Freeze–Thaw Response of Permeability and Absorption Channel Structure and Moisture Distribution in Different Coal Ranks

Abstract

Low-permeability coal seams are widely distributed in China, with significant differences in coal ranks and properties. Identifying an effective method for nitrogen fracturing is an urgent challenge. To study the impact of coal ranks on fracturing, lignite, bituminous coal, and anthracite were used in liquid nitrogen freeze–thaw experiments. Low-field nuclear magnetic resonance was used to measure T₂ curves, porosity, and pore throat distribution during the freeze–thaw process. The fractal characteristics of pore microstructure and the dynamic evolution of unfrozen water were analyzed. The results indicate that liquid nitrogen freeze–thaw promotes pore development in coal of all ranks. Lignite, with its high moisture content and abundant pore structure, showed the most significant transformation effect, followed by bituminous coal and anthracite. After a single freezing–thawing cycle, the pore growth rates of lignite, bituminous coal, and anthracite are 135.98%, 104.17%, and 53.65%, respectively. Additionally, the transformation effect on different types of pores shows different characteristics. The distribution of adsorption pore throats slightly decreases, while the increase in distribution of permeable pore throats follows the order: lignite > bituminous coal > anthracite. The fractal dimension D_A of adsorption pores is less than 2, indicating no fractal characteristics, while the fractal dimension D_S of permeable pores is greater than 2.9, showing significant fractal characteristics. During the freezing process, lignite exhibits the greatest decrease in unfrozen water content, while during the thawing process, all three coal samples show a sudden increase in unfrozen water content, with bituminous coal showing the smallest increase, only 1836.49.