Discovery of a surge in alteration depth of wellbore cement exposed to high concentration CO2 from 28 days to 56 days

Abstract

Efficient and secure storage of CO₂ in deep subsurface formations is currently an essential strategy for mitigating atmospheric CO₂ emissions. To guarantee the long-term integrity of the wellbore system, it is crucial to prevent the leakage of injected CO₂ through the wellbore. This study investigates the structural evolution of wellbore cement in static reaction with CO₂-saturated brine for 7, 14, 28, and 56 days under the conditions that mimic geologic CO₂ storage. Micro-CT was used to scan the samples both before and after the reaction. The threshold segmentation method was employed to distinguish the heavily-altered and less-altered regions of the specimens. The results indicated that the carbonation and alteration depths of the cement increased with the extension of reaction time. The alteration depth gradually increased during the initial 28 days, followed by a surge from 0.86 mm (28 days) to 3.21 mm (56 days). The SEM results indicate that the pores within the carbonate layer became larger and more pores appeared after 28 days of reaction, compared with those observed after 7 days of reaction. The primary cause of the sudden alteration depth increase after 28 days was that the carbonate layer had been defected, and the reaction fluid was able to penetrate the carbonate layer and react with the cement interior after 28 days of reaction, thereby expanding the range of the internal dissolved layer. Therefore, the carbonate layer may fail to protect the cement from CO₂ attack if the carbonate layer has defects. The prediction results based on the best-fit curve show that the carbonation front and alteration front of the wellbore cement reach 14.34 and 17.38 mm according to Fick's second law, while the Elovich equation estimates them to be 1.23 and 2.56 mm after 30 years of CO₂ attack. Those results are higher than the values reported in previous studies. The compressive strength of the wellbore cement is observed to decrease after carbonation, with a reduction of 13.7 % and 22.2 % compared to the initial compressive strength after reaction for 7 and 14 days, and has a very small decrease from 14 days to 56 days. In summary, this study reveals the possibility of a gradual loss of integrity of the carbonate shell in the exterior region of the cement exposed to CO₂, which increases the risk of wellbore cement damage.