Characteristics of fluid-fluid displacement in model mixed-wet porous media: patterns, pressures, and scalings

TL;DR

This study uses numerical modeling to analyze how mixed wettability in porous media affects fluid displacement patterns, pressures, and scalings, revealing complex dependencies on wettability fractions and contact angles.

Contribution

It introduces a dynamic pore network model to systematically explore the effects of mixed wettability on displacement patterns and pressure signatures in porous media.

Findings

Displacement pattern fractal dimension varies with wettability fraction.

Injection pressure fluctuations are linked to wettability and flow regimes.

Scaling analyses can predict pattern variations based on wetting state.

Abstract

We study the characteristics of fluid-fluid displacement in simple mixed-wet porous micromodels numerically using a dynamic pore network model. The porous micromodel consists of distinct water-wet and oil-wet regions, whose fractions are systematically varied to yield a variety of displacement patterns over a wide range of capillary numbers. We find that the impact of mixed-wettability is most prominent at low capillary number, and it depends on the complex interplay between wettability fraction and the intrinsic contact angle of the water-wet regions. For example, the fractal dimension of the displacement pattern is a monotonically increasing function of wettability fraction in flow cells with strongly water-wet clusters, but it becomes non-monotonic with respect to wettability fraction in flow cells with weakly water-wet clusters. Additionally, mixed-wettability also manifests itself in the injection-pressure signature, which exhibits fluctuations especially at low wettability fraction. Specifically, preferential filling of water-wet regions leads to reduced effective permeability and higher injection pressure, even at vanishingly small capillary numbers. Finally, we demonstrate that scaling analyses based on a weighted average description of the overall wetting state of the mixed-wet system can effectively capture the variations in observed displacement pattern morphology.