Abstract: The temperature of liquid nitrogen state is very low (-195.8℃). When it contacts with the rock mass, the cryogenic effect will cause damage to the structure of the rock mass and reduce its physical and mechanical strength parameters. Therefore, the liquid nitrogen can be used as a fracturing fluid for the fracturing of the storage reservoir. In order to study the effect of the low temperature of the liquid nitrogen on the mechanical properties of rock mass, the naturally dried rock samples and fully saturated water samples (Xinjiang limestone, Sichuan limestone and Sichuan grey sandstone) were selected and compared with the tensile and uniaxial compressive strength of rock samples before and after soaking into the liquid nitrogen with different soaking cycles. The results show that, 1) The uniaxial compressive strength and the tensile strength of the rock samples are reduced after being frozen by the liquid nitrogen. 2) Under the natural dry state, the effect of the liquid nitrogen freezing on the strength of the Xinjiang limestone is greater than that of rock samples in other regions. 3) For the saturated same type of rock samples, the effect of the liquid nitrogen freezing can be intensified under the state of water saturation and the rock damage can be induced. The impact on rock strength is significant with the increase of soaking cyclic times. The results can provide the experimental basis for further research on the liquid nitrogen fracturing mechanism.

Key words: liquid nitrogen circulating freezing, failure mode, compressive strength, tensile strength, rock damage