Salinity microemulsion systems

In summary, several phenomena occurring at the optimal salinity in relation to enhanced oil recovery by macro- and microemulsion flooding are schematically shown in Figure 18. It is evident that the maximum in oil recovery efficiency correlates well with various transient and equilibrium properties of macro- and microemulsion systems. We have observed that the surfactant loss in porous media is minimum at the optimal salinity presumably due to the reduction in the entrapment process for the surfactant phase. Therefore, the maximum in oil recovery may be due to a combined effect of all these processes occurring at the optimal salinity. 

The dynamic behavior of the PDM system in the salinity range between optimum salinity and the upper-phase microemulsion region was similar to that below optimum in that intermediate brine and microemulsion phases formed. Figure 13 shows these phases for the 1.5 gm/dl-salinity PDM system. The middle-phase type of microemulsion, being high in oil content at these salinities, grew more rapidly in the direction of the oil phase than it did at low... 

The transition from the three-phase to two-phase region in the PDM system was marked by a sudden increase of spontaneous emulsification in the oil phase. Because formation of an intermediate microemulsion ceased at this point, the emulsion drops remained near the brine interface rather than rapidly moving away to form a single-phase region above the brine. An example of this behavior is shown in Figure 15 for the 2.1 gm/dl-salinity PDM system. 

An extended experiment was performed for the 2.0 gm/dl-salinity Pm system to determine the effect on relative interface velocities of the formation of myelinic figures and the C phase. With reference to Figure. 18, the formation of a uniform layer of C phase at t1 = 12 hr1 (6 days) corresponds to a decrease in relative velocity of the brine-microemulsion interface. The layer of C phase grew uniformly after its initial formation. In Figure 18, all positions are plotted relative to the same reference position. The offset of the liquid crystal interface at t = 0 indicates brine formation due to initial mixing. 

It is possible to extend the middle phase region by increasing the concentration of carboxylic acid, coupled with an increase in alcohol concentration. The phase behavior of a mixed microemulsion system containing 5 gm/dl TRS 10—410, 1.5 gm/dl octanoic acid and 8 gm/dl IBA at 22°C is shown in Figure 17. Between 1.5 and 4 gm/dl NaCl, if only NaOH concentration is varied then upper - middle - lower phase transitions occur, apparently due to an increase in the ionization of the surfactant. Extension of salinity and NaOH concentrations in Figure 17 is expected to show upper - middle - lower + middle, and other phase transitions as observed in the oleic acid system (4). 

The system has two phases an excess oil phase and a water-external microemulsion phase. Because microemulsion is the aqueous phase and is denser than the oil phase, it resides below the oil phase and is called a lower-phase microemulsion. At a high salinity, the system separates into an oil-external microemulsion and an excess water phase. In this case, the microemulsion is called an upper-phase microemulsion. At some intermediate range of salinities, the system could have three phases excess oil, microemulsion, and excess water. In this case, the microemulsion phase resides in the middle and is called a middle-phase microemulsion (Healy et al., 1976). Such terminology is consistent with their relative positions in a test tube (pipette) with the water being the dense liquid. In the environmental sciences and engineering, however, a dense nonaqueous phase liquid (DNAPL) could be denser than water (UTCHEM-9.0, 2000). Fleming et al. (1978) used y, P, and a to name the lower-phase, middle-phase, and upper-phase microemulsions, respectively. 

The phase behavior of microemulsions is complex and depends on a number of parameters, including the types and concentrations of surfactants, cosolvents, hydrocarbons, brine salinity, temperature, and to a much lesser degree, pressure. There is no universal equation of state even for a simple microemulsion. Therefore, phase behavior for a particular microemulsion system has to be measured experimentally. The phase behavior of microemulsions is typically presented using a ternary diagram and empirical correlations such as Hand s rule. 

The argument to use a two-phase model to represent surfactant phase behavior without type III microemulsion is that experiments (Seethepalli et al., 2004 Zhang et al., 2006 Liu et al., 2008) indicate that the volume of type III microemulsion phase is small if the overall surfactant concentration is low (<0.1 wt.%). In the cases of low surfactant concentration, a type III microemulsion system was not observed by Salager et al. (1979b). The reason is that if we cannot make a sufficient number of salinity scans, and the volume of the type III microemulsion phase is small, the equilibrium phase behavior... 

As oil saturation is decreased (water saturation is increased), the acid content in the oil is decreased. Consequently, the soap molar fraction X oap is decreased, as Table 12.6 shows. As X oap is decreased, type 111 salinity limits are closer to those of surfactant. Thus, the limits are increased, and the optimum salinity is increased as well. The system is changed from type III to type I. This transition from type III to type I is exactly the salinity gradient we need. In practical alkaline-surfactant flooding, water saturation will be increased from the flood front to the upstream, and the microemulsion system will change from type III... 

Whereas Winsor III systems exhibit ultra-low interfacial tensions between the three phases and also very high solubilisation capacity, Winsor I systems have higher interfacial tensions and much lower solubilising power. At the transition between the two types of microemulsion systems, an intermediate behaviour can be found which is called supersolubilisation [47,70]. The uptake of oils into surfactant aggregates is usually enhanced by one to two orders of magnitude compared to effective micellar systems, but interfacial tension reduction is still moderate. The transition point can be adjusted by varying the salinity or organic components. 

While the middle-phase microemulsion appears at a discrete point in the temperature or salinity scan, system properties change continuously through the scan. As illustrated in Fig. 13. the interfacial tension decreases... 

A sudden increase in research effort on microemulsion systems was driven by the economic impact of the oil embargo in the early 1970s and the development of the so-called enhanced oil recovery processes that followed. The plentiful research funding available from both industry and governmental agencies resulted in an unprecedented improvement in the basic and advanced knowledge of very complex phenomena, in particular the surfactant-oil-water phase behavior in all its intricacies. It was found that the interfacial tension could be lowered to an ultralow 0.001 mN/m in many systems provided that a particular physicochemical condition was attained. It turned out that this so-called first optimum salinity, and then optimum formulation, coincides with the occurrence of three-phase behavior in which a bicontinuous microemulsion is in equilibrium with oil and water excess phases, i.e., the Winsor III case [20,21,109,110]. 

As for solvent diffusion, surfactant diffusion follows a rather general pattern for all microemulsion systems, and we illustrate it with an example (Fig. 10). As we vary the appropriate parameter (salinity in Fig. 10), changing the spontaneous curvature from positive to negative, we see at large positive Hq values (low salinity) a slow surfactant diffusion that equals the oil diffusion value. Here, clearly, surfactant diffusion describes droplet diffusion, as also verified in measurements of droplet diffusion (by self-diffusion or... 

The optimum surfactant formulation for a microemulsion system is dependent on many variables (i.e., pH, salinity, temperature, etc.). References [17,49] list some of the components in a typical formulation. The surfactants and co-surfactants must be available in large amounts at a reasonable cost. In addition, they should also be chemically stable, brine soluble, and compatible with the other formulation components. Common surfactants used are petroleum sulfonates and ethoxylated alcohol sulfates [50,51]. The degree of interfacial tension lowering depends on the... 

In summary, various phenomena occurring at an optimal salinity in relation to enhanced oil recovery by macroemulsion and microemulsion flooding are schematically shown in Figure 6. It has been demonstrated that a maximum in oil recovery correlates well with several equilibrium and transient properties of surfactant flooding in the form of macroemulsion and microemulsion systems. Results have shown that a maximum in oil recovery, a minimum in surfactant adsorption, a minimum in apparent viscosity of the emulsion, a minimum in phase separation time, an equal solubilization of oil/brine phases in middle phase microemulsion, and a minimum in interfacial tension occur at an optimal salinity of the system. 

For microemulsion systems, the Fourier transform nuclear magnetic resonance (FTNMR) method has been used extensively for self-diffusion studies of the components [28]. The nature of the composition-dependent self-diffusion coefficients of the constituents of the system saline water/SDS/butanol/toluene was determined by... 

In addition to percolation in reverse microemulsions being driven by field variables such as (disperse phase) volume fraction, temperature, and salinity, cosurfactant concentration appears also to suffice. Low-frequency electrical conductivity for the aqueous acrylamide-AOT-tol-uene reverse microemulsion system is illustrated in Fig. 6 [42]. The cosurfactant concentration = 1-2% (w/w) is annotated with the arrow (Op) in Fig. 6 and corresponds to the approximate onset of electrical conductivity percolation. The percolation threshold in conductivity occurs at ip = 3.09%. 

The ability of aminated compounds to inhibit corrosion on metallic surfaces via adsorption phenomena has been already certified. Since operations taking place at interfaces are greatly affected by variations in surface tension, aminated surfactant molecules are expected to provide even better results. This has been the case, when self-assembled micellar or microemulsion systems are used as corrosion inhibitors. In that aspect, surfactants may be used as organic corrosion inhibitors, and act by forming a protective film onto surfaces which are exposed to corrosive media, like oxygen and saline or acidic solutions. When microemulsions are used, an oil film is also adsorbed onto the surface with the surfactants tails oriented towards it, in view of the usually positive character of the surface. In the petroleum industry, the oil itself may be the nonpolar component of such systems. Figure 15.10 is a schematic of these types of films. 

In general, quite high efficiencies are reached, but microemulsified structures bring about an important effect on the inhibition ability of the surfactant molecules. In general, higher inhibition levels are attained with microemulsion systems. The deleterious effect of both salinity and temperature is also observed. 

Abstract We introduce a new technique using small-angle neutron scattering (SANS) to measure the average Gaussian curvature and the average square-mean curvature of the oil-water interface in a three-component, nearly isometric (equal volume fractions of water and oil) ionic microemulsion system. The microemulsion is composed of AOT/brine/decane. SANS measurements are made as a function of both the volume fraction of surfactant and salinity at a constant temperature,... 

Figure 1 shows the phase diagram of AOT/D2O (NaCl)/H-decane system in the salinity-surfactant volume fraction plane at a temperature 45 °C. For small surfactant volume fractions, less than 0.04, and low salinity, the system shows a 2-phase with a coexisting excess oil layer on the top and an oil-in-water microemulsion at the bottom. As salinity increases, the system goes through a three-phase region, where a middle-phase microemulsion is in coexistence with an excess oil layer on the top and an excess water layer in the bottom, and at high salinity it transforms to a 2-phase with an excess water layer in the... 

One parameter that has been discovered to be crucially important in the successful implementation of the surfactant-polymer flooding process is the salinity of the aqueous phase. As discussed previously, addition of salt to the microemulsion system induces the change from lower- to middle- to upper-phase microemulsion (Fig. 15) [33]. It was found that at a particular salt concentration, deemed the optimal salinity, a number of important parameters were optimized for the oil recovery process. The optimal salinity was found to occur when equal amounts of oil and brine were solubilized by the middle-phase microemulsion [50]. 

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

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. 

R. Leung and D. O. Shah. Solubilization and phase-equilibria of water-in-oil microemulsions. 2. Effects of alcohols, oils, and salinity on single-chain surfactant systems. J. Colloid Interface Sci., 120(2) 330-344, 1987. 

Addition of salting-out type electrolytes to oil-water-surfactant (s) systems has also a strong influence on their phase equilibria and interfacial properties. This addition produces a dehydration of the surfactant and its progressive transfer to the oil phase (2). At low salinity, a water-continuous microemulsion is observed in equilibrium with an organic phase. At high salinity an oil-continuous microemulsion is in equilibrium with an aqueous phase. At intermediate salinity, a middle phase microemulsion with a bicontinuous structure coexists with pure aqueous and organic phases. These equilibria were referred by Vinsor as Types I,II and III (33). 

The influence of sodium acetate on the phase equilibria of acrylamide microemulsions has been investigated (Holtzscherer, C. Candau, F. J. Colloid Interface Sci., in press). The interfacial tensions of the systems preequilibrated are reported versus the salt concentration in Figure 6. It can be seen that addition of sodium acetate induces a phase transition HI - H III which occurs for S = 1.2H. The intercept of the two curves which occurs in the Vinsor III domain defines an optimal salinity for the formation of bicontinuous microemulsions. 

Figure (3) shows the solubilization parameters as functions of water concentration for SDS/2- entanol ratios of 0.25 and 0.40 at 25 C. The solubilization parameters are defined as Vo/Vs and Vw/Vs, where Vo, Vs and Vw are the volumes of organic phase, surfactant and aqueous phase in the microemulsions. The parameters are related to the drop size and also interfacial torsions f7.23). The bicontinuous phase is located around the composition range corresponding to equal values of solubilization parameters. The solubilization parameters are dependent on the initial surfactant and/or cosurfactant concentration. Similar dependence has been observed in other systems as a function of salinity and pH (7.231. Conductivity measurements performed as a function of water content indicate an S-shaped curve as shown in Figure (4). This is typical of microemulsions showing transition from oil-continuous to bicontinuous to water-continuous microstructure with increasing water content. 

The Interaction of water soluble polymers with microemulsions and with surfactants will, when the components are sufficiently concentrated, often result in a phase separation or change in the phase boundaries of the mixture as a function of external variables, such as temperature or salinity. In order to arrive at a better understanding of this technologically Important phenomenon, a series of experimental studies was carried out using a variety of water soluble polymers in conjunction with model mlcroemulslon systems. The polymers used Included polyethylene oxide, polyvinylpyrrolidone, dextran, xanthan, polyacrylamide, and hydrolyzed... 

The physicochemical aspects of micro- and macroemulsions have been discussed in relation to enhanced oil recovery processes. The interfacial parameters (e.g. interfacial tension, interfacial viscosity, interfacial charge, contact angle, etc.) responsible for enhanced oil recovery by chemical flooding are described. In oil/brine/surfactant/alcohol systems, a middle phase microemulsion in equilibrium with excess oil and brine forms in a narrow salinity range. The salinity at which equal volumes of brine and oil are solubilized in the middel phase microemulsion is termed as the optimal salinity. The optimal salinity of the system can be shifted to a desired value hy varying the concentration and structure of alcohol. 

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