Molecular Dynamics Simulation of the Gas Hydrate Nucleation Process in a Coal Matrix

To investigate the mechanism by which gas hydrates nucleate within a coal matrix, a graphene slit model with properties similar to those of coal was constructed to represent the coal matrix in molecular dynamics simulations. Molecular dynamics simulations of gas hydrate formation were then conducted under different temperature and pressure conditions in both coal matrix systems and pure water systems. The results revealed that the pure water system generated significantly more clathrate cage structures than the coal system did. In the coal matrix, the evolution of density distribution reveals a competitive migration mechanism where methane transitions from "wall adsorption" to "hydrate cage trapping," ultimately concentrating in the center of fissures distant from hydrophobic surfaces. Kinetic analysis indicates that the self-diffusion coefficients of water and methane molecules in the coal system are significantly higher than those in the pure water system, confirming that the dense hydrate layer in pure water hinders mass transfer. Thermodynamically, coal systems consistently form hydrates at 250-260 K; however, above 270 K, cage formation is inhibited, evidenced by the disappearance of the second peak in the oxygen radial distribution function (RDF) and the drop in the F4 order parameter to liquid water levels. Within the 40-60 MPa range, pressure exerts minimal influence on hydrate structural order, far outweighed by temperature effects. This work provides molecular-level insights into the hydrate method for preventing coal and gas outbursts.