Evaluating the Thermal Performance of Supercritical CO2 and Water as Working Fluids in Enhanced Geothermal Systems under Various Injection Conditions

The selection of working fluid plays a crucial role in optimizing heat recovery efficiency of Enhanced Geothermal Systems (EGSs). Water has long been the conventional choice for heat extraction due to its widespread availability. In recent decades, supercritical CO2 (ScCO2) has garnered increasing attention as an alternative working fluid, offering unique benefits in geothermal applications. Given their distinct thermophysical properties, water and ScCO2 exhibit different performance in heat extraction under various injection conditions. To evaluate the thermal performance of the two fluids, we develop a field-scale single-fracture EGS model that simulates the coupled thermo-hydro (TH) processes using both water and ScCO2 as working fluids. The simulations were performed under three commonly applied injection conditions: constant pressure, constant mass rate, and constant volume rate. Results indicated under constant injection pressure, ScCO2 outperforms water in heat extraction due to its higher mass rate. In contrast, water remains the more efficient working fluid under constant mass rate and volume rate, primarily driven by its higher specific enthalpy change. In addition, thermal performances of water and ScCO2 varies under different injection parameters and reservoir properties. This study provides valuable insights into the practical application of ScCO2 as a working fluid in real-world EGSS.