Shima Parsa

Flow of multiple immiscible fluids in porous media

Flow of multiphase fluids within porous media is important in a broad range of natural and industrial applications such as CO2 sequestration and oil recovery. Most of these cases involve displacement of a non-wetting phase by a wetting fluid through the porous medium. The complex and random structure of the porous media can lead to formation of discrete ganglia of the non-wetting fluid. Mobilization of such ganglion is possible in most situations. However, some ganglion can be trapped in pores. Such phenomenon are common in oil recovery where in most oil reservoirs more than 90% of the oil is trapped in the reservoir after initial recovery. Due to complexity of the pore structure studying the displacement and trapping of the non-wetting phase is a challenging problem.

We build a 3D micromodel porous medium by lightly sintering glass beads with diameter ranging between 38 and 150 μm in a quartz capillary of cross sectional area of 1 to 9 mm2.  Using  confocal microscopy  and by matching the index of refraction of the fluids with that of the glass beads we are able to fully visualize the flow of the fluid within the 3D micromodel as shown in Fig.1. We are able to resolve the dynamics of the fluids at pore level by measuring the velocities of the wetting phase within the micromodel. We use PIV methods to extract the steady state fluid velocity by measuring the velocities of fluorescent tracer particles. These measurements such as distribution of velocities inside the porous medium provides detailed information of the dynamics of fluid flows in 3D porous media which has not been available before.

Figure 1. Schematic of a typical 3D porous media micromodel. b) Optical slices taken at different depth within a micromodel comprised of 75 μm beads. Krummel et.al  AIChEJ DOI: 10.1002/aic.14005 (2013)

Figure 2. Residual oil in place in a tertiary polymer flooding of oil (blue square) compared with waterflooding (red circles) shows an improved recovery after flow of polymer.

Polymer flooding and enhanced oil recovery

One of the methods of enhanced oil recovery (EOR) is polymer flooding. In this method after initial imbibition of oil with water, a few pore volume of a shear-thinning polymer solution at low concentration (250ppm HPAM) is injected and then water flooding is continued at larger Capillary number. This method results in a better recovery of oil as shown in Fig.2. However, the physical phenomenon responsible for the enhanced recovery is still in debate. With our ability to directly measure the dynamics of two immiscible fluids within the porous medium we can fully explore the relevant phase space of this problem.

In our measurements of the velocity distribution within the porous medium before and after polymer flooding we see that after the flow of the polymer, the distribution of velocities is broader. As shown in colormap of Fig.3, the probability of larger velocities increases and the size of the domains in which no velocity is measured (black regions) has also increased. This measurement suggests that after flow of polymer the porosity of the system changes and due to blockage of small pores the distribution of velocities and pressure within the medium explores a wider range. This can contribute to conformance control and enhanced mobilization of oil.

Figure 3.  Distribution of velocities within porous media before and after flow of 2% weight Dextran in water.