## 1 Introduction  
Coal reservoirs have complex pores and fractures. The pore characteristics of coal in these reservoirs, including porosity, pore structure, and specific surface area (SSA), can directly affect adsorption, desorption, diffusion, and seepage. Goaf caving, auxiliary ventilation equipment, and the drainage method also have a great influence on the productivity of methane [1]. These all can subsequently influence the commercial development of coalbed methane (CBM) [2]. The United States, Canada, and Australia have been successful in developing their CBM resources [3], and Poland has been working on it [4]. The production of CBM from coal in China is gradually increasing. CBM recoverable resources of anthracite in China account for approximately 25% of the country's total CBMresources, and Qinshui Basin is the most successful block for the exploitation of high-rank coal [5; 6]. The CBM production from Qinshui Basin in 2013 reportedly represented more than 85% of the entire amount of CBM production in China [7]. A number of methods have been used for investigating the characteristics of coal pore geometry, including intrusive methods, nonintrusive methods, and image analysis. The intrusive approaches comprise mercury intrusion porosimetry (MIP), low-pressure N\({}_{2}\) adsorption (LP-NA), low-pressure CO\({}_{2}\) adsorption (LP-CA), and nuclear magnetic resonance (NMR). The nonintrusive methods include small/ultra-small angle neutron scattering and computed tomography (CT). The image analysis methods primarily involve scanning electron microscopy (SEM) and the high-resolution focused ion beam technique [8; 9]. Each method, however, has limitations in characterizing the pore size distribution (PSD) of coal because of theoretical or material differences and is only able to detect pores of a certain size [10]. For example, high-pressure MIP can twist pores because of the compressibility of the coal matrix [11; 12]. LP-NA is suitable for characterizing pore with sizes between 2 and 100 nm [13], whereas LP-CA can only clearly describe pores smaller than 1.5 nm. SEM, as an intuitive detection method, does not supply quantitative data for entire specimens, whereas CT scans can only recognize macropores and micro-cleats. To overcome these limitations and deficiencies, the combined use of three investigative methods is a practical approach to accurately ascertain the comprehensive PSD of coal specimens [14; 15]. [PERSON] et al. [16] found that the pore volume (PV) of micropores is the largest, followed by those of transitional pores, macropores, and mesopores in low- and medium-rank coals (\(R_{\text{0, max}}\), 0.225-0.98%). [PERSON] et al. [17] employed an MIP test to characterize 42 coal samples collected from different low-rank coal-bearing basins, and their results showed that the transitional pore and macropore contents were the highest. [PERSON] et al. [18] investigated the characteristics of semi-anthracite and found that super-micropores were especially highly developed. [PERSON] et al. found that decreasing coal particle sizes resulted in a continuous increase of macro- and mesopore volumes and specific surface areas [19] and that mylonitic coals have more ink-bottle pores [20]. In contrast, anthracite in China has the following characteristics: highly developed super-micropores and micropores, widely distributed pore structure, minuscule capillaries, high tortuosity, and inadequate pore connectivity [21; 22]. The main factors that affect the PSD are coalification, maceral composition, and degree of structural damage [23]. The total pore volume (TPV) changes to the \"U\" type with the increase of coalification. First, a rapid decrease occurs until the maximum reflectivity (\(R_{\text{0,max}}\)) approaches 2.3-2.5%. It thereafter gradually increases with the generation of secondary pores or secondary fractures. The characteristics of different pores vary with the increase in coalification. The macro-mesopores decrease, whereas the super-micropores and micropores exhibit opposite dynamics [24].