Extending Foam Technology from Improved Oil Recovery to Environmental Remediation

Abstract

Abstract In the petroleum industry, foams are used for improved oil recovery, reducing gas breakthrough to productions wells, and diverting acid in well-stimulation treatments. A substantial body of research on foam for these applications is extended here to the high-permeabilities and low pressures of environmental remediation, through both laboratory experiments and computer simulations based on the laboratory results. New data confirm that the two steady-state flow regimes identified by Alvarez et al. for petroleum applications exist under conditions relevant to groundwater remediation. The low-quality regime is strongly shear-thinning, in agreement with Alvarez et al. The high-quality regime is more sensitive to surfactant concentration than the low-quality regime, while the low-quality regime is more sensitive to permeability. Therefore, diversion between layers differing in permeability would be more effective in the high-permeability regime, while the low-quality regime has advantages for injectivity and liquid diversion. A foam simulator can be fitted to these steady-state foam data. Simulations indicate that foam can propagate without gravity segregation over long distances at an overall average pressure gradient of well under 1 psi/ft. Moreover, a simple correlation for gravity segregation developed for petroleum applications works well for aquifers overlain by a vadose zone, as long as the assumptions of that correlation (Newtonian rheology, incompressible flow) are satisfied. Local steady state would not be achieved in the field if pressure gradient were insufficient for foam generation, however. New results indicate that at moderate ?p foam may exist in an unstable regime between strong and coarse foam. Introduction Toxic subsurface plumes of "non-aqueous phase liquid" (NAPL) can be treated in situ by injection of fluids - for instance, air in solvent vapor extraction,1 or water in pump-and-treat processes2 and surfactant-based and other enhanced remediation methods.3 All such processes suffer from potentially poor sweep efficiency.4,5 Gravity causes uneven sweep of injected fluids depending on their density. Variation in permeability throughout the aquifer and vadose zone causes uneven flow. The mere presence of NAPL in the plume reduces the relative permeabilities to water and gas locally and thereby diverts the flow of treatment fluids around the plume. Foam can improve sweep efficiency of either gas- or liquid-based remediation processes.6,7 Foam is an unstable dispersion of gas in water, partially stabilized by dissolved surfactant in the water.8,9 Foam reduces mobility, which tends to even-out flow in the presence of permeability contrasts and gravity. In some cases, foam reduces mobility more in higher-permeability media, which directly offsets the effects of permeability variations. Foam spontaneously collapses in the presence of some NAPL's, which would tend to divert flow toward a contaminant plume and deliver surfactant solution to the NAPL source. Foam has been extensively studied in connection with gas diversion in improved oil recovery (IOR) and liquid diversion in matrix acid well stimulation. The findings of these studies have not yet been fully implemented in environmental remediation, however.