SPE_114164_(Rushing)_Rock_Typing_Keys_Productivity

SPE114164RockTyping—KeystoUnderstandingProductivityinTightGasSandsJ.A.Rushing,SPE,AnadarkoPetroleumCorp.,K.E.Newsham,SPE,ApacheCorp.,andT.A.Blasingame,SPE,TexasA&MUniversityCopyright2008,SocietyofPetroleumEngineersThispaperwaspreparedforpresentationatthe2008SPEUnconventionalReservoirsConferenceheldinKeystone,Colorado,U.S.A.,10–12February2008.ThispaperwasselectedforpresentationbyanSPEprogramcommitteefollowingreviewofinformationcontainedinanabstractsubmittedbytheauthor(s).ContentsofthepaperhavenotbeenreviewedbytheSocietyofPetroleumEngineersandaresubjecttocorrectionbytheauthor(s).ThematerialdoesnotnecessarilyreflectanypositionoftheSocietyofPetroleumEngineers,itsofficers,ormembers.Electronicreproduction,distribution,orstorageofanypartofthispaperwithoutthewrittenconsentoftheSocietyofPetroleumEngineersisprohibited.Permissiontoreproduceinprintisrestrictedtoanabstractofnotmorethan300words;illustrationsmaynotbecopied.TheabstractmustcontainconspicuousacknowledgmentofSPEcopyright.AbstractThispaperpresentsawork-flowprocesstodescribeandcharacterizetightgassands.Theultimateobjectiveofthiswork-flowistoprovideaconsistentmethodologytosystematicallyintegratebothlarge-scalegeologicelementsandsmall-scalerockpetrologywiththephysicalrockpropertiesforlow-permeabilitysandstonereservoirs.Tothatend,ourwork-flowintegratesmultipledataevaluationtechniquesandmultipledatascalesusingacore-basedrocktypingapproachthatisdesignedtocapturerockpropertiescharacteristicoftightgassands.Fundamentaltothisprocessmodelareidentificationandcomparisonofthreedifferentrocktypes—depositional,petro-graphic,andhydraulic.Theserocktypesaredefinedas:●Depositional—Thesearerocktypesthatarederivedfromcore-baseddescriptionsofgeneticunitswhicharedefinedascollectionsofrocksgroupedaccordingtosimilaritiesincomposition,texture,sedimentarystructure,andstratigraphicsequenceasinfluencedbythedepositionalenvironment.Theserocktypesrepresentoriginallarge-scalerockpropertiespresentatdeposition.●Petrographic—Thesearerocktypeswhicharealsodescribedwithinthecontextofthegeologicalframework,buttherocktypecriteriaarebasedonpore-scale,microscopicimagingofthecurrentporestructure—aswellastherocktextureandcomposition,claymineralogy,anddiagenesis.●Hydraulic—Thesearerocktypesthatarealsodefinedattheporescale,butinthiscasewedefinehydraulicrocktypesasthosethatquantifythephysicalflowandstoragepropertiesoftherockrelativetothenativefluid(s)—ascontrolledbythedimensions,geometry,anddistributionofthecurrentporeandporethroatstructure.Eachrocktyperepresentsdifferentphysicalandchemicalprocessesaffectingrockpropertiesduringthedepositionalandparageneticcycles.Sincemosttightgassandshavebeensubjectedtopost-depositionaldiagenesis,acomparisonofallthreerocktypeswillallowustoassesstheimpactofdiagenesisonrockproperties.Ifdiagenesisisminor,thedepositionalenvironment(anddepositionalrocktypes)aswellastheexpectedrockpropertiesderivedfromthosedepositionalconditionswillbegoodpredictorsofrockquality.However,ifthereservoirrockhasbeensubjectedtosignificantdiagenesis,theoriginalrockpropertiespresentatdepositionwillbequitedifferentthanthecurrentproperties.Morespecifically,useofthedepositionalenvironmentandtheassociatedrocktypes(inisolation)toguidefielddevelopmentactivitiesmayresultinineffectiveexploitation.IntroductionUnconventionalnaturalgasresources—tightgassands,naturally-fracturedgasshales,andcoalbedmethanereservoirs—compriseasignificantpercentageoftheNorthAmericannaturalgasresourcebaseandthesesystemsrepresentanimportantsourceforfuturereservegrowthandproduction.Similartoconventionalhydrocarbonsystems,unconventionalgasreservoirsarecharacterizedbycomplexgeologicalandpetrophysicalsystemsaswellasheterogeneities—atallscales.However,unlikeconventionalreservoirs,unconventionalgasreservoirstypicallyexhibitgasstorageandflowcharacteristicswhichareuniquelytiedtogeology—depositionanddiageneticprocesses.Asaresult,effectiveresourceexploitationrequiresacomprehensivereservoirdescriptionandcharacterizationprogramtoquantifygas-in-placeandtoidentifythosereservoirpropertieswhichcontrolproduction.Althoughmanyunconventionalnaturalgasresourcesarecharacterizedbylowpermeabilities,thispaperaddressesonlylow-permeabilitysandstonereservoirs,i.e.,tightgassands.Theoilandgasindustryhaslongrecognizedtheimportanceofporestructure(i.e.,poreandporethroatdimensions,geometry,size,distribution,etc.)onfluidflowandstoragepropertiesinalltypesofporousmedia.Understandingtheporestructureand2J.A.Rushing,K.E.Newsham,andT.A.BlasingameSPE114164propertiesisprobablyevenmorecriticalintightgassandssincediagenesisoftenmodifiestheoriginalporestructureandreducestheaverageporethroatdiameter,typicallycausinganincreaseinbothtortuosityandthenumberofisolatedand/ordisconnectedpores.Someformsofdiagenesismayactuallyincreaseporositybycreatingsecondaryormicro-porosity.Regardlessofthetypeofdiagenesis,alltightgassandsretainsomeunderlyingtraitsofthedepositionalsystemeventhoughtheoriginalrockpropertiesmayhavebeen(significantly)altered.However,wellproductivitycannotbepredictedaccuratelybas