Adsorption-induced swelling impact on CO[formula omitted] transport in kerogen microporosity described by free volume theory

Abstract

In order to examine the impact of the flexibility of kerogens’ microstructure on the transport of adsorbed CO2, we develop a free volume model that notably accounts for adsorption-induced swelling exhibited by flexible kerogen structures. We first show that, for CO2 compared to CH4 at constant fluid molar loading and in the same temperature and confining pressure conditions, an atomistic model of microporous type I kerogen in the middle of the oil formation window exhibits slightly less swelling upon adsorption. This swelling comes with an increase in the free volume accessible to the CO2 molecules, which can then diffuse faster when the fluid loading increases, without introducing significant collective effects on the transport properties. Consequently, transport can be considered diffusive, and the self-diffusion coefficient can be used as a proxy for the collective diffusion (or transport) coefficient. Finally, as previously demonstrated for CH4, a free volume model based on the Fujita–Kishimoto theory describes the transport enhancement with CO2 fluid loading. Although CO2 interacts more strongly than CH4, the parameters of the free volume models for both fluids are shown to be similar. Together with the diffusive nature of transport, it suggests an even broader applicability of the model to the transport of light alkanes and mixtures, thus greatly simplifying transport modeling for geological applications such as carbon sequestration in shales.