Anisotropic progressive shear damage and fracture mechanisms of fractured shale based on acoustic-optical-mechanical methods

Bedding planes and natural fractures are key factors controlling the anisotropic mechanical properties of shale reservoirs, exerting a critical influence on underground engineering. However, their shear deformation and fracture evolution mechanisms remain poorly understood. In this study, synthetic shales containing bedding planes and fractures were prepared using a natural mineral reduction−stepwise compaction−mechanical similarity method. Subsequently, the evolution characteristics of macroscopic mechanical behavior, acoustic emission (AE) signals, and strain fields during the shear process of fractured shale were explored using acoustic-optical-mechanical multiphysics monitoring. Finally, the anisotropy of damage deformation, failure modes, and fracture mechanisms in shale were revealed. The test results indicate (1) synthetic shales exhibit strong similarity to natural shale, making them suitable for investigating variations in mechanical behavior; (2) cohesion and peak shear strength reach their maximum and minimum values at bedding angles (α) of 45° and 0°, respectively, while the friction coefficient and residual strength reach the two at α = 90° and 0°, respectively; (3) as α increases, a localized high-strain zone forms along the bedding planes and shear plane, leading to irregular shear fracture zones with jagged surfaces; (4) the shear fracture mechanisms in artificial shale include bedding shear fracture, bedding tensile fracture, matrix shear fracture, and matrix tensile fracture; at α = 45°, the bedding direction aligns with the σ1 direction, causing shear fracturing jointly controlled by the matrix and bedding planes; and (5) the shear failure process of fractured shale exhibits a well-coupled acoustic-optical-mechanical effect. The deformation energy density, in particular, demonstrates a strong response to the energy accumulation and release patterns. This study provides a novel experimental framework and mechanistic insights that are critical for the efficient development of deep shale gas resources, as well as for the prediction and mitigation of geological hazards.