Subsurface storage of carbon dioxide is often considered one of the most promising ways of reducing atmospheric levels of CO2. The safety of the operation is closely related to the performance of caprocks overlying the formations in which CO2 is injected. Interactions between CO2 or CO2/brine and caprocks can alter the sealing capacity which prevents upward migration of CO2. This thesis is a part of the SSC Ramore project, concerned with risk assessment related to underground geological storage of CO2. A new experimental design allowing a caprock core to be flooded with CO2 was prepared and tested at the Norwegian Geotechnical Institute (NGI). The initial aim of injecting CO2 saturated brine into a sample from the Draupne Formation had to be altered due to experimental difficulties encountered. Instead it was decided to perform a capillary breakthrough pressure test. By applying a gradually increasing pressure gradient across the sample, an apparent flow of supercritical CO2 was obtained at a breakthrough pressure of ~3.5 MPa. However, the interpretation of this as a real breakthrough pressure was made difficult by the simultaneous onset of a significant radial expansion. Geochemical effects of Draupne claystone interacting with acidic CO2-saturated brine were investigated in a batch reactor at the University of Oslo (UiO). Small amounts of powdered claystone material were reacted with solutions with pH ranging from 2 – 10. At the pH levels expected in underground geological formations after CO2 injection, the experiment indicates that most of the carbonates present in the Draupne caprock are dissolution-prone.