In Situ X-ray Diffraction Study of Na+ Saturated Montmorillonite Exposed to Variably Wet Super Critical CO^sub 2

Reactions involving variably hydrated super critical CO2 (scCO2) and a Na saturated dioctahedral smectite (Na-STX-1) were examined by in situ high-pressure X-ray diffraction at 50 °C and 90 bar, conditions that are relevant to long-term geologic storage of CO2. Both hydration and dehydration reactions were rapid with appreciable reaction occurring in minutes and near steady state occurring within an hour. Hydration occurred stepwise as a function of increasing H2O in the system; 1W, 2W-3W, and >3W clay hydration states were stable from 2-30%, 31-55 < 64%, and ≥ 71% H2O saturation in scCO2, respectively. Exposure of sub 1W clay to anhydrous scCO2 caused interlayer expansion, not contraction as expected for dehydration, suggesting that CO2 intercalated the interlayer region of the sub 1W clay, which might provide a secondary trapping mechanism for CO2. In contrast, control experiments using pressurized N2 and similar initial conditions as in the scCO2 study, showed little to no change in the d001 spacing, or hydration states, of the clay. A salient implication for cap rock integrity is that clays can dehydrate when exposed to wet scCO2. For example, a clay in the 3W hydration state could collapse by 3 Å in the c* direction, or 15%, if exposed to scCO2 at less than or equal to about 64% H2O saturation. [PUBLICATION ABSTRACT]