Optimal selection of working fluid for enhanced geothermal systems: A comparative analysis of supercritical carbon dioxide and water under various reservoir and extraction conditions

Geothermal energy is widely recognized as a highly promising clean energy due to its abundant reserve and low carbon footprint. The extraction of geothermal energy does not only depend on the stimulated reservoir volume but also depends on working fluid. Water (H2O) is usually used as the working fluid but carbon dioxide (CO2) can be a good alterative working fluid. However, the suitable conditions and extraction efficiency of CO2 compared to H2O have been unclear. This study compares the efficiency of heat extraction from an enhanced geothermal system (EGS) by using supercritical CO2 or H2O as working fluid and identifies the favorable or unfavorable conditions for each working fluid. Firstly, a thermal-hydraulic-mechanical (THM) coupling model is established and validated by two analytical solutions. Secondly, the performance of a typical EGS is evaluated with four indicators. Finally, a comparative analysis for heat extraction efficiency is performed with CO2 and H2O under different reservoir and extraction conditions. The analysis results reveal that under most conditions, the heat extraction rate of CO2 is 1.4-2.2 times that of H2O in the early stage and the final cumulative heat recovery is 0.8-1.9 times that of H2O. CO2 is more efficient in reservoirs with low permeability (no more than 2.5 mD), low injection pressure (no more than 28 MPa, or 8 MPa for pressure difference) and low temperature (no more than 35 degrees C), while H2O performs better in reservoirs with high permeability (no less than 5.0 mD), high injection pressure (no less than 32 MPa, or 12 MPa for pressure difference) and high injection temperature (no less than 40 degrees C). Furthermore, more pronounced thermal breakthrough and inadequate heat extraction are observed in reservoirs with high porosity and high injection pressure when CO2 is the working fluid.