FT-IR study of CO₂ interaction with Na⁺ exchanged montmorillonite

Carbon capture, utilization and storage (CCUS) in saline reservoirs in sedimentary formations has the potential to reduce the impact of fossil fuel combustion on climate change by reducing CO₂ emissions to the atmosphere and storing the CO₂ in geologic formations in perpetuity. At pressure and temperature (PT) conditions relevant to CCUS, CO₂ is less dense than the pre-existing brine in the formation, and the more buoyant CO₂ will migrate to the top of the formation where it will be in contact with cap rock. Interactions between clay-rich shale cap rocks and CO₂ are poorly understood at PT conditions appropriate for CCUS in saline formations. In this study, the interaction of CO₂ with clay minerals in the cap rock overlying a saline formation has been examined using Na⁺ exchanged montmorillonite (Mt) (Na⁺-STx-1) (Na⁺ Mt) as an analog for clay-rich shale. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) was used to discern mechanistic information for CO₂ interaction with hydrated (both one- and two-water layers) and relatively dehydrated (both dehydrated layers and one-water layers) Na⁺-STx-1 at 35°C and 50°C and CO₂ pressure from 0-5.9MPa. CO₂-induced perturbations associated with the water layer and Na⁺-STx-1 vibrational modes such as AlAlOH and AlMgOH were examined. Data indicate that CO₂ is preferentially incorporated into the interlayer space, with relatively dehydrated Na⁺-STx-1 capable of incorporating more CO₂ compared to hydrated Na⁺-STx-1. Spectroscopic data provide no evidence of formation of carbonate minerals or the interaction of CO₂ with sodium cations in the Na⁺-STx-1 structure.