CO2 SORPTION TO ALALOH, ALMGOH AND INTERLAYER SITES IN NA-RICH MONTMORILLONITE AT CCS P-T CONDITIONS

Carbon capture and storage (CCS) in confined sedimentary formations has the potential to reduce the impact of fossil fuel combustion by storing CO₂ in perpetuity. At PT conditions relevant to CCS, CO₂ is less dense than the pre-existing brine the more buoyant CO₂ will migrate to the top of the formation to be in contact with cap rock. Shale cap rocks are typically clay-rich and interactions between shales and CO₂ is poorly understood at relevant PT conditions. In this study we use Na-rich montmorillonite (mont) as an analog for clay-rich shale. We use neutron diffraction, excess sorption and Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy (ATR -FTIR) analyses of mont at 35°C and 50°C and from 0-200 bars to examine CO₂ interactions with mont under conditions relevant to CCS.

Excess sorption isotherms, determined gravimetrically, provide an understanding of changes in the density of CO₂ near the mineral surface. Maxima in the excess sorption isotherms were observed at a bulk density ≈0.15 g/cm³ and pressures of 58 bars (35°C) and 64 bars (50°C). Above this maxima, as the bulk density of the CO₂ increases, the amount of CO₂ sorbed to the clay decreases. Neutron diffraction measurements reveal a shift in the d(001) spacing from 12.10 Å to 12.55 Å and a decrease in the intensity of the d(001) peak, both of which are consistent with CO2 entering the interlayer region of the clay.

The same clay sample was studied using ATR-FTIR to identify the crystallographic sites on which CO₂ interacts with mont. Measurements were conducted on both hydrated and dried mont from 1-82 bars at 35° and 50°C. ATR-FTIR data show that the asymmetric stretch and bending mode of sorbed CO₂ is modified by the presence of interlayer water, but the absorption bands representing adsorbed water (3564 and 2975 cm^-1) are not affected by the presence of CO₂. Analysis of the data indicates that CO₂ adsorbs in the interlayer space and potentially sorbs onto the edges of octahedral sheets of the mont structure, depending on hydration state. The rheological properties of the caprock are likely to be affected by CO₂ - mont interactions, but further work is needed to determine if seal quality is more likely to be degraded or enhanced by this interaction.