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Abstract
Well integrity is one of the major concerns when assessing the containment of a geological CO2 storage. Whereas defect-free cement sheaths provide reliable barriers that prevent vertical migration or leakage of the stored CO2, 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 CO2 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 CO2 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.

Introduction
As man-made objects drilled through geological layers, wells are identified weak points of carbon dioxide (CO2) storage containment. It is widely accepted that a defect-free cement seal provides a reliable barrier against vertical migration or leakage of the stored CO2 (Duguid 2009). However, significant leakage can occur if a pathway opens in the cement seal. A pathway is defined as a zone of low hydraulic resistance. It can originate in the well construction – a poor cement job may not ensure the efficiency of the seal – or being the result of usual operations performed during the life of the well, such as temperature and pressure cycling (Brice Lecampion et al. 2010). The resulting defects hydraulically connect portions of the cement sheath: a micro-annulus or a crack provides vertical connection (Figure 1-a) whereas a disking crack provides horizontal connection. It becomes a leakage pathway if a continuous assembly of defects connects a pressure source and a pressure sink (Figure 1-b).

The stored fluid can flow through the pathway and alter its initial properties (Carey et al. 2010). Indeed, CO2 reacts with most of the constitutive material of the wellbore, such as casing steel or cement. For example, immersing a core of Portland cement in CO2 rich fluid will drastically affect its mechanical properties up to a complete loss of its integrity. As a result, these reactions are usually seen as a strong degradation factor when applied to well integrity. Indeed, under most conditions, the cement of the sheath will be leached away under the flow of CO2-rich fluid, opening the existing pathway further. But when integrated to the dynamics of leakage, this chemical phenomenon can have the opposite effect: the products of the reaction are released in the flow, and can, under specific conditions, re-precipitate downstream. The precipitated minerals may plug the pathway and stop the leak.

In order to evaluate quantitatively the effective leakage in an identified pathway along the cemented annulus, as well as its evolution with time, a numerical simulator has been built. After presenting its implementation, example of numerical results are produced and commented. We finally focus on a particular phenomenon of evolution of the defect hydraulic connectivity that can lead to a self-healing process. It is characterized through a dimensional analysis, in order to identify the driving mechanisms and assess the long-term stability of a pathway. This allows determining, depending on the injection condition, if the defect will heal or not.