Quantitative evaluation of hydrate-based CO2 storage in unsealed marine sediments: Viewpoint from the driving force of hydrate formation and CO2-water contact ability

•The liquid CO2-seawater system in unsealed submarine sediments is simulated.•Liquid CO2 hydrate conversion capacity is identified from driving force and contact ability.•“Comfort zone” for submarine CO2 hydrate sequestration is identified.•The influences of sediment type and CO2-seawater ratio are revealed. CO2 hydrate provides an important way for the long-term stable way for CO2 sequestration, but the current understanding of CO2 hydrate formation behavior from liquid CO2 and seawater in submarine sediments is still unclear. In this work, we constructed the unsealed submarine sediments and examined the effects of multiple factors on CO2 hydrate formation from two perspectives: driving force of hydrate formation and CO2-water contact ability. “T-p comfort zone” that CO2 could be more sequestrated as hydrate was identified, and if CO2 is injected in the appropriate area, the floating CO2 from deep sediments could always flow in the relatively “comfort zones” and eventually be more solidified. The relations of CO2-water distribution and quantity in sediments further determined the hydration rate and capacity. Due to the better dispersibility, the maximum CO2 storage density of 66.8 kg/m3 in hydrate form was obtained in silt system at 60 vol% CO2, which was 60 % higher than the sandy system. The addition of 25 wt% clay inhibited the hydrate quantity to 40 kg/m3. Our results present the friendly CO2 sequestration conditions and optimized path in unsealed marine sediments.