Multiphase displacement in porous media can be inherently unstable due to viscous / capillary instabilities. Specifically, when the displacing fluid is non-wetting, capillary instability can be triggered by slight differences in the pore size at the individual pore level. Unstable displacement often results in significant residual saturation of the wetting fluid, which is not desired for improved / enhanced oil recovery operations carried out in oil-wet formations. To overcome capillary instability, the capillary number, which is defined as the ratio between the viscous force and the capillary force, must be raised to certain geometry-dependent thresholds. Higher capillary numbers can be achieved by increasing the rate of injection, or by reducing the interfacial tension. In this study, the effect of pore geometry and interfacial tension on water-oil displacement efficiency is studied using polydimethylsiloxane (PDMS) microfluidic micromodels. These micromodels have a porosity of 0.19 and permeability from 0.13 – 0.27 × 10⁻¹² m², which are typical of sandstones. The porous media geometry included random homogeneous and heterogeneous pore networks and regular patterns made up of triangles, squares, diamonds, and hexagons. It is found that among the random networks heterogeneity decreases the displacement efficiency; among the regular patterns, the displacement efficiency decreases with increasing coordination number. Surfactant increased the displacement efficiency in all geometries. Compared with microfluidic micromodels, direct numerical simulation offers several significant advantages: flexibility in fluid and solid properties, accurate measure of pressures and saturations, and three-dimensional complex geometries. The scale of computation, however, is severely limited by the available computational power. The two approaches therefore should be combined to achieve better understanding of multiphase flows in porous media.

Xiaolong Yin received BS in Mechanics from Peking University in 1999, MS in Mechanical Engineering from Lehigh University in 2001, and PhD in Chemical Engineering from Cornell University in 2006. He conducted postdoctoral research in Chemical Engineering at Princeton University. He joined the faculty of Petroleum Engineering Department at Colorado School of Mines in 2009. He is now an associate professor and the co-director of the Unconventional Reservoir Engineering Project consortium at Colorado School of Mines. He is associate editor of the SPE Journal and the Journal of Natural Gas Science and Engineering.