The use of surfactant ionic liquids has recently gained attention for surfactant-based enhanced oil recovery.The introduction of the surfactant character in ionic liquids combines interesting properties of both types of chemicals, which are relevant for this application. An imidazolium-based surfactant has been synthesized combined with an acetate counter-ion. The aggregation effects in water were evaluated by means of surface tension and electrical conductivity. The dynamic interfacial tensions between the aqueous solutions of the surfactant ionic liquid and crude oil (Saharan blend) were evaluated by the spinning drop method. The effect of different variables has been analyzed, namely the concentration of surfactant ionic liquid, electrolytes (NaCl) and alkalis (NaOH, Na 2 CO 3 ), and temperature. Formulations of ionic liquids and alkalis were tested for the first time for enhanced oil recovery. The results obtained here improve significantly (at least one order of magnitude, most often two) previous results obtained up to now solely with ionic liquids.

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Chemical flooding with surfactants for reducing oil-brine interfacial tensions (IFTs) to mobilize residual oil trapped by capillary forces has a great potential for Enhanced Oil Recovery (EOR). Surface-active ionic liquids (SAILs) constitute a class of surfactants that has recently been proposed for this application. For the first time, SAILs or their blends with an anionic surfactant are studied by determining equilibrium phase behavior for systems of about unit water-oil ratio at various temperatures. The test fluids were model alkane and aromatic oils, NaCl brine, and synthetic hard seawater (SW). Patterns of microemulsions observed are those of classical phase behavior (Winsor I-III-II transition) known to correlate with low IFTs. The two anionic room-temperature SAILs tested were made from common anionic surfactants by substituting imidazolium or phosphonium cations for sodium. These two anionic and two cationic SAILs were found to have little potential for EOR when tested individually. Thus, also tested were blends of an anionic internal olefin sulfonate (IOS) surfactant with one of the anionic SAILs and both cationic SAILs. Most promising for EOR was the anionic/cationic surfactant blend of IOS with [Cmim]Br in SW. A low equilibrium IFT of ∼2·10mN/m was measured between n-octane and an aqueous solution having the optimal blend ratio for this system at 25°C.

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Ionic liquids derived from prolinium esters, previously described as fully green and stable, were found to decompose in the presence of water by ester hydrolysis. To avoid this problem, a new family of these biodegradable salts incorporating an alcohol instead of the ester group is proposed. From this family, two novel ionic liquids that incorporate the prolinolium cation [HOPro] and the [DS] or [DBS] anion were selected (DS=dodecylsulfate; DBS=dodecylbenzenesulfonate). Both salts are liquid at room temperature, a property not usually found in ionic surfactants, and are also chemically and thermally stable. Moreover, they are more effective in reducing the surface tension of water than the corresponding traditional surfactants in the form of sodium salts, being useful for applications related to their aggregation capacity. They were tested for surfactant enhanced oil recovery and an optimal formulation for reservoirs at high salinity and temperature, able to produce ultra-low interfacial tension, was found with [HOPro][DBS].

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The promising properties of surface-active ionic liquids (SAILs) make these salts interesting candidates for the optimization of surfactant-enhanced oil recovery (EOR) methods. The tests that should be performed at the laboratory scale before a SAIL is proposed for EOR were carried out with tributyl­(tetradecyl)­phosphonium chloride ([P4 4 4 14]­Cl). The phase diagrams with water and n-dodecane showed that the affinity of the surfactant for water is greater than that for oil, even in the presence of a high salt content. The advantage of the use of Winsor type I microemulsions in EOR is the low phase trapping/adsorption. A formulation consisting of 4000 ppm [P4 4 4 14]­Cl, 4 wt % NaCl, and 5000 ppm NaOH was able to reduce the interfacial tension between water and Saharan crude oil from 19.2 to 0.1 mN·m–1. Core-flooding experiments were carried out at room temperature and an injection rate of 2 mL/min, mimicking enhanced oil recovery with brine solutions of SAIL, NaOH, and the optimized formulation combining the two chemicals. The injection of the proposed formulation, after flooding with brine, led to an additional recovery of about 8% of the original oil in place.

The difficulty of achieving a good thermodynamic description of phase equilibria is finding a model that can be extended to a large variety of chemical families and conditions. This problem gets worse in the case of systems containing more than two phases or involving complex compounds such as ionic liquids. However, there are interesting applications that involve multiphasic systems, and the promising features of ionic liquids suggest that they will play an important role in many future processes. In this work, for the first time, the simultaneous correlation of liquid-liquid and liquidliquid-liquid equilibrium data for ternary systems involving ionic liquids has been carried out. To that end, the phase diagram for the system water + [P 6 6 6 14 ][DCA] + hexane has been determined at 298.15 K and 323.15 K and atmospheric pressure. The importance of this system lies in the possibility of using the surface active ionic liquid to improve surfactant enhanced oil recovery methods. With those and previous measurements, thirteen sets of equilibrium data of ternary systems water + ionic liquid + oil have been correlated. The isoactivity equilibrium condition, using the NRTL model, and some pivotal strategies are proposed to correlate these complex systems.Good agreement has been found between experimental and calculated data in all the regions (one triphasic and two biphasic) of the diagrams. The geometric aspects related to the Gibbs energy of mixing function obtained with the model, together with the minor common tangent plane equilibrium condition, are valuable tools to check the consistency of the obtained correlation results.