Efficient storage of carbon dioxide (CO2) in deep reservoirs will be dependent on the integrity of the natural seals above the CO2. This experimental study focuses on geochemical reactions between CO2, synthetic porewaters and Utsira caprock the sealing lithology of the Sleipner storage site. It provides data for experiments lasting 5 years, and extends the timescale of previously conducted experiments that ran for up to 14 months duration. In-situ conditions were replicated in the experimental 30 °C, at 8 MPa [80 bar]. New experimental data, confirm the previous 14-month data, with experiments pressurised with N2 showing little or no reaction. Reactions in experiments involving CO2 were dominated by initial carbonate mineral dissolution. No definitive evidence of other changes in mineralogy (including clay mineralogy), could be identified in either the CO2 or N2 experiments. In terms of the overall impact of storing CO2 at Sleipner, the results from these experiments show no indication of major deleterious, geochemical reaction processes occurring with the caprock.