Elastic Properties and Anisotropy of Shales Under Stress

Preserved shale samples from the Australian North west shelf were cored normal and parallel to bedding and tested in a triaxial apparatus while monitoring ultrasonic P- and S-wave velocities; results indicate that P- and S-wave velocities vary with increasing mean effective stress. The shale has small P-wave anisotropy and moderate S-wave anisotropy, and both are sensitive to stress variations. Considering that the recorded strains are very small, the variation of velocities has to be related to adjustment of the tiny pore-space. Intrinsic anisotropy is connected to the initial composition and fabric of the sediment and the presence of microfratures, while changes in elastic anisotropy result from the applied stresses, their orientation to the rock fabric and the degree of stress anisotropy.