Self-diffusivity, M–S and Fick diffusivity of CO(2) in Na-clay: The influences of concentration and temperature

Storing CO(2) in underground saline aquifers is an important way to reduce CO(2) emission in atmosphere, where gas/fluid diffusion in clay plays a key role in CO(2) 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 CO(2) 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 CO(2) self-diffusivity, which does not decrease with CO(2) concentration monotonously but reaches the maximum when CO(2) concentration reaches 2. Displacement distribution of CO(2) 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 CO(2) concentration. M–S and Fick diffusivities of CO(2) are calculated by MD for the first time, both of which increase with increasing CO(2) and H(2)O concentration and temperature. Based on self-diffusivity and M–S diffusivity, a quantity representing the coupling strength between CO(2) molecules is presented; it increases firstly with CO(2) concentration but begins to decrease when CO(2) concentration is beyond 2.