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

• Na-rich montmorillonite (Na-STx-1) used as an analog for clay-rich shale
• Infrared used to probe CO2 interaction with hydrated and dehydrated Na-STx-1
• Examined temperature at 35 °C and 50 °C and CO2 pressure from 0–5.9 MPa
• Dehydrated Na-STx-1 capable of incorporating more CO2 compared to hydrated Na-STx-1
• No evidence of carbonate formation or the interaction of CO2 with sodium cations

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 0–5.9 MPa. 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.