Abstract: To study the effect of thermal-mechanical coupling on fracture permeability evolution of enhanced geothermal systems (EGS), flow-stress loading and unloading coupling experiments were carried out on granite samples with rough single fracture under the normal temperature (e.g., 25 ℃) and high temperature (e.g., 150 ℃ and 180 ℃) conditions, respectively. The confining pressure of loading and unloading processes was between 5 MPa and 30 MPa. The evolution law of fracture permeability induced by elastic-plastic deformation was analyzed during stress loading and unloading. The results show that the fracture permeability has a logarithmic negative correlation with the confining pressure. When the temperature was 25, 150 and 180 ℃, the fracture permeability decreased by 78%, 90% and 92% in the loading stage, and only recovered by 63%, 26% and 19% in the unloading stage. This indicated that the rough fracture surface has occurred plastic deformation during loading and unloading processes, which leads to permanent attenuation of fracture permeability. When the temperature increased from 25 ℃ to 150 ℃ and 180 ℃, the permeability attenuation caused by plastic deformation increased from 15% to 64% and 73%, respectively. This indicated that the effect of plastic deformation on the fracture surface is related to the crushing of mineral particles, which becomes more significant in high temperatures.

Key words: enhanced geothermal system, geothermal fracture, thermal-mechanical coupling effect, flow experiment, fracture permeability