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Interfacial tension and phase behavior of surfactant-brine–oil system
Colloids and Surfaces A: Physicochem. Eng. Aspects 383 (2011) 114–119
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Interfacial tension and phase behavior of surfactant-brine–oil system Achinta Bera, Keka Ojha, Ajay Mandal ∗ , T. Kumar Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826004, India
a r t i c l e
i n f o
Article history: Received 18 January 2011 Received in revised form 15 February 2011 Accepted 10 March 2011 Available online 23 March 2011 Keywords: Phase behavior Microemulsions Interfacial tension (IFT) Ethoxylated secondary alcoholic surfactant Solubilization parameter
a b s t r a c t The phase behavior of surfactant-brine–oil system is the key factor in interpreting the performance of oil recovery by microemulsion process. Due to the well established relationship between the microemulsion phase behavior and interfacial tension (IFT), it is common in the industry to screen surfactants and their formulations for low IFT through oil–water phase behavior tests. In the present work, the aqueous phases containing different ethoxylated secondary alcoholic surfactants and electrolyte (NaCl) were contacted with synthetic oil systematically to study of their phase behavior and interfacial tension for higher degree of solubilization, which is desirable for enhanced oil recovery. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Enhanced oil recovery (EOR) by surfactant flooding has become more attractive in recent years [1–4]. Low interfacial tension at low surfactant concentrations, and acceptable adsorption levels are considered to be important design parameters in optimizing chemical systems for recovering trapped oil from petroleum reservoirs [5,6]. In enhanced oil recovery one of the most important designing factors for chemical flood is to select an appropriate surfactant formulation capable of mobilization oil without significant surfactant losses due to adsorption and phase separation in the reservoir. An optimum condition for the oil recovery is observed when the middle phase contains the added surfactant and equal amounts of oil and water [6,7]. It is now recognized that formulating surfactant/oil/brine system that exhibits desirable phase behavior is an important step in optimizing performance of microemulsion systems for enhanced oil recovery. The optimum formulation of microemulsions can best be obtained by analyzing the three phase behavior of oil–brine-surfactant system by exchanging variable parameters viz. the salinity of aqueous phase, nature of oil, type of surfactant, the surfactant hydrophilic–lipophilic balance (HLB) or the temperature. Correlations for the attainment of three phase system have been found among these variables for nonionic surfactant [8–10], and [11]. Healy et al. [12] have shown that the phase behavior of surfactant/brine/oil systems is a key factor in interpreting the performance of oil recovery by microemulsion
∗ Corresponding author. E-mail address: mandal [email protected] (A. Mandal). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.03.035
processes. By systematically varying salinity, they found low interfacial tensions and high solubilization of both oil and water in the microemulsion phase to occur in or near the salinity ranges giving three phases. Due to the well established relationship between the microemulsion phase behavior and interfacial tension, it is common in the industry to screen surfactants and their formulations for low IFT through oil–water phase behavior tests [13,14]. Several investigations [5,6,15,16] have been reported on interfacial tension behavior of microemulsions in equilibrium with both excess oil and excess brine system. They demonstrated that middle phase microemulsions exhibited the lowest interfacial tension with respect to both oil and water. In the presence of surfactant the immiscibility of water and oil may be changed to form slightly miscible phase microemulsion where both phases are present. Usually the interfacial tension between oil and brine phase is typically 20–50 mN/m [17,18]. Upon addition of an amphiphile (surfactant), some oil can be solubilized into the aqueous phase and some water can be solubilized into the oil phase. The interfacial tension generally drops to values of a few mN/m. However, under very narrow of ranges of experimental parameters and conditions, ultralow IFT (<0.01) is observable [19,20,21,22]. Winsor [23] described surfactant/oil/water microemulsions as type I (oil in water), type II (water in oil) or type III (bicontinuous oil and water in a third phase known as the middle phase microemulsion). The type III or middle phase microemulsions exhibit the lowest IFT. The influence of changes in hydrophilic–lipophilic balance (HLB) values or degree of ethoxylation of a series of ethoxylated secondary alcohols (Tergitol 15-S-5, Tergitol 15-S-7 and Tergitol 15-S-9) on the formation of multi-phase micro-emulsions has been studied. The mixture of n-heptane, benzene and toluene in different compositions were used as the synthetic oil in the present study.
A. Bera et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 383 (2011) 114–119
115
Table 1 Surfactant properties. Surfactants and molecular structures
Symbol or trade name
Hyd. No.
HLB value
Category
Tergitol 15-S-5 M.W. = 415
135
10.6
Nonionic
Tergitol 15-S-7 M.W. = 515
109
12.1
Nonionic
Tergitol 15-S-9 M.W. = 738
96
13.3
Nonionic
CmH2m+1 HC
(OCH2 CH2)5
OH
CnH2n+1 (m + n =10~14) Ethoxylated C11~14 secondary alcohol
CmH2m+1 HC
(OCH2 CH2)7
OH
CnH2n+1 (m + n =10~14) Ethoxylated C11~14 secondary alcohol
CmH2m+1 HC
(OCH2 CH2)9
OH
CnH2n+1 (m + n =10~14) Ethoxylated C11~14 secondary alcohol
In all cases middle phase micro-emulsions are formed. The solubility of oil and water in microemulsion phase has been studied by varying the salinity. The appropriate surfactants with desired composition for improved oil recovery have been predicted by characterizing the middle-phase micro-emulsions in oil–brine system. An attempt has also been paid to observe the influences of ethylene oxide number (EON) on the critical micellization concentrations of the surfactants. 2. Experimental 2.1. Materials Tergitol 15-S-5, Tergitol 15-S-7 and Tergitol 15-S-9 (with 99% purity) were used as the surfactants. All of these are nonionic surfactants purchased from Sigma–Aldrich, Germany. The details of the surfactants used in the present study have been given in Table 1. Sodium chloride (NaCl) was purchased from Qualigens Fine Chemicals, India. Benzene, toluene and n-heptane were used for synthetic oil and procured from Ottokemi, India. 2.2. Experimental technique 2.2.1. Phase diagrams The triangular pseudoternary phase diagrams are tetrahedron cuts characterized by a fixed surfactant mass, so that the apices were oil, brine (a pseudocomponent) and the surfactant. The phase diagram has been established by a conventional titration method. To measure the amount of brine required to obtain a single phase, two phase mixture of surfactant and oil system was titrated against brine. The end point of each titration is taken as the first perceptible permanent cloudiness, indicating the appearance of a second phase in the mixture. All mixtures are prepared by volume; and compositions are expressed as weight percent. All the experiments have been performed at 25 ◦ C. 2.2.2. Solubilization parameters and relative phase volume The volume of oil and brine that can be solubilized by microemulsion is of interest in characterizing a surfactant system. Healy et al. [12] expressed the amounts of oil and water solubilized by a unit of surfactant in terms of solubilization parameters (Vo /Vs and Vw /Vs ). Vs , Vo and Vw represents the volume of surfactant, oil
and water respectively in the microemulsion phase. The solubilization parameters were calculated with the assumption that all the surfactant was contained in the microemulsion phase [24]. To measure the solubilization parameters the following steps are performed: A solution of the surfactant (0.4 wt%) and sodium chloride solutions at varying concentrations were prepared separately in de-ionized water. The two-phase mixture synthetic oil and brine with 1:1 (v/v) ratio were then shaken for 60 min with desired volume of surfactant solution in Rotospin rotary mixer (Tarsons Products Pvt. Ltd., Kolkata) at 50 RPM. Thereafter the tubes were removed and left to settle for a day to attain the equilibrium condition in a specially designed rack which can hold up to several tubes. After settlement the phase volumes of the brine, microemulsion and oil phases were measured easily from the properly scaled centrifuged tubes. 2.2.3. Interfacial tension and surface tension Measurement of interfacial tension is very much useful supplementary test method for phase behavior test. It is particularly useful when only very small quantities of an experimental surfactant are available. In the present study interfacial tension between oil-microemulsion phase and brine-microemulsion phase were measured using an Auto tensiometer (Rigosha & Co. Ltd., Model: 6801ES with platinum ring) under atmospheric pressure by the ring method [25]. The platinum ring was thoroughly cleaned and flame-dried before each measurement. In all cases, more than three successive measurements were carried out, and the standard deviation did not exceed ±0.1 mN/m. 3. Results and discussion 3.1. Characteristics of pseudoternary phase diagram The construction of ternary phase diagram is very much important to select systems, which gives the concentration range for the formation of microemulsions. It is also important to prepare microemulsions with low concentrations of surfactant from the economical point of view. Fig. 1 shows the ternary phase diagram for a system comprising of synthetic oil (80 vol% n-heptane, 10 vol% Benzene and 10% Toluene), Brine (0.2 wt% NaCl) and surfactant Tergitol 15-S-7). The single phase region represents the microemulsion phase and the two-phase region represents the microemulsion
A. Bera et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 383 (2011) 114–119
0.00
Surfactant
2.5
1.00
0.25
Solubilization parameter, V/V
116
0.75
0.50
0.50
1φ 0.75
0.25
Bimodal Curve 0.00
0.25
2.0
1.5
Optimal slinity 1.0
0.5
0.0
2φ
1.0
1.00
Brine
0.50
Vw /Vs Vo /Vs
0.75
1.5
0.00
2.0
2.5
Salinity, %NaCl
1.00 Oil
Fig. 2. Solubilization parameter vs. salinity, %NaCl for 15-S-5. Fig. 1. Ternary phase diagram of oil–brine-surfactant (Tergitol 15-S-7) system.
and excess oil phase. It may be seen from the figure that a broad microemulsion region is formed, even at very low concentrations of oil and surfactant.
Solubilization parameter, V/V
The oil solubilization ratio is defined as the volume of oil solubilized divided by the volume of surfactant in the microemulsion. All the surfactant is presumed to be in the microemulsion phase. The volume of oil solubilized in microemulsion is nothing but the difference between the initial oil volume and excess oil (at the top) at equilibrium condition after proper shaking. Similarly the water solubilization ratio is defined as the volume of water solubilized divided by the volume of surfactant in microemulsion. The volume of water solubilized is found from the change of volume between the initial aqueous phase and the excess water (at the bottom). The optimum solubilization ratio occurs where the oil and water solubilization is equal as determined by drawing oil and water solubilization ratio curves from the specific data points (one data per tube) [14]. Since interfacial tensions are minimal at optimal salinity and solubilization parameters are related to interfacial tension [25], estimation of both properties is a great tool in designing the economical microemulsion flooding compositions. Increasing salinity causes the phase transition of microemulsion from lower to middle to upper phase. The solubilization parameter for oil in microemulsion, Vo /Vs, is an increasing function of salinity, whereas Vw /Vs is a decreasing function of salinity as shown in Figs. 2–4. The intersection of these functions is termed as “optimal salinity” for phase behavior. Nonionic surfactants are often characterized by their hydrophilic–lipophilic balance or HLB. The value of HLB is designating the balance of water/oil loving character. High HLB values indicate good water, or polar solvent solubility of the surfactant, while low HLB values are indicative of good solubility in nonpolar systems, such as oil. As in the case of 15-S-9, the HLB value is 13.5 which is greater than that of 15-S-5 and 15-S-7 (HLB values are 10.6 and 12.1 respectively), the solution of 15-S-9 in water is clear compared to other two. 15-S-5 with lowest HLB value forms a solution in water which is turbid in nature. The relative volume of each phase is plotted as a function of salinity for all surfactants in Figs. 5–7 respectively. This type of data set is known as “salinity scan”. Here the phases are plotted as they were visualized in a test tube, i.e., the upper phase is at the top of
3.0 2.5
Optimal salinity
2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Salinity, %NaCl Fig. 3. Solubilization parameter vs. salinity, %NaCl for 15-S-7.
4.0
Solubilization parameter, V/V
3.2. Solubilization parameters
Vw/Vs Vo/Vs
3.5
3.5
Vw/Vs Vo/Vs
3.0
Optimal Salinity
2.5 2.0 1.5 1.0 0.5 0.0 0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
Salinity, %NaCl Fig. 4. Solubilization parameter vs. salinity, %NaCl for 15-S-9.
3.0
A. Bera et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 383 (2011) 114–119
117
100
Relative Phase Volume, %
95 90
Excess Oil
85 80 75 70 65
Microemulsion
60 55 50 45
Excess Brine
40 35 30 1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
Salinity, %NaCl Fig. 5. Relative phase volume vs. salinity, %NaCl for 15-S-5. Fig. 8. Photograph of phase behavior of 0.4 wt% 15-S-7 and synthetic oil (salinity changes from 0.1 wt% NaCl to 0.25 wt% from left to right).
100
the diagram. Fig. 8 represents the typical picture of relative phases of brine–oil-surfactant mixture at varying concentration of brine. As salinity of the system decreases the volume of microemulsion phase increases, which is desirable for surfactant flooding.
Relative Phase Volume, %
90 80
Excess Oil
70
3.3. Interfacial tension and surface tension Microemulsion
60 50
Excess Brine
40 30
0.5
1.0
1.5
2.0
2.5
Salinity, %NaCl Fig. 6. Relative phase volume vs. salinity, %NaCl for 15-S-7.
It is well known that the surfactants reduce the surface tension of water by getting adsorbed on the liquid–gas interface. The critical micelle concentration (CMC), one of the main parameters for surfactants, is the concentration at which surfactant solutions begin to form micelles in large amount [26]. Surface tensions of the above three surfactants (15-S-5, 15-S-7 and 15-S-9) solutions at different concentrations were measured and plotted as a function of concentration (Fig. 9). The concentration at the turning point of the curve is critical micelle concentration. From Fig. 10 it is clear that with increasing EON, the CMC value of the surfactant increases [27]. Fig. 10 more clearly illustrates the relationship between EON and CMC value of surfactants.
100
90
Tergitol 15-S-5 Tergitol 15-S-7 Tergitol 15-S-9
65
Excess Oil
60
80
Surface Tension, mN/m
Relative Phase Volume, %
70
70
Microemulsion 60
50
55 50 45 40 35
Excess Brine 30
40 1.0
1.2
1.4
1.6
1.8
2.0
2.2
Salinity, %NaCl Fig. 7. Relative phase volume vs. salinity, %NaCl for 15-S-9.
2.4
25 0.0
0.2
0.4
0.6
Concentration of Surfactant,wt% x 10
0.8 -2
Fig. 9. Variation of surface tension with surfactant concentrations.
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0.42
oil phase gradually decreases and microemulsion phase increases. At a considerable high NaCl concentration the oil phase more or less completely converted into a biphasic system with microemulsion at the top and water at the bottom. Due to increased concentration of sodium chloride the conductivity of microemulsion increases which enhances the oil solubilization in microemulsion with increase in relative phase volume of microemulsion. The specialty of microemulsion about their stability is thermodynamic property. Microemulsions are thermodynamically stable and form without any energy input [32–34]. According to the Gibbs–Helmholtz equation (Eq. (1)), such spontaneously running processes are characterized by a negative free energy G.
0.40 0.38 0.36
CMC, wt%
0.34 0.32 0.30 0.28 0.26 0.24
G = A − TS
0.22
where G is the free energy of formation, the interfacial tension, A the change in interfacial area during the formation process, S the change in entropy, and T is the temperature. The microemulsion formation is always accompanied by a significant increase in the interfacial area A. Since the interfacial tension decreases remarkably a negative free energy is achieved when the interfacial energy (␥A) is compensated by a dramatic change in the entropy of the system, which is mainly dispersion entropy [35,36].
0.20 5
6
7
8
9
EON of Nonionic Surfactant Fig. 10. Variation of CMC with EON of nonionic surfactants.
7
4. Conclusions
Interfacial Tension, mN/m
Optimal Salinity 6
5
σmo σmw
4
3 0.5
1.0
1.5
2.0
(1)
2.5
3.0
Salinity, wt.% Nacl Fig. 11. Interfacial tensions of excess oil- microemulsion and excess brinemicroemulsion system for 15-S-9.
Ultralow interfacial tensions have been reported by several authors [28,29] in multiphase surfactant-oil–brine systems in which one of the phases is a microemulsion. The ultralow interfacial tensions of microemulsion systems are a complex function of salinities, surfactant concentrations and temperatures [30,31]. When the surfactant is made less hydrophobic, for example, by increasing salinity, the transition from lower to middle to upper phase microemulsion occurs and one or two interfaces are formed. Measurements of the micro-emulsion/oil ( mo ) and microemulsion/water ( mw ) interfacial tension have been made in the present study. Fig. 11 shows the plot of interfacial tension vs. salinity for surfactant, 15-S-9. As the salinity is increased, mo decreases, while mw increases. Whenever the middle phase microemulsion is present, both values of interfacial tension are low. The curves for mo and mw were found to intersect at usually low values of interfacial tension, and the salinity corresponding to this point is termed as the optimal for interfacial tension. 3.4. Effect of salinity on the relative phase volume of microemulsion Figs. 5–7 demonstrate that relative phase volume of different phases significantly depend on the salinity. As salinity increases the
A number of different formulations with varying compositions of oil, brine and surfactants were tested in the preparation of appropriate microemulsion systems, stabilized by three different non-ionic commercial grade surfactants. The surfactant/water/oil system forms a middle phase microemulsion in the presence of NaCl. The one-phase microemulsion region disappears completely at higher salinity. A broad microemulsion region is formed, even at very low concentrations of oil and ethoxylated secondary alcoholic surfactant. The microemulsion screening is characterized by several concurrent phenomena viz. three-phase behavior, minimum interfacial tension and high solubilization of both oil and brine in the microemulsion phase. The solubility of the surfactants in water is improved when ethylene oxide is included in the surfactant molecule. During the preparation of surfactant, increase in ethoxylation changes the solution properties from precipitation to dispersion to clear solution at high ethylene oxide number (EON). Solubilization parameters for oil and water were found to vary significantly with the salinity. From the phase behavior experiments it can be concluded that degree of ethoxylation has an important role on solubilization. The CMC values of different surfactants used in the present study was found to depend on the ethylene oxide number (EON). Results show that the interfacial tension between oil and microemulsion phase is a strong function of both concentration of surfactant and salinity. Acknowledgements The authors gratefully acknowledge for the financial assistance provided by University Grant Commission [F. No. 37-203/2009 (SR)], to Department of Petroleum Engineering, Indian School of Mines, Dhanbad, India. Thanks are also extended to all individuals associated with the project. References [1] J.G. Southwick, Y. Svec, G. Chilek, G.T. Shahin, The effect of live crude on alkaline–surfactant–polymer formulations: implications for final formulation design, Paper SPE-135357 Presented at the 2010 SPE Annual Technical Conference and Exhibition in Florence, Italy, 20–22 September, 2010. [2] R. Kumar, K.K. Mohanty, ASP flooding of viscous oils, Paper SPE 135265 Presented at the 2010 SPE Annual Technical Conference and Exhibition in Florence, Italy, 19–22 September, 2010.
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Interfacial tension and phase behavior of surfactant-brine–oil system Achinta Bera, Keka Ojha, Ajay Mandal ∗ , T. Kumar Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826004, India
a r t i c l e
i n f o
Article history: Received 18 January 2011 Received in revised form 15 February 2011 Accepted 10 March 2011 Available online 23 March 2011 Keywords: Phase behavior Microemulsions Interfacial tension (IFT) Ethoxylated secondary alcoholic surfactant Solubilization parameter
a b s t r a c t The phase behavior of surfactant-brine–oil system is the key factor in interpreting the performance of oil recovery by microemulsion process. Due to the well established relationship between the microemulsion phase behavior and interfacial tension (IFT), it is common in the industry to screen surfactants and their formulations for low IFT through oil–water phase behavior tests. In the present work, the aqueous phases containing different ethoxylated secondary alcoholic surfactants and electrolyte (NaCl) were contacted with synthetic oil systematically to study of their phase behavior and interfacial tension for higher degree of solubilization, which is desirable for enhanced oil recovery. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Enhanced oil recovery (EOR) by surfactant flooding has become more attractive in recent years [1–4]. Low interfacial tension at low surfactant concentrations, and acceptable adsorption levels are considered to be important design parameters in optimizing chemical systems for recovering trapped oil from petroleum reservoirs [5,6]. In enhanced oil recovery one of the most important designing factors for chemical flood is to select an appropriate surfactant formulation capable of mobilization oil without significant surfactant losses due to adsorption and phase separation in the reservoir. An optimum condition for the oil recovery is observed when the middle phase contains the added surfactant and equal amounts of oil and water [6,7]. It is now recognized that formulating surfactant/oil/brine system that exhibits desirable phase behavior is an important step in optimizing performance of microemulsion systems for enhanced oil recovery. The optimum formulation of microemulsions can best be obtained by analyzing the three phase behavior of oil–brine-surfactant system by exchanging variable parameters viz. the salinity of aqueous phase, nature of oil, type of surfactant, the surfactant hydrophilic–lipophilic balance (HLB) or the temperature. Correlations for the attainment of three phase system have been found among these variables for nonionic surfactant [8–10], and [11]. Healy et al. [12] have shown that the phase behavior of surfactant/brine/oil systems is a key factor in interpreting the performance of oil recovery by microemulsion
∗ Corresponding author. E-mail address: mandal [email protected] (A. Mandal). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.03.035
processes. By systematically varying salinity, they found low interfacial tensions and high solubilization of both oil and water in the microemulsion phase to occur in or near the salinity ranges giving three phases. Due to the well established relationship between the microemulsion phase behavior and interfacial tension, it is common in the industry to screen surfactants and their formulations for low IFT through oil–water phase behavior tests [13,14]. Several investigations [5,6,15,16] have been reported on interfacial tension behavior of microemulsions in equilibrium with both excess oil and excess brine system. They demonstrated that middle phase microemulsions exhibited the lowest interfacial tension with respect to both oil and water. In the presence of surfactant the immiscibility of water and oil may be changed to form slightly miscible phase microemulsion where both phases are present. Usually the interfacial tension between oil and brine phase is typically 20–50 mN/m [17,18]. Upon addition of an amphiphile (surfactant), some oil can be solubilized into the aqueous phase and some water can be solubilized into the oil phase. The interfacial tension generally drops to values of a few mN/m. However, under very narrow of ranges of experimental parameters and conditions, ultralow IFT (<0.01) is observable [19,20,21,22]. Winsor [23] described surfactant/oil/water microemulsions as type I (oil in water), type II (water in oil) or type III (bicontinuous oil and water in a third phase known as the middle phase microemulsion). The type III or middle phase microemulsions exhibit the lowest IFT. The influence of changes in hydrophilic–lipophilic balance (HLB) values or degree of ethoxylation of a series of ethoxylated secondary alcohols (Tergitol 15-S-5, Tergitol 15-S-7 and Tergitol 15-S-9) on the formation of multi-phase micro-emulsions has been studied. The mixture of n-heptane, benzene and toluene in different compositions were used as the synthetic oil in the present study.
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Table 1 Surfactant properties. Surfactants and molecular structures
Symbol or trade name
Hyd. No.
HLB value
Category
Tergitol 15-S-5 M.W. = 415
135
10.6
Nonionic
Tergitol 15-S-7 M.W. = 515
109
12.1
Nonionic
Tergitol 15-S-9 M.W. = 738
96
13.3
Nonionic
CmH2m+1 HC
(OCH2 CH2)5
OH
CnH2n+1 (m + n =10~14) Ethoxylated C11~14 secondary alcohol
CmH2m+1 HC
(OCH2 CH2)7
OH
CnH2n+1 (m + n =10~14) Ethoxylated C11~14 secondary alcohol
CmH2m+1 HC
(OCH2 CH2)9
OH
CnH2n+1 (m + n =10~14) Ethoxylated C11~14 secondary alcohol
In all cases middle phase micro-emulsions are formed. The solubility of oil and water in microemulsion phase has been studied by varying the salinity. The appropriate surfactants with desired composition for improved oil recovery have been predicted by characterizing the middle-phase micro-emulsions in oil–brine system. An attempt has also been paid to observe the influences of ethylene oxide number (EON) on the critical micellization concentrations of the surfactants. 2. Experimental 2.1. Materials Tergitol 15-S-5, Tergitol 15-S-7 and Tergitol 15-S-9 (with 99% purity) were used as the surfactants. All of these are nonionic surfactants purchased from Sigma–Aldrich, Germany. The details of the surfactants used in the present study have been given in Table 1. Sodium chloride (NaCl) was purchased from Qualigens Fine Chemicals, India. Benzene, toluene and n-heptane were used for synthetic oil and procured from Ottokemi, India. 2.2. Experimental technique 2.2.1. Phase diagrams The triangular pseudoternary phase diagrams are tetrahedron cuts characterized by a fixed surfactant mass, so that the apices were oil, brine (a pseudocomponent) and the surfactant. The phase diagram has been established by a conventional titration method. To measure the amount of brine required to obtain a single phase, two phase mixture of surfactant and oil system was titrated against brine. The end point of each titration is taken as the first perceptible permanent cloudiness, indicating the appearance of a second phase in the mixture. All mixtures are prepared by volume; and compositions are expressed as weight percent. All the experiments have been performed at 25 ◦ C. 2.2.2. Solubilization parameters and relative phase volume The volume of oil and brine that can be solubilized by microemulsion is of interest in characterizing a surfactant system. Healy et al. [12] expressed the amounts of oil and water solubilized by a unit of surfactant in terms of solubilization parameters (Vo /Vs and Vw /Vs ). Vs , Vo and Vw represents the volume of surfactant, oil
and water respectively in the microemulsion phase. The solubilization parameters were calculated with the assumption that all the surfactant was contained in the microemulsion phase [24]. To measure the solubilization parameters the following steps are performed: A solution of the surfactant (0.4 wt%) and sodium chloride solutions at varying concentrations were prepared separately in de-ionized water. The two-phase mixture synthetic oil and brine with 1:1 (v/v) ratio were then shaken for 60 min with desired volume of surfactant solution in Rotospin rotary mixer (Tarsons Products Pvt. Ltd., Kolkata) at 50 RPM. Thereafter the tubes were removed and left to settle for a day to attain the equilibrium condition in a specially designed rack which can hold up to several tubes. After settlement the phase volumes of the brine, microemulsion and oil phases were measured easily from the properly scaled centrifuged tubes. 2.2.3. Interfacial tension and surface tension Measurement of interfacial tension is very much useful supplementary test method for phase behavior test. It is particularly useful when only very small quantities of an experimental surfactant are available. In the present study interfacial tension between oil-microemulsion phase and brine-microemulsion phase were measured using an Auto tensiometer (Rigosha & Co. Ltd., Model: 6801ES with platinum ring) under atmospheric pressure by the ring method [25]. The platinum ring was thoroughly cleaned and flame-dried before each measurement. In all cases, more than three successive measurements were carried out, and the standard deviation did not exceed ±0.1 mN/m. 3. Results and discussion 3.1. Characteristics of pseudoternary phase diagram The construction of ternary phase diagram is very much important to select systems, which gives the concentration range for the formation of microemulsions. It is also important to prepare microemulsions with low concentrations of surfactant from the economical point of view. Fig. 1 shows the ternary phase diagram for a system comprising of synthetic oil (80 vol% n-heptane, 10 vol% Benzene and 10% Toluene), Brine (0.2 wt% NaCl) and surfactant Tergitol 15-S-7). The single phase region represents the microemulsion phase and the two-phase region represents the microemulsion
A. Bera et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 383 (2011) 114–119
0.00
Surfactant
2.5
1.00
0.25
Solubilization parameter, V/V
116
0.75
0.50
0.50
1φ 0.75
0.25
Bimodal Curve 0.00
0.25
2.0
1.5
Optimal slinity 1.0
0.5
0.0
2φ
1.0
1.00
Brine
0.50
Vw /Vs Vo /Vs
0.75
1.5
0.00
2.0
2.5
Salinity, %NaCl
1.00 Oil
Fig. 2. Solubilization parameter vs. salinity, %NaCl for 15-S-5. Fig. 1. Ternary phase diagram of oil–brine-surfactant (Tergitol 15-S-7) system.
and excess oil phase. It may be seen from the figure that a broad microemulsion region is formed, even at very low concentrations of oil and surfactant.
Solubilization parameter, V/V
The oil solubilization ratio is defined as the volume of oil solubilized divided by the volume of surfactant in the microemulsion. All the surfactant is presumed to be in the microemulsion phase. The volume of oil solubilized in microemulsion is nothing but the difference between the initial oil volume and excess oil (at the top) at equilibrium condition after proper shaking. Similarly the water solubilization ratio is defined as the volume of water solubilized divided by the volume of surfactant in microemulsion. The volume of water solubilized is found from the change of volume between the initial aqueous phase and the excess water (at the bottom). The optimum solubilization ratio occurs where the oil and water solubilization is equal as determined by drawing oil and water solubilization ratio curves from the specific data points (one data per tube) [14]. Since interfacial tensions are minimal at optimal salinity and solubilization parameters are related to interfacial tension [25], estimation of both properties is a great tool in designing the economical microemulsion flooding compositions. Increasing salinity causes the phase transition of microemulsion from lower to middle to upper phase. The solubilization parameter for oil in microemulsion, Vo /Vs, is an increasing function of salinity, whereas Vw /Vs is a decreasing function of salinity as shown in Figs. 2–4. The intersection of these functions is termed as “optimal salinity” for phase behavior. Nonionic surfactants are often characterized by their hydrophilic–lipophilic balance or HLB. The value of HLB is designating the balance of water/oil loving character. High HLB values indicate good water, or polar solvent solubility of the surfactant, while low HLB values are indicative of good solubility in nonpolar systems, such as oil. As in the case of 15-S-9, the HLB value is 13.5 which is greater than that of 15-S-5 and 15-S-7 (HLB values are 10.6 and 12.1 respectively), the solution of 15-S-9 in water is clear compared to other two. 15-S-5 with lowest HLB value forms a solution in water which is turbid in nature. The relative volume of each phase is plotted as a function of salinity for all surfactants in Figs. 5–7 respectively. This type of data set is known as “salinity scan”. Here the phases are plotted as they were visualized in a test tube, i.e., the upper phase is at the top of
3.0 2.5
Optimal salinity
2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Salinity, %NaCl Fig. 3. Solubilization parameter vs. salinity, %NaCl for 15-S-7.
4.0
Solubilization parameter, V/V
3.2. Solubilization parameters
Vw/Vs Vo/Vs
3.5
3.5
Vw/Vs Vo/Vs
3.0
Optimal Salinity
2.5 2.0 1.5 1.0 0.5 0.0 0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
Salinity, %NaCl Fig. 4. Solubilization parameter vs. salinity, %NaCl for 15-S-9.
3.0
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117
100
Relative Phase Volume, %
95 90
Excess Oil
85 80 75 70 65
Microemulsion
60 55 50 45
Excess Brine
40 35 30 1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
Salinity, %NaCl Fig. 5. Relative phase volume vs. salinity, %NaCl for 15-S-5. Fig. 8. Photograph of phase behavior of 0.4 wt% 15-S-7 and synthetic oil (salinity changes from 0.1 wt% NaCl to 0.25 wt% from left to right).
100
the diagram. Fig. 8 represents the typical picture of relative phases of brine–oil-surfactant mixture at varying concentration of brine. As salinity of the system decreases the volume of microemulsion phase increases, which is desirable for surfactant flooding.
Relative Phase Volume, %
90 80
Excess Oil
70
3.3. Interfacial tension and surface tension Microemulsion
60 50
Excess Brine
40 30
0.5
1.0
1.5
2.0
2.5
Salinity, %NaCl Fig. 6. Relative phase volume vs. salinity, %NaCl for 15-S-7.
It is well known that the surfactants reduce the surface tension of water by getting adsorbed on the liquid–gas interface. The critical micelle concentration (CMC), one of the main parameters for surfactants, is the concentration at which surfactant solutions begin to form micelles in large amount [26]. Surface tensions of the above three surfactants (15-S-5, 15-S-7 and 15-S-9) solutions at different concentrations were measured and plotted as a function of concentration (Fig. 9). The concentration at the turning point of the curve is critical micelle concentration. From Fig. 10 it is clear that with increasing EON, the CMC value of the surfactant increases [27]. Fig. 10 more clearly illustrates the relationship between EON and CMC value of surfactants.
100
90
Tergitol 15-S-5 Tergitol 15-S-7 Tergitol 15-S-9
65
Excess Oil
60
80
Surface Tension, mN/m
Relative Phase Volume, %
70
70
Microemulsion 60
50
55 50 45 40 35
Excess Brine 30
40 1.0
1.2
1.4
1.6
1.8
2.0
2.2
Salinity, %NaCl Fig. 7. Relative phase volume vs. salinity, %NaCl for 15-S-9.
2.4
25 0.0
0.2
0.4
0.6
Concentration of Surfactant,wt% x 10
0.8 -2
Fig. 9. Variation of surface tension with surfactant concentrations.
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0.42
oil phase gradually decreases and microemulsion phase increases. At a considerable high NaCl concentration the oil phase more or less completely converted into a biphasic system with microemulsion at the top and water at the bottom. Due to increased concentration of sodium chloride the conductivity of microemulsion increases which enhances the oil solubilization in microemulsion with increase in relative phase volume of microemulsion. The specialty of microemulsion about their stability is thermodynamic property. Microemulsions are thermodynamically stable and form without any energy input [32–34]. According to the Gibbs–Helmholtz equation (Eq. (1)), such spontaneously running processes are characterized by a negative free energy G.
0.40 0.38 0.36
CMC, wt%
0.34 0.32 0.30 0.28 0.26 0.24
G = A − TS
0.22
where G is the free energy of formation, the interfacial tension, A the change in interfacial area during the formation process, S the change in entropy, and T is the temperature. The microemulsion formation is always accompanied by a significant increase in the interfacial area A. Since the interfacial tension decreases remarkably a negative free energy is achieved when the interfacial energy (␥A) is compensated by a dramatic change in the entropy of the system, which is mainly dispersion entropy [35,36].
0.20 5
6
7
8
9
EON of Nonionic Surfactant Fig. 10. Variation of CMC with EON of nonionic surfactants.
7
4. Conclusions
Interfacial Tension, mN/m
Optimal Salinity 6
5
σmo σmw
4
3 0.5
1.0
1.5
2.0
(1)
2.5
3.0
Salinity, wt.% Nacl Fig. 11. Interfacial tensions of excess oil- microemulsion and excess brinemicroemulsion system for 15-S-9.
Ultralow interfacial tensions have been reported by several authors [28,29] in multiphase surfactant-oil–brine systems in which one of the phases is a microemulsion. The ultralow interfacial tensions of microemulsion systems are a complex function of salinities, surfactant concentrations and temperatures [30,31]. When the surfactant is made less hydrophobic, for example, by increasing salinity, the transition from lower to middle to upper phase microemulsion occurs and one or two interfaces are formed. Measurements of the micro-emulsion/oil ( mo ) and microemulsion/water ( mw ) interfacial tension have been made in the present study. Fig. 11 shows the plot of interfacial tension vs. salinity for surfactant, 15-S-9. As the salinity is increased, mo decreases, while mw increases. Whenever the middle phase microemulsion is present, both values of interfacial tension are low. The curves for mo and mw were found to intersect at usually low values of interfacial tension, and the salinity corresponding to this point is termed as the optimal for interfacial tension. 3.4. Effect of salinity on the relative phase volume of microemulsion Figs. 5–7 demonstrate that relative phase volume of different phases significantly depend on the salinity. As salinity increases the
A number of different formulations with varying compositions of oil, brine and surfactants were tested in the preparation of appropriate microemulsion systems, stabilized by three different non-ionic commercial grade surfactants. The surfactant/water/oil system forms a middle phase microemulsion in the presence of NaCl. The one-phase microemulsion region disappears completely at higher salinity. A broad microemulsion region is formed, even at very low concentrations of oil and ethoxylated secondary alcoholic surfactant. The microemulsion screening is characterized by several concurrent phenomena viz. three-phase behavior, minimum interfacial tension and high solubilization of both oil and brine in the microemulsion phase. The solubility of the surfactants in water is improved when ethylene oxide is included in the surfactant molecule. During the preparation of surfactant, increase in ethoxylation changes the solution properties from precipitation to dispersion to clear solution at high ethylene oxide number (EON). Solubilization parameters for oil and water were found to vary significantly with the salinity. From the phase behavior experiments it can be concluded that degree of ethoxylation has an important role on solubilization. The CMC values of different surfactants used in the present study was found to depend on the ethylene oxide number (EON). Results show that the interfacial tension between oil and microemulsion phase is a strong function of both concentration of surfactant and salinity. Acknowledgements The authors gratefully acknowledge for the financial assistance provided by University Grant Commission [F. No. 37-203/2009 (SR)], to Department of Petroleum Engineering, Indian School of Mines, Dhanbad, India. Thanks are also extended to all individuals associated with the project. References [1] J.G. Southwick, Y. Svec, G. Chilek, G.T. Shahin, The effect of live crude on alkaline–surfactant–polymer formulations: implications for final formulation design, Paper SPE-135357 Presented at the 2010 SPE Annual Technical Conference and Exhibition in Florence, Italy, 20–22 September, 2010. [2] R. Kumar, K.K. Mohanty, ASP flooding of viscous oils, Paper SPE 135265 Presented at the 2010 SPE Annual Technical Conference and Exhibition in Florence, Italy, 19–22 September, 2010.
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