Molecular Dynamics Simulation Study of N2/CO2 Displacement Process of Methane Hydrate

In this paper, we construct three kinds of hydrate replacement models: N-2, CO2 and N-2-CO2. The molecular dynamics (MD) method is used to study the dynamic process of N-2-CO2 replacement for natural gas hydrate. The replacement efficiency of N-2/CO2 is simulated, and the influence of gas replacement on the stability of the water cage is assessed. The results show that CH4 molecules in the driving hydrate are all enriched in the gas-liquid interface or gas region. Compared with the replacement of methane hydrate with CO2, when N-2-CO2 mixed gas is used to replace methane hydrate, the diffusion rate of CO2 is increased by 2 times, and the replacement efficiency of hydrate is significantly improved. The radial distribution function and free energy calculation results show that although the barrier of the water cage is significantly reduced after replacement, the water molecules in the cage still have difficulty jumping over the free energy barrier compared with the thermal motion energy, so the water cage structure remains highly stable.