Effect of layer orientation on acoustic emission characteristics of anisotropic shale in Brazilian tests

Shale bedding affects the constituent heterogeneity and the anisotropy of mechanical behaviour. To investigate the damage evolution and acoustic emission (AE) characteristics during failure process of anisotropic shale, the MTS 815 rock mechanics test system and the PAC AE test system were employed to perform Brazilian tests on shale samples, which revealed the effect of different layer inclination angles between the loading direction and the bedding plane. The tensile stress at the disc center was obtained based on transversely isotropic theory, and then the AE evolution process and energy release characteristics were further analyzed. Results observe a considerable influence from different layer orientations on shale failure modes and AE properties. The consistency between the failure state and the AE spatial distribution is preliminary confirmed. The AE signals were concentrated near the loading points as well as the crack kinking points, and accumulated more in the tensile-shear region rather than in the pure tensile region. The AE characteristics also reflect vividly the anisotropic split behaviors, among which the central spilt failure produced relatively weak and stable AE signals during the whole damage process. Of particular interest is that the crack initiation point might be captured by the AE source with the maximum ring counts. To a certain extent, the experimental results probe the layer effect on AE properties of fracture initiation and propagation, providing possible implications for the interpretation and analysis of microseismic monitoring in field fracturing.