Numerical Study of Hydraulic Fracture Propagation in Orthotropic Shale Reservoirs by Using the XFEM

Most reservoir matrices exhibit inherent orthotropy thatmay havea great impact on the propagation behavior of a hydraulic fracture.Modeling of fracture propagation in orthotropic materials by utilizingthe extended finite element method (XFEM) remains a challenging issue.In this work, an XFEM model with special crack tip enrichment functionsis established to further study the propagation mechanism of hydraulicfractures in orthotropic reservoirs. The modified fracture propagationcriterion considering rock toughness anisotropy is introduced to reformulatethe fracture propagation process and clarify the propagation path.The interaction integral method for an orthotropic material is adoptedto calculate the mixed-mode stress intensity factors. Then, the presentmodel is verified against the analytical results in the literatureand then applied to simulate the fracturing process under differentorthotropy conditions. Results show that the rock orthotropy, suchas material angle or elastic modulus ratio, has a strong effect onthe propagation trajectory and distribution of hydraulic fractures.In addition, the in situ stress will weaken the influence of rockorthotropy on fracture propagation if the direction of maximum insitu stress is different from the principal axis of the larger elasticmodulus. The findings can provide in-depth understanding of the hydraulicfracturing process in orthotropic formations.