Experimental investigation on the breakdown behaviours of sandstone due to liquid nitrogen fracturing

Liquid nitrogen (LN) fracturing is an efficient technology for the development of unconventional gas resources. Due to continuous injection of cryogenic LN, the reservoir temperature decreases and then cooled rock regions are gradually generated during fracturing process. To investigate the breakdown behaviours of sandstone induced by LN fracturing, laboratory simulation experiments were performed with a self-designed setup. The experimental results showed that rising initial rock temperature greatly reduced the breakdown pressure of sandstone, whereas hardly affected the fracture complexity. Under cryogenic conditions, both the compaction and cracking effects exerted influence on the pore structure of sandstone. Thus, the change in breakdown pressure depended on which effect played a domain role. Although the cryogenic conditions probably made the breakdown pressure of sandstone increase, the fracture complexity was expected to be significantly improved. In addition, the initial rock temperature increasing played a positive role in improving the fracture complexity under cryogenic conditions. For initial or heating sandstone samples, the fracture pattern mainly presented in the form of bi-wing fractures. However, for cryogenic samples, the fracture patterns contained both local fractures and complex fractures. The former was generated with lower breakdown pressure while the latter was generated with higher breakdown pressure. Additionally, the gas leak regions were observed in cryogenic samples during LN fracturing. Under cryogenic conditions, the sandstone used was favourable to generate more complex fractures and the permeability of rock matrix in unbroken region was proved to be improved due to LN fracturing. Collectively, fracturing with LN could improve the fracture complexity and the permeability of rock, which was very beneficial for well production.