Experimental investigation on permeability and energy evolution characteristics of deep sandstone along a three-stage loading path

The dynamic evolution of high in- situ stress with underground has a significant effect on the mechanical behaviour of rocks, including the evolution of energy and permeability. Therefore, three-stage loading and stress-seepage coupling tests based on the initial high in- situ stress reduction were conducted on deep sandstone using the RTR-1500 HTHP rock triaxial test system; the three stages are the initial high in- situ stress reduction stage, constant axial pressure - unloading confining pressure stage and axial loading stage. The stress-strain, energy and permeability evolution curves of deep sandstone during the entire deformation process were obtained. Then, the permeability and energy evolution characteristics were identified. The results are as follows: (1) The evolution trends differed substantially in terms of the peak strength of deep sandstone under the same pre-set confining pressure of unloading with different simulated depths. Three types of volumetric strain curves of deep sandstone were identified. (2) A significant competitive evolution mechanism was revealed between the elastic energy and the dissipation energy density, and the ratio of them showed an evolution trend that followed an inverted “S” curve with the change of the volumetric strain, which initially decreased, subsequently increased and finally decreased. (3) The relationships of the permeability with the input energy and the dissipation energy density of deep sandstone were both in good agreement with the nonlinear growth exponential model. The relationship between the permeability and the elastic energy density was in good agreement with the inverted “U” curve evolution model.