PROCEEDINGS, Thirty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 31 - February 2, 2011 SGP-TR-191 UPDATING THE CLASSIFICATION OF GEOTHERMAL RESOURCES Colin F. Williams1, Marshall J. Reed1, and Arlene F. Anderson2 1. U.S. Geological Survey Menlo Park, CA 94025 e-mail: colin@usgs.gov 2. Department of Energy Geothermal Technologies Program Washington, DC ABSTRACT Resourceclassificationisakeyelementinthe characterization,assessmentanddevelopmentof energyresources,includinggeothermalenergy. Stakeholdersatalllevelsofgovernment,withinthe geothermalindustry,andamongthegeneralpublic needtobeabletouseandunderstandconsistent terminologywhenaddressinggeothermalresource issuessuchaslocation,quality,feasibilityof development,andpotentialimpacts.This terminology must encompass both the fundamentally geologicalnatureofgeothermalresourcesandthe practicaltechnologicalandeconomicaspectsof resource exploitation while remaining understandable tothebroadcommunityofnon-specialists.Inthe UnitedStates,theclassificationsappliedto geothermalresourcesareprimarilythelegacyof resourceassessmentandcharacterizationstudies conductedinthe1970sduringatimeofrapid developmentandnewinterestingeothermalenergy. Thatmanyofthestandardsdevelopedduringthat time are still in use today is a testament to the quality oftheoriginalwork.However,developmentsover thepast30years,especiallyadvancesingeothermal technology,haveexpandedthescopeofexploitable geothermalresourcesbeyondtheearlier classifications.Aspartofitsongoingworktoassess geothermal energy resources in the United States, the U.S.GeologicalSurvey(USGS)isworkingwiththe DepartmentofEnergy(DOE),thegeothermal industry,andacademicpartnerstodevelopanew geothermalresourceclassificationsystemthatwill reflectthecurrentstateofknowledgeregarding geothermal technology and serve as a effective means ofcharacterizing,quantifying,assessingand communicatingimportantaspectsofgeothermal energypotential.Thispaperdescribesthescopeof the effort as well as initial progress in establishing the new classification terms. BACKGROUND Geothermalenergyresourcesarecharacterizedby geologicsettings,intrinsicproperties,andviability forcommercialutilization.Coherentframeworksfor classifyingtheseresourcesarenecessaryfora numberofpurposes,includingresourceassessment, exploration,development,andreporting(e.g., AGCC, 2008). The diversity of both the nature of the geothermalresourceanditsexploitationpresentsa challenge in the context of resource classification, as definitionsandconceptsthatserveonepurposemay beinadequatefororevencounterproductivewhen usedforotherpurposes.Classificationhasbeena featureofgeothermalinvestigationsfromanearly date,withlargenumbersofpublicationseither devoted to or touching on the topic during the 1960s and1970s(e.g.,White,1965;Whiteetal.,1971; KrugerandOtte,1973).Someapproacheshave continuedinuseuntilthepresent.Forexample,the basicframeworkforgeothermalresource characterizationandassessmentdevelopedby MufflerandCataldi(1978)isfoundationaltorecent resourceassessmentsbytheUSGSandother organizations(Williamsetal.,2008a,b).However, withtechnologicalchanges,theinternationalgrowth ofthegeothermalindustry,amoredetailed understanding of geologic and tectonic processes, and the potential development of new types of resources, itistimelytorevisitandpotentiallyrevisethese earlierapproachestogeothermalresource classification.Withintheinternationalgeothermal community,thevariedneedsandclassification systemsdevelopedtomeetthoseneedsmakeit unlikelythatanyonesystemwillmeetallpotential requirements,letalonebeadopteduniformlybyall concerned. IntheUnitedStates,theGeothermalSteamActof 1970gaveresponsibilityforassessinggeothermal resourcestotheUSGS,withDOE(andits predecessorERDA)collaboratingandproviding significantfinancialsupport.TheUSGS-DOE classificationeffortoutlinedhereispartofthose ongoingassessmenteffortsandhastwoprimary objectives.Firstistodeveloparevisedandupdated classificationsystemthatmeetstheneedsofcurrent USGSandDOEresourcecharacterizationand assessmentactivities.Secondistoprovidetoolsfor translatingamongthetermsanddefinitionsusedby different systems so there can be clear and consistent communicationofbothqualitativeandquantitative resource assessment results.Withinthiscontext,weproposethefollowing guidelinesfordevelopinggeothermalresource classifications.Specifically,ageothermalresource classification system should 1. be simple and logical for effective communication with and understanding by both experts and non-experts, 2. be valid from scientific and technical perspectives, 3. meet clearly identified needs for categorized resource information, 4. be easily translated into other classification systems, 5. avoid misconceptions that may unintentionally adversely affect commercial activities, 6. eliminate gaps or overlaps betweencategories, and7. avoid unnecessary (and potentially incorrect) predictions by focusing on fundamental physical and chemical properties of resources rather than on aspects of utilization that can be altered by evolving technological, regulatory and economic factors. CURRENTGEOTHERMALRESOURCE ASSESSMENT TERMINOLOGY InthispaperwefollowearlierUSGSgeothermal resource studies in using the terminology adopted by Muffler and Cataldi (1978) for the subdivision of the geothermalresourcebase.Thesesubdivisionsare easilyillustratedthroughamodifiedMcKelvey diagram(Figure1),inwhichthedegreeofgeologic assuranceregardingresourcesissetalongthe horizontalaxisandtheeconomic/technological feasibility (often equivalent to depth) is set along the verticalaxis(MufflerandCataldi,1978).USGS geothermalassessmentsconsiderbothidentifiedand undiscoveredsystemsandutilizethefollowing definitions. The geothermal resource base is all of the thermal energy in the Earths crust beneath a specific area,measuredfromthelocalmeanannual temperature. The geothermal resource is that fraction oftheresourcebaseatdepthsshallowenoughtobe tappedbydrillingintheforeseeablefuturethatcan be recovered as useful heat economically and legally atsomereasonablefuturetime.Similarly,the geothermalreserveistheidentifiedportionofthe resourcethatcanberecoveredeconomicallyand legallyatthepresenttimeusingexistingtechnology (Muffler and Cataldi, 1978). Figure 1: McKelveydiagramrepresenting geothermalresourceandreserve terminologyinthecontextofgeologic assuranceandeconomicviability. ModifiedfromMufflerandCataldi (1978). Onefundamentalaspectofgeothermalresource classification that has not been consistently addressed isthedevelopmentofaclearandconcisedefinition ofageothermalsystem.Inoilandgasassessments, conceptualdefinitionsofpetroleumsystemshave expandedtoencompassthenaturalhydrocarbon-fluidsysteminthelithospherethatcanbemapped andincludestheessentialelementsandprocesses neededforoilandgasaccumulationstoexist (MagoonandSchmoker,2000).Similarly,an important goal in geothermal resource assessments is to understand the process by which an exploitable, or potentiallyexploitable,geothermalreservoirforms and evolves over time. The AGI Glossary of Geology (Neuendorfetal.,2010)includesanearlierUSGS definitionofageothermalsystemasanyregionally localized geological setting where naturally occurring portions of the Earths thermal energy are transported closeenoughtotheEarthssurfacebycirculating steamorhotwatertobereadilyharnessedforuse. Thisdefinitioncoversnaturalhydrothermalsystems

butisnotapplicabletoconductivegeothermal resourcesorEnhancedGeothermalSystems.To broadenthisdefinition,weproposeaprovisional alterationofthisdefinitionasfollows.Ageothermal systemisanylocalizedgeologicsettingwhere portionsoftheEarthsthermalenergymaybe extracted from a circulating fluid and transported to a pointofuse.Ageothermalsystemincludes fundamentalelementsandprocesses,suchasfluid and heat sources, fluid flow pathways, and a caprock orseal,whicharenecessaryfortheformationofa geothermal resource. Akeyrequirementofgeothermalclassificationfor resourceassessmentsistoprovidealogicaland consistentframeworkforquantifyingthegeothermal resourceinthecontextofthegeneraldefinitions given above. The following examples highlight issues in the classification of natural convective geothermal systems formed through the free convection or forced convection/advectionofwaterinthesubsurface, naturalconductivegeothermalsystems,whichare basedontheexploitationoftemperaturesatdepth developedthroughtheconductivegeothermal gradient,andEnhancedorEngineeredGeothermal Systems(EGS),whichcanbedevelopedinboth convective and conductive environments.EXAMPLESOFGEOTHERMALRESOURCE CLASSIFICATION Temperature and Thermodynamic Properties Asthemeansbywhichgeothermalenergyis identifiedandutilized,temperatureisafundamental measureofthequalityoftheresourceand consequentlyistheprimaryelementofmost classificationsystems.ForUSGSassessments, geothermalsystemshavebeendividedintothree temperatureclasses:low-temperature(150oC)(WhiteandWilliams,1975; Muffler,1979;Williamsetal.,2008b).High-temperaturesystemsincludebothliquid-andvapor-dominatedresources.Moderate-temperaturesystems are almost exclusively liquid-dominated, and all low-temperaturesystemsareliquid-dominated.Allthree temperatureclassesaresuitablefordirectuse applications,butingeneralmoderate-andhigh-temperaturesystemsareviableforelectricpower generation.Systemsattheupperendofthelow-temperature range can be exploited for electric power generationifsufficientlylowtemperaturesare availableforcoolingtheworkingfluidinabinary power plant. Otherthermalclassificationsystemshavebeen proposed, with most focusing on dividing geothermal resourcesintoasimilarsetofthreeor,moresimply, twoclassesthatdefineaprogressionfromlowto hightemperature(orenthalpy)geothermalresources (Figure2).Ineachcasethetemperature/enthalpy boundariesaresetattemperaturesthoughttobe significant in either a thermodynamic or an economic utilization context. This approach has been refined to the greatest degree by Sanyal (2005), who proposed a seriesofdivisionsfocusedonthermalboundariesof significancetothegeothermaldeveloper.For example, the boundary at 100 C is tied to the boiling point of water, whereas that at 190 C is related to the abilityofgeothermalwellstoself-flow(asopposed to requiring pumping of reservoir fluids).Figure 2: Exampleclassificationsofgeothermal resources by temperature (Celsius). Lee(2001)recognizedtheapparentlyirreducible diversityofapproachestotemperature/enthalpy classification and proposed a system of classification in terms of specific exergy (or available work), which hasbeenusedincalculationsofelectricpower generatingpotentialingeothermalresource assessments (e.g., Brook et al., 1979; DiPippo, 2005; Williams and Reed, 2008) and is defined as [ WH h00(sWH s)] (1) E mWHh T 0 where sWH is the entropy of the produced fluid and s0 is the entropy at the reference temperature (the triple point of water in Lees analysis). The use of specific exergyhasanadvantageinrelatingdirectlytothe relevantpropertiesoftheproducedgeothermalfluid atthewellhead.Lee(2001)developedaspecific exergyindexastheratioofthespecificexergyofa givengeothermalsystemtothespecificexergyin saturated steam at a pressure of 9 MPa. The resulting low,mediumandhighqualityclassesprovidedby Lee (2001) are logical in the context of utilization as wellasfundamentalpropertiesofsteam,butthe communicationofthoseclassestothenon-specialist iscomplicatedbytherequirementtoreferto thermodynamicpropertiessuchasenthalpyand entropy,aswellastheneedtohaveaccesstoboth temperatureandpressureestimatesforactualor potential conditions at the wellhead. Consequently,althoughtherearesolidreasonsfor varieddivisionsintermsoftemperatureorother thermodynamicproperties,thereisnocompelling reasonforonesetofdivisionstobechosenover another.Thefundamentalrequirementofsimplicity isstillvalid,asistheneedtoavoidimbeddingany setofclasseswithassumptionsaboututilizationthat may be contradicted by changes in the technology of exploitation.Onepossiblesolutionmaybetoavoid classifyinggeothermalresourcesbytemperature altogether and simply state the range of temperatures orotherthermodynamicpropertiesusedinthe particularassessmentapplicationwhilerecognizing thatfuturetechnologicaldevelopmentsmayalter boundariesanddefinitions.Resolvingthisissue requiresfurtherworkanddiscussionwithinthe geothermal community.Geologic Setting and Fluid Chemistry Understandingandcharacterizingthegeologic controlsongeothermalresources,hasbeenan ongoingfocusofinvestigations,whetherinthe contextofplatetectonics(e.g.,Muffler,1976; Heiken,1982)orspecificprocesses,suchas volcanism (e.g., Henley and Ellis, 1983; Wohletz and Heiken,1992).Renewedinterestingeothermal energyexplorationanddevelopmenthasledto updatedeffortstocharacterizeresourcesina geologic/tectoniccontext(e.g.,Walkeretal.,2008; Erdlac et al., 2008; Brophy, 2008). A key element in thesestudiesistherecognitionthatthegeologic settingofageothermalsystemhasafundamental influenceonthepotentialtemperature,fluid composition,andreservoircharacteristics.Examples ofthisinfluenceareshowninFigures3aand3b. Figure3ashowsanexamplefault-hosteddeep circulation(oramagmatic)geothermalsystem,in whichthegeothermalfluidoriginatesasmeteoric waterthatgainsheatthroughcirculationtodepth within a fault zone (Reed, 1983). Figure 3b shows the scope of geothermalmanifestations within an island-arcvolcano(HenleyandEllis,1983)withvaried temperaturesandfluidchemistriesoccurringwithin differentpartsofthevolcanicedifice.Eachmodel incorporates the essential elements of the geothermal systemasdefinedaboveandprovidesacontextfor interpretingfieldobservations.Forexample,inthe volcaniccontext(Figure3b),chemicalanalysescan identify geothermal waters as dominantly acid (SO4), neutralchloride(Cl),orsodasprings(HCO3) (Giggenbach, 1988), and these water types, as well as otherchemicalcharacteristics,canberelatedto different processes in the geothermal system (Henley and Ellis, 1983).(a) (b) Figure 3:(a) Schematic representation of a normal fault-hostedgeothermalcirculation system.ModifiedfromReed(1983).(b) Schematicrepresentationofgeothermal systemsformedwithinavolcanicarc environment.ModifiedfromHenleyand Ellis (1983). Properly defined, geologic conceptual models such as thesehavesignificantvalueforgeothermal assessment,explorationanddevelopmentby providingquantitativeconstraintsoncharacteristics oftheresource.Evenrelativelysimple classifications,suchaswhethergeothermalsystems arerelatedtomagmaticactivityornot,highlight importantdifferencesinresourcepotential.Figure4 showsthedistributionofestimatedandmeasured reservoirtemperaturesandvolumesforidentified geothermal systems in the United States (Williams et al.,2008a,b),dividedintothoseassociatedwith activeorrecentmagmatismandthosederivingheat solelyfromdeepcirculationinregionsofelevated heatflow.Asexpectedgiventhehightemperature, shallowheatsourcesandthepotentiallylarge volumesofpermeable,youngextrusiverocks,the magmaticgeothermalsystemsare,onaverage, higherintemperatureandlargerinvolumethanthe deep circulation or amagmatic systems. (a) (b) Figure4:(a)Histogramshowingthetemperature distributionofidentifiedgeothermal systems in the United States differentiated aseithermagmatic(red)oramagmatic (blue).(b)Histogramofestimated reservoir volume for the same geothermal systems. (Williams and DeAngelo, 2008) ConductiveGeothermalResourcesandEnhanced Geothermal Systems As noted by Hochstein (1988), classification systems developedtoprovideaframeworkforassessing convectivegeothermalresourcesarenotnecessarily applicabletoconductiveresourcesinsedimentary formations.Mostresourceclassificationsdeveloped forsedimentarygeothermalresourcestodatehave beenfocusedonthemethodofexploitation,suchas dedicated pumping from sedimentary aquifers (Reed, 1983),coproductionwithoilandgas(McKennaet al.,2005),self-flowfromgeopressuredformations (Muffler,1979),andevengeothermalproduction coordinatedwithCO2sequestration(Pruess,2006). Althoughtheutilizationscenariosforthese sedimentaryresourcesarediverse,thereis consistency in the geologic and thermal environment, astheresourcesareassociatedwiththe predominantlyconductivesettingofsedimentary basins.Consequently,inongoingassessmentand classification work, we will consider these as a single broad class of geothermal resources. By contrast, EGS cover essentially the entire range of geothermalenvironmentsfromreservoircreationin lowpermeabilityandporositycrystallinerockat depththroughhighporosity,lowpermeability sedimentary formations to augmented production in a producingconvectivegeothermalreservoir.Inorder to provide a consistent framework for assessing EGS resourcesasdistinctfromnaturally-occurring geothermalresources,weproposethefollowing provisionaldefinition.EnhancedGeothermal Systemscomprisetheportionofageothermal resourceforwhichameasureableincreasein production over its natural state is or can be attained throughmechanical,thermal,and/orchemical stimulationofthereservoirrock.Inthisdefinition there are no restrictions on temperature, rock type, or pre-existinggeothermalexploitation.(Inorderto avoidconfusionwithabbreviations,Enhanced GeothermalSystemsandEngineeredGeothermal Systems are considered synonymous.) SUMMARY AND FUTURE PLANS Theabovediscussionlaysoutproposeddefinitions andapplicationsforcertainaspectsofgeothermal resourceclassification,suchasthedefinitionsfor geothermalsystemsandEnhancedGeothermal Systems. In addition, as part of ongoing work in this areawewillbuildonexistingschemestoproposea singlecomprehensivegeologic/tectonicframework forgeothermalresources.Wewillalsocontinueto evaluate revisions to classifying geothermal resources fromathermodynamicperspective,butthediverse approachestakentodateindicatethatthereare potentiallyirreducibledifferencesinclassifyingon thebasisoftemperature/enthalpy.Anticipatingthat different classification approaches will remain in use withintheinternationalgeothermalcommunity,we willproduceasetofspreadsheet-basedtoolsfor translatingamongvariousclassificationsystemsfor potentialusebythoseutilizinggeothermaldata repositories.Incorporatingpublicdomainsoftware for calculating thermodynamic properties, these tools will provide users with the ability to enter basic data fromfieldsites,suchastemperature,pressure,and fluidchemistryandthenobtaininformationon classificationofthesystemaccordingtoexisting schemes.Weexpectthenewclassification informationandtheassociatedtoolstobepublicly available by the end of 2011. ACKNOWLEDGEMENTS TheauthorsacknowledgethesupportoftheUSGS Energy Resources Program and the DOE Geothermal Technologies Program, as well as helpful discussions withSteveRichardoftheAZGeologicalSurveyon geothermal definitions. REFERENCES Axelsson,G.andGunnlaugsson,E.,2000, Background:geothermalutilization, managementandmonitoring,inLong-term monitoringofhigh-andlow-enthalpyfields underexploitation,WorldGeothermal Congress 2000 Short Course, Japan, p. 3-10. AustralianGeothermalCodeCommittee,2008,The Geothermal Reporting Code, Adelaide, 26p. Bendritter,Y.,andCormy,G.,1990,Possible approach to geothermal research and relative costs,inDickson,M.H.,andFanelli,M., eds., Small Geothermal Resources: A Guide toDevelopmentandUtilization,UNITAR, New York, p. 59-69. Brook,C.A.,Mariner,R.H.,Mabey,D.R.,Swanson, J.R., Guffanti, M., and Muffler, L.J.P., 1979, Hydrothermalconvectionsystemswith reservoirtemperatures90C,in,Muffler, L.J.P.,ed.,Assessmentofgeothermal resourcesoftheUnitedStates-1978:U.S. Geological Survey Circular 790, p. 18-85. Brophy,P.,2008,Geothermalexploration techniques,GeothermalResourcesCouncil WorkshoponGeothermalReservoir Evaluation, http://www.geothermal.org/Powerpoint08/B rophy.ppt. DiPippo,R.,2005,GeothermalPowerPlants: Principles,ApplicationsandCaseStudies, Elsevier, Oxford, 450p. Erdlac,R.J.,Gross,P.,McDonald,E.,2008,A proposednewgeothermalpower classificationsystem,Transactions, GeothermalResourcesCouncil,v.32,p. 379-384. Giggenbach,W.F.,1988,Geothermalsolute equilibria:derivationofNa-K-Mg-Ca geoindicators,Geochimicaand Cosmochimica Acta, v. 52, p. 2749-2765. Heiken, G., 1982, Geology of geothermal systems, in Edwards,L.M.,Chilingar,G.V.,Rieke,III H.H.,Fertl,W.H.,eds.,Handbookof GeothermalEnergy,GulfPublishing Company, Houston, p. 177-217. Henley,R.W.,andEllis,A.J.,1983,Geothermal systems, ancient and modern: a geochemical review,EarthScienceReviews,v.19,p.1-50. Hochstein,M.P.,1988,Assessmentandmodelingof geothermalreservoirs(smallutilization schemes), Geothermics, v. 17, n. 1, p. 15-49. Kruger,P.,andOtte,C.,1973,GeothermalEnergy: Resources,Production,Stimulation, StanfordUniversityPress,Stanford, California, 360p. Lee,K.C.,2001,Classificationofgeothermal resourcesbyexergy,Geothermics,v.30, p.431-442. Magoon,L.B.,andSchmoker,J.W.,2000,Thetotal petroleum system the natural fluid network thatconstrainstheassessmentunit,Chapter PS,U.S.GeologicalSurveyWorld PetroleumAssessment2000Description and Results, USGS Digital Data Series 60. McKenna,J.R.,Blackwell,D.D.,Moyes,C.P.,and Patterson,P.D.,2005,Geothermalelectric powersupplypossiblefromGulfCoast, Midcontinentoilfieldwaters,Oil&Gas Journal, v. 103, n. 33, p. 34-40. Muffler,L.P.J.,1976,Tectonicandhydrologic controlofthenatureanddistributionof geothermalresources,Proceedings,Second U.N.SymposiumontheDevelopmentand UseofGeothermalResources,v.1,p.499-507. Muffler,L.P.J.,1979,Assessmentofgeothermal resourcesoftheUnitedStates-1978:U.S. Geological Survey Circular 790, 163 p. Muffler,L.P.J.,andCataldi,R.,1978,Methodsfor regionalassessmentofgeothermal resources:Geothermics, v. 7, p. 53-89. Neuendorf, K.K.E., Mehl, J.P., Jr., and Jackson, J.A., eds.,2010,TheGlossaryofGeology,5th edition,AmericanGeologicalInstitute,800 p. Nicholson,K.,1993,GeothermalFluids,Springer Verlag, Berlin, XVIII, 264 p. Pruess, K.,2006,Enhancedgeothermalsystems (EGS) using CO2 as working fluid a novel approachforgeneratingrenewableenergy withsimultaneoussequestrationofcarbon, Geothermics, v. 35, n. 4, p. 351-367. Reed,M.J.,ed.,1983,Assessmentoflow-temperaturegeothermalresourcesofthe United States-1982: U.S. Geological Survey Circular 892, 73 p. Sanyal,S.K.,2005,Classificationofgeothermal systemsapossiblescheme,Proceedings, ThirtiethWorkshoponGeothermal ReservoirEngineering,StanfordUniversity, Stanford, California, p. 85-88. Walker,J.D.,Sabin,A.E.,Unruh,J.R.,Combs,J., and Monastero, F.C., 2005, Development of geneticoccurrencemodelsforgeothermal prospecting,Transactions,Geothermal Resources Council, v. 29, p.309-313. White, D.E.,1965,Geothermalenergy,U.S. Geological Survey Circular 519, 17p. White,D.E.,Muffler,L.J.P.,andTruesdell,A.H., 1971,Vapor-dominatedhydrothermalsystems comparedtohot-watersystems,Economic Geology, v. 66, p. 75-97. White,D.E.,andWilliams,D.L.,1975,Assessment ofgeothermalresourcesoftheUnitedStates 1975,U.S.GeologicalSurveyCircular726, 155p. Williams,C.F.,M.J.Reed,R.H.Mariner,J. DeAngelo,andS.P.Galanis,Jr.,2008a, Assessmentofmoderate-andhigh-temperature geothermalresourcesoftheUnitedStates,U.S. Geological Survey Fact Sheet 2008-3082, 4p. Williams, C.F., M.J. Reed, and R.H. Mariner, 2008b, AreviewofmethodsappliedbytheU.S. GeologicalSurveyintheassessmentof identified geothermal resources, U.S. Geological SurveyOpen-FileReport2008-1296,27p., [http://pubs.usgs.gov/of/2008/1296/] Williams,C.F.,andJ.DeAngelo,2008,Mapping geothermalpotentialinthewesternUnited States,Transactions,GeothermalResources Council, v. 32, p. 181-188. Wohletz,K.and Heiken,G.,1992,Volcanologyand GeothermalEnergy,Berkeley,CA,UCPress. [http://ark.cdlib.org/ark:/13030/ft6v19p151/]