Microsecond molecular dynamics of methane-carbon dioxide swapping in pure and saline water environment

This work aims at proposing the nondestructive methane-carbon dioxide (CH₄-CO₂) replacement mechanism as an ecofriendly energy production technique from the natural gas hydrate reserves in seafloor and permanently frozen grounds. Although the experimental data is widely available in literature, this replacement mechanism has not been elucidated at molecular level. In this contribution, we perform the microsecond level molecular dynamic simulations to evaluate two different CH₄-CO₂ replacement mechanisms: (i) direct CH₄ displacement from hydrate structure, and (ii) dissociation of existing methane hydrate followed by a reformation of mixed CH₄-CO₂ hydrates. For this, we analyze CH₄-CO₂ replacement in three different modes i.e., CO₂ as a replacing agent in (i) absence of free water molecules, (ii) presence of free water molecules, and (iii) presence of salt ions and free water molecules. Despite slow kinetics in the first mode, pure CO₂ is observed to replace the methane more efficiently, while in the second mode, CO₂ forms a new mixed hydrate layer on the existing seed crystal. However, in the third mode, salt ions help in destabilizing the methane hydrate and allow CO₂ to form the hydrates. This proves that salt ions are favorable for CH₄-CO₂ replacement.