Coupled thermal model for geothermal exploitation via recycling of supercritical CO2 in a fracture–wells system

Abstract

Geothermal exploitation via recycling of supercritical CO2 (CO2–EGS) in a fracture–wells system is a new method, which can reduce the injection pressure, and improve heat transfer efficiency. In this study, a coupled thermal model for this system is proposed, considering the coupling effects of the temperature and pressure on the CO2 flow and heat transfer. The simulation results show that the thermal breakthrough time is approximately 40 d when the well separation distance is 230 m. After thermal breakthrough, the fracture outlet temperature decreases from 260 °C to 100 °C, and the heat mining rate decreases from 4.6 MW to 3.0 MW over the following 2 months. However, as the deep formation continues to replenish energy into the wellbores and fracture, the decreasing trend in the temperature gradually slows down, and it reaches a quasi-equilibrium state after one year. Owing to the low heat conduction rate of the formation, thermal utilization depth is only approximately 15 m from the wellbore, and 50 m from the fracture after 20 y. Comprehensive analysis shows that the heat mining rate reaches a maximum at an injection rate of 36 kg/s in this case. This model can provide an accurate evaluation and design optimization method for CO2–EGS in a fracture–wells system.