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

Abstract

CO 2 uptake by smectites can cause swelling and self-stressing in shallow clay-rich caprocks under CO 2 storage P-T and constrained conditions. However, little data exist to constrain the magnitude of the effects of CO 2 -H 2 O-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 CO 2 ; 10 MPa fluid pressure; 40 °C). Overall, the flow of dry CO 2 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 CO 2 was performed. This permeability change appeared reversible upon re-establishing dry CO 2 flow, suggesting fracture permeability was dominated by water uptake or loss from the smectite clay, with CO 2 -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 CO 2 -rich fluid depends on the initial water activity in the clay material versus the water activity in the CO 2 bearing fluid. This has important implications for assessing the self-sealing potential of fractured and faulted clay-rich caprocks. Highlights• Permeability of potential clay-rich caprock (Opalinus Claystone) was systematically measured using variably wet CO 2 versus He. • Through-flow of dry CO 2 caused a decrease in permeability of intact Opalinus Claystone and simulated smectite fault gouge by up to > 1 order, as composed to dry He. • Fractured Opalinus Claystone and simulated gouge showed a permeability reduction of up to 3 orders of magnitude with through-flow of wet CO 2 . • Permeability decrease by through-flow of variably wet CO 2 appeared reversible upon re-establishing dry CO 2 flow. • The initial water activity in the clay material versus the water activity in the CO 2 bearing fluid determines change in permeability upon CO 2 flushing. Keywords Shale permeability • CO 2 storage • Clay swelling • CO 2 -H 2 O-smectite interactions * M. Zhang