Micellar polymers

Whereas the above systems which give rise mainly to phase separated superstructures in aqueous systems are now fairly well understood, the situation for more hydrophilic and thus watersoluble amphiphilic polymers is less clear. Such polymers are often referred to as micellar polymers because they have properties similar to surfactant micelles [18-23], and thus similar superstructures are implicitely assumed. 

Compared to low molecular weight amphiphiles, the size of polymeric amphi-philes allows for much more diverse arrangements of the hydrophilic and hydrophobic segments, as exemplified in Fig. 1. Accordingly, micellar polymers are characterized by versatile molecular architectures, giving rise to distinct subgroups. Diblock-copolymers with a clear separation of the hydrophilic 

Alternatively, the hydrophilic and hydrophobic groups may be scattered all over the macromolecule. Here, the amphiphilic character of micellar polymers results from the presence of many independent, surfactant-like structural units which are covalently linked. This is realized in polymers which bear a limited number of ionic groups in their otherwise hydrophobic backbones - corresponding to a longitudinal linkage (Fig. le) - or in polymers with functional side-chains - corresponding to a lateral linkage of surfactant units (Fig. If). 

The structural similarities of the individual polymer fragments with low molecular weight surfactants are paralleled by the similarities of two important 

Despite these similarities, other properties of polysoaps can differ considerably from the ones of standard surfactants, as exemplified by their intramolecular aggregation and the usually missing critical micelle concentration CMC [24, 46, 51, 52, 65, 75-78], 

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Microemulsions became well known from about 1975 to 1980 because of their use ia "micellar-polymer" enhanced oil recovery (EOR) (35). This technology exploits the ultralow iaterfacial tensions that exist among top, microemulsion, and bottom phases to remove large amounts of petroleum from porous rocks, that would be unrecoverable by conventional technologies (36,37). Siace about 1990, iaterest ia the use of this property of microemulsions has shifted to the recovery of chloriaated compounds and other iadustrial solveats from shallow aquifers. The latter appHcatioa (15) is sometimes called surfactant-enhanced aquifer remediation (SEAR). 

An alternative to this process is low (<10 N/m (10 dynes /cm)) tension polymer flooding where lower concentrations of surfactant are used compared to micellar polymer flooding. Chemical adsorption is reduced compared to micellar polymer flooding. Increases in oil production compared to waterflooding have been observed in laboratory tests. The physical chemistry of this process has been reviewed (247). Among the surfactants used in this process are alcohol propoxyethoxy sulfonates, the stmcture of which can be adjusted to the salinity of the injection water (248). 

Micellar/polymer (MP) chemical enhanced oil recovery systems have demonstrated the greatest potential of all of the recovery systems under study (170) and equivalent oil recovery for mahogany and first-intent petroleum sulfonates has been shown (171). Many somewhat different sulfonate, ie, slug, formulations, slug sizes (pore volumes), and recovery design systems were employed. Most of these field tests were deemed technically successful, but uneconomical based on prevailing oil market prices (172,173). 

Amoco developed polybutene olefin sulfonate for EOR (174). Exxon utilized a synthetic alcohol alkoxysulfate surfactant in a 104,000 ppm high brine Loudon, Illinois micellar polymer small field pilot test which was technically quite successful (175). This surfactant was selected because oil reservoirs have brine salinities varying from 0 to 200,000 ppm at temperatures between 10 and 100°C. Petroleum sulfonate apphcabdity is limited to about 70,000 ppm salinity reservoirs, even with the use of more soluble cosurfactants, unless an effective low salinity preflush is feasible. 

E. A. Knaggs and J. W. Hodge, Petroleum Sulfonates—Key Process Chemicals in Micellar Polymer Oil Recovery Systems, American Chemical Society,... 

Micellar flooding, 13 628 Micellar-polymer (MP) chemical enhanced oil recovery systems, 23 531 Micellar-polymer enhanced oil recovery (EOR), 16 429... 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

Figure 5 Polymer-NC complex showing an idealized micellar polymer shell (40% octylamine-modified poly(acrylic acid)). Upon addition of the titania precursor, the NC/polymer adducts are linked to each other through the titania network...

Enhanced oil recovery (EOR) is a collective term for various methods of increasing oil recoveries that have been developed since about 1970. Up until about 1980, the use of surfactants in EOR was more or less synonymous with "micellar/polymer" flooding, in which surfactants are used to decrease the interfacial tension between "oil" and "water" from 10 dyne/cm to < 0.01 dyne/cm. 

Early researchers sought to choose appropriate surfactants for mobility control from the hundreds or thousands that might be used, but very little of the technology base that they needed had yet been created. Since then, work on micellar/polymer flooding has established several phase properties that must be met by almost any EOR surfactant, regardless of the application. This list of properties includes a Krafft temperature that is below the reservoir temperature, even if the connate brine contains a high concentration of divalent ions (i.e., hardness tolerance), and a lower consolute solution temperature (cloud point) that is above the reservoir temperature. 

Micellar-polymer flooding and alkali-surfactant-polymer (ASP) flooding are discussed in terms of emulsion behavior and interfacial properties. Oil entrapment mechanisms are reviewed, followed by the role of capillary number in oil mobilization. Principles of micellar-polymer flooding such as phase behavior, solubilization parameter, salinity requirement diagrams, and process design are used to introduce the ASP process. The improvements in ""classicaV alkaline flooding that have resulted in the ASP process are discussed. The ASP process is then further examined by discussion of surfactant mixing rules, phase behavior, and dynamic interfacial tension. 

Micellar-polymer flooding relies on the injection of a surfactant solution to lower interfacial tension to ultralow levels, on the order of 10 mN/m. The resulting increase in capillary number allows the recovery of residual oil from porous media. The term micellar is used because the concentrations of injected surfactant solutions are always above their critical micelle concentration. That is, they are always above the concentration at which micelles form. 

Field Application. The micellar-polymer process for enhanced oil recovery has been used in many field trials. Petroleum sulfonates are the most commonly used surfactant 41, 42). Other surfactants have been used, such as ethoxylated alcohol sulfates 43) and nonionic surfactants mixed with petroleum sulfonates 44). 

The pilot area used for this test was relatively small, 0.71 acres. However, the test was a technical success, recovering 68% of the water-flood residual oil. The pilot began in 1982 and ended in November 1983. Since that time, Exxon has initiated two other micellar-polymer floods in the Loudon field, one a 40-acre pilot and the other an 80-acre pilot. 

Many of the basic concepts of micellar-polymer flooding apply to alkaline flooding. However, alkaline flooding is fundamentally different because a surfactant is created in the reservoir from the reaction of hydroxide with acidic components in crude oil. This reaction means that the amount of petroleum soap will vary locally as the water-to-oil ratio varies. The amount of petroleum soap has a large effect on phase behavior in crude-oil-alkali-surfactant systems. 

Phase Behavior. The use of phase-behavior diagrams in surfactant-enhanced alkaline flooding is more complicated than in micellar-polymer flooding for several reasons. One reason is that phase behavior is very sensitive to the water-to-oil ratio employed. From surfactant mixing rules, varying the amount of oil present will vary the amount of petroleum soap... 

Micellar-polymer flooding and alkali-surfactant-polymer flooding both rely on the injection into a crude-oil reservoir of surfactants or surfactantforming materials. Emulsions may be injected into the reservoir, or they may be formed in the reservoir, but their properties will change as they travel through the reservoir to eventually flow from a producing well after weeks or months. 

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