Enhanced Geothermal Systems (EGS) with CO2 as Heat Transmission Fluid - A Scheme for Combining Recovery of Renewable Energy with Geologic Storage of CO2

Abstract

It has been suggested that enhanced geothermal systems (EGS) may be operated with supercritical CO2 instead of water as heat transmission fluid (D.W. Brown, 2000). Such a scheme could combine recovery of geothermal energy with simultaneous geologic storage of CO2, a greenhouse gas. At geothermal temperature and pressure conditions of interest, the flow and heat transfer behavior of CO2 would be considerably different from water, and chemical interactions between CO2 and reservoir rocks would also be quite different from aqueous fluids.

This paper summarizes our research to date into fluid flow and heat transfer aspects of operating EGS with CO2. (Chemical aspects of EGS with CO2 are discussed in a companion paper; Xu and Pruess, 2010.) Our modeling studies indicate that CO2 would achieve heat extraction at larger rates than aqueous fluids. The development of an EGS-CO2 reservoir would require replacement of the pore water by CO2 through persistent injection. We find that in a fractured reservoir, CO2 breakthrough at production wells would occur rapidly, within a few weeks of starting CO2 injection. Subsequently a two-phase water-CO2 mixture would be produced for a few years, followed by production of a single phase of supercritical CO2. Even after single-phase production conditions are reached, significant dissolved water concentrations will persist in the CO2 stream for many years. The presence of dissolved water in the production stream has negligible impact on mass flow and heat transfer rates.