Alkali interfacial tension

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

A second field evaluation of the ASP process has been initiated in Wyoming. Additionally, an ASP field project has been designed for the Peoples Repubhc of China. The appHcability of the ASP process to a variety of reservoirs has yet to be fully determined. AppHcation of alkali and alkali polymer flooding has been limited to cmde oils having discernible acid numbers, wherein the alkali produced cmde oil soaps which in combination with alkali resulted in providing low interfacial tensions. The ASP process appears to be suitable for cmde oils with nil acid numbers (177), and hence should have broad apphcabdity. 

The surface and interfacial tension of a great number of ester sulfonates has been measured by Stirton et al. [26-28,30]. The values of the surface tension of 0.2% solutions at 25 °C are in the range from 25 to 50 mN/m and from 2 to 20 mN/m for the interfacial tension. In the group with the same number of C atoms the pelargonates and laurates have the lowest values. Among the esters of the same a-sulfo fatty acid, the surface and interfacial tension decreases with increasing molecular weight of the alcohols. Surface tension values also depend on the cation. For the alkali salts the values decrease from lithium to sodium to potassium. 

In buffered surfactant-enhanced alkaline flooding, it was found that the minimum in interfacial tension and the region of spontaneous emulsification correspond to a particular pH range, so by buffering the aqueous pH against changes in alkali concentration, a low interfacial tension can be maintained when the amount of alkali decreases because of acids, rock consumption, and dispersion [1826]. 

Double integration with respect to EA yields the surface excess rB+ however, the calculation requires that the value of this excess be known, along with the value of the first differential 3TB+/3EA for a definite potential. This value can be found, for example, by measuring the interfacial tension, especially at the potential of the electrocapillary maximum. The surface excess is often found for solutions of the alkali metals on the basis of the assumption that, at potentials sufficiently more negative than the zero-charge potential, the electrode double layer has a diffuse character without specific adsorption of any component of the electrolyte. The theory of diffuse electrical double layer is then used to determine TB+ and dTB+/3EA (see Section 4.3.1). 

Figure 5. Interfacial Tension versus Alkali Concentrations for the Acid (A1 to A3) Fractions of Crude Oil and Shale Oil.

Meyerf has measured the interfacial tension of amalgams of the alkali metals against aqueous solutions of their salts, obtaining MeddeL fr. K. Vetensk. Nohelinstitut, ii. 17,1911. t Zeit. PTiys. Chem. iixx, 321, 1910. 

Meyer s results also resemble those of Schmidt in the peculiarity that the rapid fall in the value of the surface tension does not begin at the very lowest concentrations of solute. The first addition of alkali metal indeed produces little effect on the interfacial tension, and there is a point of inflexion on the concentration surface tension at its steepest part. This behaviour appears to be characteristic of amalgams the explanation is not clear and the phenomenon deserves further investigation. 

In acid solution as far as Ph = 5 the interfacial tension is constant but with increasing alkalinity it falls. In the case of fatty acids the tension becomes vanishingly small when the Ph exceeds 8 and the acid dissolves in the alkali in the form of micelles (see Ch. ix). 

Alkali compounds are used in the Surtek process to reduce the interfacial tension between the oil phase and the aqueous phase. In addition, an alkaline agent neutralizes rock and clay surfaces and reduces the amount of exchangeable calcium and magnesium ions from the soil surface. Both of these functions reduce surfactant and polymer adsorption into the soil matrix. 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

The presence of natural organic acids in some crude oils, especially asphaltic crude oils, may eliminate the need for expensive surfactants. These acids react with strong alkali (usually NaOH) to form petroleum soaps. These soaps diffuse into the oil-water interface, decrease interfacial tension, and form emulsions. Many researchers have used dilute alkali solutions (—0.1 wt% NaOH) to form stable oil-in-water emulsions containing up to 75% oil (i, 3). 

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. 

Surfactant Mixing Rules. The petroleum soaps produced in alkaline flooding have an extremely low optimal salinity. For instance, most acidic crude oils will have optimal phase behavior at a sodium hydroxide concentration of approximately 0.05 wt% in distilled water. At that concentration (about pH 12) essentially all of the acidic components in the oil have reacted, and type HI phase behavior occurs. An increase in sodium hydroxide concentration increases the ionic strength and is equivalent to an increase in salinity because more petroleum soap is not produced. As salinity increases, the petroleum soaps become much less soluble in the aqueous phase than in the oil phase, and a shift to over-optimum or type H(+) behavior occurs. The water in most oil reservoirs contains significant quantities of dissolved solids, resulting in increased IFT. Interfacial tension is also increased because high concentrations of alkali are required to counter the effect of losses due to alkali-rock interactions. 

Dynamic Interfacial Tension. Crude-oil-alkali systems are unusual in that they exhibit dynamic interfacial tension (Figure 11). A solution of 0.05 wt% sodium hydroxide in contact with David Lloydminster crude oil initially produces ultralow values of IFT. A minimum value is reached, after which IFT increases with time by nearly 3 orders of magnitude, measured in the spinning drop tensiometer. Taylor et al. (57) showed that dynamic inter-facial tension can also occur in crude-oil-alkali-surfactant systems. Figure 11 shows interfacial tension versus time for a solution containing 1 wt% sodium carbonate, and the same solution containing 0.02 wt% of Neodol 25-... 

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... 

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. 

Many investigators have observed that the lowest interfacial tensions between a crude oil and alkali frequently occur at very low concentrations of alkali (Nelson et al., 1984). Lieu et al. (1982) reported that the concentration range in their cases was in the region of 0.2% sodium hydroxide. Green and Willhite (1998) also reported that the concentration range is in the 0.1 wt.%. The... 

At the concentrations of alkali above that required for minimum interfacial tension, the systems become overoptimum. The excess alkali plays the same role as excess salt. When synthetic surfactants are added, the salinity requirement of alkaline flooding system is increased. NEODOL 25-3S is such a synthetic surfactant used by Nelson et al. (1984). Figure 12.4, shown earlier, is a composite of three activity maps for 0, 0.1, and 0.2% of NEODOL 25-3S as a synthetic surfactant for 1.55% sodium metasilicate with Oil G at 30.2°C. We can see in the figure that without the synthetic surfactant, the active region of this system is below the sodium ion concentration supplied by the alkali. However, with 0.1 and 0.2% of NEODOL 25-3S (60% active) present, the active region is above the sodium ion concentration supplied by the alkali, so additional sodium ions must be added to reach optimum salinity. 

Interfacial tension against electrode potential curves have a parabolic shape with a maximum value which depends on the nature and concentration of the electrolyte (see fig. 10.1). Detailed results for the mercury aqueous solution interface were initially reported by Gouy [7, G5]. Examination of these data for the alkali metal halides shows that the interfacial tension depends markedly on the nature of the electrolyte at positive potentials. On the other hand, the variation with electrolyte at negative potentials is rather small. It follows that the anions in the electrolyte strongly affect the interfacial tension when they predominate in the double layer. 

These experiments show that it is possible to achieve positive results using EOR after a thorough investigation of the nature of mineral rock constituents of the oil reservoir and the choice of the surfactant delivery method. The dynamic interfacial tension is crucial in EOR. Using a model acidic oil, alkali solutions and surfactants at an optimum ratio, ionised water and surfactant adsorb simultaneous onto the interface, resulting in low dynamic interfacial tension [229]. Combined adsorption of surfactant (alkyl propoxyethoxy sulphate) and polymer (xanthan) was studied in [230]. 

Several micellar-polymer flooding models as applied to the EOR are discussed in [237]. It is noted that the co-solvent ordinarily used in this process considerably influences not only the microemulsion stabilisation, but also the removal of impurities in the pores of the medium. The idea of using an alkali in micellar-polymer flooding is discussed in [238] in detail. The alkali effect on the main oil components was studied aromatic hydrocarbons, saturated and unsaturated compounds, light and heavy resin compounds and asphaltenes. It is demonstrated that at pH 12 surfactants formed from resins allow to achieve an interfacial tension value close to zero. For asphaltenes, such results are achieved at pH 14. In the system alkali solution (concentration between 1300 to 9000 ppm)/crude oil at 1 1 volume ratio a zone of spontaneous emulsification appears, which is only possible at ultra-low interfacial tensions. 

There exist natural surface-active substances in crude oil, such as petroleum acids and asphaltenes. The ionized acids formed by the reaction between the petroleum acids and the alkali can decrease the interfacial tension [1,5-7] and accelerate the thinning and breakdown of the film. At the same time, the asphaltenes can adsorb on to the interface and improve the stability of the film. When the film thickness is small enough (< 100 nm), it can keep this value for a long time because of the stabilization of the asphaltenes in the oil. In our study, almost all crude oil/alkali systems have this drainage process, and the crude oil/brine systems do not show it. So we can conclude that the drainage is correlated with the components, which have the interactions with alkaline solutions. 

Juang and Liu [74,75] presented that the interfacial tensions between water/ -hexane and water/toluene in the synthesis of ether-ester compounds by PTC could be measured. These two-phase systems contained PT catalyst, an aqueous phase reactant, and/or alkali. The interfacial data could be well described by the Gibbs adsorption equation coupled with the Langmuir monolayer isotherm. 

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