Self-diffusivity, M-S and Fick diffusivity of CO2 in Na-clay: The influences of concentration and temperature

Storing CO2 in underground saline aquifers is an important way to reduce CO2 emission in atmosphere, where gas/fluid diffusion in clay plays a key role in CO2 leakage and migration. Various diffusivities, self-diffusivity, Maxwell-Stefan (M-S) diffusivity and Fick diffusivity, in clay interlayer are investigated by molecular dynamics (MD). Self-diffusivity varies with CO2 concentration, and reaches the maximum value at 2 molecules/unit-cell. High fluid concentration leads to clay swelling, thereby increasing self-diffusivity. However, the fractional free volume of clay explains the trend of CO2 self-diffusivity, which does not decrease with CO2 concentration monotonously but reaches the maximum when CO2 concentration reaches 2. Displacement distribution of CO2 molecules is analysed to explore the microscopic diffusion mechanism, which is characterised by logarithmic normal distribution. The mean value of such distribution further explains the self-diffusivity dependence on CO2 concentration. M-S and Fick diffusivities of CO2 are calculated by MD for the first time, both of which increase with increasing CO2 and H2O concentration and temperature. Based on self-diffusivity and M-S diffusivity, a quantity representing the coupling strength between CO2 molecules is presented; it increases firstly with CO2 concentration but begins to decrease when CO2 concentration is beyond 2.