Ultrasonic velocity, attenuation, and mechanical behavior of Longmaxi bedded shale under uniaxial compressive tests

Abstract

Knowledge of ultrasonic wave velocity and attenuation related with bedding orientations plays an important role for successful downhole, hydraulic fracturing, and surface seismic data interpretation. Real-time ultrasonic wave measurements in load direction have been conducted on Longmaxi shale specimens during compressive fractured process in this study. The relations between mechanical properties and ultrasonic response of shale with various bedding angles are investigated. The P- and S-wave velocities increase with increasing stress to the peak strength, which can be ascribed to the closure of microcracks and pores perpendicular to the wave propagation direction. And then they decrease with sudden fall of stress at the post-peak stage that is controlled by the coalescence of newly generated microcracks to macroscopic fractures. The P-wave velocity is more sensitive to the bedding angles and axial stress than S-wave velocity. The axial stress weakens the velocity anisotropy of shale. The P-wave attenuations decrease (quality factor values increase) for 0°, 15°, and 30° shale specimens under pressure, whereas the first increment and then less variation occur for 45°, 60°, 75°, and 90° specimens. The difference may be caused by the bedding scattering that is more pronounced when the beddings are more inclined and perpendicular with respect to the wave propagation direction. The P-wave quality factor has a close relationship with the peak strength for each inclination specimen. What’ s more, the failure modes have been illustrated based on the wave velocity and attenuation anisotropy.