The basic idea of the "Carbon dioxide Capture and Storage (CCS)" technology is to store CO2 produced by fossil fuel combustion and industrial processes in the deep geological underground, rather than releasing it into the atmosphere. To have a beneficial effect on climate and to prevent interaction between the surplus CO2 and the biosphere, the CO2 needs to remain safely underground for a sufficiently long time, of the order of at least 10,000 years, although it is expected to remain contained for much longer time periods in properly selected reservoirs. To ensure and verify the safe geological containment of CO2 underground, monitoring of CO2 storage site performance is mandatory. This includes, among other things, monitoring the injection process, tracking the CO2 plume migration in the reservoir and installing monitoring systems to give (early) warning in the case of CO2 leakage, i.e. CO2 leaving the storage complex. Not only do the impacts of the CO2 itself need to be considered, but also potential associated impacts due to co-injected incidental substances ("impurities"), mobilised substances, displaced migrating saline formation water and pressure increase following CO2 injection.

The main objective of this report is to identify and review monitoring methods for a performance assessment of geological CO2 storage sites. This report discusses state-of-the-art monitoring techniques, introduces general concepts and gives recommendations for procedures to set up site-specific monitoring plans. This is complemented by an overview of monitoring applications employed at demo or pilot CO2 storage sites or in field tests. There is a special focus on establishing site-specific monitoring plans, with two examples selected to represent the two major storage options in Europe and worldwide, namely saline aquifers (Romanian example) and depleted gas fields (Slovakian example). Finally, recommendations for future research and development activities are derived.