Flow Units： From Conventional to Tight Gas to Shal

SPE132845FlowUnits:FromConventionaltoTightGastoShaleGasReservoirsRobertoAguilera,SchulichSchoolofEngineering/UniversityofCalgaryCopyright2010,SocietyofPetroleumEngineersThispaperwaspreparedforpresentationattheTrinidadandTobagoEnergyResourcesConferenceheldinPortofSpain,Trinidad,27–30June2010.ThispaperwasselectedforpresentationbyanSPEprogramcommitteefollowingreviewofinformationcontainedinanabstractsubmittedbytheauthor(s).ContentsofthepaperhavenotbeenreviewedbytheSocietyofPetroleumEngineersandaresubjecttocorrectionbytheauthor(s).ThematerialdoesnotnecessarilyreflectanypositionoftheSocietyofPetroleumEngineers,itsofficers,ormembers.Electronicreproduction,distribution,orstorageofanypartofthispaperwithoutthewrittenconsentoftheSocietyofPetroleumEngineersisprohibited.Permissiontoreproduceinprintisrestrictedtoanabstractofnotmorethan300words;illustrationsmaynotbecopied.TheabstractmustcontainconspicuousacknowledgmentofSPEcopyright.AbstractProcessordeliveryspeedrepresentedbytheratioofpermeabilityandporosityisshowntoprovideacontinuumbetweenconventional,tightandshalegasreservoirs.Thesurprisingresult,basedoncoredatafromvariousNorthAmericanbasins,leadstodistinctiveflowunitsforeachtypeofreservoir.TheapproximateboundarybetweenviscousanddiffusiondominatedflowisestimatedwithKnudsennumber.Viscousflowispresent,forexample,whenthearchitectureoftherockisdominatedbymegaports,macroports,mesoportsandsometimesmicroports(port=porethroat).Diffusionflowontheotherhandisobservedatthenanoportlevel.Resultsfromthisresearchcomparewellwiththeobservationthatgrainsandporesaresmallerinshalesascomparedwithtightandconventionalgasformations.Theprocessspeedconcepthasbeenusedsuccessfullyinconventionalreservoirsforseveraldecades.However,theconceptpresentedinthispaperfortightgasandshalegasreservoirs,withthesupportofcoredata,isnew,andpermitsdifferentiatingbetweenviscousanddiffusiondominatedflow.Thisisvaluable,forexample,inthosecaseswheretheformationtobedevelopediscomposedofalternatingstackedlayersoftightsandsandshales,orwheretherearelateralvariationsduetofacieschanges.Itisconcludedthatthereissignificantpotentialintheuseofprocessspeedaspartoftheflowunitcharacterizationofunconventionalgasreservoirs.IntroductionFigure1showstheworldwidelocationofconventionalsiliciclasticandcarbonatereservoirsconsideredinthispaper.Theoriginalstudy,conductedbyEhrenbergandNadeau(2005),presentsausefulcompilationofporosity,permeability,lithology,geothermalgradientsandpre-upliftdepthsfortheAlbertabasininCanadaandotherpetroleumreservoirsaroundtheworld.Figure2presentsthelocationoftightgasreservoirsinCanadaandtheUnitedStates(Holditch,2001;PTAC,2006).InpetroleumprovincesinNorthAmericasometightgasreservoirsarefoundinbasin-centeredorcontinuousgasaccumulations.Othersarefoundinlowpermeabilityreservoirsinconventionalstructural,stratigraphicorcombinationtrapsusuallyreferredtoas“sweetspots”.Tightgasreservoirsarebestrepresentedbyatleastdualporositymodels.Figure3displaysthelocationofNorthAmericanshalegasplays(ZiffEnergy,2009).Shalegasreservoirsarebestrepresentedbyatleastquadrupleporositymodelsasshowninthispaper.Thisobservationleadstovolumesoffreegasinplacethatareprobablelargerthanconsideredpreviously.Figure4illustratesaworldgasresourcepyramidshowingglobalendowmentforconventionalgas,tightgas,shalegasandcoalbedmethanereservoirsasestimatedbytheGFREEteamattheUniversityofCalgary(AdaptedfromAguileraetal.,2008).Endowment,asusedinthispaper,isthesummationofcumulativegasproduction,reservesandundiscoveredgas.Thetotalnaturalgasendowment,excludingnaturalgashydrates,isgigantic,at45,000tcf,outofwhich67%isestimatedtobeintightandshalegas.TightgasendowmentintheUnitedStatesandCanadaisestimated450and105tcf,respectively.Thesevolumesrepresent2SPE132845only7%oftheoriginalgasinplace.ShalegasendowmentintheUnitedStateshasbeenestimatedpreliminaryatapproximately240and70tcfbytheGFREEteam.Theendowmentofnaturalgas,thecleanestburningfossilfuel,willsupplymarketneedsforseveraldecades.Thepyramidshowsthatunconventionalgasisassociatedwithverylowpermeabilities.Assuch,successfulproductionofunconventionalgaswillincludeincreasesinproduction,prices,activationindices,researchandtime.Ontheotherhandthepyramidshowsdecreasesinprocess(ordeliveryspeed)andporethroatapertures,whicharesomeofthekeytopicsdiscussedinthispaper.ProcessordeliverySpeedProcessordeliveryspeed,i.e.,theratioofpermeabilityandporosity,providesarelativeindicationofstorageandhowquicklyfluidscanmovethroughporousmedia.Theconcepthasbeenshowntobepowerfulforcharacterizingconventionaloilandgasreservoirsinvariouslithologies(Chopraetal.,1987,Gunteretal.,1997),andforpredictingrecoverablehydrocarbonvolumes(PickettandArtus,1970).Itisthusanoptimumconceptforlinkinggeology,petrophysicsandreservoirengineeringaspermeabilityandporosityarestudiedindetailandusedbyallofthesedisciplines(Aguilera,2004).Theprocessspeed(k/ø)isanimportantsegmentofthehydraulicdiffusivity(theotheronesbeingtotalcompressibilityandfluidviscosity),,whichispartofthediffusivityequation.Thisequationisattheheartoffluidflowcalcula