Abstract: Liquid nitrogen fracturing is a technique using liquid nitrogen as a fracturing fluid to stimulate the reservoir. During the treatment of liquid nitrogen fracturing, the cryogenic nitrogen is injected into formation, which leads coal rocks damage and increases coal rocks permeability. To reveal the laws of permeability variation after cryogenic nitrogen injection, the permeability tests are conducted on coal rocks before and after cryogenic treatment with pressure pulse decay method. By comparing the permeability results of dry and saturated coal rocks at different temperatures, it is found that the permeabilities of coal rock are all improved after 30 minutes cryogenic treatment respectively at -10 ℃, -25 ℃, -40 ℃ and liquid nitrogen (-195.8 ℃). Saturated coal rocks have a higher increasing rate of permeability than dry coal rocks. With the decrease of temperature, the permeability of coal rocks increases by a large amplitude. An exponential relation between the increasing rate of coal rock permeability and the temperature is obtained. The test results are analyzed to reveal the principles of coal rocks damage and permeability enhance influenced by low temperature.

Key words: liquid nitrogen, coal rock, rock damage, cryogenic treatment, permeability