Analysis of dynamic thermal behaviors for multi-stage hydraulic fracturing treatments in horizontal shale oil and shale gas wells

Accurate prediction of dynamic thermal behaviors during multi -stage hydraulic fracturing is profound which can avoid performance deterioration of fracturing fluids or failure of fracturing tools caused by severe temperature variation. This paper establishes a transient heat transfer model for multi -stage hydraulic fracturing in horizontal shale oil and shale gas wells (MFHWM), considering the real construction process of multi -stage hydraulic fracturing, the turbulent flow state, the heat source of viscous dissipation and the complex wellbore structure, which enables the accurate simulation of thermal behaviors in the early time of multi -stage hydraulic fracturing with large flow rate. The verification results of measured temperature data and simulation data prove the MFHWM has good precision. Numerical simulations indicate that the temperature distribution exhibits the characteristics of cyclic "cool -down" and "warm -back" and "stair -step" during the multi -stage hydraulic fracturing. In the "cool -down" period, the temperature drops sharply within the first hour and then the deceleration slows down. In the "warm -back" period, the temperature recovers exponentially relying on the shut-in time. The "stair -step" is caused by changing of heat transfer mechanism from convection to conduction. Moreover, the cyclic cooling has little effect on the total temperature drop and subsequent stages are not significantly cooler than previous stages. Besides, primary factors affecting temperature distributions of wellbore-formation system are discussed thoroughly. Results indicate that the viscous heat source cannot be ignored for multi -stage hydraulic fracturing and the temperature difference exhibits cyclic variation when considering and without considering the viscous heat source. In addition, the minimum temperature during each -stage -fracturing operation decreases first and then increases as the injection rate of fracturing fluid increases because of the compromise between the viscous heat source and forced convection. Furthermore, temperature distributions can be controlled by changing the injection temperature of fracturing fluid. As the injection temperature increases, a positive proportional relationship exists between the minimum temperature of fracturing fluid and the injection temperature. MFHWM can be a standalone tool to analyze temperature distributions of multi -stage hydraulic fracturing. It can also be used to provide better boundary conditions for stress analysis model of casing and cement sheath.