Study on the Wetting Mechanisms of Different Coal Ranks Based on Molecular Dynamics

The exploration of coal wettability is not only of paramount significance in the mitigation of coal dust and the development of coalbed methane, but it also provides crucial technical support for realizing the geological storage of CO2 within the &lsquo:dual-carbon&rsquo: background. Molecular simulation serves as an effective means by which to investigate coal wettability at the microscopic level. This study employed a molecular dynamics simulation to investigate the wettability of coal across 13 distinct coal ranks. Through the analysis of trajectory files, and the incorporation of experimental data during the modeling process, the mechanisms governing the evolution of wettability were revealed. The results demonstrated that the contact angle on the surface of coal increases with the elevation of coal rank. The molecule relative concentration analysis revealed that, with increasing coal rank, the overlap range between water droplets and the coal slab decreases, the height increases, and the diffusion degree of water molecules decreases, which are outcomes consistent with the results of the contact angle measurement. The contact angle was strongly correlated with the number of hydrogen bonds and secondarily correlated with the numbers of carbonyls, hydroxyls, and ether oxygens. The formation of hydrogen bonds was notably correlated with the number of hydroxyls, followed by that of ether oxygens, while its correlations with carbonyls and carboxyls were comparatively weaker. The contact angle exhibited positive correlations with vitrinite reflectance and carbon content, while showing negative correlations with oxygen content, H/C, and O/C. Additionally, it demonstrated positive associations with total sp2 carbon (fa), aromatic carbon (fa&prime:), and non-protonated aromatic carbon (faN), and negative associations with aliphatic carbon (fal) and methylene carbon (falH). Understanding the variations in wettability among different coal ranks can provide a foundational model and theoretical basis for further exploration of the complex interactions among coal, gas, and water across various coal ranks.