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The combined effects of gravitational forces and wettability on displacement and trapping have not previously been investigated experimentally in realistic 2D porous media. Here we report the first use of representative 2D micromodels that accurately reproduce characteristic geometric, morphological, and topological properties of 3D sand packs. We use these representative micromodels to compare the wettability dependence of displacement front morphology and trapping processes using selected fluid pairs at capillary numbers between 10^-6 and 10^-5. The contact angles studied ranged from 30° (normal imbibition) to 150° (strong drainage). The impact of gravity was dramatic: In the horizontal flow case, only one finger developed, i.e. longitudinal displacement was always favored, whereas gravity created a more compact displacement front with broad lateral extension, and this lateral movement created loops, leading to by-pass trapping. As a result, the trapping efficiency rose from 7% to 29%. We observed universal scaling of the cluster size distribution demonstrating that our micromodels are statistically representative. The universal 2D-scaling exponent of 2.05 was obtained with an acceptable error smaller than 5%. Our experimental results obtained by fluorescence microscopy showed (i) precursor corner flow for weak imbibition at θ = 51°, i.e. above the critical contact angle of α_c = 45° (Zhao et al., 2016). and (ii) a new wetting transition from corner flow to full duct flow under neutral wettability conditions, i.e. at θ_c ≈ 90°; and (iii) above this second critical contact angle, i.e. under strong drainage conditions, we again observed corner flow (specifically, core-annular flow).

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