Phase equilibria of 1-hexyl-3-methylimidazolium acetate with water and oil
Introduction
In oil recovery, at the end of water flooding, the residual oil is in the form of immobile globules distributed through the pores of the rocks. An ultra-low interfacial tension between crude oil and aqueous phases is required to allow the oil mobilisation and enhance oil recovery. This can be achieved in the presence of a suitable surface active agent. Surfactant Enhanced Oil Recovery (EOR) is based on a multicomponent multiphase system, generated by the injection of an optimal formulation, whose displacement is complicated by factors of phase behaviour, dispersion, and adsorption.
Surfactant solution phase behaviour is strongly affected by the presence of salt in the reservoir. At a low salinity, the surfactant is soluble in the aqueous phase and some oil is solubilised in the core of the micelles. A lower-phase microemulsion is formed in equilibrium with an excess oil phase essentially free of surfactant. At high salinity, the system separates in an oily upper-phase microemulsion in equilibrium with an excess water phase. At some intermediate salinity, the system forms three phases; an excess oil phase, a middle-phase microemulsion where the surfactant solubilises equivalent proportion of water and oil, and an excess water phase [1]. This salinity is considered optimal because it produces the lowest interfacial tension.
Many surfactants that show ultra-low interfacial tension and excellent microemulsion phase behaviour with crude oils are not sufficiently soluble at optimal salinity to give clear, stable, aqueous solutions [2]. Poor surfactant solubility may result in very high surfactant retention in the well. The use of hydrophilic co-solvents or co-surfactants helps to dissolve the surfactant in the brine and to reduce surfactant retention. Moreover, the use of these compounds could be a key factor for promoting good phase behaviour during a surfactant flooding operation.
One of the main advantages of Ionic liquids (ILs) is that they can be tailored to have a specific property or to be used in a specific application [3]. With this in mind, some researchers have proposed the use of surface active ILs as surfactants for EOR [[4], [5], [6], [7]]. Bera and Belhaj [4] present a state of the art-review of the use of ILs as alternatives to surfactants in EOR. Several ILs have been found active in reducing interfacial tension as well as changing the reservoir rock surface wettability, however up to now an ultra-low interfacial tension (∼10−3 mN/m) has not been achieved using a single IL. Recently, some works propose the use of blends of ILs and traditional surfactants [8,9]. Rodríguez-Escontrela et al. [8] propose the use of a blend IOS15-18/1-dodecyl-3-methylimidazolium bromide (mass ratio = 8/2). A low equilibrium interfacial tension of ∼2·10−3 mN/m was obtained between n-octane and seawater. Jia et al. [9] propose the blend n-dodecyl-n-methylpyrrolidinium bromide/SDS (molar ratio of 1:2.5) obtaining an interfacial tension of ∼4 × 10−3 mN/m between model oil (n-decane or toluene) and water.
The work on ILs as co-surfactants in combination with traditional surfactants for EOR applications has not practically started. Only one recent work was found in literature [10] where these salts are used with traditional surfactants to decrease surfactant adsorption onto crushed core samples. The use of these salts as co-surfactants in EOR formulations could improve the solubility of the surfactant in the injectable aqueous solution, avoid surfactant retention and increase oil recovery. They also could allow slugs to be tailored for high salinity and temperature. Moreover, as ILs are salts, they could be used to modify the optimal salinity of a surfactant leading to a change in phase behaviour. For these reasons, this research should be encouraged.
As a first step in the analysis of the possibilities of using the ionic liquid 1-hexyl-3-methylimidazolium acetate ([C6mim][OAc]) as co-surfactant in EOR, in this work, the phase behaviour of the aforementioned IL with water and different model oils (n-octane, cyclohexane and toluene) is determined at 298.15 K and 323.15 K. Data are correlated for each ternary system and all the data sets with the Non-Random Two Liquid activity coefficient model [11]. The consistency of the binary coefficients obtained is tested according to the method of Marcilla et al. [12].
Section snippets
Chemicals
Information about all the chemicals used in this work can be found in Table 1. Water was purified via double distillation. Several organic compounds were used to mimic different oils. n-Octane and toluene with nominal purities >99.9 wt% and >99.5 wt%, respectively, were purchased from Sigma-Aldrich. Cyclohexane with a nominal purity >99.5 wt% was purchased from Riedel-de Haën. All these chemicals were used as received, with the only precaution of having molecular sieves into the bottles for
Results
In a previous work [13], the possibility of using the ionic liquid [C12mim][OAc] as surfactant for EOR was analysed. It was found that this surfactant could drastically reduce the interfacial tension water/oil, however the values obtained for this property were found far from the ultra-low values required in EOR. Thus, the idea of using these ionic liquids as co-surfactants rather than surfactants came up. Blesic et al. [14] showed that [Cnmim]Cl ionic liquids with n > 8 form micellar
Data correlation
Mole fractions are correlated using the NRTL [11] activity coefficient model. The value of the non-randomness parameter α is pre-fixed at 0.1, 0.2 and 0.3, and the value that achieves the minimum deviations from experimental data is selected.
Following the method of Sϕrensen and Arlt [17], two objective functions are used in the correlation. First, Fa, which does not require any previous guess for parameters, and after convergence these parameters are used in the second function, Fb, to minimise
Conclusions
The ionic liquid [C6mim][OAc] does not possess surfactant character. As a preliminary study on the possibility of using this salt as co-surfactant in surfactant or micellar flooding EOR processes, the behaviour of this ionic liquid with water and different kinds of oils was studied.
The water + [C6mim][OAc] pair is completely miscible which is an important advantage focusing on the preparation of optimal formulations for EOR. The ionic liquid could be the base to prepare clear aqueous solutions
Acknowledgements
The authors acknowledge the Ministry of Economy and Competitiveness (Spain) for financial support throughout project CTQ2015-68496-P (including European Regional Development Fund advanced funding).
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