Experimental Approach to Understanding Water-Rock Interaction in Enhanced Geothermal Systems

Abstract

Batch- and flow-reactor autoclave systems were used to investigate granite and quartz dissolution processes, with pure water, under sub- and supercritical conditions (up to 600?C, 60MPa), to evaluate geothermal reservoir dynamics in a potential, deep-seated, enhanced geothermal system. Development of engineered reservoirs for Hot Dry Rock (HDR) geothermal energy extraction depend on the maintenance of (fracture) surfaces areas for heat exchange, yet precipitation of quartz (as a result of retrograde solubility at >380?C), albite and other minerals occur, which effectively ëplugí the system. The supercritical region, above the critical point for water (- 374îC, 22 MPa), is not a homogeneous state, but comprises ëliquid-likeí and ëvapour-likeí regions, based on the dissolution behaviour of granite and quartz. Solvent properties in the liquid-like, ëhigh-pressureí supercritical region are similar to subcritical water. In contrast, dissolution reactions in the vapour-like, ëhigh-temperatureflow pressureí supercritical region are weak, and provide optimum conditions for energy utilization of an artificial HDR reservoir.