Phase Equilibrium of CO ₂ Hydrate in South China Sea Sediments: Implications for Marine Carbon Sequestration

The formation of the CO2 hydrate during marine CO2 sequestration is significantly influenced by the sand composition and salt content of oceanic sediment. In this work, the phase equilibrium of the CO2 hydrate was systematically investigated using a microdifferential scanning calorimeter (μ-DSC) under a simulated oceanic reservoir (278.15–295.15 K, 4–6 MPa). The oceanic reservoir environment was reconstructed using South China Sea (SCS) sediment with water content from 27 to 54%. To further investigate the effect of gas impurities on the hydrate phase equilibrium, comparative experiments were conducted using both a CO2-dominant gas mixture (80% CO2/20% N2) and pure CO2. The results reveal a nonmonotonic relationship between water content and CO2 hydrate phase equilibrium in SCS sediment: a leftward shift of the equilibrium curve with decreasing water content (45–54% range) and a rightward shift with decreasing water content (42–45% range). This indicates that both excessively high and low water contents are unfavorable for the hydrate formation, with the optimal condition observed at a 45% water content. In addition, liquid CO2 is more difficult to convert into a CO2 hydrate than gaseous CO2, and liquid CO2 failed to form a hydrate in the SCS sediment with a 27% water content. Under identical temperature conditions, the CO2-dominant gas mixture requires a higher equilibrium pressure compared to that of pure CO2. Moreover, with CO2 in a gas mixture, higher sediment water content is shown to enhance hydrate stability. These findings provide significant insights for site selection and CO2 injection strategies in marine CO2 sequestration.