Stabilizing fluid–fluid displacements in porous media through wettability alteration

We study experimentally how wettability impacts fluid–fluid displacement patterns in granular media. We inject a low-viscosity fluid (air) into a thin bed of glass beads initially saturated with a more-viscous fluid (a water/glycerol mixture). Chemical treatment of glass surfaces allows us to control the wetting properties of the medium and modify the contact angle θ from 5 deg (drainage) to 120 deg (imbibition). We demonstrate that wettability exerts a powerful influence on the invasion morphology of unfavorable-mobility displacements: increasing θ stabilizes fluid invasion into the granular pack at all capillary numbers. In particular, we report the striking observation of a stable radial displacement at low capillary numbers, whose origin lies on the cooperative nature of fluid invasion at the pore scale.