Understanding anisotropic mechanical properties of shales at different length scales: In situ micropillar compression combined with finite element calculations

Keller, Lukas M.; Schwiedrzik, Jakob J.; Gasser, Philippe; Michler, Johann

Journal of Geophysical Research: Solid Earth 122 (2017)

From microstructural observations and experimental work it is known that shales consist of a mechanically weak porous fine-grained clay matrix with embedded and mechanically strong silt/sand grains. Thereby, the respective contents of weak and strong constituents control bulk mechanical properties. In addition, the clay matrix is characterized by a preferred orientation of clay platelets, which are a major control on the bulk anisotropy of shales. To date, little is known about the micromechanical properties of the fine-grained porous clay matrix, which is particularly true in case of its micromechanical anisotropy. Such information can, however, only be assessed on the microscale. Therefore, the drained micromechanical properties parallel and perpendicular to bedding were investigated by means of compressing micropillars with a flat punch indenter in a scanning electron microscope. Microscopic failure mechanism was found to be anisotropic: (i) in case loading was parallel to bedding it occurred by a combination of localized shearing, kinking/buckling of elongated clay aggregates, and bedding parallel splitting and (ii) for loading perpendicular to bedding failure occurred mainly by localized shearing. The measured stiffness of the drained porous clay matrix perpendicular (Ev) and parallel (Eh) to bedding was about 8 GPa and 30 GPa, respectively. Using these stiffness values as input in voxel-based finite element modeling and in combination with realistic microstructures, which are characterized with different contents of “soft” and “hard” constituents, revealed that the measured high microscale anisotropy Eh/Ev = 3.75 is crucial in understanding the bulk anisotropy of clay rocks.