Free Volume Model for Transport in Flexible Kerogen of Source Rock's Organic Matter

We build a model of transport of adsorbed fluid within the microporous network of kerogen's porosity, especially accounting for the adsorption-induced swelling exhibited by flexible kerogen structures. This model, based on Fujita-Kishimoto free volume theory that was historically developed for swellable polymers, is built over extensive results for the self-diffusion coefficients obtained by molecular dynamics calculations for a representative molecular model of kerogen designed to study the importance of flexibility effects on the properties of kerogen. To do so, we first highlight that transport within flexible kerogen incorporating the coupling between the dynamics of the fluid molecules and the kerogen matrix atoms does not introduce any significant collective effects in the usual long time limit. Then, we show that despite the slightly anisotropic diffusion properties, averaging over all the dimensions can still be performed in order to model the behavior of the transport properties with the amount of adsorbed fluid. Lastly, we link the increase of the self-diffusion coefficients and that of the accessible free volume with the fluid loading via the Fujita-Kishimoto model. We conclude by commenting on the evolution and significance of the model parameters over a broad range of thermophysical conditions.