Effects of phase-transition heat on fracture temperature in self-propping phase-transition fracturing technology

The thermal flux curve of phase-transition fluid (PF) was tested using differential scanning calorimetry, based on which a reaction kinetics model was established to reflect the relationship between phase transition conversion rate, temperature and time. A temperature field model for fractures and rock matrix considering phase transition heat was then constructed, and its reliability was verified using previously established temperature field models. Additionally, the new model was used to study the effects of different injection parameters and phase-transition fracturing performance parameters on the temperature variations in fractures and matrix. The study indicates that, at different positions and times, the cooling effect of the injected cold fluid and the exothermic effect during the phase transition alternately dominate the temperature within the fracture. At the initial stage of fracturing fluid injection, the temperature within the fracture is high, and the phase transition rate is rapid, resulting in a significant impact of exothermic phase transition on the reservoir rock temperature. In the later stage of injection, the fracture temperature decreases, the phase transition exothermic rate slows, and the cooling effect of the fracturing fluid on the reservoir rock intensifies. Phase transition heat significantly affects the temperature of the fracture. Compared to cases where phase transition heat is not considered, when it is taken into account, the temperature within the fracture increases to varying degrees at the end of fluid injection. As the phase transition heat increases from 20 J/g to 60 J/g, the maximum temperature rise in the fracture increases from 2.1 °C to 6.2 °C. The phase transition heat and PF volume fraction are positively correlated with fracture temperature changes, while specific heat capacity is negatively correlated with temperature changes. With increasing injection time, the temperature and phase transition rate at the fracture opening gradually decrease, and the location of the maximum phase transition rate and temperature difference gradually shifts from the fracture opening to about 10 m from the opening.