This paper studies the apparent relation between the ultimate strength and the elastic properties, anisotropy, and composition of shale rock. The study uses a series of Finite Element Models created in the numerical analysis software ABAQUS. The model considers a representative volume of shale rock adopting a binary mixture of a soft phase (clay and kerogen), and a stiff phase (quartz, feldspar, pyrite, and carbonates). The geometry of the finite element model is similar to the theoretical representation of the clay and kerogen shale constituent as an ellipsoidal inclusion in a stiff matrix. This method describes how the far field stress which is the average stress acting on the total volume of rock is partitioned among its stiff and soft constituents. The volume fraction of the ellipsoidal inclusion controls the clay and kerogen volume, while the aspect ratio of this inclusion controls the anisotropy of the rock. The Young's Modulus predicted by the FE models match very well with those predicted from the semi-analytical Differential Effective Medium (DEM) model when the same elastic properties are adopted for the inclusion and matrix. The DEM investigates the correlation between the internal stresses acting on each rock constituent and the mechanical properties of the composite rock. Two groups of analysis are performed to investigate failure anisotropy. The first group used MC model for both stiff and soft component and the second group used MCC for the soft component. Results explain the mechanical anisotropy by depicting the failure mechanisms in both horizontal and vertical samples.
The strength and deformability of shales are fundamental properties relevant to shale gas production. Understanding how different factors affect these mechanical properties is of great importance to successful exploration and production from unconventional reservoirs. Sone and Zoback (2013a, 2013b) showed a clear correlation between ultimate strength and stiffness of the shale gas and the percentage of clay and kerogen content. Another factor crucial to the characterization of shales is its mechanical anisotropy and how it is related to the orientation of bedding planes in the fabric.
Although it is not easy to accurately predict rock strength from petrophysical parameters (Chang et al., 2006); Sone and Zoback (2013b) showed that the elastic modulus seems to be a reasonable indicator of rock strength and ductility. An explanation for that is not readily available because of the fundamental differences in time scale and strain magnitude between elastic deformation and rock failure. In addition, both elastic modulus and ultimate rock strength are found to be consistently higher in horizontal samples (compression direction is parallel to bedding) than in vertical samples (compression direction is perpendicular to bedding).