A Thermal-Poro-Elasto-Plastic Coupled Model of Hydraulic Fracturing in Deep Reservoirs

The production of deep oil and gas reservoirs faces the problem of ‘three highs’ (high temperature, high pressure, and high in-situ stress), resulting in the transformation of rock mechanical properties from elasticity to plasticity. The hydraulic fracturing process in deep reservoirs involves complex multi-physical effects, and the rock mass may exhibit extensive plastic deformation. The prevailing fracture propagation models based on the linear elastic fracture mechanics are no longer applicable to predict the evolution process of hydraulic fractures in deep formations. In this paper, a thermal-hydro-elasto-plastic coupled model of hydraulic fracture propagation in deep reservoirs is established based on the elasto-plastic theory, Bio’s theory, and local non-thermal equilibrium theory. The Drucker-Prager plasticity model, Darcy’s law, cubic law and cohesive zone model are employed to describe the plastic deformation, matrix flow, fracture flow and evolution of hydraulic fractures, respectively. The nonlinear stress field with strong discontinuities is solved by combining the finite element method and return-mapping method. The discontinuous pressure field and temperature field are simulated by the discrete fracture model and the finite volume method. A dual-layer iterative procedure is developed to solve the strong nonlinear coupling problem based on the fixed-stress split method, sequential iteration method, Picard iterative method and Newton-Raphson iterative method. Then, the proposed model is verified against analytical solutions. Finally, a series of numerical cases are performed to investigate the influences of thermo-hydro-mechanical coupling on hydraulic fracture propagation. Results show thermal stress generated by the temperature difference between fracturing fluid and rock can reduce fracture extension pressure, and cooling effect can reduce the adverse effect of rock plasticity on hydraulic fracturing. In addition, the increase in fluid temperature during the fracturing process leads to a decrease in viscosity, which can also reduce fracture extension pressure.