Research on the micro-scale method for testing the mechanical anisotropy of shale

Abstract

The mechanical anisotropy of shale is an important basic parameter for the study of in-situ stress, sidewall stability and hydraulic fracture propagation. In view of the limitations and low success rate for sample preparation in macro-scale tests, we chose clastic rocks and fragmentized blocks which were easily found as the subjects for micro-scale tests, and used continuous stiffness measurements to carry out the nano-indentation tests on the shale samples with horizontal bedding and vertical bedding. The test data were analyzed on three main mineral matrices which were identified by the classification rule of hardness. And the hardness and young's modulus of the shale clay matrix were calculated by contact stiffness measurements and the fracture toughness by energy method. The test results proved the reasonability and convenience of the classification rule of hardness in processing nano-indentation data. Shale clay matrix is anisotropic at the nano-scale while the mechanical parameters at the nano-scale are related to the bedding direction. The anisotropy of each mechanical parameter varies in intensity. The anisotropy of young's modulus is weak, while that of fracture toughness is strong. The fracture toughness with horizontal bedding is 80% of that with vertical bedding.

Keywords

• shale
• mechanical properties
• anisotropy
• nano-indentation
• hardness
• young's modulus
• fracture toughness