Making reliable estimates of gas adsorption in shale remains a challenge because
the variability in their mineralogy and thermal maturity results in a broad distribution
of pore-scale properties, including size, morphology and surface chemistry. Here, we
demonstrate the development and application of a hybrid pore-scale model that uses
surrogate surfaces to describe supercritical gas adsorption in shale. The model is based
on the lattice Density Functional Theory (DFT) and considers both slits and cylindrical pores to mimic the texture of shale. Inorganic and organic surfaces associated
with these pores are accounted for by using two distinct adsorbate-adsorbent interaction energies. The model is parameterised upon calibration against experimental
adsorption data acquired on adsorbents featuring either pure clay or pure carbon surfaces. Therefore, in its application to shale, the hybrid lattice DFT model only requires
knowledge of the shale-specific organic and clay content. We verify the reliability of
the model predictions by comparison against high-pressure CO2 and CH4 adsorption
isotherms measured at 40 ◦C in the pressure range 0.01–30 MPa on four samples from
three distinct plays, namely the Bowland (UK), Longmaxi (China) and Marcellus shale
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(USA). Because it uses only the relevant pore-scale properties, the proposed model can
be applied to the analysis of other shales, minimising the heavy experimental burden
associated with high pressure experiments. Moreover, the proposed development has
general applicability meaning that the hybrid lattice DFT can be used to the characterisation of any adsorbent featuring morphologically and chemically heterogeneous
surfaces.