Elastic Properties of Shales: Influence of Stress and Anisotropy

We used undrained multi-stage triaxial tests to evaluate how the ultrasonic wave velocities and their anisotropy changed with increasing isotropic and differential stress conditions. In addition, the impact of stress orientation with respect to fabric orientation was evaluated. An array of ultrasonic transducers allowed to measure five independent wave velocities which were used to calculate the elastic properties of the shale. Results indicate that in this shale P- and S-wave velocities vary with stress in a different manner dependent on the maximum stress orientation with respect to the fabric orientation. Where the maximum stress is normal to bedding, Vpv and Vs1 increase monotonically with increasing effective stress. However Vph and Vsh decrease during individual loading stages but increase from stage to stage as confining pressure increases. The reverse occurs when the microfabric is parallel to the maximum principal stress. Where the maximum stress is bedding normal, velocity anisotropy decreases as differential stress increases; when maximum stress is fabric parallel, anisotropy increases. Intrinsic anisotropy is related to the initial composition and fabric of the sediment, while changes in elastic anisotropy result from the applied stresses, their orientation to the rock fabric and the degree of stress anisotropy