Rock elasticity as a function of the uniaxial stress: laboratory measurements and theoretical modelling of vertical transversely isotropic and orthorhombic shales

ABSTRACT We apply a rock‐physics model that describes the relationship between the effective stress and rock elasticity. We experimentally obtain and analyse a data set containing one vertical transversely isotropic and one orthorhombic shale sample. The vertical transversely isotropic symmetry of the first sample is caused by the layered structure of the rock. The seismic orthorhombicity of the second sample could be explained after microscopic analysis of thin section, which demonstrates an imperfect disorder of inhomogeneities. Both samples were loaded uniaxially in a quasi‐static regime. During the loading, we measured stress‐dependent seismic velocities and sample deformations. For the analysis of the stress‐dependent velocities and stiffnesses, we modelled the measured data set using a recent generalization of the porosity deformation approach. Comparison of the experimentally determined and numerically modelled data supports the applicability of the theory and helps in the interpretation of experimentally obtained data. In agreement with the theory, uniaxial stress increases the elliptic component of the seismic anisotropy and does not impact the anellipticity parameter. We demonstrate the distinct influence of the stiff and compliant porosities on the stress sensitivity of the elastic properties.