Assessing the possibility of CO2 leakage is one of the challenges for geological carbon sequestration. Injected CO2 can react with wellbore cement, which can potentially change cement composition and transport properties. In this work, we develop a reactive transport model based on experimental observations understand and predict the property evolution of Cement in direct contact with CO2-saturated brine under, diffusion controlled conditions. The model reproduced the observed zones, of portlandite depletion and calcite formation. Cement alteration is initially fast and slows down at later times This work also quantified the role of initial cement properties, in particular the ratio of the initial portlandite content to porosity (defined here as phi), in determining the evolution of cement properties. Portlandite-rich cement with large phi values results in a, localized "sharp" reactive diffusive front characterized by calcite precipitation, leading to significant porosity reduction, which eventually clogs the pore space and prevents further acid penetration. Severe degradation occurs at the cement-brine interface with large phi values. This alteration increases effective permeability by orders of magnitude for fluids that preferentially flow through the degraded zone. The significant porosity decrease in the calcite zone also leads to orders of magnitude decrease in effective permeability, where fluids flow through the low-permeability calcite zone also leads to orders of magnitude decrease in provides a valuable tool to link cement-CO2 reactions with the evolution of porosity and permeability. It can be used to quantify and predict long-term wellbore cement behavior and can facilitate the risk assessment associated with geological CO2 sequestration.