Comparison of EGS Thermal Performance with CO2 and Water as Working Fluids

The primary objective of this investigation was to compare the thermal performance of an enhanced geothermal system (EGS) with supercritical carbon dioxide (CO2) and water as working fluids. A major consideration in this study is the possibility that fractures may have spatial variations in aperture and it is relevant to understand the impact of the heterogeneous aperture distribution on EGS performance using CO2 in place of water. The system being modeled is a doublet EGS system. First a study was done to determine how the mass flowrates of CO2 and water could impact thermal performance. The fracture was considered to be parallel plates. The results showed that there was an optimal water mass flowrate above which the rate of heat extraction was constant. Such a deduction could not be made for CO2, as the mass flowrate was determined not to be a sufficient metric for comparing the two fluids. By choosing a mass flowrate that ensured the same energy input for CO2 and water, a comparative study on the two fluids could be done. The study showed that if the fracture aperture is smooth i.e., modeled as parallel plates, for the same energy injected, CO2 results in higher energy produced compared to water. However, if the fracture has spatial variations in aperture, in the long term, water is a better working fluid for EGS. This is because as the temperature in the system drops, the viscosity of water increases much more than that of supercritical CO2. This is advantageous in that the mobility of water reduces, creating more contact with the hot rock and consequently more heat extracted from the rock. CO2 on the other hand is impacted more by channeling due to its low viscosity and higher mobility compared to water.