Measurement of the anisotropic elastic properties of shale: uncertainty analysis and water effect

The static anisotropic elastic properties of shales can be measured by performing uniaxial compression on core specimens with bedding planes of different inclination angles (or anisotropy angles). However, these measurements differ when different numbers of specimens are tested, mainly because of the heterogeneity of shales. This study aimed to quantify the uncertainties of the anisotropic elastic properties of a shale when testing different numbers of specimens with distinct anisotropic angles. Three sets of room-dried core samples were prepared, and each set consisted of seven specimens with incremental anisotropic angles of 15 degrees. Uniaxial compression measurements were used to determine the anisotropic elastic properties for a given number (2-7) of specimens in each set. The results indicated that the uncertainties of the determined anisotropic elastic properties decreased monotonically as the number of measurements was increased. The results also suggested that at least five specimens with different anisotropic angles should be tested in order to obtain accurate elastic properties (uncertainty <10%). An additional set of wet specimens of the same shale were also tested and analyzed. It was found that water absorption significantly decreased Young's moduli, increased Poisson's ratios, and had a limited effect on the shear modulus. The wet specimens also displayed a slightly stronger anisotropic ratio of the elastic moduli compared to the room-dried specimens. Finally, the alteration of the anisotropic elastic properties of shales upon water absorption was tentatively related to the mineralogy and microstructure heterogeneity. The findings of this study have major implications for many geoengineering operations where the anisotropic elastic properties of shales must be considered.