Seepage characteristics of porous media under stress-gas pressure coupling

Compact geological formations are ideal locations for the construction of underground high-pressure gas storage facilities for compressed air energy storage. The lining and external rock masses, acting as porous media, serve as the sealing structure for the storage reservoir, bearing high-pressure gas of up to 15 MPa. The objective of this study is to investigate the seepage characteristics of porous media under coupled stress-gas pressure conditions. Through macro and microscopic tests, the mechanical properties and microstructural features of concrete and sandstone were comparatively analyzed. The effects of confining pressure and gas pressure on the gas permeability and porosity of the porous media, as well as the seepage variation patterns, were investigated and compared with other scholars' results. The findings indicate that microscopic analysis reveals that the pore structure of fine sandstone is significantly denser than that of medium sandstone and concrete. Under the same pressure conditions, the steady-state flow rate of fine sandstone is an order of magnitude lower than that of medium sandstone, while the permeability of concrete is approximately 4 to 10 times higher than that of medium sandstone. Gas porosity increases with increasing gas pressure. The unsteady seepage process can be described by the established exponential model, with the seepage process divided into rapid decline, slow decline, and zero sections. This study provides guidance for selecting seepage parameters in high-pressure gas permeability theories and numerical models.