Simplified
local density (SLD) model has been widely used to describe
the gas adsorption behaviors in porous media. However, the slit pore
geometry and constant pore width associated with the SLD model may
fail to represent the heterogeneous pore network structure in shale.
In this study, a new method to integrate the SLD model with the slit
and cylindrical pore structures as well as the pore size distribution
(PSD) is proposed and validated by the grand canonical Monte Carlo
(GCMC) simulations and the experimentally measured adsorption of methane
on shale with complex pore network. Comparison results show that reasonably
good agreement is achieved between the SLD model and GCMC simulations
for both the gas adsorption isotherms and discrete-density profiles
in multiwalled carbon nanoslit and nanotube. The corresponding average
absolute percentage deviations (% AADs) are below 0.3 and 9.3 for
gas adsorption isotherm and discrete-density profile, respectively.
In addition, the SLD model coupled with the PSD of slit and cylindrical
pores ranging from micro- to macropores properly characterizes the
measured excess adsorption of methane on Wolfcamp shale core sample
with % AADs between 1.7 and 3.6. It is found that when the pore volume
is fixed, the gas adsorption isotherm and gas density profile are
heavily dependent on the pore geometry and pore size. Furthermore,
integrating the PSD into the SLD model can guarantee the valid identification
of the adsorbed- and free-gas regions in flow channels with different
sizes based on the gas density profiles. The findings of this study
shed light on the effects of pore structure on gas adsorption in nanopores
and enable us to precisely evaluate and predict the gas adsorption
behaviors in slit and cylindrical pores over a wide range of pore
sizes.
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