Supercritical carbon dioxide (sCO2) can be used to increase the yield from gas reservoirs in a process known as enhanced gas recovery (EGR). Determining the potential of a reservoir to produce gas is highly complex as the gas may be found adsorbed to organic matter, in fissures and pores, and adsorbed to mineral surfaces. Recent studies on clay minerals indicate that between 20% and 85% of shale gas may come from desorption over the life of a reservoir. Gas-in-place estimations must consider the pore structure and chemical composition of the substrate, as well as the effect of water either from the formation or dissolved in sCO2 on the adsorption/desorption processes. High-level ab initio electronic structure calculations free of adjustable parameters can provide insight into the structure and dynamics of surface complexes relevant to EGR. We address adsorption/desorption issues in this work using first principles Molecular Orbital/Density Functional Theory to model two clay minerals, Na-montmorillonite and kaolinite, and their interactions with CO2 and CH4 molecules. Energy minimizations are performed with the M06-2X functional, well suited for non-bonding interactions, followed by determination of the infrared vibrational frequencies. The free energies of adsorption of the clay models, the adsorbates, and the adsorbate-substrate clusters are determined, and the difference used to provide results for ΔGads. The values obtained are contrasted with data derived from experimental adsorption isotherms. Preliminary calculations with selected input structures indicated that the adsorption of CH4 to a clay surface is less favored that that of CO2 and that the presence of water molecules, modeled explicitly, leads to significant differences. The outcomes depend strongly on the clay model, the adsorbate, and the initial adsorbate-substrate cluster. Small differences in the initial geometry did not produce much change in the adsorption of CO2 on kaolinite, which remained unfavorable, but significantly changed the interaction of this adsorbate with Na-montmorillonite with outcomes as different as physisorption and mineralization. In this work we explore the causes for these different outcomes by implementing a systematic approach to sampling the surface and by determining the activation energy and the rate constant for the adsorption/desorption processes.

Influence of humidity on adsorption/desorption structure and dynamics in enhanced gas recovery

251st National Meeting (2016)

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