Anisotropic Damage to Hard Brittle Shale with Stress and Hydration Coupling

Acoustic-wave velocities of shale rocks with different coring angles were tested by an acoustic-emission experiment under different confining pressures and soaking time of drilling fluid. Effects of stress and hydration coupling on the acoustic-wave velocities, elastic parameters, and anisotropic damage were analyzed and investigated. The following results were obtained: (1) Acoustic-wave velocities of shale rocks are related to the confining pressure, soaking time, and coring angles. (2) Both Young's modulus and Poisson's ratios increase with confining pressure under the same soaking time; under the same confining pressure, the changes of Young's modulus and Poisson's ratios with time are not as obvious as the confining pressure, but it shows that the Young's modulus decreases, while the Poisson's ratios increase. (3) With increasing confining pressure, the Thomsen coefficient epsilon showed an increasing trend, whereas the Thomsen coefficient gamma exhibited the opposite trend; further, the anisotropy coefficient of P-wave (epsilon) is larger than the anisotropy coefficient of S-wave (gamma). (4) Damage parameters parallel to bedding are greater than those perpendicular to bedding; when the confining pressure increases, the fracture pores gradually close, and both vertical and horizontal damage parameters are reduced.