Comparison of P- and S-wave Velocities in a Transversal Isotropic Sandstone and a Shale under Uniaxial Loading Condition

On two anisotropic rocks we performed measurements of ultrasonic velocities under low axial stress but with high resolution in order to understand the role of compliant porosity as used by theoretical models (piezosensitivity theory).

From both investigated rocks (a South African Sandstone and an oil shale from south Germany) we used a HTI and a VTI sample. In the HTI sample the symmetry axis is perpendicular, in the VTI sample parallel to the loading axis. In both rocks a positive dependency on the stress of velocities parallel to loading axis is observed, whereas velocities measured perpendicular to loading axis don’t change significantly. The increase of velocity with stress is more pronounced in the sandstone samples. The increase of especially the p-wave velocity with axial stress in the VTI samples is higher than in HTI samples concurrently the change of anisotropy with stress depends on the relation between uniaxial load and sample symmetry axis. Furthermore we show that under uniaxial load an originally HTI sandstone turns into a rock with orthorhombic symmetry. We explain the effects by the closure of compliant porosity in axial direction, considering that the compliant porosity in sandstone differs from the one in the shale.