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In this study, the effects of the different concentrations of hydrochloric acid, hydrofluoric acid, and acetic acid and a surfactant on the physicochemical characteristics of coal, such as pore diameter distribution, pore fractal dimension, and chemical structures were studied. The wettability performance of the reagent-modified coal was proposed. The results demonstrated that the mineral dissolution rate of HF in coal sample was much higher than those by HCl and HAC treatment, which increases the surface roughness of coal. With the increase of the concentration of multicomponent acid solution, the number of micropores decreased and the number of macropores increased. Moreover, both fractal dimensions D1 and D2 of the coal sample treated with the multicomponent acid comprising 6% HCl, 6% HF, and 6% HAC (#3) were the smallest. This shows that compound reagent #3 is available to enhance the pore size distribution with a better effect than the other five ones. Compared with the raw coal (#7), treatment with high concentrations of HCl (#4) significantly decreased the contact angle on coal (#4), whereas treatment with high concentrations of HF or HAC (#6 or #5), significantly increased it.

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The adsorption behaviors of multiphase fluids in coal are vital for estimating their transfer and flow mechanisms. In this study, taking the classical coal structures as instances, the impacts of maturities and moisture contents of coal on the methane adsorption capacities and thermodynamic properties were analyzed and determined by molecular simulations. Specifically, the simulated pressures were varied from 0 to 16 MPa, and three temperatures (298 K, 323 K, and 348 K) were considered. In addition, the competitive adsorption capacities of water/methane molecules were evaluated and the critical value of moisture content affecting the isosteric adsorption heat of methane was determined. Furthermore, a quantitative relationship between the gas adsorption amount and adsorption equilibrium pressure under a three-phase “solid–gas–liquid” action was established. The simulated results showed that taking the intermediate-rank coals as examples, the absolute adsorption capacities of methane molecules on different moist coals reduced by 28.3%, 35.1%, 46.9%, and 62.3%, respectively (at uniform pressure–temperature status) in comparison to their dry status. In addition, when the moisture content was lower than 1.4 wt%, the average isosteric adsorption heat remained almost stable. However, when the moisture content exceeded 1.4 wt.%, the average isosteric adsorption heat increased sharply with the increase in the water content. Finally, linear function formulas yielded a good fitting accuracy range for the low- and intermediate-rank coals; in comparison, for the high-rank coal, an exponential function formula obtained a better fitting accuracy than a linear one.

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