Numerical study on fracture vertical propagation and bedding plane slip in multilayered shale reservoirs

Multilayered shale reservoirs typically contain numerous mechanically weak bedding planes (BPs), which strongly influence the vertical propagation of hydraulic fractures (HFs). However, the mechanisms governing HF vertical propagation and the dynamic growth behavior of BPs in multilayered shale reservoirs remain poorly understood. To address this issue, this study develops a three-dimensional (3D) fracture propagation model that integrates BP slip and fluid leakoff. HF propagation is simulated using a planar 3D model, while BP slip is characterized by a lumped pseudo-3D model. The accuracy of the 3D fracture model is validated with an extended finite element model. Subsequently, the effects of fluid viscosity and interlayer stress differences on HF vertical propagation and BP slip are systematically analyzed. Numerical results reveal that HF height and width increase with higher fluid viscosity, while HF widths become discontinuous when crossing BPs. A greater interlayer stress difference leads to a more pronounced variation in HF width on both sides of the BP. Low-viscosity fluids enhance BP slip, though the BP sliding zone length generally remains below 1 m. In contrast, fracturing with high-viscosity fluids overcomes stress constraints on HF height, effectively increasing vertical stimulation.