Experimental investigation of the effects of clay content and compaction stress on the elastic properties and anisotropy of dry and saturated synthetic shale

Abstract

Anisotropy in shales is an important issue in exploration and reservoir geophysics, and it has been proven extremely difficult to correlate anisotropy in natural shale by means of a single variable (in this case, clay content or compaction stress) because of the influence of multiple factors, such as water content, total organic carbon content, and complex mineral compositions. Thus, we used quartz, kaolinite, calcite, and kerogen extract as the primary materials to construct two sets of synthetic shale samples, each with a different clay content by weight and a different compaction stress. Ultrasonic experiments were conducted to investigate the anisotropy of velocity and mechanical properties in dry and saturated samples of our synthetic shales. The results reveal that the velocities decrease with clay content by weight and increase with compaction stress and that these changes are significant at low compaction stress. The velocity anisotropy of the samples increases with clay content and compaction stress due to the increasing alignment of the clay platelets. S-wave anisotropy is more sensitive to the clay content or compaction stress than P-wave anisotropy. The dynamic Young’s modulus [Formula: see text] of the samples decreases with clay content and increases with compaction stress, whereas Poisson’s ratio [Formula: see text] increases with clay content and decreases with compaction stress. Young’s modulus perpendicular to the symmetry axis is always larger than that parallel to the symmetry axis, but Poisson’s ratio perpendicular to the symmetry axis may be larger or smaller than that parallel to the symmetry axis, which indicates that mechanical properties have obvious anisotropic behavior. The elastic properties and anisotropy are also affected by fluids; the values of elastic and mechanical anisotropy parameters in saturated samples are significantly lower than those in dry samples.