CO2 hydrate formation characteristics inside seawater residual/saturated sediments under marine CO2 storage scenes

Marine CO2 storage by hydrate method is a promising technology expected to achieve permanent CO2 storage. Here, this study focuses on the formation kinetics and morphological characteristics of CO2 hydrate under two typical scenes with seawater residual zone (50 % porewater saturation) and seawater saturated zone (100 % porewater saturation). Four kinds of porous media were selected to simulate sediments with different permeability. The results indicate that in the seawater residual zone, hydrates are mostly flaky and form granular cementation, while in the seawater saturated zone, hydrates form from the pore center and are distributed as granular filling. Poor pore connectivity and gas-liquid contact in the seawater saturated zone hinder further hydrate formation, resulting in final hydrate saturation being at least 25 % lower than that in the seawater residual zone. Additionally, the final gas consumption and water-to-hydrate conversion decreased with increasing particle size. The maximum gas consumption was 118 mmol CO2/mol H2O, with only 28 % of the water converting into hydrate. These findings are significant for marine CO2 storage under different porewater saturation and particle size conditions.