Characteristics of a Hot Dry Rock (HDR) geothermal reservoir were deduced by matching a geochemistry model which incorporates rock dissolution and displacement of an indigenous pore-fluid to chemical analyses of recirculating geothermal fluid . Two reservoir s were created between a pair of wells by hydraulic fracturing. The geometry of these reservoir systems, as well as operational conditions during flow experiments and general heat extraction and waterless performance are described in a preceeding paper by Murphy et a l . ( l ) . The first of the reservoirs was evaluated by a 75-day flow test designated as Segment 2. Subsequent operations required the cementing of the injection well to stop a leak behind the casing, and this cementing operation closed the connection to the first reservoir . The second, larger reservoir was evaluated with two flow tests -- the first had a duration of 24 days and is called Segment 4 and the second, called Segment 5, will end on December 19,1980 after 284 days of operation. Descriptions of the fluid geochemistry in the first reservoir have already been presented by Grigsby and Tester(21,and a preliminary analysis of the Segment 4 test has also been published(3). discussion will be mainly concerned with the geochemistry of the second reservoir . Of particular interest in a HDR geothermal reservoir is the relationship between the mixed-mean temperature of the geothermal fluid as measured with a downhole thermistor and the temperatures given by the silica and the Na-K-Ca geochemical thermometers. As will be shown, the temperatures predicted by applying the chemical geothermometers directly to the produced fluid composition at a given time do not represent the true current reservoir temperature.

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