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.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology
- Organic Chemistry