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.

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Fluid invasion, displacement of one fluid by another in porous media, is important in a large number of industrial and natural processes. Of special interest is the trapping of gas and oil clusters. We study the impact of wettability on fluid pattern formation and capillary trapping in three-dimensional glass beads packs (d mean = 1 mm) during fluid invasion at capillary numbers of 10 −7 using μ-CT. The invading fluid was water, and the defending fluid was air. The contact angle of the glass beads was altered between 5°and 115°using Piranha cleaning and silanization. We analyzed the front morphology of the invading fluid, the residual gas saturation, the fluid occupation frequency of pores, and the morphology and statistics of the trapped gas clusters. We found a sharp transition (crossover) at a critical contact angle θ c = 96°. Below θ c the morphology of the displacement front was flat and compact caused by the strong smoothing effect of cooperative filling. Above θ c the morphology of the displacement front was fractal and ramified caused by single bursts (Haines jumps). Across this dynamical phase transition the trapping efficiency changes from no trapping to maximal trapping. For θ > θ c the experimental results show that invasion percolation governs the fluid displacement. Strong indicators are the universal scaling behavior of the size distribution of large clusters (relative data error ε data < 1%) and their linear surface-volume relationship (R 2 = 0.99).

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We study the impact of pore structure and surface roughness on capillary trapping of nonwetting gas phase during imbibition with water for capillary numbers between 10−7 and 5 × 10−5, within glass beads, natural sands, glass beads monolayers, and 2‐D micromodels. The materials exhibit different roughness of the pore‐solid interface. We found that glass beads and natural sands, which exhibit nearly the same grain size distribution, pore size distribution, and connectivity, showed a significant difference of the trapped gas phase of about 15%. This difference can be explained by the microstructure of the pore‐solid interface. Based on the visualization of the trapping dynamics within glass beads monolayers and 2‐D micromodels, we could show that bypass trapping controls the trapping process in glass beads monolayers, while snap‐off trapping controls the trapping process in 2‐D micromodels. We conclude that these different trapping processes are the reason for the different trapping efficiency, when comparing glass beads packs with natural sand packs. Moreover, for small capillary numbers of 10−6, we found that the cluster size distribution of trapped gas clusters of all 2‐D and 3‐D porous media can be described by a universal power law behavior predicted from percolation theory. This cannot be expected a priori for 2‐D porous media, because bicontinuity of the two bulk phases is violated. Obviously, bicontinuity holds for the thin‐film water phase and the bulk gas phase. The snap‐off trapping process leads to ordinary bond percolation in front of the advancing bulk water phase and is the reason for the observed universal power law behavior in 2‐D micromodels with rough surfaces.