Experimental laboratory determination of the dynamic elastic properties of wet, drained shales

The ultrasonic anisotropic elastic properties of drained, saturated shales were measured as a function of confining pressure. Two shales were characterized in this study: a Jurassic outcrop shale retrieved under the sea in a saturated state and a Kimmeridge Clay shale cut from a North Sea borehole. Both shales were highly anisotropic, both texturally, as revealed by scanning electron microscopy analysis, and elastically, as measured ultrasonically in the laboratory. The strongest anisotropy was seen in the Kimmeridge Clay shale, where up to 38% compressional wave anisotropy (Thomsen's parameter epsilon) and up to 58% shear wave anisotropy (Thomsen's parameter gamma) were observed. In addition, for both shales, epsilon and gamma were found to decrease as a function of increasing confining pressure, with the pore pressure drained to atmosphere, while the anellipticity (deviation of the slowness surfaces from ellipses) was found to be positive and decreased as a function of increasing confining pressure. Accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure, from 10.5 to 8.5% for the Jurassic shale, The decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid-filled porosity.