Numerical study of the effect of natural fractures on shale hydraulic fracturing based on the continuum approach

Abstract Because of the complexity of shale reservoirs and multiple physical coupling processes, numerical implementation remains a challenge in engineering applications. Unlike conventional reservoirs, shale reservoirs always contain two main discontinuities: bedding interface and natural fractures. The existence of these discontinuities is the key reason a fracture network is formed. Therefore, the numerical model should contain both bedding interface and natural fractures. In this paper, the fracturing propagation process is described based on continuum theory. In contrast to other research, an anisotropic strength criterion is adopted to consider the anisotropic properties of shale, including the failure of both shale matrix and discontinuities. The criterion is described by the Mohr-Coulomb model combined with tensile failure. When fractures are generated in the shale rock mass, the stiffness and permeability of the fractured rock are homogenized in an element, and volume fracturing is represented by the deformation of three orthogonal fractures in the rock element. Simultaneously, the anisotropic permeability is also enhanced. Hence, based on this continuum approach and finite element method, a three-dimensional model for shale hydraulic fracturing simulation is presented. This model fully considers the direct coupling of fluid flow and rock deformation during hydraulic fracturing. To verify the anisotropic strength criterion, a shale compression test is conducted. Through a comparison of the numerical and experimental results, the applied criterion can describe the shale compressive strength anisotropy. In addition, the KGD analytical model is also adopted to verify the performance of the developed model for hydraulic fracturing. Finally, several cases of shale hydraulic fracturing are simulated, and the results show the geometry of the fracture region under different conditions. Furthermore, the impacts of three factors on hydraulic fracturing geometry are considered, including natural fractures, spacing of perforations and differential in situ stress. The influence of these factors on the complexity and scale of the fracturing region is also discussed.