Evolution of gas permeability and porosity of sandstone under stress loading and unloading conditions

In-depth study of the permeability of rock mass in high-pressure gas environment is of great significance for the site selection and construction of compressed air energy storage underground storage. The purpose of this study is to investigate the effects of multiple stress loading and unloading on the gas permeability and porosity of sandstone. The steady-state flow and porosity under stress loading and unloading conditions were measured. Based on the confining pressure sensitivity coefficient and permeability recovery rate, the permeability change characteristics of the sample before and after stress loading and unloading were analyzed. According to the test results, the empirical relationship between sandstone permeability and porosity with confining pressure is derived. The evolution law of permeability and porosity under repeated cyclic loading and unloading was analyzed and compared with the existing research results. The results show that high-pressure gas promotes pore expansion or dilation, with porosity increasing by about 27.3% at 10 MPa compared to 2 MPa. During 55 cycles of loading and unloading, there is little noticeable change in permeability with respect to confining pressure starting from the 22nd cycle. Repeated loading leads to a decrease in the sensitivity of permeability and porosity to changes in stress. Permeability is more sensitive to changes in stress than porosity. In the experimental stress range, the relationship between permeability and porosity under high air pressure was expressed by power function equation. This study provides a theoretical basis for the site selection and sealing design of gas storage caverns.