Investigation of Wyoming Bentonite Hydration in Dry to Water-Saturated Supercritical CH4 and CH4/CO2 Mixtures: Implications for CO2-Enhanced Gas Production

Abstract:

Injection of CO₂ into low permeability shale formations leads to additional gas recovery and reduces the flux of CO₂ into the atmosphere, thus combining a strong economic incentive with a permanent storage option for CO₂. Reduced formation transmissivity due to clay swelling is a concern in CO₂-enhanced gas production. Clay minerals partly determine the physical (i.e. permeability, brittleness) and certain chemical properties (i.e. wetting ability, gas adsorption) of shales, and montmorillonites are of particular interest because they swell by the uptake of species in their interlayer.

In this study, the hydration and expansion of Na-, Cs-, and NH₄⁺-saturated montmorillonite (Na-, Cs-, and NH₄-SWy-2) in high-pressure (90 bar) and moderate temperature (50 °C) methane, carbon dioxide, and CO₂/CH₄ mixtures (3 and 25 mole% CO₂) were investigated using in situ IR spectroscopic titrations, in situ XRD, in situ MAS-NMR, and ab initio electronic structure calculations. The overarching goal was to better understand the hydration/expansion behavior of Na-SWy-2 in CO₂/CH₄ fluid mixtures by comparison to Cs-, and NH₄⁺-saturated clays. Specific aims were to (1) determine if CH₄ intercalates the clays, (2) probe the effects of increasing dissolved CO₂ and H₂O concentrations, and (3) understand the role of cation solvation by H₂O and/or CO₂.

In pure CH₄, no evidence of CH₄ intercalation was detected by IR for any of the clays. Similarly, no measurable changes to the basal spacing were observed by XRD in the presence of pure CH₄. However, when dry Cs- and NH₄-SWy-2 were exposed to dry fluids containing CO₂, IR showed maximum CO₂ penetrated the interlayer, XRD indicated the clays expanded, and NMR showed evidence for cation solvation by CO₂, in line with theoretical predictions. IR titration of these clays with water showed sorbed H₂O concentrations decreased with increasing dissolved CO₂, suggesting competition for interlayer residency by CO₂ and H₂O. For Na-SWy-2, on the other hand, CO₂ intercalated the clay and was at a maximum only after a minimum sorbed H₂O was achieved. Further increases in sorbed H₂O led to displacement of intercalated CO₂. These findings demonstrate that complicated H₂O and CO₂ intercalation processes could lead to permeability changes that directly impact methane transmissivity in shales.