Abstract
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.
Highlights
Carbon dioxide (CO2) storage in underground saline aquifers is a promising way to reduce CO2 emission in the atmosphere
Two important diffusion coefficients that are closely related to gas transportation, namely, Maxwell–Stefan diffusion coefficient (MDC) and Fick diffusion coefficient (FDC), have not been investigated
The results show that thermodynamic factor always increases with increasing CO2 concentration
Summary
Carbon dioxide (CO2) storage in underground saline aquifers is a promising way to reduce CO2 emission in the atmosphere. On the one hand, owing to their porous (layered) structure, the clay minerals have remarkable capacity of adsorbing CO22–4. They provide a large amount of space for storing CO25. Gas leakage and environmental impact are the main problems found after risk assessment of CO2 storage These problems are closely related to fluid (gas) transportation in clay. Yang et al used molecular dynamics (MD) to study the structure and self-diffusion coefficient (SDC) of CO2 in uncharged clay-like slit pores[24]. All the above-mentioned MD simulations mainly investigated the SDCs. Two important diffusion coefficients that are closely related to gas transportation, namely, Maxwell–Stefan diffusion coefficient (MDC) and Fick diffusion coefficient (FDC), have not been investigated. The effects of gas concentration, water concentration and temperature on SDC, MDC and FDC are discussed in detail
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