Acidification modification is an effective method to enhance coal permeability for improving coal seam gas (CSG) extraction. The effect of acidification modification varies with the mineral types and contents of different rank coals. Therefore, it is necessary to optimize acidification reagent systems for permeability enhancement in different rank coals. In this study, this optimal reagent system was developed, with components including single acid, surfactant, catalyst, ethanol concentration, and composite acid. Coal samples of different ranks were acid-treated, and the changes in permeability, mineral composition, wettability, and pore–fracture structures were analyzed. Results show that the optimal acidification reagent for high-rank coal is 3% citric acid + 1% formic acid + 1.6% potassium chloride + 0.3% Tween 20 + 30% ethanol, with a reaction time of 6 h. For low-rank coal, the optimal reagent is 4% citric acid + 1% formic acid + 1.6% potassium chloride + 0.3% SDS + 20% ethanol, with a reaction time of 9 h. For medium-rank coal, the optimal reagent is 2% citric acid + 1% formic acid + 1.6% potassium chloride + 0.3% Tween 20 + 10% ethanol, with a reaction time of 9 h. In terms of acidification enhancing permeability, the medium-rank coal with high ash content shows the best performance, followed by the low-rank coal and the high-rank coal. The blended reagents reduce the contact angle between the reagent and the coal sample, resulting in superhydrophilicity. After acidification, the diffraction peaks of calcite and dolomite nearly disappear, and the diffraction peak intensity of kaolinite decreases significantly. Mineral dissolution creates numerous dissolution pores and cracks on the coal surface, improving the connectivity between the pores and fractures. This optimal reagent system provides a valuable reference for applying acidification modification technology to low-permeability coal seams.
Fan et al. (Sat,) studied this question.