Effects of Salt Ions on the Methane Hydrate Formation and Dissociation in the Clay Pore Water and Bulk Water

摘要

Gas hydrates in marine sediments are promising energy resources, while an effective recovery of methane from clay pores relies on a comprehensive appreciation of the hydrate evolution inside and outside the pore especially at a saline environment. Molecular dynamics simulations were conducted to investigate the methane hydrate formation and dissociation in the sodium montmorillonite (Na-MMT) interlayer with fresh water and saline water, respectively, by characterizing the distribution and transportation of methane and ions (K+, Na+, and Ca2+), the overall and local four-body structural order parameter, and the radial distribution functions. Results indicated that it was much easier to form methane hydrates in the bulk water than in the pore water, while the hydrates in the pore region were more readily dissociated than in the bulk region. The effects of salt ions on the hydrate formation were opposite in these two regions, which highlighted the role of the salting-out effect and the ion exchange between bulk water and pore water on the hydrate formation dynamics. It also demonstrated the priority for the hydrate dissociation from the contact area between MMT edge and bulk water because this region is more favorable for the distribution of salt ions and is more susceptible to be perturbed by the diffusion of salt ions from the pore water. Overall results provided theoretical supports for better understanding the microscopic mechanisms for the methane hydrate evolution at the heterogeneous environment with salt ions.