The transition from incoherent to coherent buoyancy‐driven gas flow is investigated in two‐dimensional tanks filled with glass beads using a high‐resolution optical‐gravimetrical setup. Both a grain‐size (dk)‐ and flow rate (Q)‐dependent transition are observed in the gas flow pattern. Standard quasistatic criteria do not explain the experimental results, since they do not take into account the competition between stabilizing friction forces and destabilizing capillary and gravitational forces. Conceptualizing the steady state tortuous gas flow as core‐annulus flow and applying Hagen‐Poiseuille flow for a straight capillary, we propose a flow rate and grain‐size‐dependent stability criterion that accounts for the experimental results and is used to classify the experiments in a dk‐Q diagram.

[1] 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.

A multi-scale optical imaging technique was developed allowing for the 2D observation of two phase flow in porous media at two different scales simultaneously: Using two coupled cameras, a 2D flow cell (0.5×0.5 m 2 ) is recorded entirely at the bench scale and at the pore scale with a spatial resolution of 0.5 and 0.01 mm, respectively. The technique is applied to study channelized gas flow in saturated glass beads. We analyze the phase distribution at the pore scale and derive a pixel-based method for the measurement of saturation at the larger scale. This method assumes linearity between the mean reflected light intensity and the local gas saturation if averaging is performed over representative areas (REV). The REV depends on the irregularity of the local pore structure and has a lower limit at the correlation length of the porous medium (somewhat above the size of the glass beads) and an upper limit which correlates with the width of gas channels. These limits could be quantified through optical analysis. The optical approach to estimate phase saturations was validated by gravimetric analysis where a characteristic ratio between the optically observed flow cell wall and the saturation within the bulk material was identified, which corresponds to the expectation based on geometrical considerations of the glass bead packing. Considering a transient flow experiment the optical method is demonstrated to be able to quantify the temporal evolution of the residual and the convective gas phase. We conclude that the new technique provides a valuable tool to improve our quantitative understanding of multiphase phenomena across different scales.

No abstract