Simulation of thermo-hydro-mechanical coupled heat extraction process of geothermal reservoir under varied geological conditions with CO2 as working fluid

Abstract

The use of supercritical CO2 as a working fluid instead of H2O in an enhanced geothermal system (EGS) for extracting heat from hot dry rock geothermal resources has garnered significant attention. The objective of this study is to investigate how different geological conditions affect the heat extraction behavior of supercritical CO2 and to reveal the efficiency differences between supercritical CO2 and H2O as working fluids in each geological scenario, providing deeper insight into reservoir management. A series of 3D coupled thermo-hydro-mechanical models were established. Besides, the impact of initial temperature, pressure, porosity, permeability, and stress field on the heat extraction of EGS with different working fluids are also analyzed and compared. Results indicate that, at the same circulating volumetric flow rate, CO2-EGS generally exhibits an earlier thermal breakthrough time, lower net energy production, and reduced heat extraction rate compared to H2O-EGS. Taking into account the extended reservoir lifespan and the relatively small pressure difference between injector and producer in CO2-EGS, CO2 is a more suitable working fluid than H2O. Furthermore, the performance of CO2-EGS is primarily affected by the initial temperature, porosity, and permeability distribution pattern, rather than the initial pressure. On the one hand, a reservoir with low initial temperature, porosity and homogeneous permeability were found to be more conducive to enhancing the average net energy production. On the other hand, reservoir vertical permeability anisotropy can have a more profound impact on CO2-EGS efficiency compared to horizontal y permeability anisotropy. Additionally, this study also examined the impact of in situ stress field. It was found that, compared to H2O-EGS, both isotropic and anisotropic in situ stress fields have a weak effect on the performance of CO2-EGS, which can be attributed to the similarity of the channelization degree of the flow field under different stress conditions.