Combined Effects of Ocean Water Salinity and Nanosized Kaolinite Clay on Kinetics and Phase Equilibrium of CO2 Hydrates

Rising atmospheric CO2 concentrations intensify the need for carbon capture and sequestration (CCS) technologies. Hydrate-based CO2 sequestration in deep-ocean sediments can be a potential solution for long-term stable CO2 storage. However, the presence of salts and nanosized clay sediments can impact hydrate-based CO2 sequestration in deep oceans by altering kinetics as well as thermodynamics of the CO2 hydrate. Despite its relevance, the coupled effect of ocean water salinity and nanosized clay on CO2 hydrate stability has received limited attention. Here, we systematically investigate the combined effects of synthetic ocean water and nanosized kaolinite clay (80–150 nm) on CO2 hydrate formation kinetics and thermodynamics by varying clay loading from 5 to 30% by mass ratio with respect to synthetic ocean water. Experimental results show that ocean water salinity inhibits CO2 hydrate formation. In contrast, nanosized clay appears to partially offset the thermodynamic inhibition caused by seawater salinity, leading to an apparent improvement in hydrate stability. At 2 MPa, CO2 hydrates formed at ∼3.0 °C in ocean water, and the addition of 30% nanoclay mass ratio increased the equilibrium temperature by ΔT ≈ 0.22 °C relative to ocean water alone. However, higher clay loadings result in a nonmonotonic gas uptake trend, increasing from 30.4 mmol/mol H2O at 5% to 37.6 mmol/mol H2O at 20% loading before decreasing at higher clay loadings. This trend reveals a trade-off between enhanced thermodynamic stability and storage capacity. Together, the qualitative and quantitative experimental results provide critical insight for advancing hydrate-based CO2 sequestration in deep-ocean sediments.