In Situ ¹³C and ²³Na Magic Angle Spinning NMR Investigation of Supercritical CO₂ Incorporation in Smectite–Natural Organic Matter Composites

Abstract

This Article presents an in situ NMR study of clay–natural organic polymer systems (a hectorite–humic acid [HA] composite) under CO 2 storage reservoir conditions (90 bar CO 2 pressure, 50 °C). The 13 C and 23 Na NMR data show that supercritical CO 2 interacts more strongly with the composite than with the base clay and does not react to form other C-containing species over several days at elevated CO 2 . With and without organic matter, the data suggest that CO 2 enters the interlayer space of Na–hectorite equilibrated at 43% relative humidity. The presence of supercritical CO 2 also leads to increased 23 Na signal intensity, reduced line width at half height, increased basal width, more rapid 23 Na T 1 relaxation rates, and a shift to more positive resonance frequencies. Larger changes are observed for the hectorite–HA composite than for the base clay. In light of recently reported MD simulations of other polymer–Na–smectite composites, we interpret the observed changes to be due to an increase in the rate of Na + site hopping in the presence of supercritical CO 2, the presence of potential new Na + sorption sites when the humic acid is present, and perhaps an accompanying increase in the number of Na + ions actively involved in site hopping. The results suggest that the presence of organic material either in clay interlayers or on external particle surfaces can significantly affect the behavior of supercritical CO 2 and the mobility of metal ions in clay-rich reservoir rocks.