Experimental study on the breakdown mechanism of high temperature granite induced by liquid nitrogen fracturing: An implication to geothermal reservoirs

To reveal the breakdown mechanism of dry hot rock (HDR) induced by liquid nitrogen (LN) fracturing, the laboratory tests were performed on high temperature granite using specialized apparatus. The breakdown pressure and fracture morphology of high temperature granites subjected to different heating temperatures and thermal treatments were analyzed. The results showed that the breakdown pressure of granite decreased with heating temperature increasing. As the heating temperature increased from 25 °C to 300 °C, the breakdown pressure of high temperature granite decreased by 40.63%. In addition, the failure mode mainly presented in the form of tensile fractures, which developed into bi-wing fractures along the hole axis. For heating temperature higher than 220 °C, the high temperature granite presented an increase in fracture complexity with the increased heating temperature. After pre-cooled with LN, the breakdown pressure was lowered and the fracture complexity was enhanced. For example, the breakdown pressure of LN-treated sample was 4.61%-27.70% lower than heated sample. Under cryogenic conditions induced by LN, the failure mode mainly presented in the form of randomly distributed tiny cracks and holes, which made the breakdown pressure decreased by 8.67%-59.46%. LN fracturing could cause multiple cracking effects including thermal fracturing (i.e., thermal shock, cryogenic damage and cryogenic cracking) and pressure-induced fracturing on HDR. Importantly, the thermal fracturing effect could reduce the breakdown pressure and improve the fracture complexity.