On the Relative Effectiveness of H2O and CO2 as Reservoir Working Fluids for EGS Heat Mining

Abstract

Theoretical calculations are presented which examine the heatsweeping effectiveness obtainable by circulating fluids through a deep geothermal reservoir. A classical “five-spot” well pattern is employed in all cases. Fluids considered are H2O, CO2, and combinations (with injected CO2 displacing in-situ H2O and the reverse). The reservoir itself is at high pressure (? 300 bars), is of relatively low porosity (1%), and all of the reservoir’s permeability arises from the presence of numerous widely-separated fractures (average fracture spacings considered range from 5 to 80 meters) that penetrate the otherwise impermeable country rock. The initial reservoir temperature is modest (200ºC). These conditions are representative of those likely to be of interest for Enhanced Geothermal Systems (EGS) projects. For this study, the figure of merit is taken to be the fraction of the total initially-available thermal energy that is recovered before produced fluids reach abandonment temperature. The present results suggest that, all else being equal, heat sweeping effectiveness will be maximized if H2O is used as the working fluid. The effectiveness of a pure-CO2 system with no H2O present is slightly lower. If H2O is injected into a CO2-filled system, the performance is almost the same as the all-H2O case (and better than that of the all-CO2 case). But displacement of native hot H2O with colder CO2 results in substantially earlier thermal breakthrough in the production wells and significantly less heat recovery. Reasons for these differences in thermal sweep efficiency are examined.