Effects of Mineral Composition and Lamina on Mechanical Properties and Fractures of the Wufeng–Longmaxi Shale in the Luzhou Area of the Southern Sichuan Basin

Abstract

Because natural fractures are crucial for the hydraulic stimulation and development of the fracture network of a shale gas reservoir, understanding the influence of mineral composition and lamination on the occurrence of fracture and mechanical properties of shale is significant. To clarify the influence of mineral composition and lamination on the occurrence and mechanical properties of shale, combined analyses of X-ray diffraction, uniaxial compression, triaxial compression, and rock slice were performed on shale samples obtained from the lower submember of the first member of the Longmaxi Formation in the Luzhou area of the Sichuan Basin. After compression testing, the mineral characteristics and fracture morphologies of the Luzhou shale were analyzed based on the results and the characteristics of rock mechanics and fractures of the Luzhou shale owing to different mineral compositions and laminar development characteristics were summarized. The mineral composition of the shale in the study area primarily comprised quartz, feldspar, calcite, and clay minerals. Under uniaxial compression, the mechanical property, Young’s modulus, exhibited a positive correlation with the quartz content. The compression of the shale samples produced one to two shear fractures running through them, and the secondary fractures produced small angles around the main fractures or developed along the laminae. The laminated layer enhanced shale anisotropy and fracture trapping abilities. Although the oblique intersection of fractures at 45° along the laminated direction can produce and develop cracks more along the laminated direction than along the vertical direction, the high clay content of the shale will weaken this phenomenon. In summary, the commonly available shale was classified into four types based on its mineral content and the degree of lamination development; the commonly available high-quality shale has high brittleness and developed laminae. This study contributes to a better understanding of the rock mechanics and fracture evaluation of shale reservoirs.