Abstract
Geologic storage of CO2 requires that the caprock sealing the storage rock is highly impermeable
by CO2. Swelling clays, which are important components of caprocks, may react with CO2 under
volume change, potentially impacting the seal quality. The interactions of scCO2 with Na
saturated montmorillonite clay containing a sub-single 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 low pressure
increasing CO2 adsorption with decreasing temperature is observed while the high-pressure
sorption properties exhibit weak or no temperature dependence.
Supercritical fluids, sorption phenomena, carbon dioxide, carbon sequestration, caprock
integrity
