Study on dynamic fracture growth mechanism of continental shale under compression failure

Microscale and nanoscale fractures induced by stress transfer can effectively improve pore connectivity, which can provide significant benefits by improving well productivity in shale reservoirs. In this research, representative samples of four shale lithofacies are investigated. The rock deformation and fracture propagation of shale samples under stress are numerically simulated by integrating various technical approaches, such as characterization of the mineral composition and distribution, digital core modeling, micromechanical tests, and finite element method-based simulations. The results show that the numerical model developed in this research can accurately replicate the microscale characteristics of the heterogeneity of shale and thus effectively clarify the effects of control of the composition, morphology, and distribution of minerals and structural defects on induced fractures in rock masses. Under uniaxial compression, the failure mechanism of rock is dominated by tensile fracturing, which generates tensile fractures parallel to the loading direction. However, compression‒shear fractures perpendicular to the loading direction also form due to the presence of lamination seams. The content of brittle minerals in different lithofacies is relatively stable, but the characteristics of the morphology and distribution of such minerals are highly differentiated, which leads to significant differences in mechanical properties and failure mechanisms. Both laminated felsic and diamictic shale (LFS and LDS) can present high fracture complexity in cases of low fracture energy. In argillaceous shale (AS), fractures initiate at low strengths, yet fracture propagation requires high energy. In contrast, the fracture initiation pressure and fracture propagation energy consumption of dolomitic shale (DS) are both high and unfavorable for the propagation of induced fractures. This research provides a technical approach for studying the formation mechanism and dynamic evolution of induced fractures, and the corresponding findings are of great theoretical significance for the identification of sweet spots and design of shale reservoirs for fracturing applications.