Gas Transport Modeling in Organic-Rich Shales with Nonequilibrium Sorption Kinetics

The presence of a large number of nanopores and complicated gas storage and transport mechanisms pose challenges for understanding fluid transport in organic-rich shales with a wide presence of nanopores. We proposed a diffusion-based model for gas transport in organic-rich nanoporous media by considering nonequilibrium sorption. The model consists of two sets of governing equations for bulk diffusion and Knudsen diffusion in the free phase, and surface diffusion in the sorbed phase. The two governing equations are connected by a source/sink term which is described by the sorption kinetic model. A newly-developed effective diffusion coefficient characterizes the delayed free-phase mass transfer due to the reduced effective pore volume and sorption. We applied the developed diffusion model to quantitatively analyze the experimental data from Xenon uptake into a Marcellus shale sample. The temporal and spatial gas concentration distribution were obtained from X-ray micro-CT images. The model was able to match the spatial profiles of Xenon concentration within the core sample. The model can capture the concentration peak due to the sorption kinetics. The results show that surface diffusion significantly contributes to the total mass transport of Xenon in the Marcellus shale sample. The sorbed phase can occupy up to 30% of pore space and the free-phase diffusion coefficient is significantly reduced by up to 40%. Therefore, neglecting the pore space occupied by the sorbed phase results in overestimated diffusion coefficients and mass concentrations.