Thermal characteristics analysis with local thermal non-equilibrium model during liquid nitrogen jet fracturing for HDR reservoirs

Highlights: • The thermal characteristics in HDR fracturing by liquid nitrogen is studied. • The model is carried out by the thermal-hydraulic-mechanical coupling method. • The LTNE model is employed to simulate the rapid heat transfer in porous rock. • Effects of five key parameters on rock temperature are analyzed in detail. This paper presents a numerical analysis of thermal characteristics in bottom-hole rock during liquid nitrogen jet fracturing for hot dry rock (HDR) reservoirs. The standard κ-ε and Darcy models are employed to calculate the fluid flow in fluid region and porous-rock region respectively. The heat transfer between flowing fluid and rock matrix is simulated by the local thermal non-equilibrium model. The thermal stresses are evaluated based on thermo-elastic model. Furthermore, a sensitivity study is conducted to investigate the influences of different factors on heat transfer efficiency in porous rock. The results show that the fluid has a relatively high velocity in the rock region impacted by high-speed jet. The flow transport in porous rock can significantly improve the cooling performance of hot rock. The thermal stresses induced by temperature gradient are huge enough to create thermal cracks in formation. The parametric analysis indicates that increasing the values of ambient pressure, rock permeability and convection coefficient can promote the heat transfer in porous rock obviously, while the parameters of pressure drop and rock porosity have a slight impact on the cooling rate. The results in this study would offer guidance for the design of HDR fracturing with liquid nitrogen jet.