Thermo-hydro-mechanical (THM) evolution law and development of permeability and pore structure of enhanced geothermal systems at ultra-high temperatures

The evolution law and fluctuation mechanism of permeability and pore structure of stimulated reservoirs in hot dry rocks (HDR) under the coupled thermo-hydro-mechanical environment (THM) in ultra-high temperature are of great significance to the production capacity of enhanced geothermal systems (EGS). This paper designed a testing apparatus to take the effects of the THM processes into account by coupling high temperatures and the high triaxial stress. A series of flow tests were performed on a HDR under the coupled THM environment at ultrahigh temperatures, i.e., temperature range of 25 to 650 degrees C, triaxial stress of 25 MPa, and seepage pressure of 6 MPa, and ultra-high thermal treatments, i.e., temperature range from 25 to 650 degrees C. Then, the micro-physical parameters of the specimens treated in various conditions were obtained via mercury intrusion porosimetry, thin-section petrography and micro X-ray computed tomography observations. The experimental results show that (1) in the temperature range of 25 to 400 degrees C, the microscopic physical parameters increase monotonously with the increase of temperature. However, the microscopic physical parameters of specimens subjected to the coupled THM condition are lower than those obtained after considering only the thermal treatment, owing to the inhibition effect of the triaxial stress on the thermal expansion; (2) in the temperature range of 400 to 550 degrees C, the microscopic physical parameters subjected to the coupled THM condition fluctuated slightly with the increase of temperature which is due to the strong thermo-elastic-plastic fluidity and the competitive effect of closing and reopening of pores/fissures; (3) in the temperature range of 550 to 650 degrees C, the microscopic physical parameters are much larger than those obtained after the thermal treatment owing to the severe deterioration of rock mass caused by the coupling THM process. The results obtained is of great significance for the assessment of stability, safety and site selection of high-temperature rock projects such as EGSs.