Impact of N-Methyl-2-pyrrolidone Erosion on Surface Functional Groups and Pore Evolution in Coals of Different Ranks

In low-permeability coal reservoirs, utilizing the organic solvent N-methyl-2-pyrrolidone (NMP) has emerged as an effective approach to improving the coal pore structure and enhancing coalbed methane productivity. However, the exact mechanisms of how solvent erosion alters functional groups and develops pores remain incompletely understood. This study utilized Fourier transform infrared spectroscopy and low-field nuclear magnetic resonance to assess the impact of NMP on the functional groups and pore structures of lignite, bituminous coal, and anthracite. The results indicate that a 6 h treatment with NMP led to an increased proportion of oxygen-containing functional groups in all coal samples, accompanied by a decrease in hydroxyls and aliphatic hydrocarbons. The aromaticity of the coal samples was enhanced to varying degrees, most notably for lignite. In terms of pore modification, the porosity of lignite and bituminous coal increased by 84.82 and 43.56%, while anthracite experienced a porosity increase of 3.04%, indicating a diminished effectiveness of NMP as the coal rank increased. These findings suggest that NMP selectively dissolves specific organic molecules in coals, thereby enhancing pore connectivity and promoting a transition from micro- to meso- and macropores. These findings highlight the potential of NMP in enhancing coalbed methane production and advance our understanding of the mechanisms behind solvent erosion.