Investigation on the thermal cracking of shale under different cooling modes

Abstract

Thermal stimulation method can be effectively utilized to enhance the flow performance in tight porous media with an induced thermal shock (TS) using the injection of cryogenic fluids. In order to reveal the thermal cracking behavior and the mechanism of shale under different cooling modes, thermal cooling laboratory experiments of shale samples were carried out in the current paper. Combined the numerical simulations and laboratory experiments, the thermal cracking behavior of shale under different cooling modes was studied. The laboratory experiments results show that the macroscopic thermal cracking tended to occur under thermal shock at a temperature of more than 400 °C. A heat transfer numerical simulation method under different cooling modes was established to analyze the temperature field distribution and the variation in the temperature gradient of shale from the perspective of heat transfer. For different cooling modes, the evolutionary trend of temperature gradient, thermal shock factor and dynamic thermal stress formed within the shale sample was consistent. The maximum values always occurred near the surface of the sample. Thermal shock factor, which can describe the ability of heat to cause damage to rocks, was introduced. The thermal shock factor could realize the quantitative classification of thermal cooling capacity. According to the evolutionary pattern of thermal shock factor, the specific time of the most serious internal fracture of shale samples could be determined. Dynamic thermal stress generated inside the shale under different cooling modes was also calculated to analyze the cause of shale cracking. The difference in the convective heat transfer coefficient of various cooling media was closely related to the fracture propagation. Thermal stimulation will be an efficient and a promising means to generate more complex fracture networks for the development of unconventional tight shale reservoirs using the combination of formation heat treatment (FHT) and thermal shock (TS).