Investigation on the anisotropy of meso-mechanical properties of shale rock using micro-indentation

Indentation provides a powerful tool for measuring the mechanical properties of shale fragments at the nano- or micro-scale, allowing test with shale cuttings and avoiding the difficulties of coring in horizontal wells, but the meso-mechanical properties and their anisotropy are rarely investigated. Therefore, three types of Longmaxi shale fragments were collected to perform micro-indentation tests, and the meso-mechanical properties, such as hardness, elastic modulus, uniaxial compressive strength (UCS), fracture toughness, and brittleness, in both the bedding plane normal (BPN) and bedding plane parallel (BPP) directions were determined. Finally, the anisotropy of different mechanical properties was compared, and the relationships among mechanical properties, clay minerals, and brittle minerals were discussed. The results indicated that the hardness, elastic modulus, UCS, and brittleness in the BPP direction are higher than those in the BPN direction, while the fracture toughness is opposite. All the mechanical properties in the BPP direction are in positive proportion to those in the BPN direction for these three shales. The hardness, elastic modulus, UCS, and brittleness of the Fuling shale are higher or slightly higher than those of the Changning shale, followed by the Weiyuan shale. All the mechanical properties in both the BPP and BPN directions increase with the brittle minerals. The anisotropy in elastic modulus increases with increasing clay content, the anisotropy in brittleness decreases with increasing clay content, because the anisotropy depends on both the anisotropic clay minerals and the oriented geometric factors of the solid phase.