Modeling the effects of microscale fabric complexity on the anisotropy of the Eagle Ford Shale

Abstract

Microscale fabric influences the elastic properties of rock formations. The complexity of the microscale fabric of shale results from composition, platy clay minerals, kerogen, and their preferred orientation patterns. This microscale fabric is also the likely cause of the elastic anisotropy of the rock. In this paper, we have developed a comprehensive three-step rock-physics approach to model the anisotropic elastic properties of the Upper Eagle Ford Shale. We started with anisotropic differential effective medium modeling, followed by an orientation correction, and then a pressure adjustment. This method accounts for the microscale fabric of the rock in terms of the complex composition, shape, and alignment of clay minerals, pore space, and kerogen. In addition, we accounted for different pressure-dependent behaviors of P- and S-waves. Our modeling provides anisotropic stiffnesses and pseudologs of anisotropy parameters. The modeling results match the log measurements relatively well. The clay content, kerogen content, and porosity decreased the rock stiffness. The anisotropy increases with kerogen content, but the influence of clay content was more complex. Comparing the anisotropy parameter pseudologs with clay content shows that clay content increases anisotropy at small concentrations; however, the anisotropy stays constant, or even slightly decreases, as the clay content continues to increase. This result suggests that the preferred orientation of clay clusters is preserved at low clay concentration but vanishes at high clay concentration. This method could also be applied to other shales with carefully chosen parameters to model anisotropic elastic properties.