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1TightGasReservoirs–AnUnconventionalNaturalEnergySourcefortheFutureG.C.NaikSummaryWiththedeclineofproductionandincreaseindemandoffossil-fuel,economicallyproducinggasfromunconventionalsources(tightgas,coalbedmethane(CBM),andgashydrate)isagreatchallengetoday.Thelargevolumeandlong-termpotential,attractivegaspricesandunprecedentedinterestinworldmarkets,bringstheunconventionalgasintotheforefrontofourenergyfuture.Tightgasexistsinundergroundreservoirswithmicrodarcy-rangepermeabilityandhaveahugefuturepotentialforproduction.Fourcriteriathatdefinebasin-centeredgasaccumulations,includinglowpermeability,abnormalpressure,gassaturatedreservoirsandnodowndipwaterleg.Althoughtightgassandsareanimportanttypeofbasin-centeredgasreservoir,notallofthemareBasin-centeredgas(BCGAs).Aconcertedtechnologyefforttobothbetterunderstandtightgasresourcecharacteristicsanddevelopsolidengineeringapproachesisnecessaryforsignificantproductionincreasesfromthislow-permeability,widelydispersedresource.Gasproductionfromatight-gaswellwillbelowonaper-wellbasiscomparedwithgasproductionfromconventionalreservoirs.AlotofwellshavetobedrilledtogetmostoftheoilorgasoutofthegroundinunconventionalreservoirsExplorationeffortsinlow-permeabilitysettingsmustbedeliberateandfocusonfundamentalelementsofhydrocarbontraps.Understandinggasproductionfromlowpermeabilityrocksrequiresanunderstandingofthepetrophysicalproperties-lithofaciesassociations,faciesdistribution,insituporosities,saturations,effectivegaspermeabilitiesatreservoirconditions,andthearchitectureofthedistributionoftheseproperties.Petrophysicsisacriticaltechnologyrequiredforunderstandinglow-permeabilityreservoirs.Improvementsincompletionanddrillingtechnologywillallowwellidentifiedgeologictrapstobefullyexploited,andimprovementsinproductpricewillallowsmalleraccumulationsorlower-ratewellstoexceedeconomicthresholds,butthisistrueinvirtuallyeverypetroleumprovince.WellClustersandOnsiteWasteManagementarethekeycomponentsofNewTechnologyConceptsfortightgasdevelopmentGeologists,engineers,loganalysts,andotherprofessionalshavetocometothecommontablewithaneedtobetterunderstandandpredictreservoirpropertiesinlow-permeabilityreservoirsandusethatinformationinresourceevaluation,reservoircharacterizationandmanagement.Thereisnofearofrunningoutofoilornaturalgas.Anenormousvolumeofunconventionaloilandgaswillbetheretofillthegaponceconventionaloilbeginstodeclineinthenext5to20years.2Introduction:TheKeyWordsThetitle“TightGasReservoirs–AnUnconventionalNaturalEnergySourcefortheFuture”containsafewkeywordslikeTightGasReservoirs,UnconventionalEnergySourceandFuture.Thefirstgroupofwords‘TightGasReservoirs’saysaboutthetypeofreservoirsandthenaturalresourcecontainedinitwhereasthesecondgroupi.e‘UnconventionalEnergySource’spellsaboutthescaleofeconomicsofexploitationwiththepresenttechnologicalknow-howandthelastword‘Future’dealswiththetimeframe.Whenlookedintotality,itspeaksaboutthetypeofnaturalenergyresourcethatisbeingfocusedbythegeoscientistsandtheenergyplannerworld-overasanalternativetothealreadydecliningsourceoffossilfuel.Thepresentstudypurportstomakeaglobalreviewofthevariousworksandcurrentresearchesrelatingtotightgasreservoirsandgainasolidscientificbackgroundinthisaspect,thentoapplytheseideastoapracticalevaluationofopportunitiesinIndia.The‘essenc’ofthestudyisasystematicreviewofmajortightgasplaysinthedifferentpartsoftheglobe.Geologicalcharacteristics(depositionalenvironments,lithologies,diagenesis),fracturepotential,reservoirdevelopment,resourcedensity,andoverallresourceprizewillbeaddressed.Theoverviewwillincludedefinitionsoftightgasreservoirsandrelatedconceptssuchasbasin-centredgasandtheDeepBasinversusconventionalresourceparadigms.Theconceptofreservoirsweetspots–bothstratigraphicandstructural–willbesummarized.Aspectrumoftightgasplaytypeswillbedescribedtoprovideaframeworkofreferenceincomparingspecificplays.Thecentralthemetothepaperwillbetoassesswhetherthisisarealpossibilityormaybesimplya‘pipedream’,overthemedium-term(20yearsormore).FromConventionaltoUnconventionalReservoirs:theFutureoftheOilandGasBusinessConventionalreservoirsarethosethatcanbeproducedateconomicflowratesandthatwillproduceeconomicvolumesofoilandgaswithoutlargestimulationtreatmentsoranyspecialrecoveryprocess.Aconventionalreservoirisessentiallyahigh-tomedium-permeabilityreservoirinwhichonecandrillaverticalwell,perforatethepayinterval,andthenproducethewellatcommercialflowratesandrecovereconomicvolumesofoilandgas.Ontheotherhand,anunconventionalreservoirisonethatcannotbeproducedateconomicflowratesorthatdoesnotproduceeconomicvolumesofoilandgaswithoutassistancefrommassivestimulationtreatmentsorspecialrecoveryprocessesand3technologies,suchassteaminjection.Typicalunconventionalreservoirsaretight-gassands,coal-bedmethane,heavyoil,andgasshales.Figure1depictstheresourcetriangleofboththeconventionalandunconventionalresources.Unlikeconventionalreservoirs,whicharesmallinvolumebuteasytodevelop,unconventionalreservoirsarelargeinvolumebutdifficulttodevelop.Increasingpriceandtheimprovedtechnologyaretheke