Research on cyclic liquid nitrogen fracturing technology for enhancing coalbed methane drainage and its application prospect

Hydraulic fracturing technology is widely used in fracturing coal seam and improving efficiency of coalbed methane drainage, but due to large water consumption, environment pollution by fracturing fluid and other problems, it is difficult to be used in water-deficient areas where are rich in coalbed methane. Based on conventional liquid nitrogen fracturing technology, a cyclic liquid nitrogen fracturing technology is proposed through altering the single injection mode of liquid nitrogen, and application of the technology in enhancing coalbed methane drainage is prospected. The technology uses cyclic injection mode of liquid nitrogen, which can make liquid nitrogen to fulfill micro-crack space continuously, so as to form complex crack networks. The principle of fracturing coal-rock of the technology is coupling mechanism of cyclic cumulative damage, low temperature fracturing and gas fracturing: ① During cyclic injection process of liquid nitrogen, different deformation responses appear among grains with different collocation, and accumulated damages among grains could weaken coal body strength of pressure resistance in local area to some degree, so as to generate new cracks and extend the primary cracks. ② Cold shock of liquid nitrogen to coal-rock leads to break of inner structure of coal-rock, so a large number of micro-cracks are generated and continuously extended with cyclic injection of liquid nitrogen. Meanwhile, water-ice phase transition caused by low temperature property of liquid nitrogen could further damage inner structure of coal-rock. ③ The generated high-pressure gas by vaporization of liquid nitrogen could make coal-rock volume to repeatedly extend and shrink, which further enhances growth and connection. Cyclic liquid nitrogen fracturing technology, which takes liquid nitrogen as fracturing fluid, greatly relieves dependence on water resources, and has no pollution to environment. The technology can be applied in coalbed methane drainage.