Salinity macroemulsions

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. 

Transient Processes. There are several transient processes such as formation and coalescence of oil drops as well as their flow through porous media, that are likely to occur during the flooding process. Figure 12 shows the coalescence or phase separation time for hand-shaken and sonicated macroemulsions as a function of salinity. It is evident ithat a minimum in phase separation time or the fastest coalescence rate occurs at the optimal salinity (53). The rapid coalescence could contribute significantly to the formation of an oil bank from the mobilized oil ganglia. This also suggests that at the optimal salinity of the system, the interfacial viscosity must be very low to promote the rapid coalescence. 

The flow through porous media behavior of various macroemulsions was studied by measuring the pressure drop across a porous medium (Figure 13). It is obvious that a minimum in pressure drop occurs near the optimal salinity of the surfactant formulation. One can conclude that the interfacial tension is an important parameter which influences the pressure drop across porous media (53). 

Figure 14 shows a very interesting and an important correlation between the rate of coalescence in macroemulsions and the apparent viscosity in the flow through porous media. It was observed that a minimum in apparent viscosity for the flow of macroemulsions in porous media coincides with a minimum in phase separation time at the optimal salinity. This correlation between the phenomena occurring in the porous medium and outside the porous medium allows us to use coalescence measurements as a screening criterion for many oil recovery formulations for their possible behavior in porous media. It is. very likely that a rapidly coalescing macroemulsion may give a lower apparent viscosity for the flow in porous media (53). 

Figure 12. Effect of salinity on the phase separation or coalescence rate of hand-shaken and sonicated macroemulsions.

The method developed originally for microemulsion formulation (Section II above) has been adapted (Salager, 1983, 2000) to macroemulsion formation. In this method, the value of the left-hand side of equation 8.10 or 8.11 is called the hydrophilic-lipophilic deviation (HLD). When the value equals zero, as in Section II, a microemulsion is formed when the value is positive, a W/O macroemulsion is preferentially formed when it is negative, an O/W macroemulsion is preferentially formed. The HLD is similar in nature to the Winsor R ratio (equation 5.2) in that when the HLD is larger than, smaller than, or equal to 0, R is larger than, smaller than, or equal to 1. The value of the HLD method is that, on a qualitative basis, it takes into consideration the other components of the system (salinity, cosurfactant, alkane chain length, temperature, and hydrophilic and hydrophobic groups of the surfactant). On the other hand, on a quantitative basis, it requires the experimental evaluation of a number of empirical constants. 

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. 

The results of our earlier investigation using spin-labelling technique to understand the structural aspects involved in the various emulsions, support the theory that water-external macroemulsions exist below optimal salinities and oil-external types exist beyond optimal salinity. In addition it was found that microemulsions coexisted with macroemulsions and were of the same type in the sonicated emulsions. These findings are further complemented by electrical conductance and bulk viscosity data. 

The electrical conductivity results discussed in section 1 have shown that the macroemulsions seem to exsit as W/o type below 1.8% NaCl and as 0/W type above 1.8% NaCl for all aqueous to oil phase ratios. This result together with the pressure drop data thus suggest that pressure drop associated with the flow of macroemulsions increase with the increase in the amount of the dispersed phase irrespective of whether it is oil or aqueous phase. It is suggested that the anomalous behavior at 2% NaCl could only be attributed to optimal salinity effects. Rheological data for this composition and for those compositions in the transition region... 

Discontinuities in electrical conductivity values demonstrate the existence of a phase inversion process in macroemulsions. For 1 1 (aqueous oil) emulsions at 25°C, phase inversion of O/W to W/0 occurs around optimal salinity (1.5% NaCl). 

Three-Phase Displacement. Provided that the alcohol free surfactant system shows a classic phase behavior, II( —) -> III II( -f) by increasing the salinity without forming gels or stable macroemulsions during the phase transitions, the chemical flood can be performed as a three-phase flood using a negative salt gradient. 

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