As organic porous medium, coal has a naturally well-developed pores and cracks system with obvious fractal characteristics, which is the main place for gas storage and migration. Understanding the fractal characteristics of the pore structure is helpful to reveal the gas storage and migration mechanism, hence to provide theoretical support for the prevention and control of gas disasters, the development and utilization of coalbed methane resources, and the geological storage of greenhouse gases. High-pressure mercury injection (HPMI), low-pressure nitrogen adsorption (LPGA-N₂) and scanning electron microscopy (SEM) were used to characterize multi-scale pore structure of coal samples with different ranks. Based on the Frenkel-Halsey-Hill (FHH) fractal theory, the Menger sponge model and the Pores and Cracks Analysis System (PCAS), the pore volume complexities (D_v), the coal surface irregularities (D_s) and pore distribution heterogeneities (D_p) were accessed, respectively. Combined with high-pressure isothermal gas adsorption experiment, the effect of the three fractal dimensions on the gas adsorption ability was also analyzed. The results indicate that the pore structure of coal mass has obvious fractal characteristics, and the fractal dimension of pore structure reflects the complexity of the dual pore networks. The pore structure fractal dimension is largely affected by the coal metamorphism degree. As the metamorphism degree increases, the fractal dimension D values of coal pore structure show an asymmetric U-shaped trend, i.e., gently decrease first and then sharply increases. The three fractal dimensions have different effects on the gas adsorption ability of coal. Langmuir volume V_L has an obvious positive correlation with D_s values, but less correlation with D_v and D_p. While, Langmuir pressure P_L is mainly affected by the combined action of D_v and D_p.
 

multi-scale pore structure,fractal characteristics,gas adsorption ability,different rank coals