The quantitative evaluation of pore-fractures which links to methane storage space and transportation channels is of great significance for underground gas extraction and coalbed methane development. This study is aimed at revealing the surface roughness and structure irregularity of typical bituminous coal samples using the combination of SEM, box-counting method, and low-pressure N₂ adsorption (LPN₂A). Pore-fracture fractal dimension is calculated based on the SEM images, and its influencing factors involving coal maturity, tectonic deformation, coal components, and pore structure have been explored in depth. The results indicate that pore morphology varies greatly among these samples. Some wedge-shaped macopores (diameter > 50 nm) and bottle-shaped pores are identified in bituminous C coals, and macropores are the dominant pore types, while macropores only account for less than 23% of pore volume in the high-rank bituminous coals, suggesting that mesopores take a leading role in coal with rank of bituminous A. These samples have different pore-fracture fractal dimensions with D ranging from 1.28 to 1.94, and D is distributed in sequence: bituminous A > bituminous B > bituminous C. The ash content plays a vital role in structure irregularity, and it can reduce the fractal dimension due to the filling in the coal pore-fractures. However, as the increase of burial depth and tectonic deformation, a large proportion of macropores and fractures are transformed into smaller pores including mesopores and micropores, leading to the growing surface heterogeneity. This work is expected to provide a better understanding of complex surface characteristics in coal.

定量评价连接瓦斯储存空间和运输通道的孔隙裂缝对地下瓦斯开采和煤层气开发具有重要意义。本研究旨在结合扫描电镜、箱数法和低压N ₂吸附（LPN ₂一个）。基于扫描电镜图像计算孔隙裂缝分形维数，深入探讨了煤成熟度、构造变形、煤组分、孔隙结构等影响因素。结果表明，这些样品的孔隙形态差异很大。烟煤C煤中发现了一些楔形大孔（直径> 50 nm）和瓶形孔，大孔是主要的孔隙类型，而大孔仅占高级烟煤孔隙体积的不到23% , 表明中孔在烟煤 A 级煤中起主导作用。 这些样品具有不同的孔隙裂缝分形维数, D范围从 1.28 到 1.94, D其分布顺序为：烟煤A>烟煤B>烟煤C。灰分含量对结构不规则性起着至关重要的作用，由于填充在煤的孔隙裂缝中，它可以降低分形维数。然而，随着埋深和构造变形的增加，很大比例的大孔隙和裂缝转化为包括中孔和微孔在内的较小孔隙，导致地表非均质性不断增强。这项工作有望更好地理解煤中复杂的表面特征。