Abstract: Considering the service environment of the CO₂ sequestration wellbore, a wellbore model containing a cement corrosion layer was developed. Based on the finite element method, the development process of micro-annulus at the cement plug-formation interface was simulated, the mechanism of micro-annulus expansion and the main controlling factors under the fluid drive were clarified, and the method of wellbore sealing enhancement was proposed. The simulation results show that the supercritical CO₂ accumulation will cause a change in the stress at the cement bonding surface. When the stress at the interface changes from compressive to tensile, and the stress value is higher than the bonding strength, micro-annulus will be generated in the wellbore. If the cement stiffness and bonding strength decrease after corrosion, the wellbore sealing decreases; the expansive additive can increase the initial compressive stress of the bonding surface and then enhance the wellbore sealing. When the expansion rate is 0.5‰, the micro-annulus propagation pressure rises by 43.7%, the micro-annulus length decreases by 42.7%, and the wellbore sealing is significantly enhanced.