Geothermal Reservoir Energy Recovery : A Three-Dimensional Simulation Study of the East Mesa Field

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Summary This paper describes the reservoir simulation model developed for East Mesa field properties and summarizes the reservoir engineering evaluation of the reserves and field performance. Geothermal reservoirs should be evaluated in terms of efficient "energy mining" rather than fluid recovery. The results indicate the behavior of the field under various scenarios of development designed to operate a 64-MW electric power plant. Introduction The East Mesa geothermal field is in an area of anomalously high heat flow on the east flank of the Salton Trough at the southeast corner of the Imperial Valley in California. Twenty-four wells have been drilled thus far, including 10 by Republic Geothermal Inc. in the northern portion, five by the U.S. Bureau of Reclamation in the central area, and eight by Magma Power Co. to the south (Fig. 1). Because of the already extensive investigations, a great deal is known about the East Mesa reservoir and its properties. Republic now is proceeding with commercial development at the northern end of the field, starting with a 64-MW power plant project.Geothermal resource development differs most significantly from petroleum resource exploitation in that an additional factor is involvedie., the recovery of thermal energy. The problems of fluid flow in a porous media are, of course, similar to the petroleum industry problems, but the ultimate recovery is measured in terms of energy mining efficiency. The produced fluid acts only as a transport mechanism and may be cycled through the reservoir to extract heat from the rock as many times as is economically feasible.This paper presents the reservoir model developed for Republic's East Mesa properties and summarizes the reservoir simulation study performed to optimize the exploitation of this resource. Reservoir Properties Petrophysical Properties Logs, ditch cuttings, and cores show that the upper 10,000 ft (3048 m) of stratigraphic section at East Mesa is a sequence of deltaic clastic sediments that includes abundant amounts of fine- to medium-grained sandstones, fine- to coarse-grained siltstones, and lesser amounts of mudstones. Mineralogically, the sands are compositionally equivalent to Colorado River delta sands, being mostly quartz (70%), lithic fragments (20%), and feldspars (10%). Post-depositional alterations are principally the addition of authigenic carbonate and quartz as overgrowths and veinlets and the conversion of the detrital phyllosilicates to illite and chlorite.The basic petrophysical properties of sand/shale discrimination, porosity (phi), and permeability (k) employed in the model were obtained from log analyses for eight existing wells in the Republic area (i.e., Producers 16–29, 16–30, 38–30, 58–30, and 78–30/78-30RD and Injectors 18–28 and 52–29). Table 1 summarizes net sand thickness (h), average sand porosity (phi) weighted for the presence of shales, and geometric mean sand permeability (k) for four depth-interval groupings. The data show good sand development in the wells between 2,500 and 7,000 ft (762 and 2134 m). Although the properties vary considerably between wells, they generally improve away from the center of the temperature anomaly, show a gradual decline with increasing depth, and exhibit rapid deterioration below 7,000 ft (2134 m). JPT P. 735^

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