Anisotropic crack evolution and fractal failure mechanism of Lushan shale under compression: insights from acoustic emission

The Lushan shale, located in the Upper Sinian Formation of China, is a promising zone for commercial shale gas development. Understanding the crack evolution and fractal failure mechanisms of anisotropic Lushan shale under compression is critical for ensuring safe drilling and optimizing reservoir reconstruction. In this study, uniaxial compression tests were conducted on Lushan shale samples with varying bedding angles (θ) ranging from 0° to 90°, using acoustic emission (AE) techniques to monitor crack propagation and failure processes. The results reveal that both uniaxial compressive strength (UCS) and cumulative AE counts initially decrease and then increase with increasing bedding angle, reaching a minimum at θ = 60°. During the initial loading phase, microcracking activity predominates, as indicated by a rising AE amplitude (b-value); however, as the stress nears 70-80% of the UCS, the b-value undergoes a sharp decline, signaling a corresponding surge in large-magnitude cracking events and the onset of rock instability. Regardless of varying bedding angles, the failure of Lushan shale under compression is primarily dominated by tensile fractures, accounting for over 70% of the total fracture events. Three failure modes of anisotropic shale were mainly identified: through-layer failure, layer-slip failure, and tensile splitting failure. Fractal statistical analysis reveals that the fractal dimensions of AE signals correlate with shale fragment distribution, indicating that microcracking evolution directly influences macroscopic failure patterns, providing a potential framework for assessing rock stability.