Study on the Mechanical Properties and Failure Characteristics of Heterogenous Shale Based on CT Scanning

Summary Shale reservoirs, as a significant type of unconventional reservoir, have always been a focal point in oil and gas exploration and development. The precise determination of shale mechanical properties is fundamental to the stimulation of shale oil and gas reservoirs. The heterogeneity of rock has a significant impact on its mechanical properties. Computed tomography (CT) scanning technology is an important method for observing the internal microstructure of rocks, and digital cores constructed based on CT scans can truly reflect the heterogeneity of shale. Numerical models of shale were established using image processing technology; the basic mechanical parameters of minerals were obtained through nanoindentation experiments; and mineral content was determined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Uniaxial and triaxial compression simulations were conducted to study the impact of mineral composition and porosity on shale mechanical performance. The results indicate that the mechanical properties of shale are the outcome of the combined effects of pore distribution, mineral arrangement, and porosity. Initial natural pores significantly influence the initiation and expansion of fractures during the loading process. For models with obvious through-going joints, fractures mainly expand along the joint planes. For models with uneven pore distribution, fractures start at the pores and expand along the loading direction, eventually connecting different pores, leading to failure. In cases where a certain type of mineral is abundant or concentrated in the mineral composition, its mechanical properties will be significantly influenced by that type of mineral. In this simulation model with a high quartz content, the direction of fracture propagation during fracture was altered by the quartz. Porosity also has a significant impact on mechanical properties. As porosity increases, the model’s compressive strength decreases. Under triaxial loading conditions, at lower confining pressures, the model primarily fails due to tensile stresses; as the confining pressure increases, the proportion of tensile failures decreases, while the proportion of compressive failures increases. Models with a high content of quartz maintain a relatively stable proportion of tensile failures under different confining pressures. Meanwhile, dolomite in the model, due to its strong deformation capability, is better able to withstand tensile stress initially, but as loading continues, the proportion of tensile failures gradually increases. The composition of shale plays a crucial role in determining its mechanical properties, serving as a key reference for analyzing the mechanical behavior of shale.