Middle Phase Microemulsion Formation Research Articles

Dilute surfactants for wettability alteration and enhanced oil recovery in carbonates

The effectiveness of dilute nonionic surfactant was evaluated in this study for wettability alteration and enhanced oil recovery (EOR) in carbonates by performing a suite of measurements such as aqueous stability, surface tension, phase behavior, interfacial tension (IFT), zeta potential, contact angle, and microscopic dynamic oil liberation. Two different brines, low salinity and high salinity, were used at 0.1 wt% surfactant concentration. These measurements were conducted at elevated temperature using the rock and crude oil samples from a carbonate reservoir. The dilute concentration of polymer has also been used together with surfactant in both high salinity and low salinity brines to quantify the effects of wettability alteration on microscopic oil liberation efficiency in carbonates.The compatibility of nonionic surfactant was demonstrated in high salinity brine at reservoir temperature. The critical micelle concentrations were found to be very low in both low salinity and high salinity brines. The phase behavior results showed the formation of middle phase microemulsion and correspondingly low interfacial tensions in high salinity brine. The contact angle data indicated the ability of nonionic surfactant to significantly alter the wettability of carbonate from oil-wet to either intermediate wet or less oil-wet in high salinity brine whereas only marginal wettability alterations from oil-wet to less oil-wet were obtained in low salinity brine. The increasing negative zeta potentials and the alteration of charge polarity from positive to negative were observed at crude oil-brine and calcite/brine interfaces, respectively due to surfactant in the high salinity brine. The microscopic equilibrium degree of crude oil liberation from carbonate surface was found to be about 20% higher with high salinity surfactant-polymer solution when compared to the low salinity surfactant-polymer solution. Such higher oil release efficiency obtained with high salinity surfactant-polymer solution can be attributed to the beneficial effects of surfactant induced wettability alteration and IFT reduction mechanisms in the high salinity brine to favorably alter both capillary and adhesion forces for efficient EOR in carbonates.

Phase-behavior-based surfactant–contaminant separation of middle phase microemulsions

This research studied separation of contaminants solubilized in middle phase microemulsions by shifting the contaminant-rich microemulsions from Winsor type III to type I systems. Precipitation-based exchange of polyvalent cations (Al3+ and Ca2+) with equivalent amounts of a monovalent cation (Na+) broke the middle phase microemulsions. Most of the contaminants solubilized in the middle phase microemulsion separated as free phase contaminant due to the much lower solubilization capacity of the resulting type I system. The phase transitions between Winsor type III and type I systems were reversible with the precipitation and re-dissolution of the polyvalent cations, allowing subsequent reuse of the surfactant solution. The phase transitions brought about by deprotonation/protonation of oleic acid, an organic additive used to promote formation of middle phase microemulsions, were also studied. The neutral form of oleic acid was more effective than its dissociated anion for promoting the formation of middle phase microemulsions, but oleic acid in the middle phase microemulsions could not be deprotonated by adding aqueous OH−. On the other hand, the oleate anion could be protonated even when it was in middle phase microemulsions because of its relative distribution in the systems. While exchange of surfactant counterions could separate contaminant solubilized in middle phase microemulsions, transformation of oleic acid in middle phase microemulsion systems did not produce the desired result.

Solubilization and Microemulsification of Chlorinated Solvents Using Direct Food Additive (Edible) Surfactants

AbstractSurfactant enhanced subsurface remediation is being evaluated as an innovative technology to expedite contaminant extraction from the subsurface. Regulatory approval of this technology will likely be enhanced by use of surfactants with FDA direct food additive status (“edible” surfactants). This research establishes edible surfactant systems capable of solubilizing (via micellar partitioning) and microemulsifying (via middle phase microemulsions) chlorinated solvents (PCE, TCE, and trans 1,2‐DCE). Micellar partition coefficients with edible surfactants are observed to be comparable to values previously reported for other surfactants, with solubilization increasing aqueous concentrations by one to two orders of magnitude for the chlorinated organics. Middle phase microemulsion formation is dependent on surfactant structure and cosurfactant concentration. Solubility enhancement in the middle phase systems (microemulsification) is at least one to two orders of magnitude higher than solubilization for the same surfactant concentration, but is much more sensitive to the surfactant system and the contaminant. In addition, successful microemulsion formation is seen to be a function of ground‐water temperature and hardness, indicating the need to consider these and additional factors for successful design and implementation of surfactant enhanced subsurface remediation. This research thus establishes a variety of edible surfactant systems that can significantly expedite subsurface remediation of chlorinated solvents, and illustrates the importance of proper selection and design of surfactant systems.

Microemulsion formation with nonionic surfactants. Effect of temperature on middle phase microemulsion formation in nonionic surfactant/alkane/water systems.

The effect of temperature on microemulsion formation was studied in C12H25O(CH2CH2O)4H/alkanes/water systems. A three-phase region composed of water, alkane, and microemulsion phases appeared with rise in temperature. The temperature, at which a middle phase microemulsion forms, rose as the alkane carbon number was increased. The diameter of the dispersoid particles in the middle phase microemulsion changed as the temperature was raised. At the temperature which gave minimum diameter of the droplets, moisture content registered 50% and the density intermediate between those of water and oil. It was found that the diameter of the dispersoid particles in the middle phase microemulsion is minimized at the temperature at which hydrophilic-lipophilic properties of the system reach perfect eqilibrium.

Formation of middle-phase microemulsions in multicomponent systems containing petrostep-465 and ethoxylated tall oils

The range of salinities over which certain Petrostep-465/ethoxylated tall oil/cosurfactant/hydrocarbon/brine systems form middle-phase microemulsions was determined. The presence of the nonionic surfactant, in cases where n-decane is the hydrocarbon and 2-butanol is the cosurfactant, increases the optimal salinity of the system. Some broadening of the salinity range over which low interfacial tensions are observed is also found. Phase behavior for systems containing P-465 as the only surfactant shows order of mixing effects.

Formation of middle-phase microemulsions using surfactants derived from a renewable resource: Ethoxylated tall oils

The range of salinities over which a large number of multicomponent systems containing ethoxylated tall oils or tall-oil derivatives form middle-phase microemulsions was determined. n-Octane and n-decane were used as the hydrocarbons; 2 methyl-l-propanol and 2-butanol as the cosurfactants. It is possible, with an appropriate choice of surfactant, to produce systems with optimal salinities from 0 to ca. 17 wt% NaCl. Phase volume diagrams are presented for 24 of the systems investigated; interfacial tension measurements for selected systems show tensions in the millidyn/cm range at the optimal salinities.

The effect of counterions on coacervation and solubilization in oil-external and middle-phase microemulsions

The effect of hydrated radii, valency, and concentration of counterions on the coacervation of aqueous petroleum sulfonate solutions and on the solubilization capacity of oil-external and middle-phase microemulsions was investigated. The critical electrolyte concentration (CEC) for coacervation increased with Stokes' hydrated radii of monovalent counterions. The CEC for CaCl 2 was much lower than that predicted by either the Stokes' hydrated radii or the ionic strength. For mixed electrolytes containing NaCl and CaCl 2, it was concluded from the shift in CEC that 1 mole of CaCl 2 is equivalent to 16 to 19 moles of NaCl. The changes in relative concentrations of NaCl and CaCl 2 for coacervation exhibited additive behavior. The maximum solubilization of brine in oil-external microemulsions occurred at a specific salt concentration. For mixed electrolytes containing NaCl and CaCl 2, the shift in electrolyte concentration for maximum solubilization showed that 1 mole of CaCl 2 is equivalent to about 4 moles of NaCl. These results suggest that the equivalence ratio of CaCl 2 to NaCl is strikingly different in aqueous solutions and oil-external microemulsions. For solubilization in middle-phase microemulsion containing mixed NaCl and CaCl 2, it was concluded from the shift in optimal salinity that 1 mole of CaCl 2 is equivalent to about 16 moles of NaCl. Here also the changes in NaCI and CaC1 2 concentrations showed additive behavior. The equivalence ratio of CaCl 2 and NaCl appears to be independent of oil chain length in the present study. As shown by the equivalence ratio of CaCl 2 to NaCl, the formation of middlephase microemulsions appears to be similar to coacervation of aqueous surfactant solutions and quite different from the solubilization of water in oil-external microemulsions.

The effect of H 2O and D 2O on colloidal properties of surfactant solutions and microemulsions

The effects of hydration on solubilization and colloidal phenomena have been studied by comparing the differences between two oil-brine-surfactant-alcohol systems containing H 2O or D 2O. The H 2O system was significantly more hydrophilic than the D 2O system in relation to coacervation of aqueous surfactant solutions, formation of middle phase microemulsions, surfactant partitioning in oil and brine, and solubilization capacity of brine in oil-external microemulsions. The two oil-brine-surfactant-alcohol systems appeared to be macroscopically similar if the salinity of the D 2O system was decreased by about 0.3% NaCl. Electrical resistance and dielectric relaxation measurements showed that the surface charge density of H 2O-in-oil microemulsions was three times higher than that of the corresponding D 2O-in-oil microemulsions. The higher surface charge density of the H 2O system caused its higher interfacial alcohol concentration, which was confirmed by alcohol titration measurements. It is interesting to note that the interfacial alcohol concentration of the two systems can have similar values if the salinity of D 2O is decreased by about 0.3% NaCl. The higher surface charge density of the H 2O system can be explained by the lower (more negative) free energy of hydration of surfactant ions and counterions in H 2O than in D 2O, which leads to a higher degree of dissociation of the surfactant polar groups in the H 2O system. An expression is given to estimate the free energy of transfer (from H 2O to D 2O) of interfacial surfactant anions from surface charge density measurements.