Mechanical properties of the Silurian Longmaxi Formation shale, southern Sichuan Basin and its microfracturing mechanisms

To reveal the mechanical properties and microfracturing mechanisms of shales, we investigate shales in the Silurian Longmaxi Formation in the southern Sichuan Basin using X-ray diffraction (XRD) analysis, together with triaxial compression, micro-computed tomography (micro-CT) and scanning electron microscopy (SEM) tests. The results indicate that the mechanical properties and reservoir physical properties of shale are significantly influenced by its mineral composition and confining pressure. Specifically, an increase in the brittle mineral content enhances its elastic modulus and peak stress, indicating positive correlations between these mechanical properties and the brittle mineral content. In contrast, higher clay mineral content increases the shale plasticity while reducing its rock strength. As confining pressure increases, fractures in the shale gradually close, and the pores deform and contract, leading to reduced porosity. A higher confining pressure results in greater compression. Two dominant types of fractures form during shale damage: boundary fractures occurring at interfaces between mineral grains and internal fractures occurring within grains. For shales with similar mechanical properties, increasing confining pressure shifts their dominant fracture type from boundary to internal fractures. Additionally, higher confining pressure causes the boundary and internal fractures to evolve into fracture zones, leading to an increased fracture density in the shale.