Assessing the effect of oriented structure characteristics of laminated shale on its mechanical behaviour with the aid of nano-indentation and FE-SEM techniques

Variations in the mechanical properties of oriented shale within the nano-to-micro scale are crucial to determine the deformation and fracture of shales. However, the relationships between the oriented structure characteristics of shale and its mechanical properties need further investigation. In this study, the Yan-Chang #7 shale in the Ordos Basin is selected to elucidate the micro-mechanical properties of different shale grains using nano-indentation experiments. Results show that pyrite grains exhibit the highest elastic modulus, followed by quartz, feldspar, calcite, dolomite, clay minerals, and organic matter. Additionally, the experiments demonstrate an abrupt shift at the contact edge of the grains due to high heterogeneity. By applying a modified Mori-Tanaka method, we upscale the micromechanical characteristics of grains to the matrix. The size, flatness of grains, and structure orientation entropy (SOE) of shale indicate a good correlation with the mechanical properties of shale or grains. A larger size, more flatness of grains, and lower SOE result in smaller values of the elastic modulus of shale. The layered structure of shale samples creates a weak surface between the layers, making them more susceptible to fracturing and deformation when subjected to stress.