Stress-Strain-Sorption Behaviour of Smectites Upon Exposure to Dry and Wet CO₂

The swelling-shrinkage behavior of smectites induced by interlayer uptake or sorption of CO₂ and H₂O has been investigated with increasing interest recent years, primarily because of its potential impact on the sealing efficiency of clay-bearing caprocks overlying CO₂ storage reservoirs. To get a better understanding of the stress-strain-sorption coupling in smectite exposed to supercritical CO₂, we performed multiple stepwise axial loading and unloading, oedometer-type experiments on ∼1 mm thick discs of pre-pressed Na-SWy-1 and of Ca-SAz-1 montmorillonite. Initially air-dry (AD) samples were first tested in the presence of wet CO₂ (20% RH) at 10 MPa pressure, and in the vacuum-dry (VD) state in the presence of pure (dry) CO₂ at 10 MPa. The samples were incrementally loaded and unloaded at 40°C, employing effective axial stresses ranging from 0.5 to 44 MPa. Control tests using wet and dry He or Ar instead of CO₂, were performed to distinguish strains due to loading-related CO₂ sorption/desorption from purely poroelastic effects. All samples saturated with CO₂ exhibited 30–65% lower apparent stiffness moduli than when saturated with He or Ar, showing that CO₂ adsorption/desorption altered the mechanical response of pre-pressed smectites. Relative to the He and Ar tests, swelling strains of a few % (corrected for poroelastic effects) were measured for AD Na-SWy-1 smectite exposed to wet CO₂, decreasing from 4.9 to 3.8% with increasing effective axial stresses in the range 1.6–36.2 MPa. AD SAz-1 material exhibited similar tends. VD samples tested with dry CO₂ showed much smaller relative swelling strains (0.5–1.5%), which also decreased with increasing applied effective stresses. The experimental data on relative swelling strain versus effective stress are well fitted by a recent thermodynamic model for stress-strain-sorption behavior in coal. Results derived from model fits indicate that smectite-rich rocks have significant storage capacity for CO₂ at shallow depths (up to 1.5–2 km) through CO₂ sorption by the clay minerals. However, this component of storage capacity is reduced by more than 80% with increasing burial depth beyond 3 km. The model provides a first step towards modelling stress-strain-sorption effects in smectite rich caprocks penetrated by CO₂, though further refinements are needed for broader application to the smectite-CO₂-H₂O system.