Coupled thermo-hydro-mechanical cohesive phase-field model for hydraulic fracturing in deep coal seams

A coupled thermal-hydro-mechanical cohesive phase-field model for hydraulic fracturing in deep coal seams is presented. Heat exchange between the cold fluid and the hot rock is considered, and the thermal contribution terms between the cold fluid and the hot rock are derived. Heat transfer obeys Fourier's law, and porosity is used to relate the thermodynamic parameters of the fracture and matrix domains. The net pressure difference between the fracture and the matrix is neglected, and thus the fluid flow is modeled by the unified fluid-governing equations. The evolution equations of porosity and Biot's coefficient during hydraulic fracturing are derived from their definitions. The effect of coal cleats is considered and modeled by Voronoi polygons, and this approach is shown to have high accuracy. The accuracy of the proposed model is verified by two sets of fracturing experiments in multilayer coal seams. Subsequently, the differences in fracture morphology, fluid pressure response, and fluid pressure distribution between direct fracturing of coal seams and indirect fracturing of shale interlayers are explored,and the effects of the cluster number and cluster spacing on fracture morphology for multi-cluster fracturing are also examined. The numerical results show that the proposed model is expected to be a powerful tool for the fracturing design and optimization of deep coalbed methane.