Hydraulic fracture propagation at weak interfaces between contrasting layers in shale using XFEM with energy-based criterion

The propagation behavior of hydraulic fractures (HFs) in unconventional shale formation is influenced by anisotropic fracture toughness and interaction with existing nature fractures (NFs) or bedding planes (BPs). This complex propagation behavior has not been fully understood. We propose a novel energy-based fracture propagation criterion for predicting HFs' propagation at weak interfaces between contrasting layers. Based on He-Hutchinson's theory, our proposed criterion takes HF's directional variation of energy release rate into account, thus providing a detailed analysis of HFs' possible crack paths. The deflection, crossing, and offset behaviors of HFs' interacting with weak interfaces can be determined. Besides, the stress singularity at the fracture tip is solved by applying practical enrichment functions through the extended finite element method (XFEM). Then, the stress intensity factors (SIFs) are calculated directly by considering all components of the fracture tip enrichment functions without extra post-processing. This technique guarantees the accuracy of the calculation of the SIFs, which is significant to determine the propagation direction in the proposed criterion. Typically, for HFs propagation at the weak interfaces with arbitrary direction, the influence of intersection angle, fracture toughness ratio, and rock property on the crack behavior are investigated and discussed meticulously. By combining theoretical and numerical methods , our model can further reveal the interaction mechanism between HFs and different types of weak interfaces in shale, which can help to optimize the design of hydraulic fracturing means and facilitate the generation of complex fracture networks . • An energy-based criterion is proposed for hydraulic fractures propagating at weak interfaces with contrasting properties. • The deflection, crossing, and offset of the hydraulic fractures interacting with the weak interface can be determined. • The stress intensity factors are calculated directly using the XFEM without extra post-processing.