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The impact of water saturation on the elastic anisotropy dispersion in the Wellington shale at seismic frequencies

The anisotropic behaviour of shales is commonly associated with the properties of a transversely isotropic medium, which are determined by five elastic constants such as five independent components of the compliance or stiffness matrix. In this study, we utilize the laboratory low-frequency technique based on stress-strain relationship to measure the dispersion of five independent stiffness tensor components and Thomsen’s anisotropy parameters of shale samples saturated with water at four different values of humidity in the range from 12% to 97.5% (12, 44, 72 and 97.5%). We have investigated three shale samples from the Wellington formation cored along the horizontal, vertical and 45°-inclination directions with respect to the bedding plane at seismic frequencies between 0.1 Hz and 100 Hz. The obtained experimental data show an increased softening of the samples, which manifests itself in reduction of the transversely isotropic Young’s moduli and Thomsen’s parameters of elastic anisotropy e and y, no noticeable changes in parameter S were found. We also observed large reductions in normal and shear stiffness tensor components with saturation. When the samples were saturated at a relative humidity of 97.5 %, the softening at the higher frequencies was partly compensated by the modulus dispersion. We presume that the weakening of the elastic moduli and components of the stiffness tensor is caused by the significant percentage of water-swellable smectite in the Wellington shale.