CO2 hydrate sequestration in unsealed submarine sediments: A 4D pore-scale experimental investigation

CO₂ hydrate sequestration in marine sediments has been identified as a safe, large-scale, long-term carbon removal method. Influenced by the high pressure and low temperature, the dynamic formation of CO₂ hydrate has not been fully investigated. We thus design and construct a low-field nuclear magnetic resonance (LF-NMR) apparatus to investigate the in-situ dynamic process of CO₂ hydrate formation. The temporal-spatial evolution of CO₂ hydrate formation is analyzed to reveal the generation and distribution of CO₂ hydrate as a function of pressure and initial water saturation. The results reveal that CO₂ hydrate mainly forms in macropores, where water-to-hydrate conversion rate exceeds 81 %, while the conversion rate is below 27 % in micropores. The spatial distribution of CO₂ hydrate exhibits strong heterogeneity, and the hydrate formation preferentially occurs where the ratio of gas-water volume is 0.2–0.8. Increased injection pressure improved the heterogeneity, as evidenced by the increased heterogeneity index from 6.18 to 12.8. The increased injection pressure cannot enhance CO₂-to-hydrate conversion at similar water saturation levels although improving the conversion rate of water-to-hydrate. Regardless of the initial water saturation, lower injection pressure has a higher CO₂-to-hydrate conversion, approximately 95.2 % (3 MPa) and 86.5 % (4 MPa), respectively. This study advances the understanding of CO₂ hydrate formation dynamics and demonstrates that lower injection pressure is more favorable for hydrate-based marine CO₂ sequestration strategies.