Molecular modeling on transportation of CO2 in montmorillonite: Diffusion and permeation

CO2 storage in underground saline aquifers is helpful to reduce CO2 emission in the atmosphere, where gas/fluid diffusion and permeation in clay mineral plays a key role in CO2 leakage and underground migration. CO2 Permeability and different fluid diffusivities in clay mineral (montmorillonite, Mt) interlayers are investigated by molecular dynamics (MD). Both CO2 and H2O self-diffusivities increase with water concentration and temperature but show a maximum at the CO2 concentration of 2 molecule/unit-cell unconventionally. The fractional free volume of Mt increases with CO2 concentration but begins to decrease if CO2 concentration exceeds 2, thus giving the reason for the above unusual CO2 self-diffusivity variation. Displacement distribution of CO2 molecules is found to be characterized by logarithmic normal distribution. The mean value of such distribution further supports the self-diffusivity dependence on CO2 concentration. M-S and Fick diffusivities of CO2 are positively related to CO2, H2O concentration and temperature. CO2 permeability is calculated by MD for the first time, which increases with CO2 pressure and H2O concentration but exhibits a turning point at temperature 360 K due to low CO2 solubility at high temperature.