Anisotropic characteristics of acoustic emission and the corresponding multifractal spectrum during progressive failure of shale under cyclic loading

The failure mechanism of shale under cyclic loading is useful for understanding fatigue hydraulic fracturing in reservoirs. The anisotropic characteristics of this mechanism were investigated in this study by conducting acoustic emission (AE) monitoring tests on shale specimens with seven different bedding inclination angles (θ) under uniaxial cyclic loading and unloading. The results showed that during the cyclic loading path, except for shale specimens with bedding inclination angles of 0° and 45°, the cumulative number of AE events and cumulative AE energy at each loading level tend to decrease and then increase during failure process; the Felicity ratio decreases overall with increasing the cycle numbers. Different trends in the number of AE events and the cumulative AE energy induced by tensile or shear fracture with increasing θ were observed. The bedding plane was found to not only affect the shale fracture mechanism but to also significantly impact the magnitude of the fracture energy release rate for the same fracture mechanism. The b value and spatial evolution of AE revealed the evolution of fractures in shale. Both tensile and shear AE events under cyclic loading and unloading were mostly generated by micro- and medium-scale fractures, regardless of θ. The AE time series parameters of anisotropic shale exhibited multifractal characteristics. The multifractal spectrum width Δα effectively reflected the nonuniformity of the AE energy parameters for failure in specimens with different θ. The difference in the fractal dimension Δf in specimens with increasing θ was found to effectively characterize the occurrence probability of large-energy AE events.