Deformation and CO2 gas permeability response of sandstone to mean and deviatoric stress variations under true triaxial stress conditions

The magnitude and direction of geo-stress undergoes complex changes following disturbance by artificial excavation in underground engineering. Tests were conducted using the self-made multi-functional true triaxial geophysical apparatus to investigate the influence of stress Lode angle on deformation, deformation modulus, and permeability of sandstone under true triaxial stress conditions (sigma(1) > sigma(2) > sigma(3)). The results indicate that principal, volumetric, and deviatoric strains, as well as permeability varied with the stress Lode angle, and eventually increased/decreased at different levels. For constant deviatoric stress and variable mean stress, the plastic strain decreased with increasing M (ratio of deviatoric stress q to mean stress p), whereas the permeability increased. For constant mean stress and variable deviatoric stress, the principal, volumetric, and deviatoric strains, as well as the permeability decreased first and then increased with the increase in M; the rock generated fracture surface when M was relatively large. The gas flow and stress axis directions significantly influence the value and changing trend of the permeability. With increasing stress Lode angle, the permeability increased first and then decreased. In our stress condition testing, the permeability value and variation range were maximum when the gas flow direction was parallel to sigma(1); when it was parallel to sigma(3), the permeability demonstrated an upward trend, but with a smaller increase range. Conversely, when it was parallel to sigma(2), the permeability decreased and the variation range was minimized. Finally, based on the analysis of the experimental results, we developed a permeability model that is expressed by both volumetric and deviatoric strains.