Mechanisms of fracture propagation and deflection in sandstone-shale interlayers: a 3D discrete lattice study

Sandstone-shale-sandstone interbedded reservoirs are common in unconventional oil and gas development, yet the mechanisms of fracture propagation and deflection induced by interfacial shear stress during hydraulic fracturing remain poorly understood. To address this, a three-dimensional discrete lattice–fluid network coupled model was established and analytically validated to reveal the fracture propagation and deflection behavior driven by interfacial shear stress. The effects of formation dip angle and fracture initiation position were systematically investigated. The results indicate that under parallel stacking conditions, fractures exhibit regular double-wing propagation, and the dip angle has a negligible influence on deflection behavior. Conversely, in stacking configurations, the fracture deflection angle increases nonlinearly with the dip angle, rising from 3.7° at a 5° dip to 8.6° at a 12.5° dip. Fracture networks tend to propagate along the normal direction of the sandstone, a trend that intensifies at steeper dips. Interfacial shear stress is identified as the dominant mechanical factor controlling deflection by regulating the stress intensity factor ratio through the dip angle, thereby determining the propagation path. This study elucidates the mechanism by which interlayer dip angle differences drive mixed-mode fracture propagation via interfacial shear, and establishes a quantitative prediction model and a response-surface-based design chart for fracture deflection angles, along with optimized engineering recommendations, providing theoretical and practical foundations for multi-layer stacked reservoir fracturing optimization.