Synergistic Coupling of CO₂ and H₂O during Expansion of Clays in Supercritical CO₂–CH₄ Fluid Mixtures

Abstract

We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na+-, NH4 +-, and Cs+-montmorillonites (SWy-2) in supercritical fluid mixtures of H2O, CO2, and CH4. Building on our prior work with Na-clay that demonstrated that H2O facilitated CO2 intercalation at relatively low RH, here we show that increasing CO2/CH4 ratios promote H2O intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO2 intercalated and expanded the Cs-clay even in the absence of H2O, while increasing fluid CO2/CH4 ratios inhibited H2O intercalation. The NH4-clay displayed intermediate behavior. By comparing changes in the HOH bending vibration of H2O intercalated in the Cs-, NH4-, and Na-clays, we posit that CO2 facilitated expansion of the Na-clay by participating in outer-sphere solvation of Na+ and by disrupting the H-bond network of intercalated H2O. In no case did the pure CH4 fluid induce expansion. Our experimental data can benchmark modeling studies aimed at predicting clay expansion in humidified fluids with varying ratios of CO2 and CH4 in real reservoir systems with implications for enhanced hydrocarbon recovery and CO2 storage in subsurface environments.

Interlayer Cation Polarizability Affects Supercritical Carbon Dioxide Adsorption by Swelling Clays

“…here suggest that expansion upon contact with wet scCO 2 is largely due to the net uptake of H 2 O in the smectite interlayers accompanied by displacement of interlayer CO 2 (Figures3 and S7), consistent with the assumption that H 2 O outcompetes CO 2 for interlayer adsorption sites at higher scCO 2 H 2 O activities [21][22][23][24][25][26][27][32][33][34]…”

“…The physical and chemical responses of smectites in contact with anhydrous and wet CO 2 at high pressures have received considerable attention in the past decade. − For a typical smectite with charges of −0.2 to −0.5 per formula unit, there is a general agreement in the literature that the energy barrier to intercalate a dry Na + saturated smectite with anhydrous scCO 2 is significant unless the basal spacing of the interlayer is comparable to a monolayer hydrated (basal spacing ∼12.5 Å; noted 1WL in this paper) state. ,,,,,,,,,,− Such a basal spacing is attainable when the interlayer cation is large (e.g., Cs + with a 6-coordination ionic radius of 1.67 Å) or when a small amount of H 2 O is present in the interlayer (enough to produce a basal spacing similar to the 1WL case). However, inclusion of additional water to produce a bilayer hydrate (2WL) state or greater can expel intercalated CO 2 from the interlayer, with the final CO 2 content dependent on the amount of available H 2 O, the charge on the smectite layer, the identity of the charge-balancing cation in the smectite interlayers, and the extent of structural fluorination on the octahedral sheet. ,− ,,− , This trend in CO 2 adsorption behavior has been experimentally observed above the critical point for different model smectites, including Na + , Ca 2+ , K + , NH 4 + , and Mg 2+ montmorillonites − ,− and for Na + , Cs + , Ni 2+ , and Ca 2+ hectorites. ,, …”

“…+ saturated smectites where the collapsed state of the NH 4 -hectorite (10.72 Å) and NH 4 -montmorillonite (10.39 Å) expanded immediately upon exposure to dry scCO 2 (Figures S1 and S2), consistent with what has been reported previously for NH 4 -montmorillonite. 24 Overall, the NH 4 -hectorite consistently maintained a larger d(001) basal spacing, ranging from 12.10 Å (0% RH) to a maximum of 12.92 Å (100% RH). The Pb 2+ saturated smectites experienced the greatest expansion upon exposure to variably wet scCO 2 and a large initial interlayer expansion during exposure to dry scCO 2 .…”

“…The number of moles of cations was determined from the mass of dry clay, the molecular weight of dry clay, and the cation stoichiometry. The H 2 O/cation mole ratio at 0% RH was determined from the linear correlation between the H 2 O/cation mole ratios calculated from transmission IR with the integrated absorbances of the HOH bending mode of sorbed H 2 O from the ATR-IR measurements …”

“…Bowers et al show that smectites bearing small cations with high hydration energies preferentially adsorb H 2 O as opposed to CO 2 , leading to H 2 O-dominated interlayers at modest H 2 O activities in scCO 2 . However, ions with large radii and low hydration energy seem to interact readily with CO 2 or H 2 O and exhibit larger CO 2 /H 2 O ratios for their interlayers when exposed to variably wet scCO 2 . ,, That being said, Bowers et al and Loring et al report very high interlayer CO 2 fractions for Cs-smectites compared to all the other cation-saturated smectite data at that time, with Bowers et al noting that Cs + is the ion with the largest polarizability that had been studied to date. A recent 2 H variable temperature NMR study of hectorite exchanged with several monovalent and divalent cations suggests that cation polarizability may also be important for understanding the behavior of H 2 O in the interlayer .…”