Dynamic analysis of heat extraction rate by supercritical carbon dioxide in fractured rock mass based on a thermal-hydraulic-mechanics coupled model

Heat production from geothermal reservoirs is a typical heat transfer process involving a cold working fluid contacting a hot rock formation. Compared to the thermal-physical characteristics of water, supercritical CO2 (scCO2) has a higher heat storage capacity over a wide temperature-pressure range and may be favored as a heat transfer fluid. Singularly characteristic of scCO2-based heat extraction is that the hydraulic-thermal properties of the scCO2 vary dramatically and dynamically with the spatial pressure gradient during unsteady-state flow along fracture. This highly nonlinear behavior presents a challenge in the accurate estimation of heat extraction efficiency in scCO2-based EGS. In this paper, a thermal–hydraulic-mechanical (THM) coupled model is developed by considering deformation of the fractured reservoir, non-Darcy flow and the varying thermal-physical properties of scCO2. The proposed model is validated by matching the modeling temperature distribution with published data. The results show that during continuous injection of scCO2, the fracture first widens and then narrows, ultimately reopening over the long term. The sequential fracture deformation behaviors are in response to the combined impacts of mechanical compression and thermally-induced deformation. By controlling the injection parameters of the scCO2, it is found that the heat extraction rate is positively correlated to its pore pressure or mass flow rate. The heat extraction rate can be significantly enhanced, when the inlet temperature of scCO2 is below its critical temperature. As a result, the heat increment recovered per unit mass of scCO2 decreases as the hot rock is gradually cooled. Meanwhile, the heat increment recovered per unit mass of scCO2 decreases by increasing the inlet temperature of scCO2 or its mass flow rate, but increases as the outlet pressure rises. Furthermore, multi-linear regression indicates that controlling the inlet temperature of the scCO2 can significantly improve the thermodynamic efficiency of heat extraction.