Geothermal Energy: The Energy-Water Nexus

are resources that are likely to exist based upon heatflow maps and surface manifestations but have notbeen verified. Near-field EGS resources areassociated with identified hydrothermal resources butmay require additional stimulation to be exploited.Deep EGS resources are hot rock formations found atdepths greater than 4 km and require stimulation tocreate fractures for fluid circulation for powergeneration.The resolution of location information availablewithin the NREL supply curve data set for thegeothermal resources varied depending upon theresource type. For identified hydrothermal and nearfield EGS resources, specific latitude and longitudelocations are given. Unidentified hydrothermalresources are specified at the state level. Deep EGSresources are specified by temperature and depthalong the region code for both the National EnergyModeling System (NEMS) model. These regioncodes cover many states. In order to perform analysisbased upon USGS HUC 4 basins, the unidentifiedhydrothermal and deep EGS resources wereinterpolated to increase the spatial resolution usingtemperature at depth maps developed by IdahoNational Laboratory and Southern MethodistUniversity (INL 2011).Both the unidentified hydrothermal and deep EGSresources were defined in the supply curve by atemperature and depth range for a given state orNEMS region code. The total area within thespecified state or NEMS region was calculated wherethe temperature was within the specified range fromthe temperature data for the specified depth for eachresource defined within the supply curve. Theseareas were then apportioned to the overlying HUC 4basins. The generation capacity for the resource wasthen allocated to these HUC 4 basins in directproportion to the calculated resource areas.Temperature data was available for depths of 3km,4km, 5km, 6km, and 10km. For depths between 6kmand 10km, and below 3km, temperatures wereinterpolated or extrapolated based upon trendscalculated from the existing data using a geo-spatialtool called a Raster Calculator.Water consumption factors based upon the LCAresults presented later in this paper were then appliedto the resources within the supply curve dependingupon system type (EGS, hydrothermal flash,hydrothermal binary). The resources selected fromthe supply curve for each scenario were selectedbased upon the estimated LCOE by selecting thelowest cost resources first until the total newgeothermal capacity defined by the scenario wasachieved. The scenarios were mapped utilizing GISsoftware to illustrate the spatial distribution of waterdemand from the various growth scenarios.A total of four growth scenarios are presented basedupon results from the Energy InformationAdministration’s NEMS integrated energy model(EIA 2011). Two scenarios are based upon a versionthat was modified to include the existing NRELgeothermal supply curve. This version of the NEMSmodel is referred to as NEMS-GPRA, forGovernment Performance and Results Act. Themodeling was performed in 2010 by OnLocation,Inc., for the DOE Geothermal Technologies Programfor its annual internal program analysis. The results,presented at the fiscal year 2010 4th-quarter meetingof the Geothermal Strategic Planning and AnalysisWorking Group (Wood and Dublin 2010), showedgrowth in geothermal electricity production of 7.9GWe by 2030 for the base supply curve and 11.5GWe for the target supply curve. A third scenario isbased upon these same modeling results for the targetsupply curve but uses a lower water consumptionfactor for EGS systems. The basis for this lowerconsumption factor is the assumption that belowground operational losses for EGS systems are madeup utilizing non-fresh water sources, limiting theimpact on fresh water resources. The forth scenario isbased upon NEMS model results presented in theEIA’s 2012 Annual Energy Outlook that showgrowth in geothermal electricity production of 3.9GW by 2035 (EIA 2012).In addition to estimating water demand, an attemptwas made to quantify the availability of water at thesame HUC 4 resolution. These estimates were basedupon data provided by Sandia National Laboratory(Tidwell 2012). The data set is currently limited tothirteen Western states, but these states overlap withthe majority of the geothermal resource in thecontinental US with the exception of some deep EGSresources.Sandia also provided estimates of water availabilitydivided into five different categories: unappropriatedsurface water, appropriated surface water, potableground water, shallow brackish ground water, andmunicipal waste water. Unappropriated surface wateravailability was determined by comparing streamflow to downstream delivery requirements whenspecific estimates were not provided directly by thestates. Appropriated surface water availability wasestimated based upon the quantity of water consumedby low value agriculture (hay and alfalfa). Apercentage of this water was assumed to be availablefor sale for higher value uses. Potable groundwateravailability was calculated based upon the safe yieldwhere pumping must be less than or equal to recharge

rates based upon USGS data. Shallow brackishgroundwater availability was estimated byaggregating data from multiple state and USGS datasets. Municipal waste water availability wasestimated based upon discharge data from the USGSand EPA (Tidwell 2012). From these data twoaggregate metrics were developed and mapped. A―total fresh water availability‖ metric was defined bycombining the unappropriated surface water,appropriated surface water, and potable groundwatervolumes. A ―total water availability‖ metric was alsodefined by combining all five categories of water.While no formal numerical analysis was performedcomparing water demand for geothermal from thevarious scenarios with the included water availabilitymetrics for this paper, a qualitative analysis isincluded which identifies some of the basins wherethe limited availability of water is most likely toimpact the development of geothermal resources.More detailed quantitative comparison of waterdemand and availability along with a focus on thecost of different water resources will be a focus ofongoing research.GEOTHERMAL WATER CONSUMPTIONDrilling and ConstructionWater is consumed in the drilling of geothermalproduction and injection wells both in drilling mudsand in the cement used to construct the wells. Wateris also consumed in the concrete often used insupport structures for pipelines that transport thegeofluid from production wells and to injection wellsand in the construction of the power plant itself.The two approaches to estimating water volumes fordrilling and constructing wells are the following: (1)estimates provided in the literature (BLM 1998;1999; 2003; 2005; 2006a,b; 2007a,b; 2009; 2010a–e;2011a–i; 2012) and (2) estimates based upon welldesigns as discussed in Clark et al. (2011). Estimatesin the literature report consumption that is twice thatof the well design estimates with at an average waterconsumption of 180,000 gallons per 1000 feet of welldepth. The literature reported maximum projectionsof daily water volumes during the drilling period (e.g.Dixie Meadows EA, Patua EA, Soda Lake EA) andtherefore are likely to be conservative estimates. Forthis reason the estimates according to well designwere incorporated into the life cycle water analysis.Although data were collected for observation andexploration wells, the life cycle water consumptionestimates were based upon total production andinjection wells as the water burden of any explorationwells that do not become production or injectionwells would likely be shared among plants developedwithin a geothermal area.Water consumption for the development of thepipeline and the power plant were determined to benegligible per lifetime energy output in the previousanalysis (Clark et al. 2011). As a result, no additionalanalysis was undertaken for this work, and theestimates from the previous report were maintainedfor the overall water consumption over the lifecycle.Stimulation and Circulation TestingAfter a well is drilled for an EGS project, it istypically stimulated. Stimulation may occur on aproduction or injection well. Stimulating a productionwell can enhance the output of the well by (1)improving near-well permeability that has beenreduced by the drilling operation clogging pathwaysor (2) opening up paths to permeable zones notintersected by the well. For injection wells,stimulation similarly enhances the injectivity of thewells. Three general types of well stimulation areused in EGS development: thermal, hydraulic, andchemical stimulation. Thermal stimulation relies onthe introduction of chilled water, and thus cold stress,to a geothermal reservoir. Hydraulic stimulationrelies on the introduction of water or a combinationof water and gel-proppant fluids to a geothermalreservoir. Chemical well stimulation techniquesinvolve the use of aqueous solutions to allow acids,bases, and chelating agents to be introduced intogeothermal reservoirs. Water is the primary additivefor all well stimulation activities.The amount of water required for well stimulationactivities is dependent upon the well-reservoirenvironment and the well stimulation method(s) used.For EGS, stimulation can consume a significantquantity of water over a short period of time. Theliterature review found a range of 1,500,000 to7,700,000 gallons required per well for stimulation(Zimmermann and Reinicke 2010; Asanuma et al.2005; Tester et al. 2006; Häring et al. 2008a; Chenand Wyborn 2009; Evans et al. 2012; Portier et al.2009; Xu et al. 2012; Michelet and Toksöz 2006;Cordon and Driscoll 2008; Schinler et al. 2010;Kitano et al. 2000; Shapiro et al. 2006a; Zoback andHarjes 1997). The average of 5,100,000 gallons wasconsistent with the 5,300,000 gal used in the previousanalysis (Clark et al. 2011). For the scenariosexamined, when amortized over the lifetime of apower plant, stimulation was found to consume asimilar volume of water as drilling and cementingwells. This is due to the assumption that onlyinjection wells would be stimulated, and that the ratioof production to injection wells is 2 to 1. For projects

where these conditions are not met, consumptionvolumes may not be as comparable.Surrounding the stimulation stage for EGS projectsare a series of flow tests that require water.Accounting for pre-stimulation, post-stimulation,short-term circulation, and long-term circulationtests, the water consumed for circulation testing issimilar to the volumes required for drilling andcementing and stimulating per lifetime energy outputfor the EGS scenarios (Tester et al. 2006). There is agreat deal of uncertainty on the water volumerequired for long-term, commercial scale circulationtesting. Projects to date have been small-scale, proofof concepts. As a result, circulation testingconsumption estimates may change in the future ascommercial scale projects are developed.Above Ground OperationsThe largest variable affecting above groundoperational water consumption is the type of coolingsystem used. The current analysis focuses primarilyon dry cooled binary systems and wet cooled flashsystems that utilize condensed geofluid for coolingwater. However, wet and hybrid cooled systems arealso often used and are therefore discussed in thissection. Cooling system selection is an importantdesign criterion and affects not only lifetime waterconsumption, but also the power generationefficiency of the power plant. While dry coolingsystems drastically reduce the water consumption forgeothermal power plants, they also come at the costof lost efficiency, especially on hot summer dayswhen power is often the most valuable.Figure 1: GeothermalOperationalConsumption Data.2008; NDWR 2012). The data are presented bycooling system type and identified by the type ofpower plant. In some cases data were provided asaggregate numbers from facilities that operate bothflash and binary systems or were projections forproposed power plants where the determination tobuild a flash or binary system had not been finalized.Wet cooled flash plants ranged from 0.7 to 3.8gal/kWh with an average of 2.4 gal/kWh. Waterconsumption from wet cooled binary plants wasslightly higher, ranging from 1.5 to 4.6 gal/kWh withan average of 3.4 gal/kWh. This difference is likelyattributed to two factors: flash plants operate withhigher temperature geofluid which makes them morethermodynamically efficient and many of the datapoints for flash systems were based upon injectionaugmentation programs which may not account

with metrics for water availability to identify potential water challenges that projects may face in areas where water scarcity is already a concern. METHODOLOGY The study was broken into two primary parts. The first part included a water LCA that estimated with water life cycle water consumption for three different power plant scenarios.