Numerical study on hydraulic fracture propagation in anisotropic shale considering bedding plane characteristics

Abstract

China possesses abundant shale gas resources. As the core method for enhancing shale gas production, hydraulic fracturing technology has seen increasing application alongside the rapid development of the industry. However, the inherent strong anisotropy of shale results in significant differences in mechanical properties between parallel and perpendicular bedding directions, leading to highly complex crack propagation patterns. This study employs the discrete element method (DEM) to construct numerical models of hydraulic fracturing in shale under varying bedding inclinations. It systematically investigates the hydraulic fracture propagation behavior in bedding shale and quantitatively analyses the effects of in-situ stress conditions and injection rates on crack propagation characteristics and failure modes. The results indicate that the stress distribution calculated by the isotropic DEM model aligns well with the analytical solution, validating the modelʼs reliability. Hydraulic fractures initiate in regions with negative tangential stresses around the wellbore and primarily propagate along bedding planes. The state of in-situ stress significantly influences crack morphology. As the lateral pressure coefficient K₀ decreases, the maximum crack propagation length decreased from 114.48 to 92.95 mm, yet interlayer propagation intensifies. The propagation direction increasingly aligns with the direction of maximum principal stress, thereby promoting the development of secondary cracks and increasing the complexity of the fracture network. Increasing the injection rate can significantly enhance maximum crack propagation length from 105.41 to 126.19 mm, reduce fracture tortuosity and roughness, and induce fracture branching to form complex fracture networks. Analysis of micro-failure modes reveals that hydraulic fracturing in shale is dominated by tensile failure. Notably, a decrease in K₀ or an increase in injection rate drives a transition in the failure mechanism from a “bedding-dominated” to a “matrix-dominated” regime, characterized by a marked increase in the proportion of matrix tensile failures. These findings provide theoretical support for optimizing fracturing parameters and enhancing reservoir stimulation efficiency in shale gas reservoirs within the Sichuan region.