Geomechanical and Ultrasonic Characteristics of Black Shale During Triaxial Deformation Revealed Using Real-Time Ultrasonic Detection Dependence Upon Bedding Orientation and Confining Pressure

The anisotropic characteristics of the P- and S-wave velocity and their velocity–pressure dependency during triaxial deformation are critical in shale gas exploration and development. However, the geomechanical and ultrasonic properties of shale under continuous triaxial deformation are still not well understood. The aim of this work is to experimentally investigate the velocity characteristics of a black shale plugged normal to bedding, parallel to bedding and at 30° and 60° to bedding, under a confining pressure of 5, 20, and 60 MPa, respectively. The in-situ ultrasonic transmission technique during sample deformation was used to reveal the velocity–pressure characteristics of the anisotropic black shale. The experimental results indicate that strength, P- and S-wave velocities, and final failure morphology present obvious anisotropy, and are influenced by the pronounced bedding plane. Before peak strength, the P-wave velocity almost increases with the sample deformation; however, the S-wave velocity first increases and then decreases, which is related to the fracture evolution. The velocity–pressure dependency analysis reveals a different propagation mechanism of ultrasonic wave through shale. Two different kinds of equations were used to fit their relationships and elevated stress. The results suggest that the static geomechanical and ultrasonic properties of black shale are affected by the rock structure and in-field stress level.