Cosolvent flooding

Cosolvent flooding is an experimental method for removing DNAPLs trapped below the water table. It involves injecting a highly concentrated aqueous mixture of solvents, such as alcohols, a chemical that is miscible with either phase in the aquifer. This process has the tendency to increase or enhance DNAPL (or LNAPL) solubility greatly, and to reduce the NAPL-water interfacial tension. Depending upon the phase behavior between the cosolvent and NAPL, a cosolvent flood can be developed to emphasize either enhanced dissolution (i.e., use of methane flooding for the dissolution of TCE) or NAPL mobilization. 

Cosolvent flooding is accomplished by the introduction of a cosolvent solution, with subsequent extraction of contaminated groundwater and NAPL. In one reported field test study that focused on enhanced dissolution, the use of about nine pore volumes of a 70% ethanol, 12% pentanol solution injected into a test cell resulted in about 81% bulk NAPL removal, with a higher removal efficiency for several other individual compounds. In another field test study, where mobilization removal was emphasized, injection of about four pore volumes of a mixture of tert-butanol and w-hcxanol into a test cell resulted in the removal of about 80% of the bulk NAPL, and higher removal efficiency of the more-soluble NAPL compounds. 

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Conventional pump-and-treat techniques are not very effective in restoring aquifers impacted by DNAPLs. This ineffectiveness is a result of the relatively low solubility of the DNAPL and the large capillary forces that immobilize the nonaqueous phase. Over the past decade, several innovative and experimental strategies have been tested for more effective recovery of DNAPLs. These strategies include the more conventional use of surfactants, and thermally enhanced extraction or steam injection. Other more experimental approaches include cosolvent flooding and density manipulations. Each of these approaches is discussed below. 

Results described in the literature have resulted in several patents, such as one for the improvement of the transport of viscous crude oil by microemulsions based on ether carboxylates [195], or combination with ether sulfate and nonionics [196], or several anionics, amphoterics, and nonionics [197] increased oil recovery with ether carboxylates and ethersulfonates [198] increased inversion temperature of the emulsion above the reservoir temperature by ether carboxylates [199], or systems based on ether carboxylate and sulfonate [200] or polyglucosylsorbitol fatty acid ester [201] and eventually cosolvents which are not susceptible for temperature changes. Ether carboxylates also show an improvement when used in a C02 drive process [202] or at recovery by steam flooding [203]. 

C. Raible. Improvement in oil recovery using cosolvents with CO2 gas floods. US DOE Fossil Energy Rep NIPER-559, NIPER, January 1992. 

The use of cosolvents to achieve neutral-buoyancy surfactant floods has also been investigated (Shook et al, 1997, 1998 Kostarelos et al, 1998). 

To evaluate the effects of cosolvent on surfactant delivery and PCE recovery, Box B was flushed with 4% Tween 80 + 5% EtOH at a Darcy velocity of 4.8 cm/hr. The surfactant/cosolvent mixture, which had a density of 0.994 g/cm3, was also representative of a neutral buoyancy flood solution (Shook et al, 1998). It is important to recognize that "neutral buoyancy" refers to density of flushing fluid after solubilization of the DNAPL. Thus, the initial density of the surfactant formulation must be less than that of the resident aqueous phase. Figure 5b shows the location and shape of the 4% Tween 80 + 5% EtOH front after flushing Box B with 0.5 pore volumes of solution. The lower density of the 4% Tween 80 + 5% EtOH solution (0.994 g/cm3) relative to the density of resident pore water (0.998 g/cm3) caused the injected solution to flow preferentially along the top of Box B (Figure 5b). This effect can become severe at low flow rates (Taylor, 1999). The... 

Micellar/polymer flooding 4807 m (equivalent to 0.33 PV), 5% YPS-3A (local snrfactant) + 3% n-bntanol (cosolvent) + 0.4% K2CO3 that is followed... 

More attractive are in situ remediation approaches, which often cost less. Cosolvents promote the mobilization of organic chemicals in soils, thus accelerating the cleanup of contaminated site. Cosolvent flushing has been developed using the same principles as those used in solvent flooding, a technique to enhance petroleum recovery in oil fields. It involves injecting a solvent mixture, mostly water plus a miscible cosolvent, into the vadose... 

In experiment DW 2, the permeability was decreased from an initial value of 15.4 Darcy to a final waterflood value of 1.5 Darcy. Column DW 5 was plugged as a result of surfactant flooding. In both experiments, a surfactant mixture with sodium dioctyl sulfosuccinate was used without enough cosolvent to prevent gels. Sodium dihexyl sulfosuccinate is less prone to forming gels and less alcohol is needed to eliminate gel when it... 

Continuous feed of hot RA vapor from the distillation column directly to the parts held adjacent to the primary condensing coil above the RA Cosolvent sump. This vapor is condensed on the part surfaces using the primary condenser to remove heat. In this way, the parts are flooded with pristine RA cosolvent as a dilution rinse fluid . ... 

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