Experimental evaluation of foam in environmental remediation
Ground water is the major source of drinking water for many people around the world. Two challenges in subsurface remediation are the removal of nonaqueous phase liquids (NAPL) located in the capillary fringe above the water table and at the bottom of an aquifer. Although several innovative technologies have demonstrated the ability to remove NAPL from source zones, geologic heterogeneities can cause significant NAPL to remain after remediation. The effect of heterogeneity can be mitigated by the application of a mobility-control fluid such as foam. The use of foam as a means of improving remediation efficiency has unique advantages. (1) Foams are inherently stiffer in region of higher permeability. (2) Foams can reduce gravity effects. (3) Foams collapse in the presence of hydrocarbon contaminants. It was found that foam has two different flow regimes in porous media: (a) the high-quality regime and (b) low-quality regime. Steady-state experiments were conducted to study the two foam-flow regimes under low pressure gradient conditions typical of subsurface remediation and identify the effect of various factors on the two regimes. Gas and surfactant solution were co-injected through sandpacks of permeability ranging from 5 to 210 darcy (hydraulic conductivity: 2x10-3 to 5x10-5 cm/sec) in vertical columns. The results confirm that the two foam-flow regimes are present in both the absence and presence of oil in porous media. In the high-quality regime, two factors weakened the foam, decreasing surfactant concentration and the presence of oil. These require an increase of liquid flow rate to maintain a given pressure gradient. In the low-quality regime, as predicted by the fixed-bubble-size model, increasing permeability requires higher gas flow rate to maintain a given pressure gradient. Shear-thinning foam behavior was consistently found in the lowquality regime, while the shear-thinning, Newtonian and shear-thickening foam behavior was observed in the high-quality regime. Lower apparent relative gas permeability was also obtained in higher-permeability porous media.