Thermodynamic Inhibition by Chlorides (KCl, NaCl, CaCl2, and MgCl2) on CO2 Hydrates: Implication on Hydrate-Based CO2 Sequestration

Hydrate-based CO2 sequestration in marine sediments has emerged as a promising strategy to mitigate global climate change due to its substantial storage capacity and operational feasibility. However, understanding the influence of chloride salts from seawater, such as KCl, NaCl, CaCl2, and MgCl2, on CO2 hydrates phase stability is essential for the practical application of subsea CO2 sequestration as hydrates. This study systematically investigates the phase equilibria of CO2 hydrates in KCl, NaCl, CaCl2, and MgCl2 solutions at concentrations of 0–400 mM, encompassing typical marine salinity conditions. The experimental results demonstrate that all four chloride salts exhibit thermodynamic inhibition effects on CO2 hydrates, with the inhibitory effect intensifying as concentration increases. MgCl2 exerts the strongest thermodynamic inhibition effect, suppressing CO2 hydrates equilibrium temperature by 0.96 K at 200 mM and 1.57 K at 400 mM concentrations under 3.0 MPa. The inhibition strength follows the order of MgCl2 > CaCl2 > NaCl > KCl, which correlates well with the ion charge and radius of cations. A thermodynamic model was developed by integrating the Hu-Lee-Sum correlation of water activity with the classical Chen-Guo model for the prediction of the thermodynamic phase equilibria for CO2 hydrates. The proposed model demonstrates high accuracy, with an average absolute deviation of pressure below 2.0% across all tested concentrations. Validation against experimental data and literature values confirms the reliability of the model and serves as a prediction tool for CO2 hydrates phase equilibria with chloride salts. This study advances the understanding of CO2 hydrates thermodynamics in saline systems, elucidating the inhibitory effects of chloride salts and the associated impact on CO2 hydrates stability. The findings provide the thermodynamic foundation for the near-future large-scale adoption of hydrate-based CO2 sequestration.