Investigating the effect of [C8Py][Cl] and [C18Py][Cl] ionic liquids on the water/oil interfacial tension by considering Taguchi method

Capillary and interfacial forces are of great influences of trapping hydrocarbon in porous media after primary and secondary recovery processes. The trapped crude oil in the reservoir can be mobilized and produced by reducing these forces. Thus, surfactant flooding, as a main enhanced oil recovery (EOR) method, is usually applied to reduce the interfacial tension (IFT) of crude oil–water system in porous medium and improves the oil recovery. This study focused on the effect of [C8Py][Cl] and [C18Py][Cl] ionic liquids (ILs), as a new family of surfactant, in combination with various salts including sodium chloride, potassium chloride, magnesium sulfate and potassium sulfate on IFT reduction. EOR injection solutions were prepared from mixing the ILs at different concentrations of 100, 250, 500 and 1000 ppm with the salts ranging from 500 to 80,000 ppm. Obtained results showed that the minimum IFT value from both ILs was achieved when the concentration of the ILs was about 1000 g/mL, and the concentrations of KCl, K2SO4, MgSO4 and NaCl were 1000, 2000, 500 and 80,000 ppm, respectively. The minimum IFTs were achieved when NaCl and ILs concentrations were the maximum and MgSO4 concentration was the minimum.


Introduction
Oil production from the matured and currently producible reservoirs is continuously declining. According to the global energy reports, the demand on oil is increasing. Discovering new oil reservoirs is difficult, and high amount of the oil is remained trapped in reservoir after the primary and secondary recovery techniques (Ali et al. 2019;Abtahi et al. 2003). Thus, in order to keep the oil production stable and increased, researchers and oil companies are focused on applications of EOR methods. Among various EOR methods, the surfactant flooding has a high potentiality in displacing the residual oil toward the production wells (Qi et al. 2014). Capillary forces have a significant role in causing oil to be trapped in the reservoir (Ahmadi et al. 2014;Ali et al. 2018a), which is mostly happened in carbonate oilwet reservoir (Ahmadi and Shadizadeh 2013;Gupta et al. 2009;Jarrahian et al. 2012;Lu et al. 2014;Pordel et al. 2012;Seethepalli et al. 2004;Bortolotti et al. 2010). Since waterflooding has shown the low performance in this kind of reservoirs, IFT reduction and wettability alteration are the main EOR mechanisms of increasing the oil recovery (Golabi et al. 2009;Ali et al. 2018b).
Surfactant flooding is basically applied to achieve the above-mentioned mechanisms, and more precisely the IFT reduction (Hezave et al. 2013a). The IFT reduction can be obtained when the Marangoni effect and mass transfer along an interface between two liquids occur through diffusion and/or convection mechanisms (Hezave et al. 2014). The IFT between water and oil can be modified using ionic liquids by increasing the length of the hydrophobic chain. Wherein, more adhesion to two different phases and more separate distance between hydrophilic and hydrophobic heads can be obtained with the longer the length of the hydrophobic chain; consequently, the trapped oil would be forced to be moved and produced (Hezave et al. 2013b). On the other hand, changing the ionic compound of the injection water can lead the oil-water IFT to another direction, such as KI, K 2 SO 4 , MgSO 4 , Na 2 SO 4 , NaCl, CaCl 2 , MgCl and KCl salts (Nowrouzi et al. 2018(Nowrouzi et al. , 2019aManshad et al. 2017). For instance, Manshad et al. reported that little dissolution of 2000 ppm of K 2 SO 4 in water can reduce IFT of water-oil system up to 57% (Manshad et al. 2016), andLashkarbolooki et al. (2014) indicated that MgCl 4 concentration with 30,000 ppm concentration can reduce the IFT by 82%.
Surfactant flooding was first introduced by Howard in 1927 to produce oil from the oil bearing sands (Howard 1927). Afterward, numerous research studies have been performed on the surfactant flooding to enhance oil recovery as shown in Table 1. During last decades, several types of conventional surfactants have studied and used in enhanced oil recovery; wherein, high amount of surface was usually lost on the surface of reservoir rocks due to its harsh condition of salinity and temperature (Hezave et al. 2013a, b). Dehghan et al. (2015) conducted an experimental study on the application of some surfactant in different conditions of reservoir, types of rocks, reservoir fluids and salinity ranges. They reported that the effect of the used surfactants on IFT reduction was interfered by the influences of the mentioned factors, and optimized when with the optimal salinity level (Daghlian et al. 2016). Rodriguez (1992) investigated the amount of surfactant adsorption on the surface of the carbonate rock, and it was identified that the surfactant adsorption was increased with increasing the flowrate at constant surfactant concentration and temperature. Currently, ionic surfactants, as green agents, are widely used to reduce the oil-aqueous phase IFTs in depleted reservoirs because of their significant characteristics including high ionic conductivity, non-flammability, high chemical and physical stability and negligible vapor pressure (Hezave et al. 2013a, b). Hezave et al. (2013a, b)

Materials
All materials for synthesizing the ILs and salts used in this study including sodium chloride (NaCl), potassium chloride (KCl), magnesium sulfate (MgSO 4 ) and potassium sulfate (K 2 SO 4 ) were purchased from MP Biomedical Company in Netherlands. The properties of these salts are given in Table 2. Dead crude oil extracted from Sarvestan oilfield located in central Iran with API of (29°) and the specifications contained in Table 3 were used.

Synthesis of ionic liquids
On the basis of the synthesis procedure ILs reported by Wasserscheid and Welton (2008), the pyridinium-based

ILs used in this study, namely [C8Py][Cl] and [C18Py]
[Cl], were synthesized by reacting 1-methylimidazolium or pyridine with excess amount of the 1-chlorododecane or 1-chlorooctane without any additional solvent in a roundbottomed flask fitted with a reflux condenser (heating and stirring at 70 °C for 48-72 h). The resulting viscous liquid was cooled down to room temperature and then washed up using diethyl ether. Finally, the clean solvent was dried overnight at 100 °C, and it is subjected to a 1 H-NMR spectroscopy analysis to investigate its purity.

Taguchi method
In order to minimize the number of the experimental tests, and save the time and the cost, the experimental work of this study was designed using Taguchi method which is widely used in planning the laboratory experiments. This method was introduced by Taguchi in the late 1940s in Japan and has been developed in the early 1960s (Wasserscheid and Welton 2008). In order to obtain the impact of the studied parameters accurately, the effective parameters must be selected precisely. The salts and ILs used in this study were selected as the main effective parameters that can influence the amount of reduction in IFT of the oil-aqueous phases. The concentration of the selected parameters was divided into four levels as shown  1 3 in Table 4. It should be noted that both ILs were used separately with the salts, and their distribution through the levels must be considered separately.

Interfacial tension measurement
Interfacial tension apparatus designed by Fars EOR (IFT400) was to measure the value of IFT in this study (Fig. 1). After hanging the droplet of the crude oil within the aqueous phase, the device takes a photograph for it by employing the axisymmetric drop shape analysis (ADSA) technique. IFT can be determined using the following equation: where Δρ is the difference in the density of the droplet and bulk fluid (g/cm 3 ), g is the gravitational acceleration of the earth (cm/sec 2 ), D is the large diameter of the droplet (1) = Δ gD 2 H (cm), and H is the droplet shape factor which is a function of S = d/D, and d the horizontal diameter calculated at a distance D above the droplet. The experimental design of the IFT test was mainly composed of two syringes connected to rotary knobs, and a cell with three thermal elements and two glass valves (one against the adjustable lighting bar and the other against 2-megapixel CCD HD camera). The device is equipped with a marker, temperature and pressure controller. IFT400 has a capability of supplying a pressure tolerance of up to 3000 psi with a precision of 0.5% full scale, a maximum allowed temperature of up to 453.15 K with a precision of ± 0.3, and measuring the IFT with the accuracy of ± 0.2-0.5 mN/m. In this study, the IFT 400 was used to measure the value of IFT between crude oil and aqueous phases including ILs and salts at ambient temperature and pressure during various time steps.

Equilibrium IFT measurement
As mentioned earlier, the Taguchi method was used to design the experimental plan of this study; accordingly, the ILs and salts used in this work were divided into four different levels of concentrations (Table 4). In each level, the concentration of salts is varied. In order to study the effect of ILs more precisely at different concentrations of 100, 250, 500 and 1000 ppm with all salts, the experimental plan was expanded to 16 tests as shown in Table 5. Both ILs were used with four different concentrations (100, 250, 500 and and NaCl at different concentrations. Figure 2a illustrates the values of IFTs between oil and aqueous phases with the presence of [C 8 Py] [Cl] and four salts at different experimental levels. The highest value of IFT about 23 mN/m was measured in test 1 (T1) between crude oil and the solution contains 100 ppm of IL, 1000 ppm of KCl, 0 ppm of K 2 SO 4 , 500 ppm of MgSO 4 and 2000 ppm of NaCl. From keeping the concentration of [C 8 Py][Cl] at 100 ppm and changing the level of salt concentrations from T1 to T4, the IFT reduction was increased with changing the Taguchi levels from 1 to 4 and 14.93 mN/m was obtained in T4. This reduction in the IFT was due to the effect of salt concentrations, especially for KCl and NaCl. However, when increasing the IL concentration to 250 ppm and keeping it constant, the IFT was firstly reduced to 13.16 mN/m in T5 with the level 1 and then increased to above 20 mN/m with changing the Taguchi levels from T5 to T8. This change in IFT value was mostly due to the effect of NaCl concentration because it was reduced from 80,000 to 40,000, 20,000 and 2000 ppm during tests T5-T8. Furthermore, when concentration of [C 8 Py][Cl] IL was increased to 500 (T9 to TT12) and 1000 ppm (T13 to T16), various values of IFTs were measured depending on the amount of salts mixed in each test (Manshad et al. 2017). For the tests, when 500 ppm of IL was used, the minimum IFT was achieved within T11 which was 12.11 mN/m. This is where the concentrations of KCl and NaCl were the maximum 10,000 and 80,000 ppm, respectively, and K 2 SO 4 was 1000 ppm. Additionally, the lower IFTs were measured when the concentration of IL was increased to 1000 ppm, and the lowest IFT of 11.03 mN/m was determined within T14; wherein, the concentration of NaCl was 80,000 ppm, that of KCl was 2000 ppm, K 2 SO 4 was 1000 ppm, and MgSO 4 was 500 ppm. Hence, NaCl was identified to be the most effective salt with [C 8 Py][Cl] IL, and the minimum IFT was achieved between oil and ionic solution of 1000 ppm [C 8 Py][Cl] IL and 80,000 ppm NaCl.
In addition, Fig. 2b Test   T1   T2   T3   T4   T5   T6   T7   T8   T9   T10   T11   T12   T13   T14   T15   T16   ILs   100  1 3 [C 18 Py][Cl]. In Fig. 2b, it is clear that the minimum value of IFT between crude oil and aqueous phase was achieved in test T14, which was 1.76 mN/m. In comparison with the highest value measured in T1, IFT was reduced by 70% from 5.89 to 1.76 mN/m. This significant reduction was mostly due to the high concentration of NaCl which was 80,000 ppm.

Dynamic IFT measurement
The dynamic data of the IFT measurements during time for both ILs are shown in Fig. 3a , in contrast to conventional surfactants, are compatible with salinity and are useful additives for reservoir conditions. • The minimum IFTs were achieved when NaCl and ILs concentrations were the maximum and MgSO 4 concentration was the minimum. • It is suggested that the effect of ionic liquids on the wettability alteration of the reservoir rock can be investigated for further studies.
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