Investigation of chemical processes in cement exposed to wet ScCO2 and CO2-saturated brine in geological CO2 storage conditions

In geological CO₂ storage conditions, wellbore cement can be exposed to both supercritical CO₂ (ScCO₂) with water vapor, and CO₂ dissolved in water. There is a lack of studies that investigate the effects of reaction environments on the extent of CO₂-induced cement carbonation and leaching. In this study, four CO₂ exposure experiments were designed with wellbore cement samples exposed to both ScCO₂ and CO₂ dissolved in water to investigate the impacts of evaporation, capillarity, diffusion, and salt deposition on cement carbonation. Severe cement carbonation after 14 and 28 days of CO₂ exposure was observed in a wet ScCO₂ phase. When water evaporation into ScCO₂ phase was minimized by a steel plate between the brine phase and the ScCO₂ phase, a strong cement carbonation in ScCO₂ phase was still visible. The reason was that imbibition and diffusion drove water to migrate from the lower section to the upper section of the cement sample to participate in the carbonation reaction. The level of cement carbonation in different CO₂ exposure environments was ranked as: wet ScCO₂ > CO₂ dissolved in water > Pure-ScCO₂. The corresponding maximum carbonation area ratios were 90 % and 38 % for the wet ScCO₂ scenario and the brine scenario, respectively, compared with a maximum carbonation area ratio of 20 % for the Pure-ScCO₂ scenario. This study implies that the most altered region in wellbore cement is at the ScCO₂—water interface, and the expansion rate of the altered region is the key to evaluate the potential for CO₂ leakage through wellbore cement.