An experimental investigation of gas permeability of a low permeability sandstone under deviatoric loading with loading/unloading cycles

Low permeability sandstones are widely found in oil and gas reservoirs. To gain a better understanding of the mechanical behavior of low-permeability sandstones, the permeability evolution of sandstone is investigated experimentally under triaxial compression loading, especially with multiple loading/unloading cycles, using two groups of experimental tests. The one is the triaxial compression tests with and without loading/unloading cycles. The other one is gas permeability tests in triaxial compression. The test results show that the stress-strain curves exhibited obvious nonlinear behavior and softening characteristics. It was demonstrated that important strain hysteresis occurred during the loading and unloading test. The permeability evolution of sandstone exhibited three phases: slow decrease, slow increase and rapid increase. Meanwhile, the permeability evolution followed the evolution of damage. The permeability under different confining pressures was different when the damage variable was approximate to 0.1. As the confining pressure increases, the permeability and the damage variable tend to be constant. The total permeability change is 0.568 when the confining pressure is 30 MPa. Moreover, a deviatoric stress threshold 0.8 sigma(p) is observed in the process of permeability evolution. When the applied stress is less than the threshold, the influence of confining pressure doesn't play a dominant role in the permeability evolution. When the applied stress is larger than the threshold, the permeability evolution depends on the combination of axial stress and confining pressure. Hydrostatic compression loading and unloading, traditional triaxial compression, triaxial cyclic loading and unloading, and permeability integration test were carried out respectively. A deviatoric stress threshold sigma(p) is identified in permeability evolution. Both axial loading and confining pressure have significant effects on the permeability evolution when the stress larger than it. The relationship between the permeability evolution and the damage variables evolution is analyzed, and the results show that under low confining pressures, the influence of the damage variable on permeability evolution is significant.