Swelling stress development in confined smectite clays through exposure to CO₂

Recent work has shown that smectite clays swell upon exposure to supercritical (SC) CO2 due to uptake in the interlayer region. In this study, we report uniaxial compaction/swelling tests performed, using a 1-D compaction cell, on pre-pressed discs of Wyoming (Na-SWy-1) and Arizona (Ca-SAz-1) montmorillonite, as well as on smectite-bearing shale. We explore the axial effective (swelling) stress generated in these materials upon exposure to 10 MPa CO2 pressure under conditions where swelling is restricted. The experiments were performed at 44 °C. In each experiment, the sample was first equilibrated with lab air (RH≈40%-60% at T=20-25°C) at the target experimental temperature. An axial normal stress of 25.9 to 40.9 MPa was then applied and the loading piston held in fixed position. This yielded an effective overburden stress of 9.6 to 24.7 MPa upon introduction of CO2 at 10 MPa, thus simulating burial depths of 0.8 to 2.0 km. Following CO2 introduction, axial swelling stresses developed rapidly, independently of the effect of increased pore pressure, attaining values of 7.1 to 12.4 MPa at equilibrium. Experiments on Na-SWy-1 montmorillonite showed that the swelling stress generated decreases with increasing initial and final effective normal stress, which suggests that overburden stress suppresses swelling (stress) development in smectite upon exposure to CO2, presumably by limiting the amount of CO2 uptake by the material investigated. The swelling stresses observed imply that CO2 penetration into caprocks and faults in geological storage systems will lead to an increase in effective normal stress components, which in turn will tend to promote closure of fractures and enhance sealing integrity. However, further work is needed to achieve a better understanding of the processes underlying the swelling of smectite caused by CO2 and to evaluate any risks posed to caprock and fault integrity by swelling-induced shear stresses.