Porous media phase trapping

The extent of trapping is determined primarily by the physical properties of the vadose zone. If the organic liquids are characterized by a low vapor pressure and a low solubility in water, they remain trapped in the partially saturated zone. In this particular case, the porous medium behaves like an inert material and the behavior of the organic liquids depends only on their own properties, with no interaction between the liquid and the solid phases. 

Once a discontinuous nonwetting phase is formed, the capillary forces opposing ganglia motion must be overcome, or the ganglia will be trapped. The maximum capillary pressure rise exhibited by ganglia of any length in a porous medium can be estimated in terms of throat radius r. and body radius... 

To develop an understanding of the emulsion flow in porous media, it is useful to consider differences and similarities between the flow of an OAV emulsion and simultaneous flow of oil and water in a porous medium. As discussed in the preceding section, in simultaneous flow of oil and water, both fluid phases are likely to occupy continuous, and to a large extent, separate networks of flow channels. Assuming the porous medium to be water-wet, the oil phase becomes discontinuous only at the residual saturation of oil, where the oil ceases to flow. Even at its residual saturation, the oil may remain continuous on a scale much larger than pores. In the flow of an OAV emulsion, the oil exists as tiny dispersed droplets that are comparable in size to pore sizes. Therefore, the oil and water are much more likely to occupy the same flow channels. Consequently, at the same water saturation the relative permeabilities to water and oil are likely to be quite different in emulsion flow. In normal flow of oil and water, oil droplets or ganglia become trapped in the porous medium by the process of snap-off of oil filament at pore throats (8). In the flow of an OAV emulsion, an oil droplet is likely to become trapped by the mechanism of straining capture at a pore throat smaller than the drop. 

If interfacial tension between two phases becomes zero, then the two phases become miscible. This result is the ultimate aim of many types of FOR to make oil-water interfacial tension equal to 0, so that a displacing fiuid can miscibly displace oil trapped in the porous medium. In practice, it is difficult to make interfacial tension approach 0 for liquids of such different characteristics as oil and water. 

The chemical flooding systems discussed in this section are without alcohol present as cosnrfactant. As pointed ont by Sanz and Pope [J 7], a major difficulty was to preclude gels, liquid crystals, macroemulsions, and precipitates along the compositional path during a chemical flood if cosurfactants/alcohols are not part of the chemical formnlation. Phase trapping and blockage of the porous medium must be avoided. 

Extraction discs (0.5 mm thick, 25 to 90 mm diameter) constitute a variation of column-based SPE. These discs allow rapid extraction of large volumes of sample, which is not possible using a small column. The discs are made of bonded-phase silica particles, a few micrometres in diameter, trapped in a porous Teflon or glass fibre matrix. The discs are operated in a similar way to a paper filter on a vacuum flask. After extraction, the analyte is recovered by percolating a solvent through the filter. The major application of this technique is the isolation of trace amounts of compound dispersed in an aqueous medium. 

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