Experimental Study of the Interaction between NaOH, Surfactant, and

May 11, 2012 - ... Surfactant, and Polymer in Reducing Court Heavy Oil/Brine Interfacial Tension ... The results show that there exists a minimum tran...
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Experimental Study of the Interaction between NaOH, Surfactant, and Polymer in Reducing Court Heavy Oil/Brine Interfacial Tension Haiyan Zhang,*,†,‡ Mingzhe Dong,§ and Suoqi Zhao∥ †

Department of Chemistry and Chemical Engineering, Qinzhou University, Qinzhou, Guangxi 535000 Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004 § Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada ∥ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 ‡

ABSTRACT: The effect of the surfactant, NaOH, and polymer and the interactions between them on the heavy oil/water interface are unveiled by studying the dynamic interfacial tension (IFT), minimal transient IFT, and total organic carbon (TOC) and analyzing the phenomenon during the measurement of IFT of heavy oil/different alkaline systems, including alkaline (A), alkaline−surfactant (AS), alkaline−polymer (AP), and alkaline−surfactant−polymer (ASP). The results show that there exists a minimum transient IFT. There is an optimal composition to achieve the minimal IFT with varying NaOH concentrations in 0.018−0.8 wt %. For different chemical solutions, the optimal composition is different. Adding polymer affects the IFT by influencing the diffusion of species to or from the interface. Despite polymer addition, adding surfactant will increase the IFT at a lower alkaline concentration because of its competitive adsorption with OH− and reduce the IFT at a higher alkaline concentration because of its synergistic effect. The synergy between the surfactant and alkaline is turned out as follows: NaOH reacts with the polar components in the oil phase to produce ionized surface-active species; then the IFT is reduced; and the oil drop is prolonged. Surfactant accelerates the diffusion of ionized species from the interface to the bulk phase, and then the polar components underneath it are exposed to NaOH; therefore, the reaction between NaOH and polar oil components can proceed to further reduce the IFT. The contraction of the oil drop after surface-active species departed can be explained reasonably by considering the influence of the composition and structure of heavy oil. surfactant reduces the IFT more effectively.11 Touhami et al.12 found that the combination of alkaline and surfactant reduced IFT more rapidly than alkaline alone. The time required to reach the minimum IFT for NaOH solution was longer than that for NaOH/surfactant mixtures. IFT is lowered more significantly by chemical reaction than by adding ready-made surfactant below the critical micelle concentration (cmc); the dynamic IFT behavior is dampened by adding the surfactant above the cmc. Chartterjee et al.13 studied the dynamic IFT of an oil and surfactant-enhanced alkaline system by a modified technique similar to the sessile drop technique. The oil used in the study was prepared with oleic acids and decane. It was suggested that the added surfactant may increase the minimal IFT when its concentration was above the cmc. There exists a minimum IFT at an optimized formula.14 Easr-El-Din et al.15,16 studied the dynamic IFT of David Loydminster crude oil with viscosity of 89.8 mPa s (31 °C dead oil) and found that crude oil/alkaline systems produced a minimal IFT of 0.02 dyn/cm at an optimum alkaline concentration (S of 0.001 wt % and A of 0.2 wt %). It was suggested that the presence of minimum IFT was due to the desorption barrier, which hindered the movement of the oleic anions into the aqueous phase. Touhami et al.5 found that the dynamic IFT was a function of acid, alkaline, and surfactant concentrations. There exists an optimal

1. INTRODUCTION The interaction between oil and alkaline is important in the enhanced oil recovery process.1 It is widely accepted that the effect of alkaline on reducing the interfacial tension (IFT) is that alkaline reacts with the polar components in oil to form surface-active species.2 These species adsorb at the interface to lower the oil/water (O/W) IFT. The dynamic characters of IFT between oil and water are caused by the reaction and diffusion of different reagents at the interface, which can reflect the roles of different species at the O/W interface to some extent. Some studies3−8 suggested that the dynamic behavior is caused by a time-dependent concentration of surface-active species at the interface. Acids in the oil diffuse to the O/W interface and react with alkaline species in the aqueous phase to produce in situ surface-active species, which lower the IFT. As the reaction proceeds, more surface-active species accumulate at the interface and, hence, IFT decreases. These activated acid ions have the tendency to partition in the original aqueous interfacial region. Adsorption and desorption of these species happens simultaneously, and their concentrations at the interface vary with time. The ionized acids may also form soap with sodium ions in the aqueous phase at high ionic strength, which may enhance the transfer of the surfactant from the aqueous phase to crude oil9 and result in increasing the IFT. For the coeffect of alkaline and surfactant, it can reduce the IFT of O/W to an ultralow level.10 Some authors suggested that the synergistic effect resulting from both alkaline and © 2012 American Chemical Society

Received: March 23, 2012 Revised: May 9, 2012 Published: May 11, 2012 3644

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this phenomenon. The way that NaOH and surfactant act with each other is revealed too.

concentration with respect to alkaline and surfactant to produce the lowest IFT. The unionized acids also help lower the dynamic IFT by adsorbing ionized acids and adding surfactant simultaneously at the interface. Kang et al.17 reported that ultralow IFT was achieved once critical concentrations of alkaline and surfactant were reached. Alkaline was suggested to lower IFT in two ways: one was to force surfactant molecules to the oil−water interface, and the other was to react with oil to produce filming materials to further increase the surfactant adsorption. Zhang et al.18 reported that the addition of NaOH implied a slight effect on the stable IFT value of the petroleum sulfonate (LH)/tetradecane system because of the lipophilicity of LH. While the transient IFT value decreased after NaOH was added, this trend is mainly controlled by the rate of diffusion and adsorption of surfactant molecules from the bulk phase to the interface. On the basis of the relationship between IFT and t−1/2, it was also suggested that the dynamic IFT process was diffusion-controlled.19 The rate-determining step of the diffusion process occurred in the aqueous phase before the minimum was reached, while after it, the rate-determining step occurred in the oil phase. Nilsson et al.20 used a spinning-drop interfacial tensiometer to measure the IFT between iso-octane and synthetic seawater as a function of the salinity and surfactant concentration with and without polymer. Their results showed that the addition of polymer did not affect the IFT between oil and water. While Nasr-El-Din et al.21 reported that the presence of a polymer affected the IFT behavior. Their results showed that, when the sodium carbonate concentration was less than 0.2 wt %, an increase in the polymer concentration led to a slight drop in IFT. At a sodium carbonate concentration of 0.2 wt %, the addition of polymer lowered the IFT minimum and long-term IFT values. Kang et al.17 also reported that the polymer has a slight effect on the IFT because it squeezes into the subphase at a higher surface pressure. It should be noted that all of the above investigations were performed on model oil or conventional oils and there are some discrepancies in the trend for different systems. While the composition of crude oil is more complicated, Cambridge et al.22 reported that the IFT behavior between synthetic and crude oils can be quite different. The structure of the acids in the oil phase also affects their interfacial activities.23 The components of heavy oil are much more complicated than conventional crude oil, including the saturated fraction, aromatic fraction, resin, and asphaltene. It is also rich in both heteroatoms and metals, and the molecular weight varies in a wide range. Heavy oil usually has a stable structure, and the interaction between the polar functional groups is important to its structure and characteristics. Therefore, reactions between the acids in heavy oil and an alkaline should be different from that of model oil and light oil. Also, different synergy should be expected from the presence of the alkaline, surfactant, and polymer. Dynamic IFT is good at studying the adsorption and desorption at the interface.24 It can reflect the effect of different components on the interfacial properties.25 In this paper, the dynamic interfacial behavior of Court heavy oil in the presence of different chemical solutions is studied using a spinning-drop interfacial tensiometer to elucidate the effect of the surfactant, alkaline, and polymer on reducing the IFT. During this process, it is observed that, under some situations, after reaching the minimum IFT, the oil drop started to contract unevenly. The composition and specialty of heavy oil are involved to explain

2. EXPERIMENTAL SECTION 2.1. Fluids and Chemicals. Oil used in this study was collected from the Court reservoir (Saskatchewan, Canada). The viscosity of the oil at ambient temperature (22.5 °C) was 1500 mPa s, as measured by a Brookfield DV-II+ viscometer (Brookfield Engineering Laboratories, Inc., Middleboro, MA). The density was 950.4 kg/m3, and the acid number was measured to be 1.4 mg of KOH/g of oil. The oil sample was centrifuged at 10 000 rpm at 35 °C for 2 h to remove water and solids. The chemicals used in this study include alkaline sodium hydroxide (ACS, Fisher Science, Ontario) and an anionic surfactant CS460 (Stepan Canada, Inc., Burlington, Ontario, Canada), and the polymer used in this study was a partially hydrolyzed polyacrylamide, AN 923 PGO (SNF Floerger, Andrézieux Cedex, France), with the hydrolysis degree and molecular weight of 25 mol % and 18 × 106, respectively. To eliminate the interference of other ions, deionized (DI) water is used to prepare all of the solutions in this study. 2.2. IFT Measurements. The dynamic IFT was measured using a model 510 spinning-drop tensiometer (Temco, Inc., Tulsa, OK). A capillary tube containing a dense phase (water phase) and a light phase (oil phase) is rotated on its longitudinal axis at a high speed. The value of the IFT is calculated according to the length and width of the oil droplet by means of built-in software. The dynamic IFT is recorded every 12 s as soon as the rotation speed reaches the set value. The measurement of IFT was stopped whenever IFT stopped varying with time or the oil drop was split into small droplets. 2.3. Total Organic Carbon (TOC) Concentration Measurement. The TOC of the aqueous phase was measured by a total organic carbon analyzer TOC-VE (Shimadzu Corporation). The measurements were used to indicate the amount of the components diffused from the oil phase to the aqueous phase. Court oil and different aqueous solutions were gently mixed at a volumetric ration of 1:5 in a glass vial. The vial was left to stand for a long time until the oil and water separated completely, and the clear aqueous phase was taken by a syringe for the TOC measurements. The total carbon (TC) concentration and the inorganic carbon (IC) concentration of the aqueous phase were measured to calculate the TOC of the aqueous phase. The presence of the surfactant in the aqueous phase promotes the partition of more organics in the aqueous phase. When the TOC is measured before and after contact with the oil, the interference of added surfactant is eliminated; therefore, the increment of TOC in the aqueous phase can be used to describe the amount of organic component partitioned from the oil phase to the aqueous phase.

3. RESULTS AND DISCUSSION 3.1. Dynamic IFT between Oil and Alkaline (A) Solution. The dynamic IFT between Court oil and NaOH aqueous solution at different alkaline concentrations in the range of 0.018−0.8 wt % is shown in Figures 1 and 2. It can be seen in the figures that the dynamic IFT curves vary with the increase of the alkaline concentration. For a NaOH concentration of 0.018 wt %, the transient IFT reaches the minimum of about 0.04 dyn/cm quickly and then increases as time proceeds. With the increase of NaOH to 0.02 wt %, there is a dramatic decrease in the IFT compared to that of 0.018 wt % NaOH, and the increase of its dynamic IFT after reaching the minimum became less significant. A further increase in the alkaline concentration to 0.05 wt % results in a gradual decrease of the dynamic IFT with time. The increase after the minimum is not as noticeable as that at lower NaOH concentrations. With the increase of the alkaline concentration from 0.1 to 0.8 wt %, as shown in Figure 2, the drop of dynamic IFT at the beginning cannot be observed and the changes of dynamic IFT with time become more static. It is also noticed that, with the 3645

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active species at the interface. The in situ-produced surfaceactive species by the reaction of alkaline with acids and esters in oil adsorbed at the O/W interface to reduce the IFT. At the beginning, the adsorption rate is higher than the desorption rate; therefore, the surface-active species accumulates at the interface. With the accumulation of surface-active species at the interface, its concentration reaches the maximum, and the minimum of dynamic IFT is achieved accordingly. Consequently, the rate of adsorption is slowed, while the desorption process is accelerated, because of the high concentration of surface-active species at the interface; therefore, the IFT starts to increase and finally reaches the equilibrium value. Bleys et al.29 suggested that it is due to the relaxation and reorientation of surface-active species at the interface. These explanations do not seem to explain, for example, at higher alkaline concentrations, higher IFT is observed and its dynamic IFT becomes less dynamic. There may be other factors affecting the behavior of dynamic IFT. 3.2. Dynamic IFT between Oil and Alkaline−Surfactant (AS) Solution. The IFTs between oil and surfactant solution with concentrations ranging from 100 to 800 ppm were measured. The IFT between the oil and surfactant solution (even at a surfactant concentration > 300 ppm) is above 50 dyn/cm. This result indicates that, for Court oil, the surfactant alone cannot reduce the IFT effectively, even at higher concentrations. When both the surfactant and alkaline are added to the aqueous solution, IFT is reduced effectively, even at lower concentrations of surfactant (100 ppm). The dynamic IFTs between Court oil and AS solution containing 100 ppm surfactant at different alkaline concentrations are shown in Figure 3. When the NaOH concentration is as low as 0.018 wt %, the IFT is about 0.34 dyn/cm, which is much higher than that of the alkaline-only system (0.04 dyn/ cm), but this value is much lower than the IFT of the oil/ surfactant system (50 dyn/cm). This result demonstrates the synergic effect between alkaline and surfactant in reducing the IFT more effectively than surfactant alone, and alkaline plays an important role in this process. Because of the competitive adsorption between the surfactant and alkaline, the concentration of OH− at the interface is reduced in comparison to that of the NaOH-only system at this alkaline concentration. There is not enough OH− to react with the acids and esters in the oil phase, and less surface-active species are produced. Therefore, at NaOH of 0.018 wt %, the IFT of the NaOH−surfactant is

Figure 1. Dynamic IFT between NaOH solution and oil, with the concentration of NaOH from 0.018 to 0.1 wt %.

Figure 2. Dynamic IFT between NaOH solution and oil, with the concentration of NaOH from 0.1 to 0.8 wt %.

increase of the alkaline concentration, the time taken to reach the minimum transient IFT becomes longer. This phenomenon is similar to the report on the effect of increasing the NaCl concentration on the dynamic IFT of the oil/aqueous phase.26 It can be ascribed to the influence of the increased ionic strength15 by increasing the NaOH concentration: a high ionic strength leads to a lower dissociation of the surfactant at the interface, which slows the decrease of IFT. The minimum of dynamic IFT between conventional oil or model oil and the aqueous phase has been reported earlier.27,28 This observation was explained by the diffusion of surface-

Figure 3. Dynamic IFT between NaOH−surfactant 100 ppm solution and oil. 3646

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Figure 4. Dynamic IFT of the AP system with a polymer concentration of 800 ppm under different alkaline concentrations.

Figure 5. Dynamic IFT of ASP under different NaOH concentrations.

higher than that of the NaOH-alone system, and because its in situ-produced surface-active species at the interface is less, the influence on the dynamic IFT caused by diffusion is slight. Therefore, the dynamic IFT behavior of the oil/NaOH− surfactant at NaOH of 0.018 wt % is less variable than that of the oil/NaOH system. When the alkaline concentration is increased to 0.04 wt %, more surface-active species are produced and the IFT is reduced to about 0.02 dyn/cm at the beginning followed by a sharp increment. The dynamic IFT curves at 0.1 and 0.2 wt % NaOH are similar to that at 0.04 wt % NaOH, except that the increment after IFT minimum is not as remarkable as that of 0.04 wt % NaOH. With a further increase of the alkaline concentration, the increasing trend of IFT after the minimum transient IFT is weakened and disappears gradually. The time for IFT staying at the minimum also becomes longer. Moreover, the time taken to reach the minimum IFT becomes longer with the increase of the alkaline concentration. The composition of chemical solution affects the time to reach the minimum IFT, as reported by Touhami et al.12 by studying the dynamic IFT of acidified model oil/ alkaline systems. This phenomenon can also be explained by the influence of the increased NaOH concentration on the diffusion of surface-active species: with the increase of the NaOH concentration, more Na+ and OH− are presented in the aqueous phase. The increased ionic strength dampens the diffusion of in situ-produced surface-active species from the interface. As a result, the reaction between alkaline and oil components is slowed; therefore, more time is needed to reach the minimal transient IFT. 3.3. Dynamic IFT of Oil/Alkaline−Polymer (AP) and Oil/Alkaline−Surfactant−Polymer (ASP) Systems. The

dynamic IFT of the oil/AP solution system is shown in Figure 4. When it is compared to the dynamic IFT of the oil/A system, it is found that the addition of the polymer causes the dynamic behavior of IFT to be more static and the increase of dynamic IFT with time proceeding after reaching the minimum of dynamic IFT disappeared. The changing trend of IFT with the increase of the NaOH concentration is not varied after the polymer is added: IFT decreases first and then increases with the increase of the NaOH concentration. With the further introduction of the surfactant to the AP system, IFT is reduced effectively (Figure 5); however, the dynamic behavior of IFT is not as remarkable as that of AS system, which is shown in Figure 3. For the ASP/oil system, with time proceeding, its IFT decreases gradually and then levels off at a constant IFT. These results demonstrate that adding polymer to both A and AS systems can weaken the dynamic features of IFT. This can be explained as follows: the dynamic feature of IFT is due to the adsorption and desorption of the surface-active species at the O/W interface, which are produced by the reaction of alkaline and oil. The existence of polymer increases the viscosity of the aqueous phase. As a result, the diffusion of alkaline and surfactant is restrained. The production of the in situ surfactant by the reaction between alkaline and polar components in oil is also slowed, as well as their desorption and adsorption. Therefore, the dynamic features of the IFT for ASP/oil and AP/oil systems are weakened in comparison to that without polymer. 3.4. Relationship between Minimal IFT and NaOH Concentration. Minimal transient IFT was observed during the measurement of dynamic IFT of Court oil and chemical solution. Taylor et al.30 applied surfactant-enhanced alkaline 3647

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flooding on Berea sandstone cores using oil with a viscosity of 34.1 mPa s. It was suggested that the oil recovery correlated with the minimal dynamic IFT better than with the equilibrium IFT. Radke and co-worker14 demonstrated that minimum IFT measured by a spinning-drop tensiometer was indicative of the lowest achievable reservoir equilibrium value. Therefore, researching the minimum IFT is crucial to understand the effect of various factors on the dynamic behavior and further determining the synthetic effects of those different factors. In this section, the minimum of transient IFT is adopted to describe the IFT of different oil/alkaline systems. When the minimum transient IFT of Court oil/NaOH is observed at different alkaline concentrations, as shown in Figure 6, it is found that the relationship between IFT with the

achieve the same alkaline concentration at the interface, a higher alkaline concentration in the bulk phase is required; therefore, the optimum alkaline concentration is increased to obtain the minimum IFT after the polymer is added. After 100 ppm surfactant is added to NaOH or NaOH− polymer solution, the IFTs are reduced dramatically, as shown in Figure 6. The IFT curves of AS and ASP systems are similar. The only difference between them is that the IFT value of the ASP system is slightly higher than that of the AS system under the same alkaline concentration, which is also due to the effect of the polymer. When the IFT curves of A are compared to AS and AP and to ASP, it is noticed that there is an apparent change on the shape of the IFT curve. For systems without added surfactant, the IFT increases with the increase of the NaOH concentration when the alkaline concentration is higher than the optimal value. After the surfactant is added, the IFT decreases with the increase of the NaOH concentration gradually and its IFT value is much lower than that of the oil/NaOH system under the same NaOH concentration. At lower alkaline concentrations (≤0.05 wt %), the addition of the surfactant increases the IFT, despite polymer addition. This can be attributed to the competitive adsorption between the surfactant and alkaline, which lowers the pH at the interface. While the NaOH concentration is low, the existence of OH− is beneficial in reducing the IFT. Therefore, at lower NaOH concentrations, adding the surfactant can increase the IFT. 3.5. Phenomena Observed during IFT Measurements Using the Spinning-Drop Method. During the measurement of the O/W IFT, some interesting phenomena are observed. First, the color of the aqueous phase close to the water/oil interface becomes darker and darker with the proceeding of the measuring process. Second, the oil drop is prolonged first and then contracted with time proceeding. The contraction of the oil drop does not go evenly, while the elongation of the oil drop at the beginning of the measuring process goes evenly. The oil drop can even be separated into several oil droplets unevenly, and they also contract as time proceeding. These phenomena are also observed for oil/ NaOH−surfactant and oil/NaOH systems. For the first phenomenon, it can be ascribed to the oil components diffusing into the water phase. For the oil/ NaOH−surfactant system, the diffusion of oil components from the oil phase to the water phase could be explained by the fact that NaOH and surfactant emulsified the oil components into the water phase. However, for the oil/NaOH system, there is no surfactant added. The diffusion could be ascribed to the diffusion from the oil phase to the water phase of the in situ surfactant formed by the reaction between NaOH and oil. However, all of the explanation mentioned above cannot explain the contraction of the oil drop (the second phenomenon). The colloid structure of heavy oil may play an important role in this process. The effect of the colloid structure of heavy oil on the contraction of the oil drop is speculated as follows: There are many polar components in heavy oil with different molecular structures.23 They are unevenly distributed in the oil phase and help to form the colloid structure of heavy oil. Most of them have some complicated structures, such as long alkane chains and functional groups.31 Because of the difference in structures, some polar components react with NaOH first to form the in situ surfactant and left the O/W interface. The O/W IFT is reduced, and the oil drop is elongated. As a result, some polar components inside the oil phase are exposed at the interface

Figure 6. IFT of Court oil and different chemical solutions as a function of the NaOH concentration.

NaOH concentration can be separated into three different stages when only NaOH is presented in chemical solution: when NaOH concentration is lower than 0.03 wt %, there is a sharp reduction on IFT with the increase of the alkaline concentration; while with the increase of the alkaline concentration in the range of 0.03−0.1 wt %, IFT increases rapidly from 0.001 to 0.031 dyn/cm; and with a further increase of the alkaline concentration to 0.8 wt %, the increase of IFT slows and finally levels off at about 0.3 dyn/cm. There is an optimal alkaline concentration (0.03 wt %) to achieve the minimum IFT. The results also demonstrate that excess NaOH is detrimental to reduce the IFT of Court oil/NaOH solution. After 800 ppm polymer is added to NaOH solution, the shape of the IFT curve is unchanged (AP in Figure 6). The difference caused by adding polymer lies in the fact that, when the NaOH concentration is lower than the optimal value (0.03 wt %), adding polymer brings up the IFT. Beyond the optimal NaOH concentration, the difference between oil/AP and oil/A is reduced with the increase of the NaOH concentration. The optimal alkaline concentration to obtain the minimal IFT (A, 0.001 dyn/cm; AP, 0.0047 dyn/cm) increases from 0.03 to 0.05 wt % after polymer is added. On the basis of the research about conventional oil and model oil,3 it was suggested that the optimal alkaline concentration reflects the required interfacial pH to approach the acid dissociation constant of the acids presented at the interface. A further increase of sodium ions raises IFT by forming undissociated soap, which has lower interfacial activity and prefers dissolving in the oil phase. The effect of the polymer is due to its effect on the diffusion by increasing the viscosity: when the alkaline concentration is lower than optimum, the diffusion of alkaline from the aqueous phase to the interface is beneficial in reducing IFT. Because of the high viscosity caused by the polymer, the diffusion of alkaline from the bulk phase to the interface is hampered. To 3648

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Figure 7. TOC of A, AS, AP, and APS solutions after contact with Court oil.

concentration needed to reduce the IFT. Second, the added surfactant increases the solubility of oil components in the aqueous phase by forming emulsion or substituting the in situformed surfactant to adsorb at the interface. At higher NaOH concentrations, the second effect of the surfactant can offset the influence of increased NaOH on constraining the diffusion of produced surface-active species; therefore, the IFT is further reduced and does not increase with the NaOH concentration.

and continue to react with NaOH. On the other hand, because of the longer alkane chain, some in situ-formed surfactants have difficulty desorbing from the interface. Their long alkane chain can act with each other, maybe join together, and lead to the contraction of the oil drop. The uneven distribution of these components in oil explains the uneven contraction and break of the oil drop. 3.6. TOC of the Aqueous Phase after Mixing with Court Oil. After mixing the aqueous phase with oil at the volumetric ratio of oil/water of 1:5, the increment in TOC of the aqueous phase is measured to unveil the interaction of the alkaline, surfactant, and polymer. For different systems, the concentration of the surfactant and polymer is 100 and 800 ppm, respectively, and the NaOH concentration varies from 0.02 to 0.8 wt %. The results are shown in Figure 7. When the curves of A and AS are compared, it is noticed that there is an optimal composition to achieve the maximal increment of TOC. The optimal concentration of the oil/AS system shifts to a higher NaOH concentration (0.1 wt %) compared to the oil/A system (0.04 wt %). Beyond the optimal concentration, the increment on TOC decreases with the increase of the NaOH concentration, and the extent of TOC reduction with the increase of NaOH for AS systems is much lower than that of A systems. When the curves of A and AP and AS and ASP are compared, it is noticed that, at higher NaOH concentrations, the TOC values are close for each pair of curves. At lower NaOH concentrations, on the other hand, there is a dramatic reduction in TOC after the polymer is added. The results also demonstrate that excess alkaline is detrimental to the diffusion of ionized oil components from oil to water because of the higher ionic strength. Adding surfactant helps the oil components diffuse from the oil phase to the water phase. Adding polymer increases the viscosity of the water phase. As a result, the diffusion of the alkaline, surfactant, and the in situ-formed surface-active species is restricted.32 Therefore, the negative effect of extra OH− on diffusion is weakened, as well as the effect of added surfactant on diffusion and the partition of surface-active species from the oil phase to the water phase. When all of the results mentioned above are combined, it can be deduced that there exists a strong interaction between NaOH and the surfactant at the O/W interface. When the alkaline and surfactant coexist in the aqueous phase, the effect of the surfactant acts in two ways: First, it competes with OH− to adsorb at the interface, which results in a higher alkaline

4. CONCLUSION All of the experimental results show that, for the oil/A system, there exists an optimal alkaline concentration at about 0.03 wt % to reach the minimum IFT of 0.001 dyn/cm. Beyond the optimal NaOH concentration, IFT increases significantly with a further increment of the alkaline concentration. The combination of the surfactant and alkaline reduces the IFT effectively, especially at higher alkaline concentrations. The colloid structure of heavy oil may be responsible for the uneven contract of the oil drop. The synergistic effect of NaOH and the surfactant is analyzed as follows: NaOH reacts with those components present in the oil phase, and the added surfactant enhances the desorption of in situ-formed surface-active species, which leads to the further reduction of IFT. Adding the polymer increases the viscosity, which restricts the diffusion of the alkaline, surfactant, and in situ-formed surface-active species.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-777-2696801. E-mail: zzhanghaiyan@yahoo. com.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Petroleum Technology Research Centre, Regina, Saskatchewan, Canada, and the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning, China, for the financial support. The authors also give thanks to Ali Deriszadeh for helping with the measurement of TOC.

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REFERENCES

(1) Sun, J.; Sun, L.; Liu, W.; Liu, X.; Li, X.; Shen, Q. Alkaline consumption mechanisms by crude oil: A comparison of sodium

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dx.doi.org/10.1021/ef300498r | Energy Fuels 2012, 26, 3644−3650