Multiple Fractal Characteristics of Nanoscale Pore Structure and Surface Morphology in Low-Rank Coal Macerals: Implications for Coalbed Methane (CBM) Exploitation

Abstract

Maceral composition critically controls nanoscale pore structure and influences methane behavior in low-rank coal reservoirs, yet the role of individual macerals remains inadequately quantified. Therefore, vitrinite, semifusinite, and fusinite of low-rank coal were characterized by atomic force microscopy (AFM) and micro-Raman spectroscopy to reveal their surface morphology, nanoscale pore parameters, and chemical structure. Bulk coal samples enriched in different maceral assemblages were subjected to low-pressure CO2 adsorption, low-temperature N2 adsorption, and high-pressure mercury intrusion to quantify the full-scale pore size distribution. Vitrinite exhibits the highest surface fractal dimension, the largest pore number, the highest micropore proportion, and the smallest mean pore size, forming an adsorption-favorable nanostructure. Fusinite has the largest mean pore size and a pore system dominated by mesopores and macropores, favoring diffusion and desorption, while semifusinite shows transitional characteristics. Raman analysis further indicates that fusinite has higher aromatic condensation and larger structural domains, whereas vitrinite is enriched in amorphous carbon and aliphatic functional groups. At the bulk coal scale, vitrinite-rich bright coal C2 is micropore-dominated, with micropores contributing 91.79% of the total specific surface area, whereas the inertinite-rich semi-dull coal develops abundant macropores but poor connectivity. The V/(V + I) ratio decreases from bright coal to dull coal and may provide a practical parameter for reservoir evaluation. Differentiated CBM exploration and development strategies are suggested. Bright and semi-bright coals are suggested to be prioritized for sweet-spot identification. Fracturing should connect the micropore-dominated matrix to the macro-fracture network in vitrinite-rich zones and bypass mineral-clogged pore throats in inertinite-rich zones.