Experimental and numerical study of gas diffusion and sorption kinetics in ultratight rocks

Mass transport in ultratight rocks is markedly different from that in typical permeable rocks due to the presence of nano-scale pores and a dual-storage mechanism in terms of free and adsorbed gas. This work provides a quantitative analysis of gas transport behavior in ultratight rocks by utilizing X-ray computed micro-computed tomography (micro-CT) imaging and numerical modeling. We conducted X-ray micro-CT core-scale experiments using high-attenuation xenon (Xe) and Marcellus shale sample to obtain temporal and spatial Xe density maps from a series of micro-CT images. We present a dual-mechanism numerical model to analyze the sorption and diffusion phenomena observed in the experiment. The numerical model considers both bulk and surface diffusion by coupling of a diffusion-based equation for free-gas transport with a surface-diffusion equation for the sorbed phase. A sorption kinetic model quantifies mass transfer between the free- and sorbed-phase. The governing equations are solved simultaneously using finite element methods. Comparisons of numerical and experimental results reveal that sorption is a non-equilibrium process in ultratight rocks and surface diffusion significantly contributes to total mass transport through nanopores. Further, results show that sorbed-phase transport a nonlinear phenomenon given the dependence of surface diffusion coefficient on concentration. Resulting transport-related parameters, such as bulk and surface diffusion coefficients and sorption rate constants, which are estimated from history matching, are consistent with literature data.

Automated summary

This study provides a quantitative analysis of gas transport behavior in ultratight rocks using X-ray computed micro-computed tomography imaging and numerical modeling. The results show that sorption is a non-equilibrium process in ultratight rocks and surface diffusion significantly contributes to total mass transport through nanopores.