On sorption and swelling of CO₂ in clays

The geological storage of carbon dioxide (CO₂) is a well-studied technology, and a number of demonstration projects around the world have proven its feasibility and challenges. Storage conformance and seal integrity are among the most important aspects, as they determine risk of leakage as well as limits for storage capacity and injectivity. Furthermore, providing evidence for safe storage is critical for improving public acceptance. Most caprocks are composed of clays as dominant mineral type which can typically be illite, kaolinite, chlorite or smectite. A number of recent studies addressed the interaction between CO₂ and these different clays and it was shown that clay minerals adsorb considerable quantities of CO₂. For smectite this uptake can lead to volumetric expansion followed by the generation of swelling pressures. On the one hand CO₂ adsorption traps CO₂, on the other hand swelling pressures can potentially change local stress regimes and in unfavourable situations shear-type failure is assumed to occur. For storage in a reservoir having high clay contents the CO₂ uptake can add to storage capacity which is widely underestimated so far. Smectite-rich seals in direct contact with a dry CO₂ plume at the interface to the reservoir might dehydrate leading to dehydration cracks. Such dehydration cracks can provide pathways for CO₂ ingress and further accelerate dewatering and penetration of the seal by supercritical CO₂. At the same time, swelling may also lead to the closure of fractures or the reduction of fracture apertures, thereby improving seal integrity. The goal of this communication is to theoretically evaluate and discuss these scenarios in greater detail in terms of phenomenological mechanisms, but also in terms of potential risks or benefits for carbon storage.