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

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


Jin, Z. - Presenter, University of Alberta
Zhang, M., University of Alberta
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.