Impact of Time-Varying Cement Degradation on the Borehole Cement Sheath Integrity in a Supercritical CO ₂ Environment

Well integrity in composite well systems is an important safety issue in supercritical carbon sequestration. Although many studies have been devoted to evaluating the failure risks associated with composite well systems, time-varying cement properties have typically been ignored in numerical modeling despite a significant amount of experimental evidence demonstrating that material degradation occurs in CO2-enriched environments. In this study, thermal–mechanical modeling with time-varying cement parameters is performed to evaluate the impact of cement degradation on stress distribution due to casing pressure and/or changes in temperature. A coupled thermal–mechanical problem was solved using the finite-element method. The model was verified by comparing the results to previous studies that did not consider material degradation. Then, the impact of cement degradation was explored. The results demonstrated that tensile hoop stress is sensitive to cement degradation due to a decrease in the elastic modulus and an increase in the Poisson’s ratio. There were also significant differences found in terms of radial stress when cement degradation was considered as compared to when it was not. Therefore, time-varying material properties should be considered when evaluating the integrity of cement sheaths in supercritical CO2 sequestration environments.