Formation characteristics and leakage termination effects of CO2 hydrate cap in case of geological sequestration leakage

Geological sequestration of carbon dioxide (CO2 ) has been considered one of the most effective strategies against global warming. The greatest concern on the stored CO(2 )in sub-seabed sediments is leakage risk and can be solved by the plugging effect of CO(2 )hydrate cap, which is derived from the capillary force change by hydrate crystal formation inside pores. This study experimentally simulated CO(2 )upward leakage process in water -containing sediments and investigated the plugging characteristics of formed hydrate cap via magnetic reso-nance imaging (MRI) and flow characteristic analysis. Different CO(2 )flow rates (0.3-4.0 ml/min) and initial pressures (1.8-3.0 MPa) were employed for experimental conditions, and the hydrate cap appeared with no CO(2 )efflux any longer after hydrate formation for several minutes. It is found that both slow flow of CO(2 )and high pressure are beneficial for the formation of hydrate cap, and the strength of hydrate caps formed in all cases is confirmed by 10.0 MPa pressure test without any CO(2 )leakage. In addition, the spatial water distribution and the hydrate cap location inside the sediments are analyzed by multi-level MRI images and pressure evolution calculation, respectively. Ultimately, this study conducted a CO2 -water flow case and found that the strength of hydrate cap increases with the continuous formation of hydrates. Approximately 27.8% of hydrate saturation is a watershed of the plugging strength of CO(2 )hydrate cap. This study provides experimental evidences for the plugging effect of hydrate cap on terminating CO(2 )leakage and is of great significance for the scheme design and risk assessment of CO(2 )geological sequestration.