Evidence of self-sealing in wellbore cement under geologic CO2 storage conditions by micro-computed tomography (CT), scanning electron microscopy (SEM) and Raman observations

Highlights: • Reaction experiments between CO₂-saturated brine and wellbore cement samples with an open hole at the center were conducted. • CT scanning results showed precipitation of high-density CaCO₃ and formation of low-density amorphous silica. • CaCO₃ precipitation contributed to self-sealing of the cement, reducing the risk of CO₂ leakage through wellbore cement. • Raman observation identified the CaCO₃ produced as calcite. In this study, reaction experiments between CO₂-saturated brine and wellbore cement samples cured under different pressures were conducted to study microstructural and mineral composition changes using micro-computed tomography (micro-CT), scanning electron microscopy (SEM) and Raman spectroscopy. The CT images of post-CO₂ exposure cement samples showed a dissolution-precipitation-dissolution pattern at the exterior of the samples. The dissolution in the inner hole of the samples, however, was not significant. Instead, only CaCO₃ precipitation was observed in the inner hole. CaCO₃ precipitation in the inner hole contributes to self-sealing of the cement, which reduces the risk of CO₂ leakage through wellbore cement. According to CT and SEM observations, a higher curing pressure caused more precipitation of CaCO₃, which favored cement self-sealing when exposed to CO₂. The appearance of a CO peak and the disappearance of a –OH peak after reaction with CO₂ were observed by Raman spectroscopy, which was attributed to cement carbonation that converted Ca(OH)₂ into CaCO₃. This study provides solid evidence of cement self-sealing due to cement carbonation under geologic CO₂ storage circumstances.