Stress-dependent seismic anisotropy of shales

A simple theory for the stress-dependent seismic anisotropy of shales can be obtained in terms of a second-rank tensor and a fourth-rank tensor that depend on the orientation distribution of contacts between clay platelets. The theory allows the normal and shear stiffness of the contact regions between clay platelets to be obtained as a function of stress from measurements of seismic P- and S-wave velocities for shales. The ratio of the normal-to-shear compliance, B N /B T , of the contact regions between clay particles is found to be sensitive to the saturation state of the shale. Inversion of velocity measurements for fully saturated shales shows a low value of B N /B T when compared with measurements on air-dry shales, consistent with the expected reduction in normal compliance in a fluid-saturated, low-permeability rock. For all shales considered, B N /B T is found to be less than unity. The contacts between clay particles are therefore more compliant in shear than in compression.