Performance characterization of wellbore cement containing different CO2-resisting additives under geologic CO2 storage conditions

Abstract

CO₂ geological utilization and storage (CGUS) is an effective method for reducing greenhouse gas (GHG) emissions and mitigating their global warming effects. Wellbore cement is a crucial component of the CGUS system, but it exhibits chemical instability when exposed to CO₂-rich environments, which can lead to potential CO₂ leakage from wellbores. Maintaining the integrity of the wellbore cement is essential for preventing CO₂ leakage, ensuring the long-term containment of CO₂ in subsurface formations, and ensuring the success of CGUS projects. The objective of this study is to evaluate the impact of different CO₂-resisting additives on the corrosion depth, compressive strength, and the pore structure of wellbore cement when exposed to CO₂-saturated brine under geologic CO₂ storage conditions. The mineralogy, microstructure, and morphology of specimens with different additives were analyzed using XRF, XRD, SEM and micro-CT. The results show that the cement containing supercritical CO₂-modified Ca-montmorillonite (CM) was more effective in resisting CO₂ corrosion than biochar (BC), crystalline admixture (CA), and geopolymer (GP). The corrosion inhibition efficiency, when compared to the reference sample (00-RF), followed the descending order of CM (44.04%), GP (26.17%), and BC (6.49%). Adding CA to the cement not only failed to inhibit corrosion but also increased the carbonation depth within the wellbore cement by 35.78%. The analysis of CM indicates that its reinforcement mechanisms stem from micro calcite-induced carbonate growth and montmorillonite swelling, and the prevention of CO₂ infiltration is attributed to the structure of montmorillonite after supercritical CO₂ modification. Regarding compressive strength, all samples with different additives exhibited a decline after CO₂ corrosion. The sample containing supercritical CO₂-modified CM showed the smallest reduction in compressive strength (10.40%) after carbonation while the sample with GP has the highest reduction (54.63%). In summary, compared to the other additives, CO₂-modified montmorillonite has the most promising application prospect.