In situ surfactant flooding

The cost of implementing an in situ surfactant flood will vary significantly from site to site. Because costs of full-scale implementation do not exist, currently available cost estimates are based on hypothetical examples and extrapolation from field pilot tests. For typical waste sites having contamination limited to the upper 15 m (49 ft) below ground surface, estimated costs range from 1.4 million per hectare to 18 million per hectare, or approximately 90 to 990/m ( 65 to 750/yd ) of treated contaminated soil (D16070D, p. ES-4). 

Figure 9.2 Illustration of an in situ surfactant flood for the displacement and production of DNAPL contaminants from a contaminated subsurface zone. From Battelle [538], Copyright 2002, Naval Facilities Engineering Command.

Several successful field demonstrations of in situ surfactant flooding for NAPL soil remediation have been conducted [523,526,528,530,538,539] and design and implementation manuals are available [538-540],... 

Alkaline flooding is based on the reaction that occurs between the alkaline water and the organic acids, naturally occurring in some crudes, to produce in-situ surfactants or emulsifying soaps at the oil/water interface. Recent literature (i-J.) summarizes several proposed mechanisms by which alkaline water-flooding will enhance oil recovery. These mechanisms include emulsification and entrapment, emulsification and entrainment, and wettability reversal (oil-wet to water-wet or water-wet to oil-wet). Depending on the initial reservoir and experimental conditions with respect to oil, rock and injection water properties, one or more of these proposed mechanisms may be controlling. 

Tertiary oil was increased up to 41% over conventional CO2 recovery by means of mobility control where a carefully selected surfactant structure was used to form an in situ foam. Linear flow oil displacement tests were performed for both miscible and immiscible floods. Mobility control was achieved without detracting from the C02-oil interaction that enhances recovery. Surfactant selection is critical in maximizing performance. Several tests were combined for surfactant screening, included were foam tests, dynamic flow tests through a porous bed pack and oil displacement tests. Ethoxylated aliphatic alcohols, their sulfate derivatives and ethylene oxide - propylene oxide copolymers were the best performers in oil reservoir brines. One sulfonate surfactant also proved to be effective especially in low salinity injection fluid. 

A fundamental chemical process is surfactant flooding in which the key mechanism is to reduce interfacial tension (IFT) between oil and the displacing fluid. The mechanism, because of the reduced IFT, is associated with the increased capillary number, which is a dimensionless ratio of viscous-to-local capillary forces. Experimental data show that as the capillary number increases, the residual oil saturation decreases (Lake, 1989). Therefore, as IFT is reduced through the addition of surfactants, the ultimate oil recovery is increased. In alkaline flooding, the surfactant required to reduce IFT is generated in situ by the chemical reaction between injected alkali and naphthenic acids in the... 

Alkaline flooding is also called caustic flooding. Alkalis used for in situ formation of surfactants include sodium hydroxide, sodium carbonate, sodium orthosilicate, sodium tripolyphosphate, sodium metaborate, ammonium hydroxide, and ammonium carbonate. In the past, the first two were used most often. However, owing to the emulsion and scaling problems observed in Chinese field applications, the tendency now is not to use sodium hydroxide. The dissociation of an alkali results in high pH. For example, NaOH dissociates to yield OH" ... 

Alkalis react with naphthenic acid in crude oil to generate soap. The soap, an in situ generated surfactant, reduces the interfacial tension between the alkaline solution and oil. It is intuitive to infer that the main mechanism in alkaline flooding is low IFT. 

Figure 12.2 shows that the optimum salinity increases as the soap/surfactant ratio decreases. During ASP flooding, oil saturation decreases from the downstream (the displacing front) to the upstream. Because soap concentration is proportional to oil saturation, the soap/surfactant ratio would likely decrease. The soap generated in situ is a surfactant different from the injected synthetic surfactant. These two surfactants have different properties. Generally, the injected surfactant is more hydrophilic than the soap. Thus, the optimum salinity of soap is lower than that of the synthetic surfactant. As the soap/surfactant ratio decreases, the optimum salinity would increase. Consequently, the salinity upstream would likely be lower than the optimum salinity, resulting in a local Winsor 1 environment. Such a microemulsion environment is desirable. 

His research interests have included many aspects of colloid and interface science applied to the petroleum industry, including research into mechanisms of processes for the improved recovery of light, heavy, or bituminous crude oils, such as in situ foam, polymer or surfactant flooding, and surface hot water flotation from oil sands. These mostly experimental investigations have involved the formation and stability of dispersions (foams, emulsions, and suspensions) and their flow properties, elec-trokinetic properties, interfacial properties, phase attachments, and the reactions and interactions of surfactants in solution. 

Recently we have carried out laboratory tests (17, 18, 19) in which the sodium silicate was added directly to a dilute surfactant solution to recover oil. Such a process would be akin to alkaline flooding processes where a dilute surfactant is formed in-situ. In this case however the crude is lighter and does not contain the natural acids necessary to form surfactants in-situ. Therefore surfactant is injected and protected or enhanced by the sodium silicate such that a low tension waterflood is assured. Such a system is less complex and therefore more widely applicable than micellar/polymer techniques thus filling the void between the alkaline and micellar/polymer EOR processes. 

Foam Flooding Processes If the injection of the surfactant solution is followed by gas injection, it can form in-situ foam, which can improve oil recovery. Several aspects of foam flooding such as mechanism of foam flow in a porous medium, microscopic behavior of foam, bubble size, CO2 foam, steam foam and oil recovery have been discussed in the literature.Several aspects of the foam flooding process are schematically presented in Figure 7. 

The effect of oil viscosity on the displacement of oil is presented in Figure 10. In order to determine the oil displacement efficiency by foam flooding, the injection of gas phase was started at surfactant solution breakthrough. Both air and steam were employed to generate in-situ foams. The steam foam recovered more oil as compared to air foams. 

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