The economic and environmental benefits of CO2 sequestration in geological formations are well established;however, the process has significant operational and maintenance challenges.Primarily, cement well plugs are vulnerable to deterioration in high temperature and high pressure underground storage settings, and the ability to accurately predict their service performance in these unfavorable conditions is necessary for successful implementations.In this work, a coupled reactive transport model is developed using the finite element method to investigate class H cement deterioration under CO2 exposure in wellbore conditions.The numerical model involves the solution of Nernst-Planck equations for transport of relevant aqueous species in cement and fluid flow in the presence of hydration and carbonation processes.Modeled carbonation process is presented for two cases simulating high pressure and high temperature exposure conditions.