Evolution of permeability and microscopic pore structure of sandstone and its weakening mechanism under coupled thermo-hydro-mechanical environment subjected to real-time high temperature

Thermal damage mechanisms and the characteristics of performance deterioration of a repository host rock is critical for evaluating the potential of an underground coal gasification project. Previously, many studies have primarily focused on the microstructural evolution of various rock types after high-temperature treatment only. However, relatively little is known regarding the coupled thermo-hydro-mechanical (THM) behavior of rock, which is frequently encountered in geo-engineering underground regions with high-temperatures. In this study, using a self-developed universal tester with the ability of THM coupling of high temperature and high pressure, permeability evolution in sandstone under real-time high temperature (20–700 °C) and triaxial stress (hydrostatic pressure = 25 MPa) were observed. The microphysical parameters of these specimens subjected to the THM environment were then measured, and the corresponding morphological evolution processes were also assessed using micro-computed tomography (MCT). Further, for the purpose of contrast, we have also determined the microphysical parameters for the sandstone after heat treatment only. The results show that the evolution of the microscopic structures within the sandstone under the coupled THM condition is vastly different from those subjected to heat treatment only. The experimental results in this study can provide theoretical guidance for the stability of surrounding rock of a combustion cavity during in-situ coal gasification.