Surfactant-brine-oil phase behavior

The effect of polymer on the surfactant/brine/oil phase behavior has been investigated, indicating that polymer is usually present in an aqueous phase, which can be highly concentrated. Furthermore, polymer will extract water from a microemulsion phase, thus increasing the surfactant concentration in the microemulsion and shifting the invariant point M away from the brine corner. 

The surfactant-brine-oil phase behavior relevant to the microemulsion flooding process of enhanced oil recovery has been described in a number of papers in the literature (1). In these papers it has been indicated that under suitable conditions a microemulsion phase can be found to be thermodynamically stable in contact with an aqueous solution and with oil. Furthermore, it has been described how this phase behavior depends on the type of surfactant and cosurfactant, ionic composition of the brine, type of oil and on temperature. 

In connection with these experiments, data are presented on how the surfactant-brine-oil phase behavior is influenced by small amounts of a polysaccharide polymer dissolved in the brine. This polymer apparently complicates phase behavior considerably in principle a fourth axis is required in the phase diagram. However, in view of the approximate nature of the ternary representation of the surfactant-brine-oil system, we shall not attempt to draw sys-... 

In the right-hand part of the ternary diagram, where the microemulsion is in equilibrium with oil, the polymer is found as a hydrated mass on the bottom of the test tube, in fact as a third phase. The effects of increasing amounts of polymer on the surfactant-brine-oil phase behavior have been systematically studied with polymer concentrations up to 4000 ppm. Figure 4 shows the continuous shifting of the invariant point M to the right with increasing polymer concentration. 

Martin, E.D., Oxley, J.C., 1985. Effect of various chemicals on phase behavior of surfactant/brine/ oil mixtures. Paper SPE 13575 presented at the International Symposium on Oilfield and Geothermal Chemistry, Phoenix, 9-11 April. 

C. D., and Scriven, L. E., paper SPE 5811, presented at SPE-AIME Improved Oil Recovery Symposium, Tulsa, Oklahoma, March 1976, "Interfacial Tension and Phase Behavior in Surfactant-Brine-Oil Systems."... 

By the same procedure we find that with 2.0 percent surfactant in the system and 0.69 moles/kg of sodium in the brine, 0.035 moles/ kg of SDSW multivalent cations affects phase behavior to the same extent as 0.57 moles/kg of additional sodium ion. And with 0.8 percent surfactant in the system and 0.37 moles/kg of sodium in the brine, we find that 0.019 moles/kg of SDSW multivalent cations affects phase behavior to the same extent as 0.79 moles/kg of additional sodium ion. From these numbers we present in Table 2 relative effectiveness of the divalent ions in SDSW to sodium ions in changing phase behavior of the surfactant-brine-oil system under discussion as a function of overall surfactant concentration in the system. 

It is apparent from the table that the phase behavior of the system under discussion is much more sensitive to the multivalent cation concentration at low surfactant concentrations than at high surfactant concentrations. This means that the exact ionic composition of the brine in the surfactant bank is more critical near the end of a chemical flood than it is in the beginning. It means also that the effect of ion exchange on the phase behavior and, hence, on the oil displacing activity of the surfactant-brine-oil system becomes more pronounced as the chemical flood proceeds. 

In n-octane/aqueous systems at 27°C, TRS 10-80 has been shown to form a surfactant-rich third phase, or a thin film of liquid crystals (see Figure 1), with a sharp interfacial tension minimum of about 5x10 mN/m at 15 g/L NaCI concentration f131. Similarly, in this study the bitumen/aqueous tension behavior of TRS 10-80 and Sun Tech IV appeared not to be related to monolayer coverage at the interface (as in the case of Enordet C16 18) but rather was indicative of a surfactant-rich third phase between oil and water. The higher values for minimum interfacial tension observed for a heavy oil compared to a pure n-alkane were probably due to natural surfactants in the crude oil which somewhat hindered the formation of the surfactant-rich phase. This hypothesis needs to be tested, but the effect is not unlike that of the addition of SDS (which does not form liquid crystals) in partially solubilizing the third phase formed by TRS 10-80 or Aerosol OT at the alkane/brine interface Til.121. 

A correlation of the detergency performance and the equilibrium phase behavior of such ternary systems is expected, based on the results presented by Miller et al. (3,6). The phase behavior of surfactant - oil - water (brine) systems, particularly with regard to the formation of so-called "middle" or "microemulsion" phases, has been shown by Kahlweit et al. (7,8) to be understandable in teims of the... 

The cell tests consisted of three steps (1) In the first step, the cell was charged with approximately equal volumes of CO2 and an aqueous solution of the test surfactant in reservoir brine. The desired behavior was formation of an emulsion-like dispersion of the C02-rich phase in the aqueous phase. (2) In the second step, a small amount of reservoir oil was added. Desirable surfactants formed three-phase dispersions in which both the C02 rich and oil-rich phases were dispersed in the aqueous phase. (The crude oil was not miscible with CO2.) (3) In the third step of the test, the amount of oil in the cell was increased until it was somewhat larger than the volumes of CO2 and of aqueous phase. Although relatively few surfactants passed this third step, the desired dispersion structure was believed to be droplets of the C02-rich phase dispersed in the continuous oleic phase, with films of aqueous surfactant solution encasing the dispersed droplets (42,43, S. L. Wellington, Shell Development Company, personal communication, November 13, 1987). "Foaminess" tests performed under these conditions correlated with the results of flooding experiments. Both nonionic alkoxylated surfactants and their anionic sulfonated derivatives were tested by these methods (42,43). 

The other application of pore-level mechanisms exploits their dependence on dispersion type, wettability, capillary number, and capillary pressure to design surfactants that will optimize these parameters. Measurements of phase behavior, interfacial tensions, surfactant adsorption, wettability, and related parameters will be needed to fit the various requirements of different reservoirs, each of which has a unique combination of mineralogy, pore structure, temperature, pressure, oil and brine composition, etc. 

At higher EO levels, the foam volume produced by AEGS and AESo surfactants were less adversely affected by the presence of an oil phase than were other surfactants studied (Table I. Figure 1). This behavior was likely due to the formation of an oil/water emulsion which stabilized the fluid films between gas bubbles. Although foam volumes were smaller, at 75°C in three different brines, the sensitivity of AE and AES surfactants to the presence of decane decreased with increasing surfactant ethylene oxide content. 

A particularly interesting part of the pilot involved the treating of produced emulsions. Over the life of the pilot, 93% of the injected surfactant was produced at the production wells, and this situation led to serious emulsion problems. Heating the emulsion to a specific, but unreported, temperature caused the surfactant to partition completely into the aqueous phase and leave the crude oil with very low levels of surfactant and brine. The resulting oil was suitable for pipeline transportation. The critical separation temperature had to be controlled to within 1 0. At higher temperatures, surfactant partitioned into the oil, and at lower temperatures, significant quantities of oil remained solubilized in the brine. Recovered surfactant was equivalent to the injected surfactant in terms of phase behavior, and had the potential for reuse. 

Phase Behavior. The surfactant formulations for enhanced oil recovery consist of surfactant, alcohol and brine with or without added oil. As the alcohol and surfactant are added to equal volumes of oil and brine, the surfactant partitioning between oil and brine phases depends on the relative solubilities of the surfactant in each phase. If most of the surfactant remains in the brine phase, the system becomes two phases, and the aqueous phase consists of micelles or oil-in-water microemulsions depending upon the amount of oil solubilized. If most of the surfactant remains in the oil phase, a two-phase system is formed with reversed micelles or the water-in-oil microemulsion in equilibrium with an aqueous phase. 

When a surfactant-water or surfactant-brine mixture is carefully contacted with oil in the absence of flow, bulk diffusion and, in some cases, adsorption-desorption or phase transformation kinetics dictate the way in which the equilibrium state is approached and the time required to reach it. Nonequilibrium behavior in such systems is of interest in connection with certain enhanced oil recovery processes where surfactant-brine mixtures are injected into underground formations to diplace globules of oil trapped in the porous rock structure. Indications exist that recovery efficiency can be affected by the extent of equilibration between phases and by the type of nonequilibrium phenomena which occur (J ). In detergency also, the rate and manner of oily soil removal by solubilization and "complexing" or "emulsification" mechanisms are controlled by diffusion and phase transformation kinetics (2-2). 

After describing the experimental technique in the next section, we report our observations of intermediate phase formation and spontaneous emulsification in three parts corresponding to three types of equilibrium phase behavior found when equal volumes of oil and the surfactant-alcohol-brine mixtures are equilibrated. The three types are well known (8-9) and, in order of increasing salinity, are a "lower" phase, oil-in-water microemulsion in equilibrium with excess oil, a "surfactant" or "middle" phase, probably of varying structure, in equilibrium with both excess oil and excess brine, and an "upper" phase, water-inoil microemulsion in equilibrium with excess brine. 

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. 

Phase behavior studies of oil-brine-surfactant systems have shown that the ultralow interfacial tension (less than 0.01 dyne/cm) necessary for EOR is very sensitive to salinity changes (2 3). Such low tensions are obtained only within a small range of salinity near the point of "optimum salinity" where equal amounts of oil and brine are solubilized. The tolerance of ultralow tensions to divalent ions is still less. 

Hsieh, W.C. and Shah, D.O., "The Effect of Chain Length of Oil and Alcohol as well as Surfactant Alcohol Ratio on the Solubility, Phase Behavior and Interfacial Tension of Oil/Brine/Surfac-tant/Alcohol Systems," Soc. of Pet. Eng. of AIME Paper No. 6594, 1976. 

Surfactant solution phase behavior is strongly affected by the salinity of the brine. In general, increasing the salinity of the brine decreases the solubility of the anionic surfactant in the brine. The snrfactant is driven out of the brine as the electrolyte concentration increases. Fignre 7.3 shows that as the salinity is increased, the surfactant moves from the aqneons phase to the oleic phase. At a low salinity, the typical snrfactant exhibits good aqueous-phase solubility. The oil phase, then, is essentially free of snrfactant. Some oil is solubilized in the cores of micelles. 

Based on this concept, a crude oil/surfactant/brine system should have phase behavior (e.g., optimum salinity, IFT minima) similar to that of the pure alkane/ surfactant/brine system whose ACN is the same as the crude EACN. However, the concept of EACN is not practically applicable for several reasons. First, all the hydrocarbon compositions of a crade oil are not readily identified. Thus, the EACN of a crude oil cannot be calculated directly using Eq. 7.79. Second, measurement of the EACN of a crude oil requires a series of surfactant solutions to be tested to obtain individual minimum ITT. Then these surfactant solutions are tested against increasing alkane carbon numbers to find minimum IFTs. The ACN at which a surfactant solution also gives the lowest IFT for the crude oil is the EACN of the oil. Finding it is not an easy task. Third, several parameters affect the value of the EACN. Variations in EACN with alcohol cosolvent type, total WOR of the sample, and crude oil composition have been observed (Tham and Lorenz, 1981). In practice, we always select surfactants by scan tests using the actual crude oil for a specific application. 

Hsieh, W.C., Shah, D.O., 1977. The effect of chain length of oil and alcohol as well as surfactant to alcohol ratio on the solubilization, phase behavior and interfacial tension of oil/brine/sur-factant/alcohol systems. Paper SPE 6594 presented at the SPE International Oilfield and Geothermal Chemistry Symposium, San Diego, 27-29 June. 

The general pattern of microemulsion phase behavior described above is seen when the amounts of water and hydrocarbon present are comparable. However, a hydrocarbon-free mixture of surfactant and water (or brine) near optimal conditions is typically not a simple micellar solution but either the lamellar liquid crystalline phase or a dispersion of this phase in water. Starting with such a mixture and adding hydrocarbon, we sometimes find that the system passes through several multiphase regions before reaching the microemulsion/oil/water equilibrium characteristic of optimal conditions. 

Complete information on phase behavior including tie-lines and on diffusion coefficients is rarely available for oil-water-surfactant systems. Nevertheless, Raney and Miller used plausible phase diagrams for an anionic surfactant-NaCl brine-hydrocarbon system as a function of salinity to calculate diffusion paths that exhibited intermediate phase formation and spontaneous emulsification in agreement with experimental observations made using the vertical cell technique. For example. Figure 9.12 shows a diffusion path for a surfactant-alcohol-brine mixture of composition D in contact with oil for a case when initial salinity is high. An intermediate brine phase is predicted as well as spontaneous emulsification in the oil phase, both of which were, in fact, observed. 

In the AOT-brine-propane system, 2-3 transitions occur readily. However, the structure of AOT is such that the 3-2 transition is much more difficult. Furthermore, the phase behavior of AOT is not particularly temperature-sensitive. Nonionic surfactants, on the other hand, are very responsive to temperature and readily form both oil-in-water and water-in-oil phases when the surfactant size and HLB are chosen so as to be compatible with both water and oil. A plot of surfactant concentration vs. temperature has a characteristic fish shape, as seen in the work of Kahlweit et al, [46]. 

There are two bulk interfaces in middle phase microemulsions and one in lower or upper phase microemulsions. Thus, one or three values of interfacial tension (IFT) may be measured depending on system composition (1) ymo between microemulsion and excess oil phase, (2) between microemulsion and excess brine phase, and (3) >Vm between excess oil and brine phases. Phase volumes and consequently the volumes of oil (Vo) and brine () solubilized in the microemulsion depend on the variables that control the phase behavior. The solubilization parameters are defined as Vg/Vs and V JV, where Vs is the volume of the surfactant in the microemulsion phase. These parameters are easily determined from phase volume measurements if all the surfactant is assumed to be in the microemulsion phase. The magnitude of decreases as Vg/Vs increases, i.e., as more oil is solubilized. Similarly, the magnitude of decreases as Vg/Vs increases. The salinity at which the values of ymo and are equal is known as the optimal salinity based on IFT. Similarly, the intersection of Vg/Vs and V. /Vs defines the optimal salinity based on phase behavior. The optimal salinity concept is very important for enhanced oil recovery. 

H Kunieda, K Hanno, S Yamaguchi, K Shinoda. The three-phase behavior of a brine/ionic surfactant/non-ionic surfac-tant/oil system Evaluation of the hydrophile—lipophile balance (HLB) of ionic surfactant. J CoUoid Interface Sci 107 129, 1985. 

JC Noronha, DO Shah. Ultra—low interfacial tension, phase behavior and microstructure in oil/brine/surfactant/alcohol systems. AIChE Symp Ser, No. 212, 78 42—57, 1982. 

RE Anton, JL Salager. Emulsion instability in the three— phase behavior region of surfactant-alcohol-oil-brine systems. J Colloid Interface Sci 111 54—59, 1986. 

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