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

Abstract

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