Molecular Insight Into the Replacement Behavior of CO2CH4 Hydrate in Porous Media: Implications for CH4 Recovery and CO2 Storage

CO₂ replacement method is an auspicious method for the CH₄ extraction from gas hydrate and the CO₂ geological storage into sediments. The replacement of CO₂CH₄ hydrate in porous medium system is jointly affected by many factors such as heat transfer, mass transfer, and reaction. It is of great significance to deeply understand the mechanism and dynamics of different factors influencing the replacement characteristics of CO₂CH₄ hydrate in porous media. In this study, the molecular dynamics simulation method was employed to study the replacement characteristics and kinetic process of CO₂CH₄ hydrate in porous medium system with varying conditions expecting to offer significant theoretical direction and a point of reference for the CO₂ replacement method of natural gas hydrate extraction in permafrost regions in reality. The quantitative influence and internal mechanism of different factors on the replacement process of CO₂CH₄ hydrate were revealed. The results show that, in the porous medium system, when the temperature was ranged of 265–270 K and the pressure was ranged of 10–20 MPa, the replacement effect of CO₂CH₄ hydrate is the best under the initial concentration of CO₂ of 100%. It was further indicated that the replacement effect is appropriate when the initial concentration of CO₂ was ranged of 40%–60% under the case of 265 K and 10 MPa. Moreover, the result also indicated that the effects of some certain factors, including temperature, pressure, and initial concentration of CO₂ on the replacement process of CO₂CH₄ hydrate, exist slightly different. Owing to the adsorption effect of porous media on CO₂ molecules, it reduced the replacement efficiency between CO₂CH₄ hydrate. Additionally, the initial concentration of CO₂ imposed a more significant influence on the replacement of CO₂CH₄ hydrate in porous medium system considering the adsorption effect of porous. It does not mean that the higher the initial concentration of CO₂, the better the replacement effect of hydrate. The diffusion capacity of CO₂ depends on the concentration of H₂O molecules and the adsorption effect of porous media.