Molecular simulation study of methane hydrate formation mechanism in NaCl solutions with different concentrations

Abstract

The effects of sodium chloride on hydrate formation depend on its concentrations. NaCl solutions can promote hydrate formation at low concentrations and inhibit it at high concentrations. Molecular dynamics simulations were performed to study the effect of different concentrations of NaCl on hydrate formation. Results implied that the induction time of hydrate in 1 wt% NaCl solution is shorter than that in pure water. The induction phase of hydrate formation in NaCl solution with a mass concentration greater than 1% is longer than that in pure water. The induction time of hydrate prolongs with increasing salt concentration due to the NaCl reducing tetrahedral water molecules in the methane hydration layers and inhibiting methane dissolution. During the hydrate growth phase, some ions are involved in the formation of unstable hydrate cages, while disturbing and destroying the nearby cage structures, retarding the hydrate growth. Besides, ions destroy unstable structures in amorphous hydrates, thereby reducing the empty cage rate. It was found that 1 wt% NaCl promotes hydrate formation for the local accumulation effect of ions on methane. Furthermore, analysis of the methane hydration layers around chloride ion shows that chloride ions facilitate the transformation of methane hydration layers to cage structures, which also have a positive effect on hydrate formation.