Water-oil drainage dynamics in oil-wet random microfluidic porous media analogs

TL;DR

This study investigates water-oil displacement in microfluidic porous media with different geometries, revealing how surface tension and pore structure influence displacement stability and efficiency.

Contribution

It introduces experimental analysis of water-oil drainage in various microfluidic porous media analogs, highlighting the effects of pore heterogeneity and surface tension.

Findings

Reduced surface tension stabilizes displacement.

Heterogeneity leads to stronger fingering.

Small pores cause meniscus retreat due to capillary effects.

Abstract

Displacement experiments carried out in microfluidic porous media analogs show that reduced surface tension leads to a more stable displacement, opposite to the process in Hele-Shaw cells where surface tension stabilizes the displacement of a more viscous fluid by a less viscous fluid. In addition, geometry of porous media is observed to play an important role. Three random microfluidic porous media analogs were made to study water-oil drainage dynamics, featuring a pattern of randomly connected channels with a uniform width, a pattern with Gaussian channel width distribution, and a pattern with large isolated pores. The microfluidic chips fabricated using Polydimenthylsiloxane with glass covers have the internal surface treated by Trichlorosilane to achieve a uniform oil-wet condition. The aqueous phase displaces the oil phase, with a viscosity ratio of about 1:40 and a density ratio of 1:0.85. Videos 1-3 show water flooding processes. It is observed that both channel size distribution (Video 2) and heterogeneity in pore size (Video 3) lead to stronger fingers and reduced displacement efficiency. Video 4 shows that meniscus in small channels retreat as water front moves into a nearby large cavity due to the disparity in the capillary force and contact angle hysteresis. Videos 5 and 6, both taken at 100X magnification in Chip 2, show the stabilizing effect of reduced interfacial tension.