Analysis of Comparative Thermo-Hydraulic Performance of sCO2 and H2O as Heat-Exchange Fluids in Enhanced Geothermal Systems

Abstract

The relative performance of H₂O and sCO₂ as geothermal working fluids (GWFs) in liquid-dominated enhanced geothermal systems (EGSs) was investigated in this study. Such systems rely on the injection of GWFs (geothermal working fluids) to sustain geothermal energy recovery, which is dominated by conduction-based heat exchange from the rock to the GWF in the hydraulic fracture. H₂O is currently the only GWF considered for EGS operations, but supercritical CO₂ has been proposed as a potential GWF because of its lower density and viscosity, which lead to the hypothesis of potentially significant thermal energy recovery. However, H₂O appears to have an initial advantage because of its significantly higher thermal conductivity. We compared the performance of H₂O and SCO₂ as GWFs in a 3D stencil (minimum repeatable element) of an EGS involving a hydraulic fracture connecting the injection and the production wells, the main body of the EGS rock that provides the heat source, and boundaries that are sufficiently distant from the main body of the main body of the rock to maintain constant pressure and temperature conditions over a 30-year period of EGS operations In our studies we considered variations in the initial reservoir temperature, in the injection method (at a constant-rate and at a constant bottomhole pressure) and in the reservoir permeability, in an effort (a) not only to compare the EGS performance of H₂O and sCO₂ as GWFs but also (b) to determine the conditions (if any) under which sCO₂ can be more effective than H₂O. The results of the study indicated the overwhelming superiority of H₂O as a GWF under any and all of the conditions covered by the study, producing fluids at dependably much higher temperatures and yielding invariably drastically higher energy recovery than sCO₂ despite the consistently higher GWF injection and production rates attained with sCO₂.