The Impact of Wettability and Surface Roughness on Fluid Displacement and Capillary Trapping in 2‐D and 3‐D Porous Media: 2. Combined Effect of Wettability, Surface Roughness, and Pore Space Structure on Trapping Efficiency in Sand Packs and Micromodels

Abstract

A comprehensive understanding of the combined effects of surface roughness and wettability on the dynamics of the trapping process is lacking. This can be primarily attributed to the contradictory experimental and numerical results regarding the impact of wettability on the capillary trapping efficiency. The discrepancy is presumably caused by the surface roughness of the inner pore-solid interface. Herein, we present a comparative μ-CT study of the static fluid-fluid pattern in porous media with smooth (glass beads) and rough surfaces (natural sands). For the first time, a global optimization method was applied to map the characteristic geometrical and morphological properties of natural sands to 2-D micromodels that exhibit different degrees of surface roughness. A realistic wetting model that describes the apparent contact angle of the rough surface as a function surface morphology and the intrinsic contact angle was also proposed. The dynamics of the trapping processes were studied via visualization micromodel experiments. Our results revealed that sand and glass beads displayed opposite trends in terms of the contact angle dependence between 5°and 115°. Sand depicted a nonmonotonous functional contact angle dependency, that is, a transition from maximal trapping to no trapping, followed by an increase to medium trapping. In contrast, glass beads showed a sharp transition from no trapping to maximal trapping. Since both porous media exhibit similar morphological properties (similar Minkowski functions: porosity, surface density, mean curvature density, Euler number density), we deduce that this difference in behavior is caused by the difference in surface roughness that allows complete wetting and hence precursor thick-film flow for natural sands. Experimental results on micromodels verified this hypothesis.

Role of Pore‐Scale Disorder in Fluid Displacement: Experiments and Theoretical Model

“…From the works of Cieplak and Robbins (1988) and Zulfiqar et al. (2020), the contact angle of our microfluidics is larger than the critical contact angle, and the unstable flooding is expected. However, as shown in Figure 3, we observe the compact displacement in uniform porous media (see Figures 3a 5 , 3a 6 , 3b 5 , and 3b 6 ).…”

“…Recently, Geistlinger and Zulfiqar (2020) and Zulfiqar et al. (2020) performed fluid‐displacement experiments in 2D and 3D porous media to further explore the wettability effect. They found that the critical contact angle for the pattern transition is significantly affected by the pore space structure.…”

“…This pore-filling event, known as overlap (Cieplak & Robbins, 1988, 1990, stabilizes the fluid-fluid interface at the pore scale and then leads to a smooth displacement front at the pore network scale. From the works of Cieplak and Robbins (1988) and Zulfiqar et al (2020), the contact angle of our microfluidics is larger than the critical contact angle, and the unstable flooding is expected. However, as shown in Figure 3, we observe the compact displacement in uniform porous media (see Figures 3a 5 , 3a 6 , 3b 5 , and 3b 6 ).…”

“…The phase diagram and the pattern transitions have been intensively investigated previously, with focus on wettability (Holtzman & Segre, 2015;Hu et al, 2018b), pore space structure Zulfiqar et al, 2020), pore-scale disorder (Z. Wang et al, 2019).…”

The Impact of Wettability and Surface Roughness on Fluid Displacement and Capillary Trapping in 2‐D and 3‐D Porous Media: 1. Wettability‐Controlled Phase Transition of Trapping Efficiency in Glass Beads Packs

“…Previous studies on capillary trapping in natural sands (estimated contact angle about 20°; Geistlinger et al, 2015) show residual gas trapping of about 15%. Future research will investigate the impact of grains with rough surface and irregular geometry on the Cieplak‐Robbins transition and will be published in Part 2 of the study (Zulfiqar et al, 2020).…”