Molecular insight into carbon dioxide hydrate formation from saline solution

Effects of marine environments on methane hydrate formation in clay nanopores: A molecular dynamics study

In nature, CH₄ hydrates are mainly buried in marine sediments. The complex marine environments on the seafloor continuously affect hydrate formation. Herein, systematic molecular simulations have been performed to investigate CH₄ hydrate formation in clay nanopore, mainly affected by several marine environmental factors, including seawater salinity, pressure and temperature. Simulation results show that these factors exert different effects on hydrate formation in the nanopore and the outside bulk solutions by affecting the mass transfer and phase separation inside and outside of the nanopore. Specifically, high salinity hinders the diffusion of CH₄ molecules from nanopores into the outside bulk solutions, promoting hydrate formation in nanopore and inhibiting hydrate formation in bulk solution; salinity has a dual effect on hydrate formation in the whole system by changing the local CH₄ concentration via the formation of the hydration of salt ions. High pressure favors the diffusion of CH₄ molecules from nanopore into outside bulk solutions, promoting hydrate formation in bulk solution and inhibiting hydrate formation in nanopore; high pressure promotes hydrate formation at the nanopore throats by increasing CH₄ concentration and reducing ion concentration therein. In contrast, temperature significantly affects hydrate formation in the system by causing phase separation, i.e. high temperature promotes the aggregation of CH₄ molecules to form nanobubbles and inhibits hydrate formation. Under high temperature conditions, the nanobubble in the nanopore gradually decomposes, while the nanobubble in the outside bulk solution grows an extra-large cylindrical nanobubble. These molecular insights into the formation behavior of CH₄ hydrates in clay nanopores are helpful for understanding the formation process of natural gas hydrates in marine sediments and the development and utilization of CH₄ hydrates.