approachesforrationalwellspacingestimationarestudiedfromfiveaspectsincludinggeologicalmodeling,drainageareasestimation,interferencewelltest,numericalsimulationandeconomicevaluation.Me-thodologyofwellspacingoptimizationfortightsandgashasbeensetupandappliedintheSuligegasfield.1MMethodologyofwellspacingoptimizationfortightsandgasThepurposeofwellspacingoptimizationistomaximizetheproducingdegreeofGIIPandrecoveryratiowithnoin-terwellinterference.Threefactorsincludingreservoirdistri-butionpattern,percolationcharacteristicsandfracturing&completionconditionsshouldbetakenintoconsiderationinwellspacingoptimization.Ifwellspacingistoowide,someinterwellgas-bearingsandswouldnotbepenetratedornotbefracturedbystimulation,resultinginlowproducingdegreeofGIIPandpoorrecoveryratio.Ontheotherhandifwellspac-HEDongboetal./PetroleumExplorationandDevelopment,2012,39(4):491–497492ingistoodense,interwellsandswouldbepenetratedbytwoormorewellsorfracturedtwiceormoreandproductionin-terferencewouldoccurwhichresultindeclineofestimatedultimaterecovery(EUR)andprojecteconomicbenefits.Thuswellspacingoptimizationshouldbeanimportantwork.Wellspacingoptimizationprocessisgivenbelowthroughgeologi-calmodeling,drainageareasestimation,interferencewelltest,numericalsimulationandeconomicevaluation.1.1GGeologicalmodelingTightsandgasreservoirsarewidelydistributedandsuper-imposedwithmulti-layersinlargeareas[5].Butinreservoirarchitectureelementstherearelithologicinterfaceandphysi-calinterfacewhichareformedinprocessofdepositionanddiagenesis.Theseinterfacesresultinsmallscalesofsingleflowunitandclusterdistributionofflowunits.Inordertorealizeeffectivecontroloverflowunitsorsands,widthandlengthofsandbodiesshouldbeestimatedaccurately.Thatis,inreservoirgeologicalmodeling,thekeyfactorsaretoaccu-ratelydeterminethescale,geometryandspatialdistributionfrequencyofgas-bearingsands.Toestablishgeologicalmodelforwellspacingoptimization,genesisforgas-bearingsandsshouldbedeterminedfirstlybasedonsedimentology,diagenesisanditsgas-bearingfea-tures.TakingSuligegasfieldasanexample,channelbarinthebraidedchannelsedimentarysystemisdeterminedtobegas-bearingsands[6].Andthenscaleandgeometrypatternofchannelbarshouldbeinvestigatedbythreemethods[7].Oneisgeologicalstatistics,inwhichcoredataandloginterpretationareusedtoworkoutnetpaythickness,andthesizeofeffec-tivesandsisestimatedusingthewidth/thicknessandlength/widthratiosofthesametypeofsedimentarysandsobtainedfromquantitativegeology.Thesecondmethodisoutcropanalogyanalysis,whichinvolvesselectingthesur-roundingoutcropsfromthesamelayer,quantitativelyde-scribingtheoutcropsin2-Dor3-D,settinguprelationshipbetweenoutcropsandundergroundtargetlayers,andestimat-ingthescaleofnetpayfinally.Forexample,fortightgassandoftheWilliamsForkFormationintheSouthPiceancebasin,adistributionmodelwasestablishedwithoutcropsdata.Thesandsareidentifiedaspointbarofmeanderingchannel,whichprovidedsolidbasisforspacingoptimization(Figure1a)[8].Thethirdmethodistoconductdensewellpilottest,inwhichinterwellreservoirstratigraphiccorrelationandsandsconnec-tivityrelationscanbethoroughlyinvestigatedthroughgeol-ogy,geophysicsandpilotproductiondata.Andthenscaleofnetpayorsandbodycanbeestimated.IntheSuligegasfield,densewellpilotrevealsthatmostofinterwellsandbodiesarepoorinconnectivityatwellspacingof400600m(Figure1b).1.2DrainageareasestimationBasedonwelltesttheory,drainageareaevaluationincludesestimatingwellcontrolledGIIPandproducingareasbyusingdynamicproductiondata,andthenoptimizingwellspacing.Ananalysismodelconsideringfracturehalflength,skinfactorandseepageboundaryetcshouldbeestablishedfirstly,andthenbematchedusingsinglewellhistoryproductiondataandreservoirparameters.Itshouldbenotedthatthereservesde-rivedfromthemodelandfromtheproductionhistoryshouldbethesameorclose.Thenthedrainageareasofgaswellcanbeestimatedandrationalwellspacingcanbedetermined.Astightgaswellsarecommonlyfractured,drainageareaseva-luationmethodswhichtakefractureintoconsiderationin-cludesfourtypicalcurveanalysismethods,namelyBlagsin-game,AGRatevs.Time,NPIandTransientdimensionlessproductionanalysis[912].Inthefourmethods,typicalcurvesofdimensionlessproductionrate,dimensionlessintegralofproductionrate,dimensionlessderivativesofproductionratevs.dimensionlesstimewithdifferentdrainageareasandfrac-Fig.1ThedistributionmodelfortightgassandsinthePiceanceBasinandOrdosBasin(Fig.1aisfromreference8).HEDongboetal./PetroleumExplorationandDevelopment,2012,39(4):491–497493turehalflengthshouldbebuilt.Historymatchingcanbecon-ductedtofitthetypicalcurveandthenfracturehalflengthanddrainageareasforparticulargaswellcanbeestimated(Figure2).Afractureanalogyanalysismodelistomatchwellbottomholepressure(WBHP)atcertainproductionrateandtoesti-matethefracturehalflengthanddrainageareas(Figure3).Becauseofitsinherentlowpermeability,strongheteroge-neityandlowseepagerate,ittakesafewyearstoreachtheseepageboundaryintightgassands.Thatis,theflowbound-aryisdynamicanditisexpanding.Thusdrainageareasandreserveestimationattheearlystageusingproductiondataareusuallysmallerthantheactualone.Besides,fracturestimula-tionisusuallyappliedintightgasdevelopmentwhichcanpenetratetheflowbarriersandthenthedrainageareasareexpandedeffectively.Inpracticalapplication,drainageareasderivedfromdrainageareasestimationandgeologicalmod-elingshouldbetestedandverifiedmutuallytogettheobjec-tiveresults.1.3InterferencewelltestInterferencewelltestreferstowelltestingthroughchang-Fig.2Blasingametypicalcurvewithdimensionlessproductionrate,dimensionlessintegralofproductionrate,dimensionlessderivativesofproductionratevs.dimensionlesstimeFig.3TypicalcurveoffracturemodelforgaswellinSuligegasfieldinggaswellproductionstatus,suchasopen/shutingaswells,orchangingtheproductionrateandobservingtheWBHPofthesurroundingwellsthroughhighprecisionandhighsensi-tivitypressuregauges.Thiskindofpressurechangerevealsthescaleofgassandseffectively[13].Toavoidinterference,rationalwellspacingshouldbemorethanthescaleofgas-bearingsands.Therefore,throughinterferencewelltest,thelowlimitofwellspacingcanbefiguredoutandinterwellconnectivitycanalsobeevaluatedthroughinfillwellpressuredata.Bycomparisonbetweentheoriginalformationpressureofnewlydrilledinfillwellsandsurroundingproducingwells,interwellconnectivitycanbeinvestigatedthoroughly.Ifinfillwellsshowpressurerelease,thewellsareconnected.1.4NNumericalsimulationNumericalsimulationonbasisofthreedimensionalgeo-logicalmodelinvolvesmodelingsinglewellproductionper-formance,predictingproductionindexes,andinvestigatingtherelationshipsbetweenwellspacingandsinglewellultimaterecoveryatdifferentwellspacingandwellarraycombina-tion[14].Ifwellspacingiswideandthereisjustonewellpro-ducinginapermeabilityandporosityunit,therewouldbenointerferenceandEURofsinglewellwouldnotchangewithwellspacing.But,ifwellspacingistooclose,interferencewouldoccurandEURwouldrapidlydecline.Figure4showstherelationshipsbetweenwelldensityvs.EURandrecoveryratio.ItrevealsthatthewelldensityattheinflectionpointatwhichEURdeclinesrapidlyshouldbetherationalone.Andnumericalsimulationrevealsrecoveryrateriseswithwelldensity.1.5EconomicevaluationEconomicevaluationshouldbeconductedtoensurethemaximumdegreeofreserverecoveryinthecurrenteconomicconditions.FirstlyEURofsinglewellshouldbeestimatedbyrestrictionofexpendituresondrilling,completionandsurfacefacilities.Relationcurveofwelldensityvs.EURofsinglewellcanbeobtainedfromnumericalsimulation(Figure4).Intersectionoftheabovetworelationcurvesshouldbethewelldensityineconomiclimitandthecorrespondingdegreeofreserverecoveryistherecoveryratioineconomiclimit[15].Generally,maximumeconomicbenefitscanbeachievedinFig.4Relationshipbetweenwelldensity,singlewellEURandrecoveryratio(modifiedfromreference15)HEDongboetal./PetroleumExplorationandDevelopment,2012,39(4):491–497494caseofmaximumwelldensitywithnowellinterference.Wheneconomicconditionsallow,welldensitycanbeinfilleduntilinterferenceoccurstogethigherrecoveryratioatthecostofsinglewellEUR.2CCasestudy2.1IntroductiontoSuligegasfieldLocatedatthenorthwestofShanbeiSlope(Figure5),theSuligegasfieldisabout40000km2.LowPermianShihezi8MemberP1x8andLowShanxi1MemberP1s1arethemainpayzones,withanaverageporosityof7%andmeanforma-tionpermeabilityof0.1u103m2.Thetargetedreservoirisdepositedinabraidedchannelsettingandchannelbaristhemaingas-bearingsand.Thecharacteristicsofchannelbarimplythegas-bearingsandsareinsmallscaleandisolateddistribution.2.2WellspacingoptimizationforSuligegasfield2.2.1GeologicalmodelingPilotproductionatwellspacingof800mhasbeencarriedoutattheearlystageofevaluationintheSuligegasfield(Figure6).Itrevealspoorconnectivityofgas-bearingsandsandinsufficientproducingdegreetoGIIP.Inordertoopti-mizethewellspacing,geologicalmodelingshouldbecon-ductedfromthefollowingthreeaspects.Thefirstoneistoconductquantitativecoredescriptioninconjunctionwithlog-gingtodeterminethethicknessofsinglechannelbarsandbody.Theresultsshowthatthethicknessofsinglechannelbarsandbodyarefrom25m.Accordingtooutcropinvesti-gationandphysicalsimulationexperimentsforbraidedchan-nelsedimentarysystem[16],thewidth/thicknessratioandlength/widthratioforchannelbarare80120and1.52.0respectively.ThusthewidthandlengthforbraidedchannelinSuligewouldbe160600mand3001200m,respectively.ThesecondaspectistoconductoutcropinvestigationinLi-ulin.StratainLiulinoutcropsareofthesamegeologicaltimewithtargetedreservoirintheSuligegasfield.Fieldmeasure-mentsshowthewidthofsinglesandis200400m.Thethirdaspectispilotdevelopmentwithchangingwellspacingat400600mtoinvestigatethedistributionofsands.TheresultsTheresultsshowthatover80%ofsandsareoflessthan600mwide.InconclusionwellspacingintheSuligegasfieldshouldbelessthan600mandwellarraylessthan1000m.2.2.2DrainageareaestimationGas-bearingsandsintheSuligegasfieldaremainlychan-nelbarofbraidedfluvialsedimentarysysteminnearellipseshape.Byusingtheabovefourtypicaldimensionlessproduc-tionchartandfractureanalogymodel,thedrainageareasforthetwentyeightpilotproductionwellswhichareputintoproductionin2002to2003wereevaluated(Table1).ForgasFig.5GeologicalandgeographicalmapforSuligegasfieldFig.6Reservoircross-sectionprofileofSu6blockinSuligegasfieldHEDongboetal./PetroleumExplorationandDevelopment,2012,39(4):491–497495Table1Drainageareasevaluationfor28pilotwellsinSuligegasfieldGaswellclassificationProportion/%Dynamicreserves/104m3Effectivefracturehalflength/mControlledarea/km2Majorsemi-axisforellipse-likedrainagearea/mMinorsemi-axisforel-lipse-likedrainagearea/mFirst-class32.1399792.70.235330220Second-class28.6232876.70.186292195Third-class39.3115772.60.155267178Average240480.20.190294196wellswhichareclassifiedasclassI,theaveragedrainageareaisestimatedtobe0.235km2,whichisinellipseshapewithamajorandminorsemi-axisof330mand220m,respectively.DrainageareaforgaswellsofclassIIisestimatedtobe0.186km2withamajorandminorsemi-axisof292mand195m,respectively.GaswellsclassifiedasclassIIIhasadrainageareaof0.155km2withthetwosemi-axisof267mand178m,respectively.Thus,theradiusofdrainageareasofthe28pilotproductionwellsfallsbetween200and300mandtherationalwellspacingshouldbe400to600m.2.2.3InterferencewelltestorpressuremonitoringToidentifyinterwellconnectivity,Su6blockisselectedforwelltest.GaswellsinSu6blockwereputintoproductionatearlystageofpilotproductionandwelldensityinSu6blockisquitedense.Sixmoreinfillwellsweredrilledinclud-ingsu6-j1,su6-j2,su6-j3,su6-j4,su6-j5andsu6-j6andwellspacingismorecloseto400m(Figure7a).Afterflowbackinfracturedwells,pressurebuiltuptestshouldbecarriedouttoinvestigatetheformationpressureandjudgewhetherthefor-mationpressureisattheoriginalvalueorhavedroppeddown.PressuremonitoringinSu6blockshowsthatformationpres-sureinwellsu6-j3andwellsu6-j4havedroppeddown.Afterflowbackfor5days,formationpressurein
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