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 a Na-saturated montmorillonite (Na-SWy-2) in high-pressure (90 bar) and moderate temperature (50 °C) methane and mixtures of methane and carbon dioxide were investigated using in situ IR spectroscopic titrations and in situ XRD. The goals were to (1) determine if the hydration/expansion behavior of the clay in supercritical methane is different than in supercritical CO₂, (2) determine if methane intercalates the clay, and (3) probe the effects of increasing CO₂ concentrations. IR spectra were collected as Na-SWy-2 was titrated with water under several fluid exposures: pure methane, 25, 50, and 75 mole% CO₂ in methane, and pure CO₂. Complementary in situ XRD experiments were conducted in the same fluids at discrete dissolved water concentrations to measure the d₀₀₁ values of the clay and thus its volume change on hydration and CH₄ and/or CO₂ intercalation. In pure methane, no direct evidence of CH₄ intercalation was detected in CH bending or stretching regions of the IR spectra. Similarly, in situ XRD indicated the montmorillonite structure was stable in the presence of CH₄ and no measurable changes to the basal spacing were observed. However, under low water conditions where the montmorillonite structure was partially expanded (~sub 1W), the IR data indicated a rapid intercalation of CO₂ into the interlayer, even with fluid mixtures containing the lowest concentrations of CO₂. Likewise, in situ XRD showed indirect evidence of CO₂ intercalation from an increase in the basal spacing from 11.8 to 12.3 Å under identical conditions. These findings demonstrate that water and CO₂ intercalation processes could lead to permeability changes that directly impact methane transmissivity in shales.