Abstract: The CO₂ stored in the underground reservoir may contact wellbores and underground concrete structures, causing carbonation reaction. To understand the severity of the carbonation reaction between CO₂ and wellbore cement/ concrete, ordinary Portland cement and class G oil well cement samples were used to react with different concentrations of CO₂. Micro-CT and XRD were used to study the changes in microstructure and mineral compositions of the cement after the reaction with CO₂. The test results show that the carbonation rate first increases and then decreases. The higher the CO₂ concentration, the more the calcite can be produced by the reaction. The Elovich equation was applied to fit the test data, and the data-fitting results predict that the long-term carbonation depths of Group 1, 2 and 3 samples after 100 days reaction will be 0.51 mm, 2.06 mm and 0.81 mm, respectively. In summary, the carbonation reaction of cement is mainly the conversion of calcium-based cement hydrates into calcite, and Class G oil well cement has better CO₂ corrosion resistance than ordinary Portland cement. The research results improve people's understanding of the reaction mechanism between cement and high-concentration CO₂, and can provide a reference for the evaluation of cement service performance in related projects.