Behaviors of hydrate cap formation via CO2-H2O collaborative injection:Applying to secure marine carbon storage

Abstract

Leakage prevention of carbon dioxide (CO₂) determines the safety of carbon marine geological storage. Hydrate caps have been proven to be one of the most effective structures for preventing CO₂ leakage, which has attracted worldwide attention. However, the structure and formation behavior of hydrate caps during CO₂ storage process are still unclear. In this study, a strategy of CO₂-H₂O co-injection was proposed to simulate the actual flow process and accelerate the hydrate cap formation rate. A series of CO₂-H₂O flow rates (0.25–10 mL/min) were employed. The results show that there will be four stages for the dynamic formation process of hydrate cap: fluid migration and diffusion, local hydrate formation, local area plugging, and hydrate cap formation. What's more, the water-gas flow ratio is inversely proportional to hydrate cap formation rate and positively proportional to the carbon storage efficiency. Meanwhile, the huge sealed capacity of hydrate cap was also proved in this work. The sequestration pressure of CO₂ under hydrate cap exceeds 12 MPa, and the maximum carbon storage efficiency increases from 53.12% to 93.30%. Moreover, the macrostructure of hydrate cap consists of two layers with a certain hydrate saturation: the zero-permeability layer and the low-permeability layer. The zero-permeability layer determined the sealed capacity of hydrate cap. These findings may provide a theoretical foundation for the prevention of CO₂ leakage in actual project of carbon marine geological storage.