Reactive transport modelling of the impact of CO2 injection on the clayey cap rock at Sleipner (North Sea)

During geological CO2-sequestration, dissolved CO2 will diffuse slowly into the lower section of the cap rock where, depending on the cap rock mineralogy, it might trigger geochemical reactions affecting crucial parameters such as porosity (and therefore possibly the sealing capacity). To assess this possibility, reactive transport modelling including reaction kinetics was performed of dissolved CO2 in the cap rock at Sleipner (37 °C, 101.3×105 Pa). Major geochemical reactions between CO2, the cap rock formation water and the cap rock mineralogy are identified and the impact on the porosity is calculated. After several years of initial carbonate dissolution, feldspar dissolution dominates over the subsequent hundreds and thousands of years, with the reaction rate strongly depending on plagioclase composition. A slight decrease of the porosity is predicted which might improve the sealing capacity of the cap rock, but this porosity change will be restricted to the lower metres, even for the most reactive case. Major uncertainties affecting the accuracy of the modelling are the selection of the primary and secondary mineral assemblages for the cap rock in the model and the poor knowledge of kinetic rate constants. Nevertheless, it is shown that using the local equilibrium hypothesis is inadequate and leads to erroneous results in this case.