Abstract: The petrophysical properties and structures of rock materials are changed when they contact with cryogenic nitrogen that has an extremely low temperature of ?195.8℃, and thus the principle can be used to improve reservoir fracturing. To study the effect of cryogenic nitrogen freezing on rock strength and mechanical parameters, different types of rock are selected, such as marble, sandstone and granite with different moisture states (dry and saturated), to freeze them with cryogenic nitrogen, and then to determine tensile strength and uniaxial compressive strength of these samples before and after freezing. The results show that the uniaxial compressive strength, tensile strength and elastic modulus of rocks are decreased after freezing with cryogenic nitrogen. When the rock sample is dry, the influence of cryogenic nitrogen freezing on the strength of marble is greater than that of red sandstone. However, when the rock sample is saturated, the influence on red sandstone is greater than that on marble. When the saturated rock is frozen with cryogenic nitrogen, there is a break point on the elastic deformation stage of stress-strain curve. For the same type of rock, the effect of cryogenic nitrogen freezing on the damage of saturated rock is more obvious than that of dry rock. Scanning electron microscope (SEM) tests are further conducted on these three types of rock and particularly the marble samples are analyzed in detail. It is found that intergranular fractures occur after cryogenic nitrogen freezing. This study provides an experimental basis for further studying the mechanism of induced fracturing by cryogenic nitrogen.

Key words: cryogenic nitrogen freezing, compressive strength, tensile strength, moisture states, rock damage