Elastic Anisotropy Reversal During Brittle Creep in Shale

We conducted two brittle creep experiments on shale samples under upper crustal conditions (confining pressure of 80 MPa at 26 degrees C and 75 degrees C). We deformed the samples to failure, with bedding oriented perpendicular to the maximum compressive stress direction, using the stress-stepping methodology. In both experiments, the failure stress was similar to 64% higher than the short-term peak strength. Throughout each differential stress step, ultrasonic wave velocities initially decreased and then gradually increased with deformation/time. The magnitude of these variations depends both on the direction of measurement with respect to the bedding and the temperature, and it is largest for velocities measured parallel to the bedding and at high temperature. Elastic wave anisotropy was completely reversed at 75 degrees C, following a limited amount of axial strain (similar to 0.6%). Scanning electron microscope investigation confirmed evidence of a time-dependent pressure solution, localized compaction, crack sealing/healing, and mineral rotation. Our observations reveal that elastic anisotropy can evolve rapidly in both time and space, which has implications on the stress state and its rotation near fault zones.