Modeling carbonation and chloride ingress in well cements

Abstract

The long-term durability of the wellbore system has become an important topic in recent decades. Specifically in a CO2 underground storage system, both the ingress of chloride and the leaking of carbon dioxide (CO2) can lead to severe degradation of the system, including corrosion of steel casing, and carbonation of the cement annulus. In this paper, a model was developed to consider the coupling effect of the carbonation reaction and chloride diffusion in well cement. The stoichiometric model was first applied to calculate the chemical compositions change of the well cement during the hydration process and the carbonation process. The chloride diffusion coefficient of carbonated well cement was calculated based on the general self-consistent (GSC) model and pore tortuosity. After that, the concentration of free chloride ions was predicted based on Fick's law, which takes into consideration released bonded chloride in the carbonated cement. The proposed model was validated based on available experimental data, and a comparison was made between the proposed model and other previously published models. The model has great significance for the safe and reliable geological storage of CO2. As described in this paper, the model can be used to predict the durability of a CO2 wellbore system and to inform the design of future systems.