Damage evolution of surrounding sandstone rock under charging–discharging cyclic loading in the natural gas storage of abandoned mines based on the discrete element method

Gas storage in abandoned mines is one way to reuse waste space resources. The surrounding rock of gas storage reservoirs in underground roadways undergoes damage and deformation under the cyclic loading of gas charging and discharging, which can pose a risk to the safety of the reservoirs. This study establishes a true triaxial numerical model of rock mass with the discrete element method (DEM) and explores the crack evolution of surrounding rock of underground gas storage during cyclic loading and unloading. Also, a damage evolution model in numerical analysis considering residual deformation is developed to explain the experimental results. As was revealed, cyclic loading and unloading resulted in fatigue damage in the specimen and caused strength deterioration of the specimen. During the loading process, the uniformly distributed force chains of the rock mass redistributed, evolving gradually to mostly transverse force chains. This contributed to the appearance of blank areas in the force chains when through cracks appear. The ratio of tensile cracks to shear cracks gradually decreases and finally stabilizes at 7:1. The damage evolution model considering residual strain can be mutually verified with the numerical simulation results. Based on the DEM model, it was found that there was a certain threshold of confining pressure. When the confining pressure exceeded 30 MPa, the deformation to ductility of sandstone samples began to accelerate, with a greater residual strength. This study provides a theoretical basis for analyzing the long-term mechanical behavior of surrounding rock of gas storage in abandoned mines.