Abstract
AbstractA 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.
Highlights
The morphology and connectivity of pore space and the wettability and roughness of the pore‐solid interface determine (i) the fluid pattern formation during multiphase flow, (ii) the geometry of the displacement front, and (iii) the capillary trapping efficiency of the defending fluid by the invading fluid.Recent research has focused on capillary trapping and the impact of wettability on fluid pattern formation. Wang et al (2019) studied the combined effect of viscous and capillary forces, local disorder, and wettability using Lattice‐Boltzmann simulations
Since both porous media exhibit similar morphological properties, we deduce that this difference in behavior is caused by the difference in surface roughness that allows complete wetting and precursor thick‐film flow for natural sands
Glass beads show a monotonous behavior, that is, a sharp transition from no trapping to maximal trapping (Sg,res ≅ 7%)
Summary
The morphology and connectivity of pore space and the wettability and roughness of the pore‐solid interface determine (i) the fluid pattern formation during multiphase flow, (ii) the geometry of the displacement front, and (iii) the capillary trapping efficiency of the defending fluid by the invading fluid.Recent research has focused on capillary trapping and the impact of wettability on fluid pattern formation. Wang et al (2019) studied the combined effect of viscous and capillary forces, local disorder, and wettability using Lattice‐Boltzmann simulations. The morphology and connectivity of pore space and the wettability and roughness of the pore‐solid interface determine (i) the fluid pattern formation during multiphase flow, (ii) the geometry of the displacement front, and (iii) the capillary trapping efficiency of the defending fluid by the invading fluid. The authors found an interesting wetting transition from no trapping to trapping with increasing contact angle for low disorder and small capillary numbers (≈10−5). This dynamical phase transition associated with changes in local growth modes at the displacement front was first discussed by Cieplak and Robbins (1988). Depending on the morphology and connectivity of the pore space, a second wetting
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