Influence of volume ratio of liquid CO2 to seawater on CO2 hydrate sequestration in submarine sediments

Abstract

CO₂ hydrate-based sequestration in submarine sediments shows great potential for carbon emission reduction. Considering the proportional relationship of CO₂ and water for hydrates formation, their existing ratio largely determines the CO₂ sequestration density and phase state. Here, this work focuses on determining the optimal ratio of CO₂ to seawater in sediments simulated with 20−40 mesh (0.42−0.85 mm) quartz sand, in order to maximize CO₂ hydrate conversion in sediments. The results show that the conversion rate of CO₂ hydrate increases with the initial water saturation, reaching 15.3% at 80% initial water saturation. The optimal CO₂ hydrate formation occurs at 30% initial water saturation, with the corresponding CO₂ storage density in hydrate form of 33.09 kg·m⁻³ and the hydrate saturation of 22.3%. However, CO₂ hydrate conversion rate is <10%, which implies that most CO₂ still exists in liquid state, despite the presence of free water. The total CO₂ sequestration density is negatively correlated with the initial water saturation, and at 10% initial water saturation, 398.73 kg·m⁻³ of CO₂ is sequestered, of which only 18.02 kg·m⁻³ is hydrated. Additionally, the lower initial water saturation corresponds to the shorter time to achieve t₉₀ of CO₂ consumption, and the water conversion rate to hydrate reaches 90% at 10% initial water saturation. In summary, adjusting the volume ratio of liquid CO₂ to seawater can effectively increase the sequestration amount of CO₂ hydrates, but methods to increase CO₂ conversion to hydrate still need to be established.