Development of swelling strain in smectite clays through exposure to carbon dioxide

Smectites are common constituents of claystones, mudstones and shales and are often present in the caprocks and faults that seal potential CO₂ storage reservoirs at depths up to 2 or even 3km. Whilst the marked swelling behaviour of smectites caused by hydration through interlayer sorption of water molecules has been extensively studied, less is known regarding possible interactions of the mineral structure with supercritical CO₂. To explore the possibility of such effects, and to assess any implications for CO₂ storage, we performed unconfined volumetric strain (dilatometry) measurements on compacted pellets of montmorillonite, a common smectite frequently present in smectite-bearing shales and fault gouges. This was done by exposing SWy-1 and Na-SWy-2 type smectite samples (Wyoming) to CO₂ in a high pressure optical cell. We used this cell to assess the strain response to exposure to CO₂ at the P-T conditions expected in relatively shallow carbon dioxide storage reservoirs, i.e. at a temperature of 45°C and CO₂ pressures up to 15MPa. Samples were heat-treated prior to exposure to CO₂ to obtain initial hydration states (d₀₀₁-spacing), determined independently using X-ray diffraction methods. Our results show that SWy-1 and Na-SWy-2 montmorillonites swell almost instantaneously (in a few seconds) to an equilibrium state when exposed to (supercritical) CO₂ at the conditions investigated, with both materials exhibiting closely similar behaviour. Swelling strain increases systematically with CO₂ pressure in the range 1-7MPa, but only slightly at higher pressures. The SWy-1 data show maximum swelling strain for an initial d₀₀₁ spacing of 11.1Ǻ, reaching 2.46±0.45% at a CO₂ pressure of 15MPa. For initial d-spacings down to the fully dehydrated state of 9.8Ǻ, swelling decreases systematically with decreasing d-spacing at all pressures investigated. Only minor swelling (0.55±0.47% at 10MPa CO₂ pressure) is found for an initial d₀₀₁ spacing of 9.8Ǻ. For smectite-bearing caprocks and faults sealing carbon sequestration reservoirs, our results imply that CO₂ penetration can be expected to cause swelling of a few percent in the affected zone, closing small fractures or joints and thus reducing bulk permeability.