Observational evidence confirms modelling of the long-term integrity of CO₂-reservoir caprocks

Storage of anthropogenic CO₂ in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO₂ escaping. Although natural CO₂ reservoirs demonstrate that CO₂ may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO₂-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO₂ reservoir-caprock system exposed to CO₂ over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ~7 cm in ~10⁵ years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO₂.