Controllable emulsion phase behaviour via the selective host-guest recognition of mixed surfactants at the water/octane interface
Graphical abstract
Introduction
It is estimated that at least half of the crude oil remains trapped in the porous medium of reservoirs after the primary and secondary operations (Amjad et al., 2010, Lake, 2014). Several typical methods, mainly including chemical method, gas drive, thermal recovery and microbial oil recovery, are well accepted for their superior efficiency in enhanced oil recovery (EOR) (Cao et al., 2018, Wu et al., 2017, Wu et al., 2018, Xie et al., 2016, Xie et al., 2018). As a significant approach in chemical methods, surfactant flooding has been investigated at the academic and industrial level (Iglauer et al., 2010, Wei et al., 2019). The surfactants can largely reduce the water/crude oil interfacial tension (IFT) to ultralow values (<10−2 mN/m) and facilitate the generation of emulsions (or microemulsions), thereby leading to the mobilization of trapped oil (Chen et al., 2004, Fletcher et al., 2015).
Many research groups have been paying more attention to novel high-performance surfactants, such as zwitterionic surfactants, gemini surfactants and biosurfactants (Bera et al., 2014a, Gao and Sharma, 2013, Jia et al., 2017a, Jia et al., 2018a, Martinez-Magadan et al., 2018, Seokju et al., 2018, Wang et al., 2015). Bera et al. investigated the IFT between different ethoxylated secondary alcoholic surfactant solutions with NaCl and synthetic oil, which was desirable for EOR (Bera et al., 2011). Recently, mixed surfactant systems with extraordinary properties have shown great potential applications in many fields (Feng et al., 2018a, Li et al., 2009, Nan et al., 2015, Tian et al., 2013). Our group reported that the mixed surfactant system comprising the cationic surface-active ionic liquid N-dodecyl-N-methylpyrrolidinium bromide (L12), and the anionic surfactant sodium dodecyl sulfate (SDS) could decrease the IFT to an ultralow value at a low surfactant concentration and largely improve crude oil recovery (Jia et al., 2017b).
Emulsification is considered an important mechanism in surfactant flooding (Bera and Mandal, 2015, Hirasaki et al., 2011, Louiza et al., 2018, Kumar et al., 2017, Pal et al., 2018, Salager et al., 2005, Salager et al., 2013, Sun et al., 2011). Oil-in-water (O/W) emulsions may promote volumetric sweep of the reservoir by plugging high permeability channels (Bryan et al., 2008, Dong et al., 2009, Yu et al., 2018). The formation of water-in-oil (W/O) emulsions with high viscosity can improve the sweep efficiency and mobility ratio of the injected fluid (Feng et al., 2018b, Liu et al., 2016). In addition, microemulsions attract considerable attention in EOR due to their extraordinary efficiency in the extraction of trapped oil (Bera et al., 2012, Bera et al., 2014b, Bera et al., 2014c, Kumar and Mandal, 2018a).
Emulsion phase inversion is a process that interconverts the dispersed and continuous phases of an emulsion. This process is dramatically affected by the salinity, the temperature, the nature of the oil, the types of emulsifiers, and the water-oil ratio (Bouchama et al., 2003, Dai et al., 2018, Kumar et al., 2015, Perazzo et al., 2015, Salager et al., 2004). In 1968, Shinoda et al. found temperature-related phase-behaviour issues during phase inversion (Shinoda and Saito, 1968). Thereafter, a large number of investigations focused on phase inversion. Woodward et al. reported that subtle structure changes in a copolymer surfactant can lead to diverse behaviours in the pH-responsive emulsions (Woodward et al., 2009). Lv et al. explained the mechanism of the emulsion phase inversion by adding hydrophilic surfactant to the hydrophobic surfactant-stabilized emulsions (Lv et al., 2014).
In our previous report, the additional β-cyclodextrin (β-CD) molecules could selectively enclose the surfactant molecules in mixed L12/SDS surfactants via host-guest interaction to effectively modify the interfacial properties of the mixed surfactants, thereby leading to a great reduction in the water/oil IFT (Jia et al., 2018b). The emulsifying ability is another significant criterion for evaluation of the interfacial activity of the surfactant system. Herein, we evaluated the effects of additional β-CD molecules on the emulsifying ability of mixed L12/SDS surfactants. The addition of β-CD molecules remarkably affected the emulsion stability and even the emulsion type. The β-CD concentrations, surfactant concentrations and L12/SDS molar ratios were changed to investigate the mechanism. Moreover, rheological properties of the emulsions were also measured.
Section snippets
Chemicals
N-Dodecyl-N-methylpyrrolidinium bromide (L12) was synthesized according to the previous literature (Jia et al., 2017b). The product purity was examined by 1H NMR spectroscopy with a Bruker Avance 300 spectrometer (Germany Bruker Company). Sodium dodecyl sulfate (SDS, >99 wt%), β-cyclodextrin (β-CD > 99 wt%) and n-octane (>99 wt%) were purchased from Aladdin Chemical Reagent Company. All the chemicals of analytical grade were used as received. Deionized water was used in all the experiments.
Measurements of electric conductivities
Results and discussion
In our previous reports, the effects of molar ratios, salinity, temperature and additional β-CD on the IFT for the mixed L12/SDS surfactant system (≤500 mg/L) were systematically investigated (Jia et al., 2017b, Jia et al., 2018b). It was found that the L12/SDS system with an optimal molar ratio (R) of 1:2.5 showed extraordinary interfacial activity, which could dramatically reduce the IFT to 4 × 10−3 mN/m. In the surfactant flooding process, the surfactant concentration should be far above the
Conclusions
In conclusion, the effects of the host-guest recognition on the interfacial activity of the mixed surfactants, particularly on the emulsion behaviour, were systematically investigated. The moderate additional β-CD could selectively enclose the extra SDS molecules via host-guest recognition to promote the intensive arrangements of mixed L12/SDS surfactants at the water/n-octane interface, thereby leading to further reduction of the IFT. Meanwhile, the lipophilicity of the mixed adsorption layers
Acknowledgements
The authors are grateful for funding from the Key Project of the National Natural Science Foundation of China (No. U1762106), the National Natural Science Foundation of China (No. 21403301) and the National Training Program of Innovation and Entrepreneurship for Undergraduates (20181032 and 20181034).
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