Rock-water interactions in hot dry rock geothermal systems: field investigations of in situ geochemical behavior

Abstract

Two hot dry rock (HDR) geothermal energy reservoirs have been created by hydraulic fracturing of Precambrian granitic rock between two wells on the west flank of the Valles Caldera in the Jemez Mountains of northern New Mexico. Heat is extracted by injecting water into one well, flowing it through the fractured region and recovering the heated water from the second well. The produced fluid is cooled on the surface and reinjected into the system. The first reservoir was formed by fracturing the injection well at a depth of 2.75 km where the initial rock temperature was 185° C. A heat-extraction experiment conducted in this reservoir was run from January 27 to April 13, 1978. A second, larger reservoir was created after cementing the fracture-to-wellbore connections at 2.75 km in the injection well and refracturing 180 m deeper. This second reservoir was tested from October 23 to November 16, 1979. During each of these experiments, samples of the geothermal fluids and gases were collected at regular intervals from the injection wellhead, the production wellhead, and at the make-up pump which provided the water from storage ponds to replace the water lost downhole by permeation into the reservoir walls. Changes in the composition of the produced fluid provide a means for studying the reservoir behaviour under normal (recirculating) operating conditions. Certain of the dissolved species appear to be derived by displacement of an indigenous pore-fluid, while others appear to be derived by dissolving minerals known to be present in the reservoir rock. In this paper we describe the results of the chemical analysis of the geothermal fluids and relate the fluid and gas chemistry to geochemical processes that result from the heat- extraction experiments. In particular, the implications of the silica and NaKCa geothermometers and the pore-fluid displacement theory are examined for insight about the long-term effects of fluid geochemistry on heat extraction from HDR reservoirs.