Injection of CO2 into low permeability shale formations leads to additional gas recovery and reduces the flux of CO2 into the atmosphere, but reduced formation transmissivity due to clay swelling is a concern. 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. In pure CH4, no evidence of CH4 intercalation was detected by IR or XRD for any of the clays. However, when dry Cs- and NH4-SWy-2 were exposed to dry fluids containing CO2, IR showed 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.

Investigation of Wyoming bentonite hydration in dry to water-saturated supercritical CH4 and CH4/CO2 mixtures: Implications for CO2-enhanced gas production

251st National Meeting (2016)

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