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Well integrity is one of the major concerns when assessing the containment of a geological CO 2 storage. Whereas defect-free cement sheaths provide reliable barriers that prevent vertical migration or leakage of the stored CO 2 , significant leakage can occur if pathways appear in the cement seal: the stored fluid can flow through and alter their initial properties. A simulator has been built to numerically investigate the leakage rate of CO 2 rich fluid through a pathway along a cemented annulus. Careful analysis of the physics and characteristic dimensions of the phenomena allows coupling of the flow, the mechanics of any defect and the chemical reactions that take place in the materials (fluid and cement). Results of the simulation, besides predicting the instantaneous flow rate and hydraulic conductivity of the defect, show different mechanisms of flow rates variation with time: in particular, coupling between flow along the defect and chemical reaction between cement and CO 2 can lead to the deposition of minerals within the defect space. This may lead to a total plugging of the pathway -a self-healing of the cement. Based on dimensional analysis and semi-analytical solution, a criterion is defined in order to assess the long-term stability of the pathway.