Enhancing CO2 sequestration safety with hydrate caps: A comparative study of CO2 injection modes and saturation effects

Leakage prevention of CO2 is crucial for the safety of submarine geological carbon sequestration, with hydrate caps identified as effective barriers against leakage. The stability and efficiency of these caps are influenced by water saturation, initial hydrate saturation, and H2O-CO2 seepage behaviors, but mechanisms governing rapid cap formation and efficient sequestration remain unclear. This study investigates hydrate cap formation under H2O-CO2 co-injection and CO2 pre-injection modes, analyzing the effects of water and hydrate saturation on sealing caps stability and CO2 sequestration efficiency. Results show that CO2 pre-injection promotes uniform hydrate distribution in sealing caps, reducing localized accumulation and enhancing sequestration efficiency. Moreover, the cap formation time negatively correlates with hydrate saturation and positively with water saturation in the pre-injection mode, while no linear relationship is observed in the co-injection mode. Optimal water saturation for rapid cap formation in co-injection mode is 40%–80 %. Compared to co-injection, CO2 pre-injection reduces cap formation time by 61.43 %, CO2 leakage by 32.56 %, and forms a pressure drop buffer region during cap failure, preventing continuous leakage and improving long-term sequestration safety. The present study provides technical support for the marine geological CO2 sequestration using the hydrate cap method.