Effects of Sulfate during CO₂ Attack on Portland Cement and Their Impacts on Mechanical Properties under Geologic CO₂ Sequestration Conditions

To investigate the effects of sulfate on CO₂ attack on wellbore cement (i.e., chemical and mechanical alterations) during geologic CO₂ sequestration (GCS), we reacted cement samples in brine with 0.05 M sulfate and 0.4 M NaCl at 95 °C under 100 bar of either N₂ or supercritical CO₂. The results were compared to those obtained from systems without additional sulfate at the same temperature, pressure, salinity, and initial brine pHs. After 10 reaction days, chemical analyses using scanning electron microscopy with a backscattered electron detector (SEM-BSE) and inductively coupled plasma optical emission spectrometry (ICP-OES) showed that the CO₂ attack in the presence of additional sulfate was much less severe than that in the system without additional sulfate. The results from three-point bending tests also indicated that sulfate significantly mitigated the deterioration of the cement’s strength and elastic modulus. In all our systems, typical sulfate attacks on cement via formation of ettringite were not observed. The protective effects of sulfate on cement against CO₂ attack resulted from sulfate adsorption, coating of CaSO₄ on the CaCO₃ grains in the carbonated layer, or both, which inhibited dissolution of CaCO₃. Findings from this study provide new, important information for understanding the integrity of wellbores at GCS sites and thus promote safer GCS operations.