Feasibility analysis of liquid CO2 injection and sequestration as hydrates in South China Sea marine sediments over 100 years

Hydrate-based CO2 sequestration (HBCS) is proposed as a promising way for storing large amounts of CO2 in solid hydrate form in marine sediments for climate mitigation. However, the interplay of liquid CO2 injection, liquid and dissolved CO2 migration, and the long-term stability of the formed CO2 hydrates in marine sediments at field-scale remain unclear and warrant investigation. To this end, we develop an in-house thermo-hydro-chemical (T-H-C) coupled numerical code for analysis of the liquid CO2 injection and CO2 hydrate formation processes in silty sandy medium in South China Sea. The fate of the injected liquid CO2 and the spatial and temporal evolution of CO2 hydrate, liquid CO2 and the dissolved CO2 are analyzed in details for the five-year CO2 injection stage and the 100-year CO2 sequestration stage. Moreover, we design a 3-by-3 sensitivity analysis on key formation thermophysical properties, i.e., permeability (k), salinity (Xs) and thermal conductivity (λwet) to elucidate their effects on CO2 hydrate stability and the CO2 sequestration efficiency. Only a small fraction of CO2 hydrate (mass ratio of 0.8 %) is formed during CO2 injection and the continuously growing CO2 hydrate cap over the 100 years is the primary CO2 storage medium. The formation of the low-permeable CO2 hydrate cap effectively restricts the upward migration of liquid CO2, while dissolved CO2 exhibits an extensive vertical migration above the CO2 hydrate cap. The conversion of CO2 hydrate from liquid CO2 increases with increasing λwet, but decreases with increasing k and Xs, following the order of significance of Xs > λwet > k. The results of this study provide important guidance for the site selection and the operational design in future large-scale HBCS adoption in South Chin Sea.