The Solubility and Kinetics of Minerals under CO2-EGS Geothermal Conditions: Comparison of Experimental and Modeling Results

Abstract

Although the thermal and hydraulic aspects of a CO2-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. A fully developed CO2-based EGS would consist of three distinct zones: (1) a central zone or “core” (Zone 1) in which all aqueous phase has been removed by production or by dissolution into the flowing CO2 stream, so that the reservoir fluid is a single supercritical CO2 phase; (2) a surrounding intermediate zone (Zone 2), in which the reservoir fluid consists of a two-phase water-CO2 mixture; and (3) an outer or peripheral zone (Zone 3), in which the reservoir fluid is a single aqueous phase with dissolved CO2. Chemical reaction processes are expected to be quite different in the three zones. Solubility and reaction kinetics of minerals are very important for understanding and predicting mineral alteration occurring in a CO2-H2O mixture (Zone 2) and under aqueous phase (Zone 3) conditions. Laboratory experiments were performed under both conditions for some major individual minerals that are likely encountered in a CO2-EGS system. In order to gain insight into solubility and kinetics, here we present modeling analyses based on the evolution of observed aqueous species concentrations. The combined experimental and modeling study as presented in this paper could enable us to predict the coupled processes of fluid flow, heat transfer, and geochemical reactions, and then to evaluate the performance, risks and benefits of a CO2-EGS operation.