Mechanical and Acoustic Response of Low-Permeability Sandstone under Multilevel Cyclic Loading-Unloading Stress Path

Low-permeability sandstone reservoirs have been widely used as a gas storage medium worldwide. Compared with the high porosity and permeability of sandstone, low-permeability may present different mechanical (deformation, damage or failure) acoustic responses under cyclic loading-unloading processes caused by high-rate injection–production underground storage. In this paper, multistage triaxial loading–unloading tests continuously increased upper limit stress were carried out on six confining pressures. The results showed that superposition stress–strain curves become much denser in process each level stress. Based variation elastic modulus alternating loads, behavior loading is divided into three stages: hardening, stability softening. According to evolution emission (AE) signal parameters, AE counts appear intensively at initial stage then gradually stabilize. peak frequency presents zonal distribution, which low-frequency, intermediate-frequency high-frequency zones. Low pressure leads small b-value. RA–AF distribution implies mixed tensile–shear cracks are generated during process, shear more obviously developed.