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.