Nuclear magnetic resonance study on the law of methane adsorption in water-bearing medium rank lump coal

To reveal the influence of water on the gas adsorption process of medium rank lump coals, four groups of medium rank coals were selected for the study, and pore analysis experiments and low-field nuclear magnetic resonance (NMR) isothermal adsorption experiments with different water contents were carried out. The microscopic pore structure characteristics of medium rank lump coals were characterized by the combined mercury-pressure method and nitrogen adsorption method; the gas adsorption process of water-bearing coal samples was quantitatively analyzed based on the NMR isothermal adsorption experimental platform, and the influence of the water endowment state on the gas content of lump coal reservoirs was discussed. The results show that medium grade coals are predominantly microporous and small pore, followed by mesoporous, with the least macrospores, and have good gas storage capacity and that the adsorption capacity increases with the degree of metamorphism. With the increase of adsorption time, the gas adsorption of the aqueous coal samples showed a trend of first increasing and then remaining constant, reaching the adsorption equilibrium within 9 h. The adsorption saturation, adsorption increment, and adsorption gas content all increased with the increase of metamorphism, while the non-adsorbed gas content decreased for the same moisture content coal sample. The saturation, increment, adsorption, and non-adsorption gas content decreased with increasing water content for the same coal sample.