Salinity surfactant concentration effects

In general, the surfactant formulations used for enhanced oil recovery contain a short chain alcohol. The addition of alcohol can influence the viscosity, IFT and birefringent structures of micellar solutions as well as coalescence rate of oil ganglia. The present paper reports the effect of addition of isobutanol to a dilute petroleum sulfonate (< 0.1% cone) solution on IFT, surface shear viscosity, surfactant partitioning, the rate of change of IFT (or flattening time) of oil drops in surfactant solutions and oil displacement efficiency. The two surfactant systems chosen for this study indeed exhibited ultralow IFT under appropriate conditions of salinity, surfactant concentration and oil chain length (11,15,19). 

Decreasing the pH of 3% NaCl (entries 2 and 3, Table 14) could decrease neutralization of crude oil organic acids. This neutralization increases both aqueous phase salinity and effective surfactant concentration. A lower effective surfactant concentration at pH 8 could account for the increased interfacial tension value. However, a similar pH change does not reduce IFT when the surfactant is AOS 1618 with a much lower di monosulfonate ratio (entries 6 and 7, Table 14). 

Adsorption of a surfactant on solids is dependent, among other things, on the structure of both the hydro-phobic and hydrophilic portions of it. There are a number of mechanisms proposed for surfactant adsorption and an understanding of the effects of the structure of the surfactant can help in elucidating the role of these mechanisms. In this study, the effect on adsorption on alumina of some structure variations of sulfonates (chain length and the branching and the presence of ethyoxyl, phenyl, disulfonate and dialkyl groups) is examined above and below CMC as a function of surfactant concentration, pH and salinity. Co-operative action between an ionic alkyl sulfonate and a nonionic ethoxylated alcohol is also studied. 

Further evidence to support the above hypothesis on the role of structure in phase separation of aqueous solutions is provided by the effect of additional alcohol. The amount of alcohol was increased from 3.0 to 5.0 gm/dl, the surfactant concentration kept constant, and the salinity varied. The addition of alcohol extended the range of salinity where the aqueous solutions are isotropic to 0.8 gm/dl NaCl. According to the above hypothesis, no phase separation should take place on addition of polymer to the isotropic solutions existing up to 0.8 gm/dl NaCl. Indeed, no phase separation was observed when as much as 1500 ppm Xanthan was added at such compositions. Thus, the addition of alcohol increases the critical electrolyte concentration for phase separation, an effect seen also by others (9). 

If we input Cjjmaxi = 0.03, Cjs axo = C33max2 = 0.06, and injected salinity C(M,KC,L) = Cii X Cse = 0.99 x 0.36S = 0.3614 meq/mL solution (not water), the effective salinity in. SALT is then exactly equal to 0.355 meq/mL water. Here, C, = 1 - C31 = 1 - 0.01 = 0.99 because the surfactant concentration is 1%. In C(M,KC,L), M denotes the well number, which is 1 for the injector in this simulation model KC denotes the component number, which is 5 for anion and L denotes the phase number, which is 1 for the injected aqueous phase. The solubilization ratios C23/C33 and C13/C33 from the simulation are the same—1 5.2. This solubilization ratio is lower than the experimental data—15.8. To improve this ratio, we reduce C33maxi to 0.03 X 15.2/1 5.8 = 0.0289 and keep the other parameters unchanged. Then we have the solubilization ratios equal to 1 5.8. Thus, we have matched the point at the optimum salinity. 

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. 

FIGURE 8.4 Effect of surfactant concentration on optimum salinity for 1 wt.% alkaline chemicals, brine, n-tetradecane, and surfactant Exxon 914-22. Source Martin and Oxley (1985). 

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. 

This section further discusses the effects of k, curves, optimum phase type, and phase viscosity. The effect of negative salinity gradient is further discussed under conditions where different relationships between optimum salinity and surfactant concentrations occur. 

Based on Eq. 12.1, optimum salinity follows the logarithmic mixing rule. Mohammadi et al. (2008) replaced the ratio of oil to surfactant concentration shown in Figure 12.5 by soap molar fraction and used the more generally effective salinity in the vertical axis. They did so because they could get these values from UTCHEM simulation models. Based on the logarithmic mixing rule, both axes in such activity maps are in logarithmic scales, and the upper and lower boundaries should be linear. 

The xs are the molar fraction of the two base surfactants in the mixture, but if they are relatively close products, e.g. nonylphenols with EON = 4 and 6, the weight fraction, which is easier to calculate, maybe used instead with insignificant error. In practice, it is preferable to use a relatively high total surfactant concentration to avoid fractionation effects, say, 2-3 wt.% at least. Note that EONbm only depends on all the other formulation variables, e.g. salinity, oil, alcohol and temperature, which are fixed in all experiments. Since neither the structure nor the a value is known, the correlation to be used for the unknown non-ionic surfactant is the one including characteristic parameter (3. 

In general, trends in MRF with surfactant concentration and foam quality are consistent for all surfactants, but the effects of brine salinity (Figure 4) and temperature (3) vary from surfactant to surfactant. Different surfactants are also affected to different degrees by the presence of an oil phase, as discussed in Chapter 4 of this book. The MRF increases with increasing permeability (Figure 5), as also noted by Lee and Heller (4) and described earlier in Chapter 5. This effect could be very beneficial to foam performance, because it leads to better mobility control in high-permeability zones. 

Figure 2 summarizes the effect of NaCl concentration, oil chain length and surfactant concentration on the interfacial tension in this system. It is evident that ultralow interfacial tension minimum occurs at a specific salinity or specific oil chain length or specific surfactant concentration. Using light scattering, osmotic pressure, surface tension, dye solubilization and various other techniques, we have shown that the ultralow inter facial tension minimum in this system correlates with the onset of micellization in the aqueous phase and the partition coefficient of surfactant near unity (14). Baviere (15) has also shown that the partition coefficient is near unity at the salinity where minimum interfacial tension occurs. 

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... 

Absolute values of M /Na" effectiveness and the dependency of those values on surfactant concentration depend upon the particular surfactant or surfactant blend. For example, for Petrostep 450 alone in the same system as the 80/20, Petrostep 450/NEODOL 25-3S, blend we have been discussing, the M /Na" effectiveness ratio rises from 21 to 67 as the surfactant concentration is lowered from 5.0 to 2.0 percent. On the ocher hand, judging from how "flat its Salinity Requirement Diagram is, the M /Na+ effectiveness ratio for NEODOL 25-3S by itself does not change much with surfactant concentration. (The midpoint salinity of NEODOL 25-3S drops only 11 percent, from 185 percent to 165 percent SDSW, as its concentration in the subject system is decreased from 5.0 to 0.8 percent.)... 

The "steepness" of a Salinity Requirement Diagram indicates how rapidly the M /Na+ effectiveness ratio changes with surfactant concentration. If that ratio did not change at all with surfactant concentration, Salinity Requirement Diagrams made with multivalent cations in the brine would parallel the "flat" diagrams, such as Figure 6, found when the only cations in the brine are sodium. 

The effect of surfactants Figure 6 shows that the microemulsion volume is proportional to the surfactant concentration as reported previously (8,10). However, in some cases it appears that the optimum salinity in a given system varies with this concentration when the surfactant used is not an isomerically pure product. It has been pointed out, for instance, by interfacial tension measurements that by increasing the petroleum sulfonate (TRS 10-80 supplied by Witco Chemical Company) concentration, the preferred alkane molecular weight is decreased. This shift occurs apparently because there is a dependence of the mean micellar and monomeric molecular weights on the total surfactant concentration (9,11). 

The surfactant chosen was Chevron Chaser GR-1080, a proprietary commercial mixture of surfactants proven to form effective mobility control foams in high salinity and hardness conditions, and in the presence of crude oil [11]. The surfactant concentration was kept at 0.5 wt% throughout the experiments. 

In a general case, displacement of oil by polymer and surfactant is effected by complex physico-chemical processes, when modeling and numerical realization of which there take place definite problems. For example, viscosity of injected solution depends on various factors, such as reservoir temperature, concentration of polymer/surfactant in solution and water salinity and etc. The model takes into account the following assumptions ... 

In the 1990s, the thmst of surfactant flooding work has been to develop surfactants which provide low interfacial tensions in saline media, particularly seawater require less cosurfactant are effective at low concentrations and exhibit lower adsorption on rock. Nonionic surfactants such as alcohol ethoxylates, alkylphenol ethoxylates (215) and propoxylates (216), and alcohol propoxylates (216) have been evaluated for this appHcation. More recently, anionic surfactants have been used (216—230). 

Typical adsorption isotherms are shown in Figs. 16 and 17. Despite the large experimental scatter, a steep increase in adsorption can be seen at low concentrations, followed by a plateau at concentrations exceeding the CMC. Similar behavior has been observed before with model surfactants [49-54] and has also been predicted by modem theories of adsorption [54]. According to Fig. 16, adsorption increases modestly with salinity provided that the calcium ion concentration remains low. The calcium influence, shown in Fig. 17, cannot be explained by ionic strength effects alone but may be due to calcium-kaolinite interactions. 

TITRATIONS FOR COMPARISON OF METHODS. The automated photometric and turbidimetric methods were compared using 30 cm3 samples of surfactant solution containing a nominal 20 mol SDBS to give an equivalence volume of 5 cm3. The effect of salinity on the titrations was studied using samples prepared containing sodium chloride concentrations of 0.0, 0.14, 0.70 and 1.46 wt%. The influence of the choice of filter (580 or 620 nm) was also investigated. 

The critical micellar concentrations of anionic/nonionic surfactant mixtures examined are low in a saline medium, so that, at the concentrations injected in practice, the chromatographic effects resulting from the respective adsorption of monomers are masked. Such surfactants propagate simultaneously in the medium in the form of mixed micelles. 

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