In situ XRD study of Ca2+ saturated montmorillonite (STX-1) exposed to anhydrous and wet supercritical carbon dioxide

Abstract

Reactions involving supercritical carbon dioxide (scCO2) and a calcium saturated dioctahedral smectite (Ca-STX-1) were examined by in situ high-pressure X-ray diffraction over a range of temperatures (50–100°C) and pressures (90, 125, and 180bar) relevant to long-term geologic storage of CO2. Exposure of Ca-STX-1 containing one water of hydration (1W) to anhydrous scCO2 at 50°C and 90bar produced an immediate increase of ∼0.8Å in the d001 basal reflection that was sustained for the length of the experiment (∼44h). Higher ordered basal reflections displayed similar shifts. Following depressurization, positions of basal reflections and FWHM values (d001) returned to initial values, with no measurable modification to the clay structure or water content. Similar results were obtained for tests conducted at 50°C and higher pressures (125 and 180bar). Exposure of Ca-STX-1 containing two waters of hydration (2W) to scCO2 resulted in a decrease in the d001 reflection from 14.48Å to 12.52Å, after pressurization, indicating a partial loss of interlayer water. In addition, the hydration state of the clay became more homogeneous during contact with anhydrous scCO2 and after depressurization. In the presence of scCO2 and water, the clay achieved a 3W hydration state, based on a d001 spacing of 18.8Å. In contrast to scCO2, comparable testing with N2 gas indicated trivial changes in the d001 series regardless of hydration state (1W or 2W). In the presence of free water and N2, the basal spacing for the Ca-STX-1 expanded slightly, but remained in the 2W hydration state. The experiments show that potential collapse or expansion of the interlayer spacing depends on the initial hydration state of the clay and scCO2, where 1W clay is stable but ≥2W layer clay loses water when exposed to anhydrous CO2. Consequently, the implications of this study depend upon the depth of the caprock. If the caprock is quite deep, then the 1W hydration state is favored and the introduction of dry CO2 could actually help seal the formation. If the caprock is located closer to the surface where 2W or 3W montmorillonite is the predominant form then the introduction of dry CO2 should result in the creation of permeability. Further, these experiments indicate that scCO2 can become intercalated within hydrated clays under conditions proposed for geologic storage of CO2 and act as secondary CO2 traps.