(413g) CO2 Adsorption in Partially Water-Saturated Kaolinite Nanopores from Molecular Perspectives in Relation to Geological Carbon Sequestration

Atmospheric CO₂ concentration has been gradually growing since the industrial revolution, leading to climate change and global warming. The excess CO₂ can also cause ocean accidification which can significantly endanger aquatic ecosystems. As a result, carbon capture and sequestion (CCS) has become utterly important for human scociety.

Thanks to the well-developed nanoscale pore structures providing substantial adsorption sites, geological carbon sequestration (GCS) in depleted tight gas/oil reservoirs is one promissing method to mitigate carbon emission (Energy & Environmental Science, 2018, 11 (5), 1062-1176). As one of the typical clay minerals in tight reservoirs, kaolinite also contains a great number of nanoscale pores with their sizes ranging from a few to hundreds of angstroms. Kaolinite is a typical 1:1 clay, which has two structurally and chemically distinct basal surfaces known as siloxane and gibbsite surfaces. While experimental measurements can obtain the total CO₂ adsorption capacity in kaolinite, it is practically impossible to distinguish the effect of two distinct kaolinite surfaces on CO₂ adsorption behaviors. In addition, kaolinite can be partially saturated with water generated during the early-sediment deposition (Applied Geochemistry, 1990, 5 (4), 397-413) and subsequent hydraulic fracturing (AAPG Bulletin, 2015, 99 (1), 143-154), which can also greatly affect CO₂ adsorption. In addition, by using molecular simulation, Xiong et al. (Langmuir, 2020, 36 (3), 723-733) has shown that water can have bridging structure in illite nanopores, which is dependent on the surface properties. How water structures in partially water-saturated kaolinite pores affect CO₂ adsorption remains unanswered.

In this work, we use molecular dynamics (MD) and Grand canonical Monte Carlo (GCMC) simulations to investigate CO₂ adsorption in partially water-saturated kaolinite nanopores with two distinct basal surfaces under GCS conditions. Without water, CO₂ presents a stronger adsorption ability on the gibbsite surface than the siloxane surface, indicating a higher CO₂ adsorption capacity in gibbsite kaolinite pores. In gibbisite kaolinite pores, water tends to form a thin film covering the surface due to strong hydrogen-bonding (H-bonding) interactions between water and surface -OH groups, while CO₂ fill the middle of pores. While CO₂ is depleted from the gibbsite surface, it tends to accumulate at the water-CO₂ interface. Unlike the gibbsite kaolinite pores, water bridges appear in siloxane kaolinite pores in line with Xiong et al. (Langmuir, 2020, 36 (3), 723-733). In siloxane kaolinite mesopores, the shape of water clusters gradually turns to be spherical ones as CO₂ pressure increases suggesting a more CO₂-wet surface, while water deformation in micropores is not obvious due to the strong confinement effects. The distribution of CO₂ in partially water-saturated siloxane kaolinite pores can be divided into six distinct regions according to its two-dimensional (2-D) density contour plots. The highest CO₂ density appears in the three-phase (water-CO₂-surface) contact areas, while it exhibits a strong tendency to accumulate in the two-phase contact regions. In general, the presence of water leads to the reduction of CO₂ adsorption capacity in both gibbsite and siloxane kaolinite pores. However, a slight enhancement is observed in siloxane kaolinite mesopores when pressure is low, due to the compensation of CO₂ enrichment at two-phase and three-phase contact regions. Overall, the presence of water is more detrimental to CO₂ adsorption in gibbsite kaolinite pores. Our work should provide important insights into CO₂ adsorption in partially water-saturated kaolinite nanopores and reveal the CO₂ sequestration mechanisms in the presence of water.