Effect of CO₂-H₂O-Smectite Interactions on Permeability of Clay-Rich Rocks Under CO₂ Storage Conditions

CO₂ uptake by smectites can cause swelling and self-stressing in shallow clay-rich caprocks under CO₂ storage P–T and constrained conditions. However, little data exist to constrain the magnitude of the effects of CO₂-H₂O-smectite interactions on the sealing properties of clay-rich caprocks and faults. We performed permeability experiments on intact and fractured Opalinus Claystone (OPA) cores (~ 5% smectite), as well as on a simulated gouge-filled faults consisting of Na-SWy-1 montmorillonite, under radially constrained conditions simulating “open” transport pathways (dry and variably wet He or CO₂; 10 MPa fluid pressure; 40 °C). Overall, the flow of dry CO₂ through intact OPA samples and simulated smectite fault gouge caused a decrease in permeability by a factor of 4–9 or even by > 1 order, compared to dry He permeability. Subsequent to flow of dry and partially wet fluid, both fractured OPA and simulated gouge showed a permeability reduction of up to 3 orders of magnitude once flow-through with wet CO₂ was performed. This permeability change appeared reversible upon re-establishing dry CO₂ flow, suggesting fracture permeability was dominated by water uptake or loss from the smectite clay, with CO₂-water-smectite interactions play a minor effect. Our results show that whether an increases or decreases in permeability of clayey caprock is expected with continuous flow of CO₂-rich fluid depends on the initial water activity in the clay material versus the water activity in the CO₂ bearing fluid. This has important implications for assessing the self-sealing potential of fractured and faulted clay-rich caprocks.