Fluid-fluid displacement in mixed-wet porous media

It is well known that wettability exerts fundamental control over multiphase flow in porous media, which has been extensively studied in uniform-wet porous media. In contrast, multiphase flow in porous media with heterogeneous wettability (i.e., mixed-wet) is less well understood, despite its common occurrence. Here, we study the displacement of silicone oil by water in a mostly oil-wet porous media patterned with discrete water-wet clusters that have precisely controlled wettability. Surprisingly, the macroscopic displacement pattern varies dramatically depending on the details of wettability alteration-the invading water preferentially fills strongly water-wet clusters but encircles weakly waterwet clusters instead, resulting in significant trapping of the defending oil. We explain this counterintuitive observation with pore-scale simulations, which reveal that the fluid-fluid interfaces at mixed-wet pores resemble an S-shaped saddle with mean curvatures close to zero. We show that incorporation of the capillary entry pressures at mixed-wet pores into a dynamic pore-network model reproduces the experiments. Our work demonstrates the complex nature of wettability control in mixed-wet porous media, and it presents experimental and numerical platforms upon which further insights can be drawn.

Automated summary

Wettability plays a fundamental role in controlling multiphase flow in porous media, and its influence has been extensively studied in uniform-wet porous media. However, the understanding of multiphase flow in porous media with heterogeneous wettability, known as mixed-wet media, is limited. In this study, we investigate the displacement of silicone oil by water in mostly oil-wet porous media patterned with discrete water-wet clusters. Surprisingly, the displacement pattern depends on the wettability alteration, where water preferentially fills strongly water-wet clusters and encircles weakly water-wet clusters, resulting in significant oil trapping. Pore-scale simulations provide insights into this counterintuitive observation by revealing the S-shaped saddle-like fluid-fluid interfaces at mixed-wet pores.