Investigation of the effect of bedding and confining pressure on the energy evolution of shale during the unloading process

To explore the effect of bedding and initial confining pressure on the energy evolution characteristics of shale during the unloading process, samples were drilled with different bedding angles, unloading tests were conducted under different initial confining pressures, and the mechanical and energy evolution characteristics of shale during the unloading process were analyzed. The results show that the stress–strain curve of the unloading test can be divided into the linear elasticity stage, the stable crack growth stage, the accelerated crack growth stage, and the post-failure stage. Critical confining pressure can show the relative strength of the rock samples. The elastic modulus and Poisson's ratio increase with an increase in axial preset load. The elastic modulus increases with the bedding angle, and the effect of the bedding angle on the Poisson's ratio is insignificant. The energy evolution of the unloading test can be divided into three stages: energy accumulation, energy dissipation, and energy release. The larger the axial preset load, the higher the critical confining pressure, the higher the elastic modulus, and the higher the Poisson's ratio. The total energy, elastic energy, and dissipation energy all increase with the increase in the initial confining pressure, and the correlation is high. Confining pressure enhances the ability of the shale sample to store elastic energy and improves the ability of the shale sample to resist internal crack propagation. The total energy, elastic energy, and dissipated energy of the samples in the failure point decrease first and then increase with the increase in the bedding angle. The maximum value can be obtained when the bedding angle is 0°. The elastic energy and dissipated energy of shale are highly heterogeneous due to bedding, and the effects of bedding should be taken into account when exploring the law of rock deformation and failure from an energy perspective.