Phase behavior salinity effects

So far we have considered only the effect of salinity on microemnlsion phase behavior. The effects of other compositional variables and temperatnre can be understood in terms of a single unifying principle. First, we summarize the results of numerous experiments by several workers in systems containing oil, brine, an anionic surfactant, and a short-chain alcohol. 

This method requires knowledge of the characteristics values for the oil and alcohol effects, which is not always the case, in particular if some natural ill-defined product like a petroleum refinery cut is used. Alternatively, it might be impossible to attain three-phase behavior in the feasible experimental range, for instance the salinity that satisfies Eq. 4 might be too high to be attainable in practice. In such a case, another variable should be changed to keep the optimum value of the scan in the feasible range, for instance the introduction of another alcohol, which would alter the value of/(A). However, this tends to introduce inaccuracies. 

The phase behavior of anionic-cationic surfactant mixture/alcohol/oil/ water systems exhibit a similar effect. First of all, it should be mentioned that because of the low solubility of the catanionic compound, it tends to precipitate in absence of co-surfactant, such as a short alcohol. When a small amount of cationic surfactant is added to a SOW system containing an anionic surfactant and alcohol (A), three-phase behavior is exhibited at the proper formulation, and the effect of the added cationic surfactant may be deduced from the variation of the optimum salinity (S ) for three-phase behavior as in Figs. 5-6 plots. Figure 16 (left) shows that when some cationic surfactant is added to a SOWA system containing mostly an anionic surfactant, the value of In S decreases strongly, which is an indication of a reduction in hydrophilicity of the surfactant mixture. The same happens when a small amount of anionic surfactant is added to a SOWA system containing mostly a cationic surfactant. As seen in Fig. 16 (left), the values of In S at which the parent anionic and cationic surfactant systems exhibit three-phase behavior are quite high, which means that both base surfactants, e.g., dodecyl sulfate... 

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. 

Diffusion studies were made using an Isopar M/Heavy Aromatic Naptha (IM/HAN) 9 1 oil mixture (Exxon). Isopar M and HAN are refined paraffinic and aromatic oils, respectively. Figure 3 shows equilibrium salinity scans measured in the laboratory for equal-volume mixtures of the surfactant solution and oil. Since room temperature varied somewhat, the effect of temperature on phase behavior was determined. As Figure 3 shows, there is a small temperature effect, especially at the lower salinities. However, it is not large enough to have influenced the basic results of the contacting experiments. Optimum salinity, where equal volumes of oil and brine are contained in the middle phase, is approximately 1.4 gm/dl. 

The effects of pH on microemulsions have been Investigated by Qutubuddin et al. (4,5) who have reported a model pH-dependent microemulsion using oleic acid and 2-pentanol. It has been shown that the effect of salinity on phase behavior can be counterbalanced by pH adjustment under appropriate conditions. Added electrolyte makes the surfactant system hydrophobic while an increase in pH can make it hydrophilic by ionizing more surfactant. Based on the phase behavior of pH-dependent systems, a novel concept of counterbalancing salinity effects with pH is being proposed. The proposed scheme for reducing the sensitivity of ultralow interfacial tension (IFT) to salinity is to add some carboxylate or similar surfactant to a sulfonate system, and adjust the pH. The pK and the concentration of the added surfactant are variables that may be... 

Figures 2(b) and 2(c) show the effect of Xanthan gum (Abbott) on the phase behavior when the polymer concentrations are 750 ppm and 1500 ppm, respectively, and salinity is varied from 0.8 to 2.2 gm/dl NaCl. The textures are significantly different from those of the polymer-free system shown in Figure 2(a). Different phases and structures observed with the PLS are indicated in Table 1. It may be pointed out that the structures in the different phases were not investigated in detail in this study.

The effect of two different polymers on the phase behavior at low salinities is shown in Figure 4. No polymer is present in Set B (blank) while Sets D and X contain Dow Pusher 500 and Xanthan (Pfizer) polymers, respectively. The solutions are simple isotropic phases at 0.1 and 0.3 gm/dl NaCl whether or not any polymer is present. 

The effect of pH on phase behavior of microemulsions has been discussed in a different paper (4). In general, an increase in pH by addition of NaOH at constant salinity makes surfactant more hydrophilic by ionizing the carboxylic acid. Therefore, under appropriate conditions, the effect of salinity which is to make the surfactant hydrophobic, can be counterbalanced by an appropriate change in pH. The amount of NaOH, or equivalently, the pH needed for an upper phase microemulsion to shift to a middle phase increases with increasing salinity. Thus, the concentrations are 0.03M and 0.1M NaOH for 2 and 7 gm/dl NaCl, respectively. The upper + middle + lower phase transitions were observed with pH adjustment for salinities less than 5 gm/dl NaCl. For higher salinities, the microemulsion remained as a middle phase even with an excess of NaOH. All the surfactant molecules are ionized in such a situation, and the salinity is too high to be counterbalanced by pH adjustment only. 

FIGURE 7.3 Three types of microemulsions and the effect of salinity on phase behavior. 

This section describes how to use Hand s rule to represent binodal curves and tie lines. The surfactant-oil-water phase behavior can be represented as a function of effective salinity after the binodal curves and tie lines are described. Binodal curves and tie lines can be described by Hand s rule (Hand, 1939), which is based on the empirical observation that equilibrium phase concentration ratios are straight lines on a log-log scale. Figures 7.15a and 7.15b show the ternary diagram for a type II(-) environment with equilibrium phases numbered 2 and 3 and the corresponding Hand plot, respectively. The line segments AP and PB represent the binodal curve portions for phase 2 and phase 3, respectively, and the curve CP represents the tie line (distribntion cnrve) of the indicated components between the two phases. Cy is the concentration (volnme fraction) of component i in phase) (i or j = 1, 2, or 3), and 1, 2, and 3 represent water, oil, and microemulsion, respectively. As the salinity is increased, the type of microemulsion is changed from type II(-) to type III to type II(-i-). C, represents the total amount of composition i. 

In this way, we can maintain exactly the same salinity as in the pipette test (optimum salinity now), although the initial salinity could be arbitrary because it will be displaced by a large volume of injected solution, and eventually it will be replaced by the injected salinity. The injected salinity must be the same as that in the pipette test. We should always check the resulting effective salinity (in the. SALT file) to confirm that the effective salinity has not been changed after the simulation is completed. The other phase behavior parameters can be left the same as the default numbers in the batch.txt because they may not affect the results at the optimum salinity. Note that the input salinity in the injection solution, C(M,KC,L), is the salinity in the injected aqueous phase, C5, which is not effective salinity. However, in. SALT, the output is the effective salinity, which is defined as... 

Variables identified as important in the achievement of the low IFT in a W/O/S/electrolyte system are the surfactant average MW and MW distribution, surfactant molecular structure, surfactant concentration, electrolyte concentration and type, oil phase average MW and structure, temperature, and the age of the system. Salager et al. (1979b) classified the variables that affect surfactant phase behavior in three groups (1) formulation variables those factors related to the components of the system-surfactant structure, oil carbon number, salinity, and alcohol type and concentration (2) external variables temperature and pressure (3) two-position variables surfactant concentration and water/oil ratio. Some of the factors affecting IFT-related parameters are briefly discussed in this section. Some other factors, such as cosolvent, salinity, and divalent, are discussed in Section 7.4 on phase behavior. Healy et al. (1976) presented experimental results on the effects of a number of parameters. 

Because alkalis provide an additional somce of electrolytes, their presence in a surfactant solntion will reduce the optimum salinity. When Martin and Oxley (1985) investigated the effects of alkalis on surfactant phase behavior, they fonnd that for petrolenm sulfonate, alkali anion had little or no effect on the phase behavior, whereas cations were effective for decreasing the optimum salinity in this order potassinm > sodium ammonium (see Figure 8.4). For the solutions with and withont alkali, the optimum salinity decreased with the snrfactant concentration. In the presence of alkaline chemicals, Martin and... 

To the best of our knowledge, the work by Gupta and Trushenski (1979) and experimental data from Nelson (1982) are the only data published so far to support the concept of a negative salinity gradient. Gupta and Trushenski, and Nelson used the same kind of surfactant with a special phase behavior. My explanation to their observation on salinity effect is that for the surfactant they used, the IFTs for both microemulsion/oil and microemulsion/water in the type III system were high. Therefore, when a lower salinity was in the drive water, low IFT was obtained because the lower salinity matched the lower optimum salinity of surfactant as the surfactant concentration was diluted. 

Martin and Oxley (1985) studied the effect of different alkalis on surfactant systems. They showed that the presence of any alkali lowered the optimum salinity of the surfactant system. This phenomenon is caused by two facts (1) alkali can provide electrolytes and (2) alkali reacts with crude oil to generate soap, and soap has lower optimum salinity (see the next section). Martin and Oxley found a linear relationship between the optimum salinity and sodium concentration. The addition of any alkali agents results in a decrease in the optimum salinity of the system. However, alkali anions have very little effect on the phase behavior. 

Pithapurwala, A.K., Sharma, R.C. and Shah, D.O. (1986) Effect of salinity and alcohol partitioning on phase-behavior and oil displacement efficiency in surfactant-polymer flooding. /. Am. Oil Chem. Soc., 63(6), 804-813. 

Table I summarizes the qualitative changes in the phase behavior of microemulsions containing ionic surfactants. Some details of the effects of different variables are available in Ref. 13 and various chapters in this book. The phase transitions are generally understood in terms of relative strengths of hydrophilic and hydrophobic properties of the surfactant film in the microemulsion. The phase behavior depends strongly on the type and structure of the surfactant. For example, microemulsions containing nonionic surfactants are less sensitive to salinity but are more sensitive to temperature than those with ionic surfactants. The partitioning of cosolvents such as alcohols between the surfactant film, the organic phase, and the aqueous phase also affects the phase behavior. Microemulsions can be tailored for specific applications by adjusting an appropriate variable. For example, as indicated in Table 1, the effect of salinity on the phase behavior can be counterbalanced by an increase in the pH of an appropriate microemulsion [18,19].

The investigation technique is the unidimensional formulation scan as in the phase behavior. studies discussed in the previous chapter. For the sake of simplicity, the scanned variable is often taken as the salinity for ionic surfactant systems, and as surfactant EON or temperature for nonionic systems, but it should be well understood that other formulation variables would produce exactly the. same effects. In the reasoning, the formulation will be referred to a.s SAD, the deviation from optimum formulation, whatever the variable used to produce the scan. 

For certain classes of anionic surfactants, other effects are also important. For example, changes in pH affect the degree of ionization and hence the phase behavior when the surfactants include organic adds, amines, or other pH-sensitive compounds. An increase in the degree of ionization produces a more hydrophilic surfactant film and hence, other things bdng equal, increases optimal salinity (Qumbuddin et al., 1984). 

Multivalent cations affect phase behavior, hence, optimal salinity, more than the effect of an equal molar quantity of monovalent cations and the multivalent to monovalent cation effectiveness ratio increases with decreasing surfactant concentration. Consequently, ion exchange during a chemical flood can influence... 

The influence of alcohols on phase behavior is known as mainly effective inside phases (1-3), for instance, by changing the optimal salinity for a given oil-surfactant system (4-6) or the surfactant partitioning between the oil and brine (7-9). Therefore, it becomes convenient to look at the problem in two separate steps ... 

In this work, salt, alcohol and surfactant effects are investigated, at different water-to-oil ratios to determine their influence on phase behavior. A linear relationship is found between the logarithm of water-oil ratio in the middle phase and the salinity. Moreover, in the middle of the range of salt or alcohol concentration giving a three-phase system, the water-oil ratio in the middle phase has a value close to unity (value defined as an optimum), whatever the overall water-oil ratio may be. 

Table 1. Effect of the salinity on the phase behavior of the brine-octane-sulfonate-isopropanol system. WOR = 1.1, sulfonate and isopropanol concentrations are 1.0 and 3.0 wt %, respectively. 

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... 

In general, flow experiments in EM-Gel packed beds show that pressure drop of emulsions increase with the increase in the amount of dispersed phase, irrespective of whether it is oil or aqueous phase. The anomalous behavior at 2% NaCl is attributed to optimal salinity effects. 

The salinity of poljmier solution can influence four major parameters of surfactant-pol)mi r flooding process, namely, interfacial tension, mobility control, surfactant loss and phase behavior. When polymer solution of various salinities are equilibrated with surfactant solution in oil, the formation of lower, middle and upper phase microemulsion has been observed (1) similar to the effect of increasing connate water salinity (2,3). In general, there is an optimal salinity (2) which produces minimum interfacial tension and maximal oil recovery (1,4). On the basis of interfacial tension alone, the salinity of polymer solution should then be designed at or near the optimal salinity of the preceding surfactant formulation. 

The phase behavior studies reported in Figure 4 were performed in a brine of similar salinity as in Figure 3, but containing additives to ensure long-term chemical stability of the polymer among these additives was 0.4% isopropanol. These additives have a marginal effect on phase behavior and the same increase in microemulsion/ aqueous phase interfacial tension has been observed. However, we did not find the drastic increases in viscosity reported in Figure 3. 

Although the phase behavior of PA is altered for monolayers on physiological, buffered saline subphases (pH = 6.9,0.15 M NaCl), the presence of SP-B protein has similar effects to those seen on a pure water subphase. For pure PA monolayers, the buffered saline subphase conditions result in a change in the mechanism of collapse. 

As already pointed out, the purpose of the phase scan is to determine the optimal temperature/ salinity that can produce microemulsion of desired type and properties. Once the optimal temperature/salinity is established, one can further investigate the effects of composition on phase behavior using the ternary phase diagram. 

This effect of temperature has been recognized first on nonionic surfactant systems, and the temperature is still the choice variable to study nonionic surfactant systems phase behavior [51,56-58]. According to the Winsor approach, it is clear that the temperature is likely to change not only Acw but all other interactions as well, a situation that does not simplify the interpretation of experimental data. Moreover, the temperature range that can be experimentally handled without complication is not very wide, because it matches the liquid state of water. This is why the electrolyte concentration (salinity) and ethylene oxide number (EON) have been often preferred by experimentalists as scanning variables for ionic and nonionic systems, respectively, as their effect is easier to predict and interpret. 

---