The geological sequestration of CO2 in abandoned coal mines is a promising option to mitigate climate changes while providing sustainable use of the underground cavities. In order to certify the efficiency of the storage, it is essential to understand the behaviour of the shaft sealing system. The paper presents a numerical analysis of CO2 transfer mechanisms through a mine shaft and its sealing system. Different mechanisms for CO2 leakage are considered, namely multiphase flow through the different materials and flow along the interfaces between the lining and the host rock. The study focuses on the abandoned coal mine of Anderlues, Belgium, which was used for seasonal storage of natural gas. Atwo-dimensional hydromechanicalmodelling of the storage site is per- formed andCO2 injection into the coalmine is simulated.Model predictions for a period of 500 years are present- ed and discussedwith attention. The role and influence of the interface between the host rock and the concrete lining are examined. In addition the impact of some uncertain model parameters on the overall performance of the sealing systemis analysed through a sensitivity analysis.