Ensuring the long-term integrity - particularly zonal isolation and durability - of CO₂ injection wells constitutes a major scientific and technological challenge, as it is essential for preventing undesirable gas migration from the wellbore into overlying formations or to the surface. The integrity of wells designed for CO₂ injection depends, among other factors, on well depth, temperature and pressure conditions, injection rate and the composition of the injected CO₂, as well as on the applied casing and cementing technologies and the type of sealing cement slurries used. Equally important are the quality and composition of the slurries, their rheological and technological properties, as well as their mechanical, phase, and microstructural characteristics, including permeability and porosity. This paper presents the results of laboratory-scale experimental investigations on the incorporation of highly concentrated graphene oxide into ordinary Portland cement-based slurries. The hardened cement slurries were examined over a nine-year period. The results indicate that graphene oxide-modified cement slurries exhibit two mechanisms that enhance their durability and performance. These improvements result from the combined effect of reduced microporosity and enhanced mechanical resistance associated with a decrease in the content and dispersion of the weakest hydration product - portlandite within the C–S–H matrix. To the best of the authors’ knowledge, this study provides the first assessment of the microstructural properties of cement slurries containing graphene oxide subjected to long-term curing over a period of nine years.