Pore-scale and continuum modeling of gas flow pattern obtained by high-resolution optical bench-scale experiments

High-resolution optical bench-scale experiments were conducted in order to investigate local gas flow pattern and integral flow properties caused by point-like gas injection into water-saturated glass beads. The main goal of this study was to test the validity of the continuum approach for two-fluid flow in macroscopic homogeneous media. Analyzing the steady state experimental gas flow pattern that satisfies the necessary coherence condition by image processing and calibrating the optical gas distribution by the gravimetrical gas saturation, it was found that a pulse-like function yields the best fit for the lateral gas saturation profile. This strange behavior of a relatively sharp saturation transition is in contradiction to the widely anticipated picture of a smooth Gaussian-like transition, which is obtained by the continuum approach. This transition is caused by the channelized flow structure, and it turns out that only a narrow range of capillary pressure is realized by the system, whereas the continuum approach assumes that within the representative elementary volume the whole spectrum of capillary pressures can be realized. It was found that the stochastical hypothesis proposed by Selker et al. (2007) that bridges pore scale and continuum scale is supported by the experiments. In order to study channelized gas flow on the pore scale, a variational treatment, which minimizes the free energy of an undulating capillary, was carried out. On the basis of thermodynamical arguments the geometric form of a microcapillary, macrochannel formation and a length-scale-dependent transition in gas flow pattern from coherent to incoherent flow are discussed.