Thermal cracking characteristics of high-temperature granite suffering from different cooling shocks

Cooling shock can be considered a potential method of causing the high-temperature rocks to crack and achieve the most efficient exploitation and utilization of geo-thermal energy in the future. However, it is important to accurately recognize the thermal cracking effect and the corresponding typical characteristics of cooling shock. Therefore, in this study, we conducted a systematic experiment on the effects of cooling shocks with different temperature gradients on the cracking of high-temperature granite. The granite samples were heated to 150, 350, 550, and 750 degrees C, and then injected with three kinds of refrigerants of 20, 0 and - 20 degrees C into the granite boreholes. Furthermore, the cracking characteristics of granite were compared by means of optical microscope. According to the experimental analysis, several conclusions could be obtained: (1) Compared with natural cooling conditions, cooling shocks of 20, 0, and - 20 degrees C resulted in no evident open cracks on the granite at 150 and 350 degrees C; however, the distribution of the micro-crack networks became denser with a decrease in the refrigerant temperature. (2) For the high-temperature granite samples at 550 and 750 degrees C, the effect of the cooling shock was significant, and localized open cracks could be observed; however, several differences were evident in the effect of granite cracking under different combinations of cooling shocks and high temperatures. For granite with the same temperature gradient, with the decrease in the refrigerant temperature, the number of inter-granular and trans-granular cracks increased, and the cooling shock enhanced the cracking effect. (3) The main factor of granite cracking was the anisotropy of the mineral particles affected by the temperature difference, in which a large amount of quartz was contained in the granite, and the effect of repeated phase transformation near 573 degrees C was remarkable. Moreover, with the temperature difference between the refrigerant and the heated samples increasing, the generated tensile forces in the outer edge of samples would increase rapidly, causing amounts of trans-granular cracks and leading to the denser micro-crack networks. This work can provide an experimental reference for understanding the effect of cooling shock on mechanical properties and cracking of high-temperature rocks.