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As an innovative technology for the stimulation of coalbed methane, liquid nitrogen cold soaking can produce more fractures in coal body and effectively increase the permeability of coal seam. The pore structure distribution of coal affects the permeability of coal and determines the flow and occurrence state of coalbed methane and other fluids in coal. This paper carries out a number of nuclear magnetic resonance tests on coal samples with different coal qualities treated by liquid nitrogen cold soaking and studies the influence law of liquid nitrogen soaking on coal pore structures. It is found that the liquid nitrogen makes the T2 spectral peak shifts to the right, and the anthracite changes from interval T2 spectrum to type T2. It indicates that liquid nitrogen cold soaking in the transformation of pore size to large size improves the pore connectivity and leads to more pore structures. With an increase in times of liquid nitrogen cold soaking, the size and number of coal pore structures gradually increase, and the increasing size is on the order of anthracite > bituminous > lignite. There is a positive correlation between the peak area and the times of liquid nitrogen cold soaking. The porosity and permeability of each coal sample increase with the times of liquid nitrogen cold soaking. From the MRI images of coal samples, it can be found that the liquid nitrogen cold soaking makes the microcracks extend and forms a crack network with other cracks, thereby causing macroscopic damage. The research results are helpful to further reveal the microscopic mechanism of liquid nitrogen cold soaking on coal damage. [ABSTRACT FROM AUTHOR]

In this paper, the pore structure and fractal characteristics of coal under liquid nitrogen cold soaking are tested for different coal qualities of anthracite, bituminous coal and lignite, and the variation patterns of BET specific surface area, BJH total hole and adsorption pore specific surface area with liquid nitrogen cold soaking times are studied. Among these coal samples, it is found that the raw bituminous coal has the most internal micropore content and the largest adsorption space for gas, followed by anthracite, and lignite is the last. After one time liquid nitrogen cold soaking, the increase of adsorption and percolation pore contents of each coal sample in the order of: anthracite > lignite > bituminous coal; and with the increase of liquid nitrogen cold soaking times, the internal pore size and number of pores of the three coal samples gradually increases, but the specific surface area of adsorption pores decreases. The reduction amplitude of the specific surface area of coal adsorption pores by one time liquid nitrogen cold soaking gradually increases with the increase of metamorphism. With the increase of liquid nitrogen cold soaking times, the average pore size, total BJH hole and percolation hole of each coal sample increases, while the BET specific surface area and adsorption pore specific surface area decreases gradually. The fractal dimension D 1 of each coal sample increases linearly with the increase of liquid nitrogen cold soaking times, and the variation of D 2 has an opposite trend. The results of this study can provide theoretical and experimental support for further application of liquid nitrogen to increase the permeability of coal seams. • Pore structure and fractal characteristics of coal under the liquid nitrogen cold soaking are tested. • Liquid nitrogen cold soaking could improve the coal pore connectivity. • Fractal dimension D 1 of coal increases linearly with the liquid nitrogen cold soaking number. [ABSTRACT FROM AUTHOR]