Investigation on the effect of confining pressure on the geomechanical and ultrasonic properties of black shale using ultrasonic transmission and post-test CT visualization

Abstract

Knowledge of the influence of confining pressure on the geomechanical and ultrasonic characteristics is essential to exploration and development of gas shale reservoirs. In contrast to the extensive studies on the physical and mechanical properties of shale, the velocities of P-wave and S-wave during continuous deformation are not thoroughly understood. In this study, a suite of triaxial compression tests were done on the shale samples collected from the Sichuan Basin and were cored with horizontal bedding structure under various confining pressure. In-situ ultrasonic transmission technique combined with post-test CT imaging analysis was used to reveal the velocity-pressure characteristics and crack patterns. The experimental results indicate that strength, elastic parameter, P- and S-wave velocities, and failure pattern are influenced by the applied confining pressure. At the pre-peak stage on the stress-strain curve, the P-wave velocity increases with elevated stress; however, the S-wave velocity first increases and then decreases, which is related to the crack damage evolution. The meso-structural changes during deformation are considered to be the main factor controlling the pressure sensitivity of the velocities. Under compression, the relatively soft pores, micro-cracks and minerals were compacted gradually, and it is the primary reason leading to the increment of velocities. Based on the CT scanning technique, the crack pattern was obtained after triaxial testing, the crack pattern analysis suggests that the fractal dimension of fracture distribution almost increases with confining pressure. What is more, the stimulated fracture density increases with increasing confining pressure. It suggests that, for the Longmaxi black shale, a high confining pressure condition is beneficial to form a complex fracture network.