Smectite–brine–CO 2 interactions: effects of interlayer chemistry, brine concentration, CO 2 pressure, and temperature

• Published in: Clays and clay minerals 2024, Vol.72, Article e17
• Main Authors: Benavides, Paolo Andre, Guggenheim, Stephen
• Format: Article
• Language: English
• ISSN: 0009-8604; 1552-8367
• DOI: 10.1017/cmn.2024.28

Smectite may impact the ability of saline aquifer–caprock systems to store CO 2 effectively, because of changes in pressure, temperature, and brine concentration induced by the injection of CO 2 . These changes influence the molar volume of smectite, affecting the short-term structural and stratigraphic trapping, or the dissolution of smectite via the long-term geochemical trapping. This study investigated the d 001 value of an interlayer-cation-exchanged smectite, Na-rich SWy-2 (Na-SWy-2), with Ca or Mg (hereafter CaSWy-2 and MgSWy-2). Molar volume experiments used X-ray diffraction and a high-pressure environmental chamber. The extent of smectite dissolution was simulated at experimental conditions by geochemical modeling using a rate equation derived from the transition state theory. CaSWy-2-CaCl 2 and MgSWy-2-MgCl 2 brine systems showed that increasing the brine concentration from 0.17 M to saturation results in a < 18% decrease in d 001 values, and increasing the temperature from approximately 33 to 150°C results in < 11% decrease. The effect of the interlayer cation shows the d 001 values of MgSWy-2 are < 0.4 Å higher compared with CaSWy-2. Geochemical modeling shows the extent of dissolution of Na-SWy-2, CaSWy-2, or MgSWy-2 is only < 1.1% in acidic conditions. Furthermore, the calculated swelling pressure needed to decrease the H 2 O sheets in the interlayer, from 3W to 2W, of MgSWy-2 and CaSWy-2 are higher compared with Na-SWy-2. The swelling pressure was approximated from the sum of the osmotic repulsive pressure, the van der Waals attractive pressure, and the hydration pressure. The data suggest that Na-SWy-2, CaSWy-2, and MgSWy-2 may affect saline aquifer–caprock systems to store CO 2 . The molar volume is affected by changes in pressure, temperature and brine concentration, or swelling pressure from the injection of CO 2 . An increase in the d 001 value of SWy-2 can enhance the sealing capabilities of a caprock by making saline aquifers less porous and less permeable and thus increasing the capability for CO 2 storage. In contrast, a decrease in the d 001 value can create cracks in a caprock and thus provide conduits for the CO 2 to escape. Furthermore, the CO 2 injection will cause a decrease in pH, causing smectite to dissolve until it reaches a steady state. However, despite acidic aquifer conditions, SWy-2 has low solubility.