Industrial Crops and Products 43 (2013) 6–14
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Formulation, optimization and application of triglyceride microemulsion in enhanced oil recovery Z. Jeirani a , B. Mohamed Jan a,∗ , B. Si Ali a , I.M. Noor a , C.H. See b , W. Saphanuchart b a b
Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia BCI Chemical Corporation Sdn. Bhd., Lot 7, Jalan BS 7/22, Taman Perindustrian Bukit Serdang, Seksyen 7, 43300 Seri Kembangan, Selangor Darul Ehsan, Malaysia
a r t i c l e
i n f o
Article history: Received 24 April 2012 Received in revised form 3 July 2012 Accepted 3 July 2012 Keywords: Microemulsion Enhanced oil recovery Chemical flooding Palm oil Triglyceride
a b s t r a c t This paper presents the determination of an aqueous phase composition of a new triglyceride microemulsion in which the triglycerides constitute the whole oil-phase of the microemulsion. Palm oil was used as the oil phase of the microemulsion. Experimental results indicate that the optimum triglyceride microemulsion was achieved when equal mass of palm oil and the aqueous phase containing 3 wt% sodium chloride, 1 wt% alkyl polyglycosides, 3 wt% glyceryl monooleate, and 93 wt% de-ionized water were mixed. The formulated composition of the aqueous phase was able to form translucent Winsor Type I microemulsion with palm oil at ambient conditions. The measured interfacial tension between the optimum microemulsion and the model oil, which is n-octane in this study, was 0.0002 mN/m. The maximum tertiary oil recovery of 71.8% was achieved after the injection of the optimum microemulsion formulation to a sand pack. The significant increase in total oil recovery (87%) suggests the effectiveness of the triglyceride microemulsion formulation for enhanced oil recovery. Its capability in recovering additional oil (4.3% of the trapped oil after water flooding) compared to a typical polymer in tertiary oil recovery indicates the efficiency of the optimum triglyceride microemulsion formulation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Chemical flooding, one of the most successful techniques to enhance oil recovery (Austad and Milter, 2000), involves the injection of a chemical to displace the oil remaining in the reservoir after waterflooding. Microemulsion is one type of formulation, which consists of oil, water, and an amphiphile mixture (Paul and Moulik, 2001). Amphiphile is a substance containing both hydrophilic and hydrophobic parts in its molecular structure. Surfactants and cosurfactants are two examples of amphiphiles. Microemulsion has been used in tertiary oil recovery since the 1970s (Putz et al., 1981; Purwono and Murachman, 2001; Bouabboune et al., 2006; Santanna et al., 2009). A microemulsion slug mobilizes the remaining oil by reducing the interfacial tension (IFT) between oil and
Abbreviations: APG, alkyl polyglycoside; NaCl, sodium chloride; IPA, isopropyl alcohol; IBA, isobutyl alcohol; NBA, normal butyl alcohol; FA8, fatty alcohol C8; FA810, fatty alcohol C8/C10; FA1214, fatty alcohol C12/C14; SA, stearyl alcohol; CA, cetyl alcohol; EGEE, ethylene glycol mono-ethyl ether; TWN20, Tween20; TWN80, Tween80; GLY, glycerol; SM, sorbitan monooleate; GM, glyceryl monooleate; SAP, saponin; GC, gas chromatography; FID, flame ionization detector; IFT, interfacial tension; PIT, phase inversion temperature; CMC, critical micelle concentration; HLB, hydrophilic–lipophilic balance; PV, pore volume; IOIP, initial oil in place; wt%, weight percent. ∗ Corresponding author. Tel.: +60 379676869; fax: +60 379675319. E-mail address:
[email protected] (B. Mohamed Jan). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.07.002
water. Microemulsion flooding has been conducted in two distinct ways. In the first method, a microemulsion system is prepared by mixing water, oil, and amphiphile(s) on the surface prior to its injection (Putz et al., 1981; Santanna et al., 2009). In the second method, a microemulsion slug is prepared in the porous media from the formation brine, residual oil, and the injected amphiphile and water (Zhao et al., 2008; Wang et al., 2010). In the 1980s chemical flooding was considered uneconomical due to low oil price, which caused a major halt in scientific publications. On the other hand, in 1990 researchers started to embark on chemical flooding due to the higher oil prices accompanied with the first gulf war. However, at that time microemulsion flooding was not considered as a common chemical flooding. Only recently numerous researchers have started conducting tests to improve microemulsion flooding efficiency by modifying the existing microemulsions in the market or developing a new microemulsion (Iglauer et al., 2009; Wu et al., 2010). It is evident that altering the chemical nature or amount of any component in a microemulsion may result in drastic changes in the IFT, type of microemulsion, and amount of the recovered oil (Kahlweit et al., 1995a; Sottmann and Strey, 1997; Sottmann and Stubenrauch, 2009). New microemulsions were formulated to enhance oil recovery by altering the surfactants and/or cosurfactants (Wang et al., 2010; Iglauer et al., 2010a,b); however, the oil-component in the microemulsion prepared on the surface plays an important role not only in the cost of microemulsion preparation
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but also in the efficiency of the microemulsion flooding due to changes in the IFT. Some researchers in literature used a specific crude oil or a fraction of petroleum such as n-octane as the oil phase of the microemulsion (Putz et al., 1981; Bouabboune et al., 2006). Despite extensive attempts made on the formulation of microemulsions with n-alkane or hydrocarbons, there is another attractive option; using triglycerides for the oil phase of the microemulsion. It is not a very common practice in enhanced oil recovery because microemulsions have typically been prepared in situ from the injected chemical slug and the remaining oil in the reservoir. However, microemulsions can be prepared outside before the injection and the oil component of the prepared microemulsion can be either hydrocarbons or triglycerides. The application of a triglyceride microemulsion in tertiary oil recovery was once tested by Santanna et al. (2009) with the use of pine oil (a kind of vegetable oil) as the oil phase of a microemulsion. A great number of triglyceride-based microemulsion formulations have also been carried out in the last decades in applications other than enhanced oil recovery (Kunieda et al., 1988; Joubran et al., 1993; Kahlweit et al., 1995a,b; Hecke et al., 2003; Komesvarakul et al., 2006; Engelskirchen et al., 2007; Do et al., 2009; Fanun, 2010; Phan et al., 2011). Triglycerides are esters of glycerol and three fatty acids. Triglycerides are the major components of palm oil. Triglycerides in palm oil are a mixture of mono-unsaturated, poly-unsaturated, and saturated triglycerides. Compared to n-alkane oil, saturated triglycerides are among the hardest oil to microemulsify since saturated triglyceride molecules have a bulky polar head and three long hydrocarbon chains (Phan et al., 2011). Therefore, the presence of unsaturated triglycerides in palm oil reduces the possibility of forming microemulsion. However, if an effective surfactant and a suitable co-surfactant are selected in the microemulsion formulation, an efficient triglyceride microemulsion with low to ultra-low interfacial tensions and a high solubilization parameter could be achieved easily. Various types of surfactants and co-surfactants such as alcohols were used to formulate triglyceride microemulsions in the literature (Kunieda et al., 1988; Joubran et al., 1993; Kahlweit et al., 1995a,b; Hecke et al., 2003; Komesvarakul et al., 2006; Engelskirchen et al., 2007; Do et al., 2009; Fanun, 2010; Phan et al., 2011). The main objective of this paper is to formulate an efficient triglyceride microemulsion on the surface to improve oil production in a tertiary recovery process after its injection. Alkyl polyglycosides were used as the surfactant of the triglyceride microemulsion. In this paper, an attempt has been made to screen and select the most effective co-surfactant from various kinds of cosurfactants such as alcohols and non-ionic surfactants. The highest cumulative tertiary oil recovery and the lowest interfacial tension between the formulated microemulsion and the model oil are the two essential criteria considered in the selection of the cosurfactant. After the co-surfactant screening, the composition of the aqueous phase of the triglyceride microemulsion was optimized to ensure minimum interfacial tension and maximum cumulative tertiary oil recovery are achieved. Finally, the efficiency of the optimum triglyceride microemulsion formulation was compared to a common commercial polymer solution in tertiary oil recovery by conducting a sand pack flooding. In all these steps, the type of the formulated triglyceride microemulsions used is Winsor Type I microemulsion, which exists in equilibrium with the excess palm oil. 2. Experimental tests 2.1. Materials The surfactant used in this work was Glucopon 650EC, a mixture of alkyl polyglycosides (APGs) having an average alkyl chain
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Table 1 Some properties of the palm kernel oil. Composition (wt%) Monounsaturated fat Polyunsaturated fat Saturated fat Density (g/ml) Viscosity (cP)
43.33 12.22 44.45 0.8142 65.39
length of 11, hydrophilic–lipophilic balance (HLB) of 11.9, and critical micelle concentration (CMC) of 0.073 g/L at 37 ◦ C (Jurado et al., 2007). It was provided by Cognis (Malaysia) Sdn. Bhd. which is now part of BASF Chemical Company. The active percentage of the surfactant solution is 50–53 wt%. Sodium chloride (NaCl, A.R. grade) and lower alkanols such as isopropyl alcohol (IPA), isobutyl alcohol (IBA), and normal butyl alcohol (NBA) were supplied by LGC Scientific, Malaysia. In addition, fatty alcohol C8 (FA8), fatty alcohol C8/C10 (FA810), fatty alcohol C12/C14 (FA1214), stearyl alcohol (SA), cetyl alcohol (CA), and ethylene glycol mono-ethyl ether (EGEE) were provided by BCI Chemical Corporation Sdn. Bhd. Furthermore, n-octane, Tween20 (TWN20), Tween80 (TWN80), glycerol (GLY), sorbitan monooleate (SM), glyceryl monooleate (GM), and saponin (SAP) were supplied by Sigma–Aldrich. Palm kernel oil was purchased from Delima Oil Products Sdn. Bhd. in Malaysia. Some properties of the palm kernel oil are tabulated in Table 1. All of the materials were used as supplied without further purification. 2.2. Triglyceride microemulsion preparation A triglyceride microemulsion was prepared by mixing 230 g of the oil phase with 230 g of the aqueous phase. The aqueous phase comprised surfactant, co-surfactant, sodium chloride, and de-ionized water at various compositions. The oil phase was pure palm oil. After adding all the components at their desired fractions, the sample was shaken with orbital shaking motion. Lab Companion SI-300 Benchtop Shaker at low shaking frequency of 100 rpm was used to avoid the appearance of emulsion phase. The triglyceride microemulsion sample was shaken for at least an hour to ensure a well-mixed and uniform mixture because of quite low aqueous solubility of APG, low frequency of mixing, and the relatively large volume of the samples. The sample was then poured into a separating funnel and left undisturbed for at least one week for equilibrium. Since palm oil is a natural component, to avoid the possibility of degradation, sample bottles used during the mixing were sealed with Viton lined screw caps. In addition, separating funnels used in the resting step were sealed with tight lubricated Teflon stopper. The microemulsion phase formed after the equilibrium was Winsor Type I or lower phase microemulsion. Then the microemulsion phase was separated from the upper (excess oil) phase in the separating funnel and was used in the next measurements. 2.3. Methodology of IFT measurements Prior to IFT measurements, the density or specific gravity of both the light and heavy phases was determined. The heavy phase was a microemulsion, which is prepared based on the aforementioned methodology and separated from the excess oil phase. The light phase was n-octane, which represents the crude oil in reservoirs (Green and Willhite, 1998). In density measurements, about 1.0 cm3 of sample was filled into the oscillating U-tube capillary of a Density/Specific Gravity Meter DMA 4500/5000 (Anton Paar, Austria) via syringe at 30 ◦ C. Measurement was repeated for all samples of the heavy and light phases.
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Fig. 1. Set-up of the experimental flooding system.
In the next step, interfacial tensions between the heavy and light phases were measured using a Spinning Drop Tensiometer SITE 4 (KRUSS, Germany) at 30 ◦ C. As a general procedure, 10 l of the light phase was injected into the rotating capillary filled with the heavy phase. During the injection, the capillary was rotated at the speed of 1000–2000 rpm. The diameter of the droplet was measured at high rotational velocity of 6000–8000 rpm after the equilibrium was reached. The system reaches the equilibrium when the diameter of the droplet did not change with time (Drelich et al., 2002). Since in all of the IFT measurements the system reached equilibrium after a very short time of about 10 s, the depletion of co-surfactant into the n-octane phase was considered negligible. Measurements were carried out in triplicate. The IFT was estimated using Vonnegut’s equation, which relates IFT to the droplet radius, rotational velocity, and the densities of the two phases (Vonnegut, 1942).
2.4. Sand pack design and microemulsion flooding Sand pack flooding tests were conducted to investigate the performance of all the formulated microemulsions in oil recovery. The sand used in these experiments was composed of SiO2 and CaO with particle size of 100 mesh or 0.150 mm. The sand was packed vertically in a glass sand pack holder 45 cm in length and 5 cm in diameter. Both sides were equipped with stainless steel sieves to prevent any sand flow. The sieve size was 300 mesh per inch with sieve opening of 0.053 mm. The sand pack was placed horizontally and flooded at atmospheric pressure and room temperature. Fig. 1 shows the experimental setup of the flooding system. The measured properties of the sand pack and floodings are listed in Table 2 (sand pack test #1). First, brine (1 wt% NaCl) was injected for about 3 pore volumes (PVs), followed by 3 PVs of n-octane to reach the connate water saturation. For the secondary oil recovery, 2 PVs of brine was injected. Then for the tertiary oil recovery, the sand pack was continuously flooded with 4 PVs of a prepared triglyceride microemulsion. The flow rates of the injections were kept constant at 0.83 ml/min, or about 2 ft/day frontal advance rate in order to mimic real field injection velocities (Iglauer et al., 2010a,b; Kumar and Mohanty, 2010). The effluents from the sand pack were collected in sample
tubes and the incremental oil recoveries were measured against time by gas chromatography (GC). The gas chromatograph (Agilent Technologies, Model 7890A) was equipped with flame ionization detector (FID) maintained at 300 ◦ C. The column was a DB-1 capillary column (J&W, 40 m × 0.18 mm inner diameter × 0.4 m film thickness). The column temperature was held at 35 ◦ C for 15 min, programmed to 70 ◦ C at 1.5 ◦ C/min and then to 130 ◦ C at 30 ◦ C/min and held at 130 ◦ C for 20 min. The carrier gas was helium at speed of 25 cm/s. The amount of recovered oil was quantitatively expressed by the term of cumulative tertiary oil recovery, which is the percentage of the volume of produced oil in the tertiary oil recovery step to the volume of residual oil (remaining oil after secondary oil recovery step). The same methodologies of sand pack microemulsion flooding and IFT measurement were conducted for all microemulsion samples to determine the optimum microemulsion formulation. The calculated cumulative tertiary oil recovery and IFT were used as the criteria of selecting the most efficient microemulsion. 2.5. Co-surfactant screening The aforementioned methodology of triglyceride microemulsion preparation was applied for various types of co-surfactants, namely IPA, IBA, NBA, TWN20, TWN80, GLY, SM, GM, FA8, FA810, FA1214, SA, CA, EGEE, and SAP at concentrations of 1, 2, 3 and 4 wt% of the aqueous phase . The concentration of APG (surfactant) and NaCl was fixed at 1 and 3 wt% of the aqueous phase. Cumulative tertiary oil recovery and IFT results were obtained for all of the Table 2 Summary of the properties of the sand pack floodings for sand pack test #1. Length Diameter Porosity Permeability Pore volume (PV) Connate water saturation Brine PV (secondary oil recovery) Residual oil Flow rate of injections Microemulsion PV (tertiary oil recovery)
45 cm 5 cm 34% 25 D 299 ml 8% 2 107.83 ml 0.83 ml/min 4
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Winsor Type I microemulsion formulations. The co-surfactant of the microemulsion with the highest tertiary oil recovery and the lowest IFT is selected as the most effective co-surfactant. 2.6. Optimization of the aqueous phase composition of the triglyceride microemulsion 2.6.1. Optimization of salinity Different microemulsion samples were prepared at various salinities ranging from 0.5 to 12 wt% of the aqueous phase. The concentration of APG (surfactant) and the selected co-surfactant was fixed at 1 and 2 wt% of the aqueous phase. Cumulative tertiary oil recovery and IFT results were obtained for all the Winsor Type I microemulsion formulations. The optimum concentration of NaCl in the aqueous phase is the salinity at which its microemulsion yields the highest tertiary oil recovery and the lowest IFT. 2.6.2. Optimization of the co-surfactant concentration Different microemulsion samples were prepared at various concentrations of co-surfactant ranging from 0.5 to 4 wt% of the aqueous phase. The concentration of APG was fixed at 1 wt% of the aqueous phase. All of the samples were prepared at the determined optimum salinity. Cumulative tertiary oil recovery and IFT results were obtained for all the Winsor Type I microemulsion formulations. The optimum concentration of the co-surfactant in the aqueous phase is the concentration at which its microemulsion yields the highest tertiary oil recovery and the lowest IFT. 2.6.3. Optimization of the surfactant concentration Different microemulsion samples were prepared at various concentrations of surfactant ranging from 0.25 to 2 wt% of the aqueous phase. The concentrations of co-surfactant and NaCl in the aqueous phase were fixed at their optimum values. Cumulative tertiary oil recovery and IFT results were obtained for all the Winsor Type I microemulsion formulations. The optimum concentration of the surfactant in the aqueous phase is the concentration at which its microemulsion yields the highest tertiary oil recovery and the lowest IFT. 2.7. Application of the optimum microemulsion formulation The same flooding procedure mentioned previously was used until the sand pack reached the residual oil saturation after secondary oil recovery by water flooding. In the tertiary recovery, 1 PV of the triglyceride microemulsion with optimized aqueous phase composition was injected to the sand pack, followed by 1 PV of polymer solution (1500 ppm xanthan gum in 1 wt% NaCl) to provide mobility control. The viscosity of the polymer solution was 27 cP at 30 ◦ C and measured using a rotational viscometer (Haake VT550 controlled-rate viscotester, Germany). The viscometer was equipped with a coaxial cylinder sensor system consisting of a set of NV cup and stainless steel rotor. Finally 1 PV of brine was injected into the sand pack as the chasing fluid to push all the injected fluids. The flow rates of injections were also kept constant at 0.83 ml/min, or about 2 ft/day frontal advance rate. The produced oil volume was recorded by GC for both secondary and tertiary oil recovery. The measured properties of the sand pack and floodings are listed in Table 3 (sand pack test #2). In the next step, the efficiency of the formulated microemulsion with optimized aqueous phase composition and the same polymer solution (1500 ppm xanthan gum in 1 wt% NaCl) in tertiary oil recovery was determined and compared. The same flooding procedure was used until the sand pack reached the residual oil saturation after the secondary oil recovery by water flooding. In the tertiary recovery, the sand pack was continuously flooded with 4 PVs of optimum triglyceride microemulsion before being flooded
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Table 3 Summary of the properties of the sand pack floodings for sand pack test #2 and #3.
Length Diameter Porosity Permeability Pore volume (PV) Connate water saturation Brine PV (secondary oil recovery) Residual oil Flow rate of injections Microemulsion PV (tertiary oil recovery) Polymer PV (tertiary oil recovery) Viscosity of the polymer solution Chasing brine PV
Sand pack test #2
Sand pack test #3
45 cm 5 cm 34% 25 D 299 ml 8% 2 107.83 ml 0.83 ml/min 1 1 27 cP 1
45 cm 5 cm 34% 25 D 299 ml 8% 2 107.83 ml 0.83 ml/min 0 4 27 cP 0
with 4 PVs of polymer solution (1500 ppm xanthan gum in 1 wt% NaCl). The produced oil was recorded by GC in tertiary oil recovery. The measured properties of the sand pack and floodings are listed in Tables 2 and 3 (sand pack test #1 for microemulsion flooding and sand pack test #3 for polymer flooding). 3. Results and discussion 3.1. Co-surfactant screening Some surfactants do not sufficiently reduce interfacial tension at the oil/water interface to form microemulsions (Myers, 1992). They may not distribute between the aqueous and oil phase properly (Alany et al., 2000). To overcome this, co-surfactant molecules are introduced to sufficiently lower the oil/water interfacial tension, fluidize the rigid hydrocarbon region of the interfacial film, and induce ideal curvature of the interfacial film (Alany et al., 2000). Co-surfactants are molecules with weak surface-active properties that are combined with the surfactants to enhance their ability to reduce the interfacial tension and promote the formation of a microemulsion (Schick, 1987). For a co-surfactant-free triglyceride microemulsion sample, which contains only 1 wt% of APG as surfactant and 3 wt% NaCl in the aqueous phase, its IFT against n-octane was measured to be 5.3658 mN/m. This value is low but not ultra-low. Thus to reduce the IFT to ultra-low values, it is essential to select an appropriate co-surfactant. Selecting an effective co-surfactant could be the first and the most critical step in synthesizing a triglyceride microemulsion for tertiary oil recovery because the type of co-surfactant might considerably influence the IFT reduction and consequently the tertiary oil recovery efficiency.
Fig. 2. The IFT of microemulsion samples containing the three lower alkanols against n-octane.
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Fig. 3. The IFT of microemulsion samples containing the Tweens against n-octane.
Fig. 4. The IFT of microemulsion samples containing the two monooleates against n-octane.
Fig. 5. The IFT of microemulsion samples containing the five fatty alcohols against n-octane.
Fig. 6. The IFT of microemulsion samples containing other types of co-surfactants against n-octane.
Fig. 7. The cumulative tertiary oil recovery of microemulsion samples containing the three lower alkanols against n-octane.
Fig. 8. The cumulative tertiary oil recovery of microemulsion samples containing the Tweens against n-octane.
Figs. 2–6 show the IFT variation of the microemulsion samples with various co-surfactants against n-octane with an increase in the concentration of co-surfactants. Figs. 7–11 show the cumulative tertiary oil recovery of the microemulsion samples with an increase in the concentration of co-surfactants. The whole set of these figures is an indicator of the performance of the co-surfactants used. In general, the observed reduction in IFT with an increase in the volume of the co-surfactants shows good performance of the cosurfactant in reducing the IFT. Increasing the amount of co-surfactant decreases the hydrophilic–lipophilic balance (HLB) number. The HLB value is an indication of the oil or water solubility of the microemulsion (Griffin, 1949). The HLB value characterizes the relative oil and water solubility of surfactants. Lower HLB number indicates the
Fig. 9. The cumulative tertiary oil recovery of microemulsion samples containing the two monooleates against n-octane.
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Fig. 10. The cumulative tertiary oil recovery of microemulsion samples containing the five fatty alcohols against n-octane.
microemulsion is more oil soluble and vice versa. It is evident that decreasing the HLB number of a nonionic mixture leads to a change in the microstructure of the microemulsion from Winsor Type I to Winsor Type III, and subsequently to Winsor Type II (Wu et al., 2001). While the microstructure is changing, the IFT decreases to a minimum before it starts to increase (Wu et al., 2001). The IFT reaches a minimum in the phase inversion temperature (PIT) point, which is fairly in the middle of Winsor Type III region (Wu et al., 2001). It was observed that for all the co-surfactants except GM, the IFT decreases with the amount of co-surfactant (or decreasing the HLB number). This trend indicates that the microstructure of the microemulsions can be Winsor Type I or Winsor Type III before the PIT at the studied range of co-surfactant. The IFT variation shows a minimum only for GM in co-surfactant range of 1–4 wt% of the aqueous phase (Fig. 4). The minimum IFT value indicates the appearance of Winsor Type III microemulsion. In other words, the microstructure of the triglyceride microemulsion with co-surfactant of GM is changed from Winsor Type I to Winsor Type III microemulsion in the presence of n-octane. Furthermore, the efficiency of the co-surfactant can also be estimated from the values of either IFT or the tertiary oil recovery. TWN80, which yields the highest IFT (Fig. 3) and the lowest tertiary oil recovery (Fig. 8) is considered as the worst choice of co-surfactant among the co-surfactants studied. In contrast, GM which yields ultra-low IFT against n-octane (Fig. 4) and the highest tertiary oil recovery (Fig. 9) is considered as the most efficient co-surfactant among co-surfactants studied. Therefore, GM is the desired co-surfactant for a triglyceride microemulsion formulation and it is selected to be one of the components of the triglyceride microemulsion.
Fig. 11. The cumulative tertiary oil recovery of microemulsion samples containing other types of co-surfactants against n-octane.
Fig. 12. IFT of microemulsion samples against n-octane for the samples at various salinities (on left-side y-axis); the cumulative tertiary oil recovery of microemulsion samples at various salinities (on right-side y-axis). In all the samples, the concentration of APG and GM was maintained at 1 and 2 wt%, respectively.
An appropriate surfactant blend for formulation of a microemulsion can be selected if the HLB of the candidate surfactant blend (surfactant and co-surfactant) approaches the required HLB of the oily component for a particular system (Schick, 1987). In addition, in the selection of surfactant blend it is more favorable if the lipophilic part of the used surfactant matches the oily component (Prince, 1977). This may be the reason that GM seems to be an effective co-surfactant for a triglyceride microemulsion. 3.2. Optimization of the aqueous phase composition of the triglyceride microemulsion 3.2.1. Optimization of salinity Six microemulsion samples were prepared at salinities of 0.5, 1, 3, 6, 9, and 12 wt% of the aqueous phase. In all the samples, the concentration of APG and GM was maintained at 1 and 2 wt%, respectively. Fig. 12 shows both the variation of IFT and cumulative tertiary oil recovery with salinity. The IFT values on left-side y-axis of Fig. 12 demonstrate that the IFT of the microemulsion samples against n-octane is fairly constant at various NaCl concentrations. The IFT was observed to be quasi-independent of salinity because the nature of the microemulsion samples is intrinsically nonionic (Kahlweit et al., 1995a; Balzer and Lüders, 2000; Iglauer et al., 2009). The mean of the IFT values was found to be 0.0037 ± 0.00005 mN/m, which is in the ultra-low region (
