Study on the characteristics of CO2 seepage and heat transfer in rough fractures based on thermo-hydro-mechanical coupling

In this paper, a rough fracture model is constructed based on fractal interpolation method. Combined with the thermo-hydro-mechanical multi-field coupling numerical simulation, the flow and heat transfer characteristics of CO2 in rock fractures are systematically studied. The effects of different fracture apertures on CO2 heat transfer are investigated. The differences of CO2 seepage and heat transfer under fixed fracture aperture and non-uniform fracture aperture with different roughness are studied. The seepage and heat transfer characteristics of CO2 in three intersecting fractures of “X” type, “Y” type and “branch” type are analyzed. The results show that the average reservoir temperatures with fracture aperture of 1 mm, 2 mm and 3 mm decrease by 10.71 K, 20.97 K and 28.07 K respectively when CO2 is injected for 10 min. It reflects the positive influence of fracture aperture on the heat transfer rate in the early stage of heat transfer. The fracture roughness has different effects on the heat transfer rate in fracture with fixed and non-uniform aperture. The “X” type fracture is the best because of its high flow rate and large contact area. The “Y” type fracture can achieve uniform heat transfer, while the “branch” type fracture forms a dominant flow path, which leads to the under utilization of local heat resources. This study deepens the understanding of the seepage and heat transfer characteristics of CO2 in rough fractures. It provides a scientific basis for the optimal design of fractures in enhanced geothermal systems.