Experimental Study on the Dilatancy Characteristics and Permeability Evolution of Sandstone under Different Confining Pressures

It is of practical significance to investigate the dilatancy and seepage characteristics of tight sandstone gas under different confining pressures for its efficient development. Therefore, fluid-solid coupling triaxial loading experiments with gas-bearing sandstone were conducted. The results showed that the gas-bearing sandstone exhibited brittle characteristics with tensile-shear composite failure. The dual logarithmic model can better characterize the sandstone strength (R-2 = 0.9952), whereas the fitting effect of the linear Mohr-Coulomb criterion is poor (R-2 = 0.9294). The dilatancy capacity of sandstone was negatively correlated with confining pressure, and the dilatancy index decreased by 38.4% in the form of its convex power function with the increasing confining pressure. The sandstone underwent significant damage dilatancy during the yielding stage, resulting in a significant permeability recovery, with an increase of 67.0%similar to 70.4%, which was greater than the decrease of 9.6%similar to 12.6% in the elastic stage. In view of the different dominant factors of permeability reduction induced by pore compaction and recovery induced by crack development, the permeability model was established with volumetric strain and radial strain as independent variables, which could better reflect the whole process of permeability evolution.