Performance analysis of enhanced geothermal system under thermo-hydro-mechanical coupling effect with different working fluids

Hot dry rock (HDR) geothermal energy is an effective alternative to traditional fossil fuels and can be exploited through enhanced geothermal systems (EGS). Compared to water, CO2 may have better performance in EGS as a working fluid. In this paper, the thermo-hydro-mechanical (THM) coupling mechanism inside the HDR reservoir was analyzed, and a dual-medium coupling model was constructed and validated using a saturated soil column problem. A numerical model of an EGS containing four different fracture networks was simulated using CO2 and water as working fluids, and the physical field evolution and heat extraction performance were compared under different working fluid conditions. The results indicate that reducing the fracture spacing and having a fracture network that is more intricate and inter-connected is more conducive to improving EGS heat extraction efficiency. When the HDR matrix is initially at temperatures of 423.15 K and 473.15 K, using CO2 as the working fluid leads to a faster decrease in outlet temperature and a sustained higher heat extraction ratio compared to water. When the initial temperature of the HDR matrix is 453.15 K, using CO2 as the working fluid results in a more stable outlet temperature. Using CO2 as the working fluid and intermittent exploitation mode may be more favorable for obtaining higher output thermal power. The findings of the study offer valuable guidance for choosing appropriate working fluids during EGS operations.