Phase-field cohesive zone modeling of hydro-thermally induced fracture in hot dry rock during liquid nitrogen fracturing

Liquid nitrogen (LN2) fracturing is regarded as a viable alternative for the efficient development of hot dry rock (HDR) resources due to its main advantage in producing complex fracture networks and lowering breakdown pressure. However, the fracturing mechanisms and major factors controlling the LN2 fracturing efficiency are still poorly understood. A thermo-hydro-mechanical-damage (THMD) coupling model is proposed to study the fracture initiation/propagation behavior of LN2 fracturing under various fracturing parameters and reservoir conditions based on a phase-field cohesive zone method (PF-CZM). The characteristic differences between LN2 and water fracturing are compared. Results indicate that the tensile stress of LN2 fracturing is concentrated at the tip of fractures. When the HDR reservoirs are undertaken lower stress level with higher initial rock temperature, the damage ratio can be enhanced. Young's modulus and thermal expansion coefficient are the key petrophysical and mechanical properties affecting the LN2 fracturing performance. The optimal pressurization rate is 0.2 MPa/s. It indicated that a balanced contribution of thermal stress and fluid pressure is crucial to the bifurcation of major fracture and forming complex fracture networks. The major findings of the study are expected to provide theoretical guidance and computational simulation basis for the LN2 fracturing on HDR reservoirs.