The significance of caprock sealing integrity for CO₂ storage

The safe long-term storage of gas/CO₂ in spatially limited underground volumes requires the combination of a structural trap with intact structural integrity and a suitable low permeability caprock (seal). The occurrence of natural gas reservoirs proves that certain lithotypes do provide efficient seals which can prevent leakage of gas to the atmosphere over long geological time periods (millions of years). In order to assess the risk of CO₂ leakage through caprocks on top of potential storage sites to the surface one has to consider both, the present sealing capacity of the rock and its likelihood to alter in contact with CO₂. The aim of this paper is to provide an overview of mechanisms affecting sealing integrity of intact (non-fractured/faulted) caprocks. Although certain caprocks can be suitable for hydrocarbons over geological time periods, CO₂ in contact with the seal may pose additional risks. Depending on the lithofacies and the amount of reactive mineral species, CO₂/water/rock interactions might alter the caprock, physical adsorption on organic matter or mineral surfaces will affect sealing integrity and interfacial properties will affect capillary entry and fluid transport behaviour.