Capturing and storing carbon dioxide (CO2) underground for thousands of years is one way to reduce atmospheric greenhouse gases, often associated with global warming. Leakage through wells is one of the major issues when storing CO2 in depleted oil or gas reservoirs. CO2-injection candidates may be new wells, or old wells that are active, closed or abandoned. In all cases, it is critical to ensure that the long-term integrity of the storage wells is not compromised. The loss of well integrity may often be explained by the geochemical alteration of hydrated cement that is used to isolate the annulus across the producing/injection intervals in CO2-related wells. However, even before any chemical degradation, changes in downhole conditions due to supercritical CO2 injections can also be responsible for cement debonding from the casing and/or from the formation, leading to rapid CO2 leakage. A new cement with better CO2 resistance is compared with conventional cement using experimental procedure and methodology simulating the interaction of set cement with injected, supercritical CO2 under downhole conditions. Geochemical experimental data and a mechanical modeling approach are presented. The use of adding expanding property to this new cement to avoid microannulus development during the CO2 injection is discussed.