A shear hardening model for cohesive element method and its application in modeling shear hydraulic fractures in fractured reservoirs

Although the cohesive element method has been widely used for modeling hydraulic fracturing, the hardening properties of shear strength and shear fracture energy of rock matrix and natural fracture with the increasing of confining pressure have not been applied to the constitutive law of cohesive element for a long time. In this study, a shear hardening model and the corresponding user subroutine USDFLD are proposed and verified. Numerical results show that the shear hardening model has an excellent performance in modeling initial geostress state and shear hydraulic fracture (SHF) propagation in fractured reservoirs. The distribution and proportion of SHF affected by natural fracture asperity, azimuth of natural fracture network, and stress field are analyzed and discussed. The results suggest that SHF is a significant part of the hydraulic fracture network in fractured reservoirs with a proportion of 18%-61%, and plays a vital role in enhancing the complexity of hydraulic fracture network. The SHF has the potential to become the main type of the fracture network, indicating that its contribution in enhancing the conductivity of fractured reservoirs cannot be ignored. The macro width of the hydraulic fracture network and the tortuosity of the main hydraulic fracture decreases as asperity increases. High stress difference is a favorable stress environment for the development of SHF. As the absolute confining pressure of fractured reservoir increases, the possibility of shear hydraulic fractures becoming the primary fracture increases significantly.