Wells are complex structures of cement and steel that stretch kilometers into the ground. They can never be removed, but need to be permanently plugged when they have performed their duty. The plugging involves filling discrete sections of the well with an appropriate plugging material, typically cement. It is important to avoid leakage through these plugs, since wells provide passages from the reservoir to the surface. Long-term integrity is especially challenging in CO2 wells, as reservoir pressure is high and since CO2 is also a buoyant and reactive fluid. In this project, the aim is to understand how to safely and efficiently plug CO2 wells to ensure long-term geological CO2 storage. This requires a better understanding of how the lifetimes of plugging materials are affected by the CO2 well downhole environment. Possible deterioration of candidate plugging materials upon CO2 exposure will be studied, together with any effects the pressure and temperature situation in a CO2 well might have on the plug solidification process. The outcome of the project will be advice on plugging materials for CO2 wells, recommended safe plug lengths and estimates of leakage rates through/along plugged wells. Suggestions will also be given on how to improve today's well plugging materials (structurally or chemically) to maximize the safety of long-term geological CO2 storage. The first half of the project has focused on understanding fundamental mechanisms that can cause leakage through or along permanent well plugs. Three candidate plugging materials are being studied, namely cement, cement with silica additions and a thermosetting polymer material. All of these have been studied with respect to placeability in the well and how they react upon prolonged exposure to CO2. An important result of the placement work is that the fluid properties of materials must be tuned to avoid creation of channels/pockets in them as they are pumped into the well. We have also published a concept involving the use of electric voltage on the casing during placement to optimize cement bonding to steel. The CO2 exposure experiments have also given interesting results, namely that both cement pores and channels are filled with CaCO3 precipitates during CO2 exposure. This can hinder leakage, and is thus a good news for CO2 storage safety. A negative finding is, however, that cement with silica additions (as is commonly used on the Norwegian Continental Shelf) has a significantly lower resistance towards CO2 than ordinary cement.