W.J. Jones M. Ruane Appendix C GEOTHERMAL ENERGY

The first problemin estimating resources of geothermal energy is the difficulty in assessingThe second problem is estimating thethe presence of deep-seatedintrusions of molten rock.potential removal of energy(form and rate) from the resource.been limited to hydrothermalBased on geologicalhasOur major commercial experienceresources, which make up a small fraction of the potential resource.surveys and extrapolationof our limited geothermal experience,several groups(Muffler and White, 1972),have estimated the extent of geothermal resources in the United States.(White, 1973), (Rex and Howell, 1973). The most recent data is shown in Table 2.1.Table 2.1Geothermal Resourcesin Quads1 Quad 1015 BtuERDA-86*USGS CIRCULAR erred-----Hydrothermal10010022104200High Temperature( 1500 C)150049002011014706350Low Temperature(90C 150 0 02304330017400Hot Dry 03970001469104411001472416231.8x10632.3x106Vapor dominatedLiquid dominatedTOTALS*(ERDA,31.7x10 631.7x10 61420-2180317000-3206001975, p. I-5)**(White and Williams,1975) as shown in (Milora and Tester,1976)***Electricity production, present or near-term technology, without regard to cost.consider that in 1976 the entire energy consumptionTo put the data in Table 2.1 in perspective,of the United States was about 98 Quads (1 Quad 1015 Btu).having a temperatureno application(Table 2.2).0rise above ambient of less tnan 15 C.for electricityproduction,There are considerabledespite the uncertainties,national energy economy.energy be extracted?These resources probably would havebut might provide process heating or district heatinguncertaintiesin estimates of this type; it seems clear that,geothermal resources are large enough to have an impact on ourThe critical questionsThese questionstraction, and conversion,Table 2.1 also does not include resourcesare how soon and by what methods can geothermalinvolve the technology of geothermal energy prospecting,which are addressedin the following sections.C-4ex-

Table 2.2Principal Utilization(other than for Electricity)of Geothermal ResourcesLOCATIONUSEIcelandSpace HeatingHungaryU.S.S.R.New ZealandKlamath Falls, OregonBoise,IdahoAir ConditioningNew panItalyLakeview, OregonPaper ProcessingPaperNew ZealandDiatomiteIcelandSaltJapanByproductsDry IceImperial Valley, CaliforniaBoronItalyCalcium ChlorideImperial Valley, California2.2Geothermal ProspectingOur scientific understandingwe do havecomesfromof the earth's geothermal processes is incomplete.surface explorationtion for oil, gas, and mineral resources,techniques and the technology developedTwo types of explorationThe knowledgefrom explora-are being used: intrusive plorationExplorationinvolves techniques which do not rely upon drillingtential geothermal resource to identify its characteristics.ration is its ability to considerinto the po-The advantage of non-intrusiveexplo-large areas in a short time, with relatively small investmentsin manpower or equipment.This advantageresults from non-intrusivemethods.is offset by the high uncertaintyand lack of detailinSeeking out geysers, fumeroles, hot springs, or lava flowsis the most direct non-intrusive method,Unfortunately,these surface manifestationsneed notalways exist, or may be deflected laterally many kilometers from their source by impermeable cappingrock structures.For example, the closest water discharges are 7 km away from the center of heatC-5

upflow in the Russian Ahvachapan Thermal SystemAerial infrared photographyand interpretationface temperature(Armstead,1973, p. 36).has been used to locate surface manifestations.Data collectioncan be difficult, and many large geothermal resources do not exhibit abnormal sur-gradients.The Geysers in California and the Larderello Field in Italy, which arethe largest producing systems in the world, exhibit"meager surface manifestations"(UNESCO, 1973,p. 36).GeochemicalPhysiochemicalcumstantialmethods have been increasingly appliedparameters such as the concentrationfeasibilityareas whereprospectinggeothermal(the movementinstrusions of molten rock may have occurred.of intrusions.a region.are cir-Usually these methods1973) (Stoker, 1975).Signs of tectonic activitymasses on the sea of magma), such as faults, increaseGravity and magnetic surveys can delineate major geologicaltures while ground noise measurementsreservoirs.new sources (Sigvaldason,resources.relies on the study of the earth's structural behavior to indicateof the floating contenentalthe possibilityof arsenic and mercury in groundwaterevidence in the planning of geothermal development.do not offer much help in locating completelyGeophysicalto evaluate potential hydrothermaland observationof microearthquakesstruc-can identify geothermalStudy of the basic rock formations also gives information about the recent activity ofSince rocks conduct heat very slowly, intrusions from hundreds of thousands of yearsago (Quaternary age) may still contain sufficient heat to warrant exploitation.2.2.2Intrusive ExplorationUsually non-intrusivepotential.explorationprovides a first-passBefore any planning for geothermalassessment of a region's geothermalfacilities begins,intrusive methods are applied inan attempt to quantify the extent of the possible resource and the characteristicsof its energysupply.Intrusive methodsempiricallymeasuringtime and money.for hydrothermalthe quantityDeep drillinghave not yet diffused upwardsresources usually involve drilling several test wells andand quality of the resource.Such testing takes considerable( 5 km) is often required since younger intrusions of molten rocksignificant amounts of heat.If successful test wells are found(one out of four typically [Milora and Tester, 1976, p. 79]), analysis of the heat content, pressure, chemical composition,and flow rates of the vapor or liquid must be performed to estimatethe reservoir size.Measurementof electric conductivitythermal sourLes.andsalinitymeasurementshas had great successof interstitialfor hydro-fluids, hydrothermalreservoirs have high conductivity.porosityDirect currentare usually made (Muffler, 1973, p. 258).Intrusive explorationfor hot dry rocks is not as well developed as with hydrothermalTest wells can be drilled to determineapplied to extract the heat.of the fracturingexperimentation,temperatureinvolve fracturing of the hot rock.depends on the extent and porosity of the fractured region.for determiningthat there will be noticeable heat above a geopressuredthermal changesin the surroundingThe successNo methods, exceptfracturing potential in a region.resources can be explored much like hydrothermal.to the viability of geopressuredsystems.gradients, but there must be artificial methodsUsually these methodsexist at presentGeopressuredcontributingin geothermal explorationBecause the earth's electrical resistance varies directly with temperature,resourceHowever, it is less likelybecause two of the major factorsresources, pressure and methane gas, generate norock.C-6

Geothermal Energy Extraction2.3Magma2.3.1removal of energy directly fromAt present there is no available technology for the controlledmoltenrock, which exists at temperaturesin excess of 650C.Only formativeresearchis under way(Milora and Tester, 1976, p. 7).2.3.2HydrothermalDepending on the temperaturesescape as steam (a vapor-dominatedprovided(Figure 2.4).and pressures in a hydrothermalsystem), or as hot water (100 C) when a path to the surface isAs in analogous gas or oil recovery operations,Thermala well drilledinto theCritical parameters of the resulting flow include the flowaquifer will provide the needed path.rate (mass/sec),aquifer, the heated water will0the thermal fluid temperature(C),and the thermal gradient(C/km).Intru-gradient values are, in general, a function of the type of resource formation.sive explorationcan identify regions of greatest thermal gradient within a given resource.fluid temperatureincreases with drilling depth since deeper reservoirsperatures.approximatethe drilling depth needed for a desired fluid temperature.since locally the gradient need not be linear with depth.capping formation.This is only an approximationComplex three-dimensionalare typical, and result from the unknown distributionssion of heated rock, the intermediaryare subject to greater tem-of 15 0 C and knowledge of the thermal gradient, one canWith a normal surface temperatureof temperatureThermaldistributionsin space of the originalintru-conductive rock, the porous rock forming the aquifer and theThe fluid flow rate depends on aquifer porosity, which limits the replacementof fluid around the well casing, aquiferpressures and the availableflow of water into the drilledregion of the aquifer.The designer can compensate for these natural parameters through choice of drilling parameters,numbers of wells, diameter of bore and depth,oil and gas drilling experienceConsiderableable and depths of over 16 km (Berman, 1975, p. 120) have been achieved.is avail-Bore. size isUp-related to well depth since the upper casing must allow the passage of later drilling sections.per casingsizes range from aboutl2 to 18 inches in inside diameter.e.g.,Spacing of wells must be farenough to avoid the extraction of heat energy at a faster rate than it can be replaced with heatfrom the earth's core.On the other hand, wells should be as close as possible to maximizetion and minimize piping costs.generalizedSpacing on the order of 200-300 manalysis, but must be designed on a case-by-casebasis.on the order of 5-10% between wellhead and point of utilizationFor most hydrothermalextrac-in aEnergy losses are typically(Armstead, 1973, p. 165).systems, it will be necessary to drill additional wells for reinjectionof the fluids removed from the aquifer.This helps to replenish the aquifer and to dispose of the highmineral content fluids after the heat has been extracted.disposal problem.has been suggestedReinjectionThis brinish water presents a surfacewells are generally simpler than extraction wells which require a per-manent casing to prevent turbulent erosion of the well shaft.can serve as reinjection wells.Since some fluid condenses,cooling purposes, the total flow down the reinjectioninjection wells are fewer than extraction wells.C-7Oftenunsuccessfulexploratorywellsis lost, or is used at the surface forwells is less than the extractionrate, so re-

Figure 2.4SchematicDiagram of a HydrothermalC-8"-F1·--l--l--·LL--.Reservoir

At the surface the flows from the various wells must be collected and fed to the conversionquipmentfor electricityproduction.presents no technical obstacles.This requires establishedpipe constructionis available.GeopressuredThe technology for extraction of geopressuredfor hydrothermalmethods andIn general, it can be said that the technology for commercialvapor dominated or liquid dominated geothermal energy extraction2.3.3e-reservoirs.which enters the well.Additional equipmentReinjectiongeothermal energyis essentiallythe same asis required to separate and handle the methaneof brine or condensed steam presents a slightly differentproblem since the original reservoir is under high pressure.Usuallyinto other formations in the ground as a disposal technique.Gases from geopressuredthe fluids are reinjectedsources arehighly corrosive to the well casings and surface piping.2.3.4Hot Dry RocksHot dry rocks must be converted into a form of hydrothermalgeothermal energy.variety of techniques such as explosive fracturing,have been proposed in the past.fracturing,resource in order to extract theirThis first involves making the rocks porous through fracturing methods.The only prospect being actively(Figure 2.5) (Blair, et al., 1975).system to hydraulicallysmall nuclear devices, or chemicaltemperature(Berman, 1975).A hole is drilled untilA second hole is drilled 10-20 mpressure water is pumped into the well to cause the rock to fracture.of connecting cracks between the two wells whichWhen operating,has designed alevels is reached and the well casing is inserted.will extract the hot water produced later.developmentinvolves hydraulicLos Alamos Scientific Laboratoryfracture hot dry rock as followsa depth with satisfactoryinvestigatedAleachingThis wellaway and sufficientA criticalhigh-feature is theserve as pathways for water.water pumped down the second well will flow to the first through the cracks and be-come heated along the way.Theoretically,thermal stresses caused by the flow of relativelywater over the hot rocks will cause further fracturing and a continual enlargementcoldof the fracturedzone (Kruger, 1975, p. 4).To date, the use of hydraulic fracturing has been demonstrated200 C but continuous energy extraction is not yet operational.a pumping pressure of 1750 psi was used (Smith, 1977, p. 2).in granite up to 9600 ft. andAt the Los AlamosThe first well was also fracturedwhen no connecting cracks developed from the fracture of the injection well.tures are ellipticalin a vertical plane and do not intersect.developed between the ellipses allowing the system to operateC-9Fenton Hill siteThe resulting frac-However, connecting cracks(Smith, 1977, p. 4).have

Figure 2.5FrArtlirEd Hnt Dry Rock GothcrmnalystemiAIRHEAlHOTccCLUIlcn' )O0I 0o00ozI11 dII11rt(.FRACTURRADIUSROCK TEMPERATUREC-10200"CD

problem can plague hot dry rock thermal recovery besides the uncertaintyAnotherpatterns.of fractureIf the fracture opens to an existing fracture or fault, the wells may lose most of theinjected water into the earth instead of collectingit at the wellhead.At Los Alamosthis has beenonly a minor problem as 90-98% of the injected water is ultimately recovered.plants for hot dry rock thermal systems may be operatingSmall (10 MWe) demonstrationunder the best conditions two to four 80 MWe plants may begin commercial operationby 1980;by 1985 (Smith,1975, p. 6).2.4Reservoir Lifetimes and Production RatesBecause the detailed structure of natural andartificialdifficult to predict their lifetime and production rates.maintainedgeothermalresources is not known, it isNatural vapor dominated reservoirsoutput above 50% of initial production rates from less than eight yearsItaly).to more than 30 years (Lardarello,systems does not exist (Milora and Tester,Hot dry rock systems can be controlledan indefinitesource of energy.formance of a hydraulicallyoutlet temperatureField experience with geopressuredthroughfractured reservoir,and hot dry rockflow rate and thus may supplyan equilibriumresults for the projectedflow rates.Drawdown Curves for a 1500 m Radius Crack withNo Thermal Stress Cracking(taken from McFarland,Time (yr)C-llper-indicating the variation of both thermal power andFigure 2.6Power and Temperature(the Geysers),1976, p. 82).Figure 2.6 shows computer-simulatedas functions ofhave1975).

Numerous problems can arise to limit well life.The most obvious problem is exhaustionof theaquifer supply of steam or hot water, or at least recession of the aquifer below the well opening.Corrosion of the well casing by corrosive fluids cancauseshutdown.If reservoir outputfallsoff at a rate such as the Geysers have experienced(drop below 50% in eight years),will be required to maintain output.6% new capacity will be needed each year as oldoutput drops off, resultingApproximatelyin nearly continuousnew drillingdrilling.2.5 Conversion Technologies2.5.1Cycle EfficiencyThe maximumefficiency of any engine converting heat to electricityof thermodynamicswhich describes an ideal Carnotnwork out 1work in 1heat engine:T12T1 final temperature(2.1) KT2 initial temperatureFor geothermalis limited by the second law Kgeneration at the Geysers, where T1 260 C (3000 K) and T 2 180 C (4530 K), n 33,8%.This is a maximumtheoreticalefficiency and can be comparedto the theoreticalefficiency of con-ventional coal-fired power plants where n 61 %.Several observationsshould be made.First, we can increase our theoreticaleither lowering the engine's final temperatureinput temperaturedeeper.sink.or by raising the input temperature.involves obtaining a higher temperatureLowering the final temperatureefficiencybyTo raise thesource of steam or water, i.e., drillinginvolves a cooling system condenser with an associatedThis could be a body of water or cooling towers (used at the Geysers).heatThe cooling systemwill have to dispose of four to six units of energy for every one converted to electricity.Second, for a fixed final temperaturehenceforth called the reinjectionthe condensedfluids will be reinjected),the theoretical maximumof geothermalfluid is a function only of the fluid's initial temperatureof these values.Finally, the simplicity of theseCandidatedirectelectrical generating equipment.CyclesFour basic thermodynamic2.8):(Figure 2.7) (Milora andsteps which prevent thecurves conceals a multitude of prac-tical problems in utilizing the geothermal fluids in conventional2.5.2assuming thatuseful work obtainable from a massTester, 1976, p. 17). Any real process will have losses and nonreversibleattainmenttemperature,cycles exist for converting geothermalenergy to electricitysteam flashing cycle, single cycle, dual cycle, and topping/bottomingcycle.(FigureAscan be seem from Figure 2.8, common elements of each cycle are the turbine, pumps, and condenser.The turbine convertsthe energy of steam (or other gases) into rotational energy for driving theshaft of the electricalThe condensergenerator(not shown).Pumps circulate liquids and gases through the system.is a heat exchanger which effectivelyreduces T1, the reinjectionreduces the back pressure at the turbine exit by condensingC-12temperature, andthe exhaust steam from the turbine.

Figure 2.7Maximum useful work or availability (AS) plotted as a function ofgeothermal fluid temperature for saturated steam and saturated water sources

APPENDIX C GEOTHERMAL ENERGY CONVERSION Page 1.0 INTRODUCTION C-1 2.0 GEOTHERMAL ENERGY SYSTEMS C-1 2.1 Geothermal Resource