Fluid-Rock Interactions in Enhanced Geothermal Systems with CO2 as Working Fluid: Modeling of Geochemical Changes Induced by CO2 Injection into the Ogachi (Japan) EGS Site

Abstract

There is a growing interest in the novel concept of operating enhanced geothermal systems with CO2 instead of water as heat transmission fluid (CO2-EGS). Initial studies have suggested that CO2 may achieve larger rates of heat extraction than water-based systems, and can offer geologic storage of carbon as an ancillary benefit. A fully developed CO2-EGS would consist of three distinct zones, (1) a central zone or “core” in which all aqueous phase has been removed by dissolution into the flowing CO2 stream, so that the reservoir fluid is a single supercritical CO2 phase; (2) a surrounding intermediate zone, in which the reservoir fluid consists of a two-phase water-CO2 mixture; and (3) an outer or peripheral zone, in which the reservoir fluid is a single aqueous phase with dissolved CO2. Fluid-rock interactions are expected to be vastly different in the zones with an aqueous phase present, as compared to the central zone with anhydrous supercritical CO2. The peripheral zone may experience a combination of mineral dissolution and precipitation effects that could impact reservoir growth and longevity. Field experiments have been conducted at the Ogachi Hot Dry Rock geothermal site, Japan, in which water with dissolved CO2 was injected into a high temperature borehole (OGC-2, 210oC) to study CO2 sequestration in solid minerals. During the experiments, aqueous chemical evolution and mineral changes were observed. The Ogachi CO2 experiments provide an analogue to study geochemical changes in the peripheral zone of a CO2-EGS system. This paper develops a reactive transport model for aqueous chemical evolution and mineral alteration induced by mixtures of CO2 and water, and investigates the development and operation of a CO2-based EGS for actual reservoir conditions.