New Structural Percolation Transition in Fractional Wet 3D‐Porous Media: A Comparative μ‐CT Study

Natural porous media possess spatial heterogeneous (fractional) wettability, which controls the multiphase flow behavior, that is, displacement and trapping. We used mixed hydrophilic (contact angle θ = 0°) and hydrophobic (θ = 100°) 1 mm sands and 1 mm glass beads as model systems for fractional wet porous media. Both porous media have the same morphological characteristics (Minkowski functions). A comparative μ‐CT study of such fractional wet systems is lacking in the literature. A previous study (Geistlinger & Zulfiqar, 2020, https://doi.org/10.1029/2019wr026826) showed a percolation transition of the invading fluid from compact to fractal displacement when the wettability changed from imbibition to drainage. This correlates to a transition from no to maximal trapping. For the first time, we observed in fractional wet glass bead packs, a second structural percolation transition with a percolation threshold of approximately 50% hydrophobic glass beads. Below this threshold, the percolating cluster consisted of hydrophilic glass beads, which resulted in compact displacement without any trapping. Above the threshold, the percolating cluster consisted of hydrophobic glass beads, which resulted in fractal displacement with maximal trapping. Trapping occurred only in hydrophobic environments/configurations, that is, when the trapped clusters were surrounded by hydrophobic glass beads. Fractional wet glass beads and sands show an opposite trapping behavior, that is, the trapping efficiency increases for glass beads, whereas it decreases for sands with increasing percentage of hydrophobic grains. Interestingly, the trapping efficiency approaches the same limit value for pure hydrophobic sands and glass beads. This demonstrates that bypass trapping is the dominant trapping mechanism, because it depends strongly on the pore connectivity. Key Points Fractional wet glass beads and sands with same morphological properties (pore space connectivity) show an opposite trapping behavior Pure hydrophobic sands and glass beads show equal trapping efficiency caused by bypass trapping (equal pore space connectivity) Structural (site) percolation transition with a percolation threshold of approximately 50% hydrophobic glass beads