Evaluation of mechanical well integrity during CO₂ underground storage

The storage of CO₂ in depleted oil and gas reservoirs, coal seams or saline aquifers is an important means considered to contribute to the mitigation of the greenhouse effect on the environment. In CO₂ underground storage projects, well integrity is a crucial factor that must be demonstrated for all wells, accessible and abandoned, affected by the injected CO₂ to ensure safety during the injection and storage of CO₂. Field experience and laboratory experiments have shown that chemical degradation due to CO₂ can affect well integrity severely only when there are existing pathways in the casing-cement-rock composite system. Therefore, the study of mechanical well integrity is of paramount significance. In this paper, analytical and numerical models are developed to assess well conditions including the micro-annulus between the cement sheath and the casing due to downhole condition changes. The parametric study shows that cement with a low Young’s modulus and high Poisson ratio has smaller micro-annulus than that with a high Young’s modulus and low Poisson ratio. When temperature and pressure decrease there is a potential for debonding of the interface between casing and cement.