CO₂ sorption to subsingle hydration layer montmorillonite clay studied by excess sorption and neutron diffraction measurements

Geologic storage of CO₂ requires that the caprock sealing the storage rock is highly impermeable to CO₂. Swelling clays, which are important components of caprocks, may interact with CO₂ leading to volume change and potentially impacting the seal quality. The interactions of supercritical (sc) CO₂ with Na saturated montmorillonite clay containing a subsingle layer of water in the interlayer region have been studied by sorption and neutron diffraction techniques. The excess sorption isotherms show maxima at bulk CO₂ densities of ≈0.15 g/cm³, followed by an approximately linear decrease of excess sorption to zero and negative values with increasing CO₂ bulk density. Neutron diffraction experiments on the same clay sample measured interlayer spacing and composition. The results show that limited amounts of CO₂ are sorbed into the interlayer region, leading to depression of the interlayer peak intensity and an increase of the d(001) spacing by ca. 0.5 Å. The density of CO₂ in the clay pores is relatively stable over a wide range of CO₂ pressures at a given temperature, indicating the formation of a clay-CO₂ phase. At the excess sorption maximum, increasing CO ₂ sorption with decreasing temperature is observed while the high-pressure sorption properties exhibit weak temperature dependence.