Synergistic Coupling of CO 2 and H 2 O during Expansion of Clays in Supercritical CO 2 -CH 4 Fluid Mixtures.

We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na ⁺ -, NH ₄ ⁺ -, and Cs ⁺ -montmorillonites (SWy-2) in supercritical fluid mixtures of H ₂ O, CO ₂ , and CH ₄ . Building on our prior work with Na-clay that demonstrated that H ₂ O facilitated CO ₂ intercalation at relatively low RH, here we show that increasing CO ₂ /CH ₄ ratios promote H ₂ O intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO ₂ intercalated and expanded the Cs-clay even in the absence of H ₂ O, while increasing fluid CO ₂ /CH ₄ ratios inhibited H ₂ O intercalation. The NH ₄ -clay displayed intermediate behavior. By comparing changes in the HOH bending vibration of H ₂ O intercalated in the Cs-, NH ₄ -, and Na-clays, we posit that CO ₂ facilitated expansion of the Na-clay by participating in outer-sphere solvation of Na ⁺ and by disrupting the H-bond network of intercalated H ₂ O. In no case did the pure CH ₄ fluid induce expansion. Our experimental data can benchmark modeling studies aimed at predicting clay expansion in humidified fluids with varying ratios of CO ₂ and CH ₄ in real reservoir systems with implications for enhanced hydrocarbon recovery and CO ₂ storage in subsurface environments.