Carbon Dioxide Attack on Wellbore Cement under Geologic Sequestration Conditions: Synthetic Brine versus Actual Formation Water

Geochemical interaction between CO₂ and wellbore cement is crucial to understand and prevent structural degradation. Here, we studied the interaction of Class H cement with actual formation water and compared it with laboratory-made synthetic brine saturated with CO₂. After 14 days of reaction at 60 °C and 100 bar CO₂, we studied the extent of chemical alterations of the cement core samples using scanning electron microscopy, along with energy-dispersive spectroscopy (SEM-EDS), synchrotron micro-X-ray fluorescence (μ-XRF) mapping, X-ray absorption spectroscopy (XAS), and X-ray diffraction (XRD). The cement sample reacted with synthetic brine, compared to the actual formation water case, exhibited more secondary Fe-bearing solids on the exterior of the core, as evidenced by a brighter orange color and a more Fe(III)-rich rim in the cross section. This difference can be attributed to the greater pH buffering capacity and stronger Fe complexation in the formation water. In contrast, both cement samples showed a similar extent of Ca dissolution and carbonation, although the carbonated layer was slightly denser and less invasive in the actual formation water case. These findings underscore the need for caution when extrapolating laboratory-based results to field conditions, particularly when synthetic brines are used in place of authentic formation waters.