Interaction of CO2 and brines with montmorillonite to 400 bars and 45oC

Carbon Capture and Storage (CCS) from large stationary CO2 point sources is considered a promising option to reduce anthropogenic CO2 emissions in short- to medium time scales. Besides a storage reservoir allowing for large storage capacity at reasonable injection rates, an impermeable caprock overlying the reservoir is indispensable to guarantee safe long-term storage. In this context, understanding the behavior of clay systems with aqueous fluids and CO2 is an essential aspect of site selection and design of CO2 storage projects. Therefore, montmorillonite was studied using an X-ray, high-pressure environmental chamber (HPEC) developed at the University of Illinois at Chicago. Two series of experiments include: (a) dry montmorillonite at various relative humidity with supercritical CO2, and (b) montmorillonite immersed in brines as slurries with supercritical CO2. Changes in the layer-to-layer spacing [d(001)] of montmorillonite under conditions of elevated pressure (P) and temperature (T) were monitored. Samples of dry montmorillonite (Clay Minerals Society Source Clay, SWy-2) were prepared and loaded in the HPEC and allowed to equilibrate with small amounts of water. Initial amounts of interlayer H2O (i.e., various relative humidity) were monitored by the d-value of the (001) diffraction peak, then CO2 was added to the HPEC. The position and shape of the (001) peak changed for most samples under low CO2 pressures (40-50 bars, 40 - 45oC). Response to CO2 by the d(001) is strongly related to the initial H2O content. For samples with an initial d(001) of ~ 10.5 Å, the peak position changed by a maximum shift of ~ 1.75 Å to higher values. Very dry (10.10 Å) and very hydrated (16.5 Å) samples did not exhibit detectable changes of the (001) peak position or shape. Experiments to P = 400 bars showed similar results to those at P = 40-50 bars. Aqueous brine (0.5 M NaCl) slurries of SWy-2 were examined also under CO2 pressure. Initially, the d(001) was at 19.5 - 20 Å. Over 36 hours at 135 - 200 bars of CO2 pressure (T = 27oC), a peak formed at ~ 28 Å. The peak remained nearly unchanged after ~150 hours. Financial support from Shell Exploration and Production B.V. and the National Science Foundation is gratefully acknowledged.