Enhanced Geothermal Systems (EGS): Comparing Water and CO2 as Heat Transmission Fluids

Abstract

It has been suggested that enhanced geothermal systems (EGS; also referred to as “hot dry rock” systems) 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 operating EGS with CO2. Our modeling studies indicate that CO2 would achieve more favorable heat extraction than aqueous fluids. The peculiar thermophysical properties of CO2 give rise to unusual features in the dependence of energy recovery on thermodynamic conditions and time. Preliminary geochemical studies suggest that CO2 may avoid unfavorable rock-fluid interactions that have been encountered in water-based systems. To more fully evaluate the potential of EGS with CO2 will require an integrated research programme of model development, and laboratory and field studies.