Molecular dynamics simulation of replacement of methane hydrate with carbon dioxide

Abstract

Molecular dynamics simulation was performed to analyse the phenomena of replacement of methane hydrate with carbon dioxide (CO2) at 270 K and 5.0 MPa for 5300 ps. The methane hydrate phase was constructed with 16 unit cells of hydrate. Every cage in the hydrate was occupied by one methane molecule. The methane hydrate phase was sandwiched between two CO2 phases. During the simulation the hydrate partially melted and liquid water phase appeared, and CO2 dissolved in the liquid water phase. The replacements were observed three times at the hydrate–liquid water interface during the simulation. In the first case, the replacement occurred at a S-cage without changing the structure. In the second case, an M-cage of methane hydrate partially collapsed, and methane and CO2 molecules exchanged. After the exchange, the cage occupied by CO2 remained in the M-cage structure. In the third case, a S-cage of methane hydrate partially collapsed, and methane and CO2 molecules exchanged. After the exchange, the cage occupied by CO2 changed to an M-cage-like structure.