Article pubs.acs.org/EF
Chemical Interactions and Demulsifier Characteristics for Enhanced Oil Recovery Applications Duy Nguyen, Nicholas Sadeghi,* and Christopher Houston Nalco Company, Sugar Land, Texas 77478, United States ABSTRACT: In this study, a design of experiments was used to investigate the importance of several parameters (alkaline concentration, anionic surfactant concentration, polymer concentration, temperature, shear rate, water cut, and salinity) and their interactions (i.e., synergism or antagonism) that govern emulsion stability in chemical enhanced oil recovery (CEOR). Emulsion stability decreased with an increase in salinity or water cut. An increasing surfactant concentration, polymer concentration, temperature, or shear rate enhanced emulsion stability. One of the main contributions for the tight emulsion from alkaline surfactant polymer (ASP) flood was the addition of alkaline. The surfactant, alkaline, and polymer decreased the size of oil droplets, increased the surface charge of oil droplets, and increased the film elasticity, thereby making oil−water separation difficult. Selected cationic surfactants (patents pending) proved much more effective than conventional non-ionic resins and polymeric cationic flocculants in separating oil-in-water emulsions. The chemistry was also investigated by studying the effect of alkyl chain length (C8−C18) of benzyl and methyl quaternary compounds (quats) on demulsifying efficiency. As the surfactant concentration in the brine decreased, the concentration of the cationic demulsifier required to separate the emulsion decreased and the optimum chain length of the cationic demulsifier also changed. The particle video microscope and focused beam reflectance measurement probes showed a significant increase of the size of oil droplets and reduction in the number of oil droplets in the presence of a cationic surfactant. This is in agreement of a decrease of the anionic charge on the surface of the oil droplets and a reduction of the film elasticity in the cationic system. Measurements of interfacial properties, such as the interfacial tension reduction rate, interfacial tension, elastic modulus, and ζ potential, at the oil/brine solution interface were also conducted. A qualitative correlation was found between the interfacial tension reduction rate, elastic modulus, ζ potential, and phase separation. The interfacial tension reduction rate decreased, ζ potential became less negative, elastic modulus decreased, and the size of oil droplets remarkably increased when a cationic demulsifier or an amphoteric demulsifier (patents pending) was added to the emulsion. However, there appears to be no direct correlation with interfacial tension. Without direct information, this preliminary correlation may provide guidelines for selecting demulsifiers for emulsions produced by chemical enhanced oil recovery.
1. INTRODUCTION Primary and secondary recovery techniques together are able to recover only about 35−50% of the oil in place. Because there is a significant amount of oil remaining in the reservoir after the primary and secondary processes have been used, chemical flooding (using surfactants, polymers, and sometimes alkali) has been one of the technologies that can be used to recover up to an additional 35%. It is estimated that several hundred chemical enhanced oil recovery (CEOR) field trials have been conducted over the past 50 years, with many occurring during the 1970s and 1980s.1 Chemical flooding today has been reinvigorated by the introduction of more cost-effective surfactants and polymers coupled with improved reservoir modeling. Globally, it is estimated that there are over 20 CEOR projects in field trial or commercial stage in 2011. Additionally, the current (April 2011) $100/barrel crude oil price environment continues to stimulate interest in new projects by super majors, national oil companies, and independents. Although there have been more than 100 field trials for EOR, most of the efforts were focused on optimization toward the surfactant in the formation. Conversely, there have been limited studies on the separation of the produced fluids at the surface. Emulsions created by chemical flooding have been extremely difficult to break because of the high concentration of © 2012 American Chemical Society
alkali, surfactant, and polymer tightly bound with the oil and water. Traditional non-ionic demulsifiers have a limited effect on the chemical emulsions. Some early 1980s chemical floods were known to require several thousand parts per million (ppm) demulsifier to break the emulsion. Many compounds have been proposed or used as demulsifiers. One type of widely used demulsifier capable of displacing significant portions of the asphaltene layers in nonchemical EOR emulsions and promoting coalescence consists of ethoxylated and/or propoxylated alkylphenol formaldehyde (APF) resins, with molecular weights of a few thousand daltons.2 As shown in Figure 1, the addition of conventional demulsifiers of the type described above produced some separation; however, it was incomplete, and the water content of the oil phase was unacceptable [11% basic sediment and water (BS&W)]. Adding aluminum or conventional cationic polymeric flocculants to the emulsion was also unsuccessful at resolving the emulsion and caused viscous sediments at the bottom. In our previous work,3 it was shown that cationic Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: November 16, 2011 Revised: January 26, 2012 Published: January 31, 2012 2742
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Figure 1. (a and b) Conventional demulsifiers versus (c) patent-pending cationic demulsifer in surfactant and polymer flood.
surfactants are effective in separation of produced emulsions from surfactant polymer (SP) and alkaline surfactant polymer (ASP) processes for chemical enhanced oil recovery (CEOR), which typically use anionic surfactants. This paper will discuss a new series of patent-pending cationic demulsifiers that has been created that addresses the unique nature of CEOR emulsions.
Table 2. Surfactant/Polymer Flooding Produced Water Formulations produced water NaCl (%) CaCl2·2H2O (%) polymer (%) surfactant (sulfate) (%) surfactant (sulfonate) (%) isobutyl alcohol (%)
2. EXPERIMENTAL SECTION 2.1. Materials. Pure cationic surfactants of the types, alkyltrimethylammonium halide (C6−C18) and alkyldimethylbenzylammonium halide (C10−C18), were obtained from Sigma-Aldrich. The structures of the cationic surfactants are shown below in Figure 2. The crude oils
1.0 0.18 0.12 0.15 0.05 0.4
Table 3. Alkali/Surfactant/Polymer Concentrations in Produced Water from ASP Flooding produced water Na2CO3 (%) NaCl (%) CaCl2·2H2O (%) MgCl2·6H2O (%) NaHCO3 (%) polymer (%) surfactant (sulfonate) (%) surfactant (sulfate) (%) isobutyl alcohol (%)
Figure 2. Cationic surfactant structure: (above) alkyltrimethylammonium and (below) alkyldimethylbenzylammonium.
1.0 1.0 0.015 0.139 0.043 0.15 0.113 0.038 0.75
Table 4. High Surfactant/Polymer Flooding Produced Water Formulations
were from the U.S.A., and their physical and chemical properties are listed in Table 1. The SP oil was received from the Permian Basin, and
produced water
Table 1. Crude Oil Physical Properties API (at 20 °C) viscosity (at 20 °C, cP) saturates aromatics resins asphaltenes
SP oil
ASP oil
30.03 13.10 15.4 56.42 4.49 1.61
28.92 17.95 50 8.7 4.3 0.3
NaCl (%) CaCl2·2H2O (%) polymer (%) surfactant (sulfate) (%) surfactant (sulfonate) (%) isobutyl alcohol (%)
1.0 0.18 0.12 0.3 0.1 0.4
to prevent uneven solubilizing of the polymer and result in consistent viscosity. The emulsion (100 mL) was produced in the lab by mixing the produced water (i.e., brine solution) containing polymer and surfactants with the oil by shaking the 6 oz prescription bottle mechanically for 10 min using a heavy-duty, two-speed Eberbach shaker. The demulsifier was added to the above emulsion, and the bottle was again shaken for another 3 min. The amounts (ppm) of demulsifiers added were based on the volume of the emulsion. The ratios of produced water/oil used were varied from 1:1 to 9:1. 2.3. Particle Video Microscope (PVM). The PVM is from Mettler-Toledo Lasentec and consists of six near-infrared (NIR) lasers, which illuminate a small area in front of the probe face. The probe records high-quality images even in dark and concentrated suspensions or emulsions in real time. Droplets between 2 and 1000 μm can be detected.
the ASP oil was received from the Illinois Basin. The polymer [partially hydrolyzed polyacrylamide (HPAM)] from the SNF Company has a molecular weight of 8 × 106 and 30% hydrolysis. Alcohol propoxylate sulfate and alkyl sulfonate surfactants were obtained from the Stepan Company. Brine solutions were prepared using various inorganic salts obtained from Sigma-Aldrich. Diethylene glycol monobutyl ether (DGBE) and isobutanol co-solvents were purchased from Aldrich Chemical. 2.2. Brine Preparation and Emulsion Generation. A synthetic SP/ASP fluid solution was prepared with the composition shown in Tables 2, 3, or 4. The polymer was slowly added to the brine and stirred overnight to allow for complete hydration. This was intended 2743
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2.4. Interfacial Tension (IFT). The equilibrium IFTs of brine and crude oil after phase separation were measured at 22 °C, using a University of Texas spinning drop tensiometer. Aliquots of oil and water were withdrawn with a 25 mL syringe equipped with a 2 in. needle. Each aqueous phase was brought to the temperature in the instrument as the continuous phase. Then, 1.0 μL drop of oil phase was introduced as the drop phase, and the system was spun at variable speeds. 2.5. Demulsification Tests. Demulsification tests were conducted in graduated 6 oz prescription bottles to allow for rapid water drop readings. All bottles used 100 mL of emulsion. After pouring the emulsion followed by chemical addition, the bottles were allowed to reach the separator-desired temperature via a water bath. Upon reaching the desired temperature, the samples were shaken via a mechanical shaker and then returned to the water bath. Water drop readings were recorded in milliliters as a function of time. Water drop values were also used to gauge emulsion stability, where a faster water drop indicated lower emulsion stability. Water quality was determined by visual comparison against the other samples. For the purpose of this study, water quality was examined on a qualitative basis. After the oil drop readings, the resolved or partially resolved oil from each bottle was analyzed for water content. Using a syringe with a needle, a small portion of the oil (about 6 mL) was withdrawn. The tip of the syringe was set to 10−15 mL above the theoretical oil−water interface as determined by the slug grindout value. This aliquot of oil was added to a graduated API centrifuge tube containing an equal volume of an aromatic solvent, and the contents were shaken by hand. The centrifuge tubes were then centrifuged on high speed for 3 min. After centrifugation, the percent residual emulsion, typically referred to as BS, and the percent W were noted for each bottle. After BS&W values were recorded, alkyl sulfonate surfactant (a chemical known to resolve the remaining emulsion) was added to the centrifuge tube. Such chemicals are generally called “slugging or knockout chemicals” and are typically low-molecular-weight sulfonate-based materials. After slugging, the tube was again shaken and centrifuged as previously described. The BS was therefore completely eliminated, and only water remained in the bottom part of the tube. The slug grindout number is reported as a percentage. Smaller values of BS&W and slug indicate drier oil. The time (in minutes) elapsed for the total volume of water to separate is taken as a measurement of the emulsion stability. 2.6. Determination of ζ Potential of Oil Droplets. The ζpotential measurements were conducted on a Horiba DT-1200, acoustic and electroacoustic spectrometer, by means of colloid vibration current (CVI). Basically, an ultrasonic wave is introduced and disturbs the double layer. The displacement of the ionic cloud creates a dipole moment. The sum of these dipole moments over many particles creates an electrical field that causes the CVI to flow. The instrument measures the CVI from which the ζ potential is determined from an empirical equation. Details about the technique can be found elsewhere.4 One of the main advantages of electroacoustics over the traditional electrokinetic methods is the elimination of dilution because the instrument can measure up to 50% volume of dispersed liquids or solids. When diluted, droplet−droplet interactions become less important, which is not true for the systems with which we are dealing. An emulsion at 70% water cut was made as described previously and sat for 2 h without agitation. The emulsion was then stirred gently with a stirring bar, and the ζ potential was measured.
and nature of the hydrophilic group, and also was inaccurate when comparing different surfactant families. In 1954, Winsor6 introduced the R ratio that describes the molecular interaction energies between the surfactant adsorbed at the interface and the water and oil phases surrounding it. Several authors7,8 have used Winsor’s R parameter and showed that a maximum in demulsification performance corresponds to a physicochemical condition for which R = 1. At such a condition, Salager et al.9,10 also observed that minimum stability occurs with emulsions made with oil−surfactant−water systems. However, the R ratio remains qualitative and is limited because the molecular interaction energies cannot be determined experimentally. In 1964, Shinoda11 developed a method based on the determination of the phase inversion temperature (PIT) of a surfactant−oil−water mixture heated under stirring. This method is comparable to the cloud-point phenomenon associated with non-ionic surfactants and is more reliable than the above methods because it takes into account various variables, such as surfactant, oil, salinity, and co-surfactant, which affect the PIT. However, the PIT method is applicable only to non-ionic surfactants because ionic surfactants are much less sensitive to the temperature. In the 1970s, the drive of EOR stimulated researchers to improve upon Shinoda’s PIT and quantify Winsor’s R parameter. Salager et al.12 introduced the hydrophilic− lipophilic deviation (HLD) concept, a dimensionless numerical expression that is expressed as a linear relationship including all formulation variables, such as the nature of the surfactant, oil nature, salinity nature and concentration, and presence of alcohol co-surfactant, as well as the temperature. The HLD has been used as a quantitative design tool in the crude oil dehydration process to determine regions where macroemulsions are likely to break easily.13 The emulsion stability is very low when the surfactant has exactly the same affinity for both oil and water phases. Such formulation for the emulsion quickest breaking has been called “optimum” formulation and has been observed for a variety of surfactant−water−oil systems.7,13−15 HLD is a dimensionless number expressed, for anionic surfactants, by the following relation:16 HLD = σ + ln S − k ACN − t(T − 25 °C) + aA
(1)
where ACN is the carbon number of the n-alkane, which is replaced by the equivalent alkane carbon number (EACN) for non-alkane oils, A and S are alcohol and salt concentrations, σ is a characteristic parameter of the anionic surfactant, which increases linearly with the length of the lipophilic tail, and k, t, and a are positive constants, which depend upon the surfactant headgroup and electrolyte. As can be seen, the HLD value takes into account not only the HLB of the surfactant but also the temperature, nature, and concentration of the co-surfactant (e.g., alcohol), the electrolyte, and the nature of the oil phase. When the HLD value is zero or very close to zero, an optimum formulation is attained, indicating that the emulsion is easy to break or very unstable. When HLD is positive or negative, the emulsion type is oil-in-water (O/W) or water-in-oil (W/O), respectively. To obtain an optimal formulation with a system that already contains an anionic surfactant(s), in which σ is negative (CEOR where HLD < 0), a demulsifier must be added to ensure the characteristic parameter of the mixture of the anionic surfactant and demulsifier results in HLD = 0 or close to zero, corresponding to minimum emulsion stability. This can be performed by adding a cationic surfactant to form ion pairs
3. RESULTS AND DISCUSSION 3.1. Effect of Formulation on Emulsion Stability. Emulsion stability depends upon both the nature of the surfactant and the oil. In 1949, the hydrophilic−lipophilic balance (HLB) concept was introduced by Griffin5 to describe the relative affinity of a surfactant for water and oil. The simplicity of the HLB concept was its main advantage, but it had several limitations, such as it did not take into account the effects of the temperature, salinity, co-surfactant concentration, 2744
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with an anionic surfactant, resulting in a positive value of σ.17 These authors showed that the ion pairs increased the detergency efficiency significantly. Interestingly, by applying the HLD concept, we have discovered that selected cationic surfactants (patents pending) accelerated separation of O/W emulsions representative of produced emulsions expected during CEOR processes and yielded oil and water phases with greatly improved quality compared to the initial emulsion. Furthermore, the concentration of demulsifier when combined with the anionic surfactant must produce HLD = 0. This is in agreement with the observation that dehydration conditions are specific and that a small change in the demulsifier concentration (above or below the optimum) or in the temperature may cause the emulsion shifting from being unstable to stable. Also, because this optimum formulation corresponds to the proper mixture of demulsifier and anionic surfactant at the interface, it depends upon the anionic surfactant concentration, as will be discussed in this paper. 3.2. Characterization of Oil. As shown in Table 1, the physical and chemical properties of the oils were measured and characterized. The American Petroleum Institute (API) gravity and viscosity were measured at ambient temperature. SARA analysis was performed to identify the total saturates, aromatics, resins, and asphaltenes. Resins and asphaltenes are known to stabilize the emulsion for water in oil emulsions and for nonEOR applications. 3.3. Characterization of Produced Water. As shown in Tables 2, 3, and 4, the concentrations of surfactants and polymer in the produced water from SP and ASP floodings are above 1000 ppm. Surfactants and polymer were mainly responsible for the stability of oil droplets. Polymer increased the interfacial film between water and oil, decreased the ζ potential (more negative), and blocked aggregation and flocculation of oil droplets. Surfactants decreased the IFT and ζ potential, making oil droplets difficult to approach and coalesce. Alkali, such as sodium carbonate, reacted with acidic components of the crude oil and converted them into natural surfactants, which can further stabilize the emulsion. These SP and ASP formulations were synthetic to test for the accurate concentrations of the components and their effects of emulsion stability. A field sample of these fluids would not have the precision needed for the concentrations of salinity, surfactant, or polymer. 3.4. Effects of Cationic Demulsifier Concentrations (Patents Pending) and Salinity. In our previous work,18 dodecyltrimethylammonium bromide (C12TAB) proved very effective at breaking the ASP emulsions at 25 °C. Figure 3
of concentration associated with low stability is so wide (100− 1000 ppm for 3% NaCl), the product is robust, thereby providing a large margin for error if overdosed. In other words, any error in dosing the product close to the optimum formulation would not cause an increase in emulsion stability. However, the addition of C12TAB beyond their optimum (>1000 ppm) resulted in an increase of the emulsion stability (e.g., 3000 ppm), perhaps because of the formation of an elastic and viscous film caused by the excess of demulsifier. We observed two phenomena when the salinity is increased from 1 to 3% (Figure 3). First, the minimum stability occurred at a broader range of the demulsifer concentration. This feature is very beneficial in the field because it indicates a robust condition. Second, below the optimum dosage, the separation time (i.e., emulsion stability) decreased drastically. Because we are dealing with oil-in-water emulsions, the HLD value is negative. To shift the HLD from negative to nearly zero (i.e., minimum stability), one can increase the salinity (see eq 1). This is in agreement with our observation. Also, an increase in salinity caused salting out of the demulsifier (C12TAB) and, particularly, the anionic surfactants (sulfate and sulfonate), resulting in adsorption of these products at the oil−water interface as opposed to being present in the water phase. This will shift the HLD from negative to nearly zero values. In other words, the increase in salinity enhanced the demulsifier efficiency. 3.5. Effect of the Nature of Oil. Figure 4 shows the SP and ASP emulsions treated with alkyltrimethylammonium bromides with different alkyl chain lengths (C8−C18). The test was run at 25 °C. The optimum chain length shifted from C8 (SP flood) to C12 (ASP flood). The molar ratio of cationic/anionic for treatment of 200 ppm octyltrimethylammonium bromide (C8TAB) is 1:3. A ratio of 1:1 or greater can cause precipitation. It has been shown that cyclization, in particular with double bonds, reduces the equivalent alkane carbon number (EACN).19 For example, the EACN is 6 for hexane, 3.5 for cyclohexane, and −3 for benzene. As shown in Table 1, the SP crude oil contains much more aromatics than saturates, while the ASP crude oil contains more saturates than aromatics. Therefore, an increase in EACN (from SP crude oil to ASP crude oil) required an increase in the hydrophobic group present in the alkyltrimethylammonium bromide. For this reason, a transition from HLD ∼ 0 (SP flood) to HLD < 0 (ASP flood) occurs for C8TAB; hence, emulsion stability increased. For the same reason, a transition from HLD > 0 (SP flood) to HLD ∼ 0 (ASP flood) takes place for C12TAB; hence, emulsion stability decreases. The water quality of the optimum-treated emulsion was considerably better than the untreated emulsion. Additional treatment can be performed on the separated water to improve the water further in conditions of especially high O/W concentrations (>1000 ppm O/W). In the field, this can be performed by chemical treatment, such as water clarifiers. It can also be via mechanical means, such as corrugated plate interceptors, hydrocyclone and membrane filters, or flotation cells. 3.6. Effect of the Temperature. Increasing the temperature from 25 to 60 °C increased the emulsion (ASP) stability for the treated and untreated samples (Figure 5). The following may explain the observation. An increase in the temperature leads to an increase in the interactions between the surfactant and the water phase, because of the dissociation of the ionic surfactant. As a result, the surfactant becomes a little more hydrophilic, causing a more negative HLD value. Another explanation could be the chemistry behavior of the propoxylated sulfate. The propylene oxide chains would cause these surfactants to become less hydrophilic with an increasing
Figure 3. Stability of the emulsions at 20% oil cut in ASP flood for 1 and 3% NaCL as a function of the C12TAB concentration.
shows the emulsion stability variation, as a funcion of the C12TAB concentration for 1 and 3% NaCl. Because the stability curve is so flattened near the minimum and the range 2745
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Figure 4. SP and ASP bottle tests for 200 ppm alkyltrimethylammonium bromide demulsifiers at 25 °C and 30% oil cut.
while another emulsion set was set at 60 °C for 1 day and then at ambient temperature for 2 days. The water quality improved for the emulsions after they were removed from the thermal exposure. This leads to the suggestion that the hindrance caused by the thermal exposure of the emulsion is not permanent. The thermal interaction only affects the emulsion while applied. 3.7. Effect of the Anionic Surfactant Concentration. The tests compare the performance of methyl quaternary demulsifiers in the normal-surfactant SP fluid (2000 ppm anionic surfactant; Table 2) to those in the high-surfactant SP fluid (4000 ppm anionic surfactant; Table 4). Tests were run at 25 °C. C8-TAB was the best performing demulsifier for the normal-surfactant SP fluid (2000 ppm anionic surfactant), but C14-TAB had the best performance for the high-surfactant SP fluid (4000 ppm anionic surfactant) (Figure 7). The alkyltrimethylammonium bromide demulsifiers were tested at a higher dosage, 300 ppm, in the high-surfactant SP fluid (Figure 8). The C8-TAB performance increased with the higher dosage; however, C12-TAB and C14-TAB still exhibited the best performance at the 4000 ppm surfactant fluid. This suggests a directly proportional relationship between the demulsifier concentration and the surfactant concentration of the SP fluid. It also appears that, with the increase in the surfactant concentration, the optimum chain length of demulsifier also increases. Rondon et al.20 reported that the optimum demulsifier concentration is proportional to the asphaltene concentration for W/O emulsions. 3.8. Effect of the Water/Oil Ratio (WOR). The effect of the WOR on emulsion stability is complicated and has been investigated by several authors.21,22 First, because the WOR is varied, the anionic surfactants (sulfate and sulfonate) and demulsifier (cationic surfactant) mixture or the ion pairs that are adsorbed at the water−oil interface also change. Second, the WOR also affect the emulsion properties, such as morphology, droplet size, and stability.23 Table 5 shows the effect of the WOR on emulsion (surfactant−polymer flood) stability at a fixed demulsifier concentration (200 ppm C8TAB). Interestingly, at this dosage, C8TAB was found to remain effective at various WORs. Conversely, the emulsion stability increased with a decrease in the WOR for the untreated sample.
Figure 5. Thermal tests for ASP emulsions.
temperature. This occurrence could also cause the observed decrease in demulsifier effectiveness at 60 °C by shifting the surfactant phase behavior or HLD value for the original emulsion system. The effect of the duration of the increased temperature on the treated and untreated emulsions was also studied (Figure 6). One emulsion set was exposed to 60 °C for 3 continuous days,
Figure 6. Conditional thermal-treated emulsions. 2746
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Figure 7. Effect of the anionic surfactant concentration in SP emulsions at 22 °C and 200 ppm demulsifier.
Figure 8. Performance of the demulsifier at a higher dosage, 300 ppm, with 4000 ppm anionic surfactant.
Table 5. Effect of Oil Cut on Emulsion Stability Treated with C8TAB at 25 °C with SP Emulsion dosage (ppm)
oil cut (%)
stability (min)
comment
200 200 200 200 200 0 0 0
50 40 30 20 10 50 30 10
8 8 8 8 8 >240 >240 20
good interface good interface good interface good interface good interface rag layer rag layer rag layer
stability became more drastic as the demulsifier chain length increased. The exception was C8TAB, which increased its performance as the dosage increased. Thus, C8TAB is more tolerant in the case of dosing past the optimum range. This stabilization of the emulsions is perhaps due to the formation of an elastic and viscous film caused by the excess of demulsifier. This film stability increases with the increased chain length. 3.11. Effect of the Demulsifier on the Size of Oil Droplets. Using a mechanical shaker, the time of emulsifying was also varied (13 and 33 min), and then the size of oil droplets was observed. The emulsion droplets were observed by an Olympus BX51 research microscope at 10× magnification and recorded with a mounted Olympus Q-Color 5 digital camera. In our recent work with cationic surfactants,18 dodecyldimethylbenzylammonium bromide (C12benzyl-DAB) proved effective at breaking the ASP emulsions at 25 °C. C12BenzylDAB was tested for oil droplet size compared to untreated emulsions. Samples were measured immediately after the mixing ended. As seen from Figures 11 and 12, the droplet size became smaller, especially for the untreated, suggesting that the emulsion was more stable with an increase in the blending time. Furthermore, the treatment with C12benzyl-DAB resulted in larger oil droplets, demonstrating a faster coelescense than the untreated emulsion, regardless of shearing duration.
3.9. Effect of Re-emulsification. After 1 day of phase separation, the untreated and treated samples (100 and 200 ppm C12TAB) were reshaken. Figure 9 shows that, for the treated samples, it took an additional 3 min to achieve 100% separation when compared to the fresh samples. The treated samples, however, produced a much faster phase separation and better water and oil quality than the untreated samples. 3.10. Effect of Demulsifier Overtreatment in Emulsions. The treatment rate of three demulsifiers was investigated by increasing the dose rate beyond the optimum dosage range (Figure 10). Testing was performed with the SP fluid and oil and ambient temperature. Salinity of SP fluid was 1% NaCl. As the treatment rate is increased beyond 200 ppm demulsifier, the emulsion stability began to increase. This increase in 2747
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Figure 9. Effect of re-emulsification for ASP emulsion at 25 °C and 20% oil cut.
time (3 min). PVM helps visualize droplets in dark and opaque systems and monitor droplet rearrangement and droplet morphology in real time. This gave a more accurate representation of the dynamic interactions between the oil and SP fluid. 3.12. IFT Measurements. IFT measurements have been investigated by several authors to give insights into the mechanisms of demulsifier adsorption at the oil−water interface.24,25 Figure 15 shows results from equilibrium IFT measurements on C6TAB, C12TAB, C16TAB, and the untreated samples in ASP emulsions at 22 and 60 °C. The appearance of the bottles of these samples after 1 day was also shown in Figure 15. It is observed that C12TAB and C16TAB exhibited the lowest IFT. The IFTs measured for these samples (0.44 dyn/cm) were about 50% smaller than those measured from the untreated and C6TAB samples (0.93 and 0.87 dyn/cm, respectively). The agreement between the IFT measurements and bottle tests is qualitative; i.e., a good separation with good water quality and low BS&W was observed for the system that exhibited the lowest equilibrium IFT. This finding is consistent with the interpretation of the optimum demulsification efficiency, in which a minimum stability is achieved when the demulsifier adsorbed at the interface exhibits the same affinity for the oil and water phases. Typically, the IFT values for microemulsion−water interfaces range from 0.01 to 0.0001 dyn/cm,26 which are much lower than that measured for C16TAB (0.43 dyn/cm). Perhaps because of the low concentration of demulsifiers used in these experiments (200 ppm), a middle (microemulsion) layer was not observed. Increasing the temperature from 22 to 60 °C produced a lower IFT for the untreated sample and a higher IFT for the C12TAB sample. This finding is consistent with more stable emulsions. 3.13. Effect of the Demulsifier on ζ Potential. Figure 16 shows the effect of the demulsifier dosage (octyltrimethylammonium bromide) on the ζ potentials of oil droplets. It can be seen that ζ potentials of the oil droplets became less negative with an increasing demulsifier concentration, indicating that the ion pairs of a cationic demulsifier and an anionic surfactant can adsorb at the oil−water interface and replace the surfactants on the surface of oil droplets. For example, ζ potential increased from −49 mV with no demulsifier to −28 mV with 400 ppm demulsifier. The decrease in electrostatic repulsion between oil droplets promotes their coalescence. Surfactant and polymer in produced water can adsorb on the surface of oil droplets and change the ζ potential. Because both the polymer and surfactants are anionic, the adsorption of these chemicals increases the density of negative electric charge on the surface of oil droplets, thereby stabilizing the oil droplets via the electrostatic stabilization mechanism. The demulsifier
Figure 10. Stability of the emulsions at 20% oil cut in SP flood as a function of the alkylTAB concentration.
Figures 13 and 14 show the particle video microscope images of the oil droplets in the SP emulsion at 30% oil cut with and
Figure 11. Effect of the blending time on the size of oil droplets. Emulsions were treated with 200 ppm dodecyldimethylbenzylammonium bromide (settling time of 0 min).
Figure 12. Effect of the blending time on the size of oil droplets for the untreated emulsions (settling time = 0 min).
without the addition of the demulsifier (C8TAB) at 0 and 60 min, respectively. The size of oil droplets for the untreated was about 50 μm and did not change much after 15 min. On the other hand, initially larger droplets were observed when the emulsion was treated with 500 ppm C8TAB and coalesced in a short period of 2748
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Figure 13. PVM images for the untreated sample in the SP emulsion at 30% oil cut.
Figure 14. PVM images for 500 ppm C8TAB in the SP emulsion at 30% oil cut.
4. CONCLUSION Ion pairs formed between the cationic demulsifiers (patents pending) and anionic surfactants may explain the accelerated separation of O/W emulsions with good oil and water quality during CEOR processes. At the optimum demulsification efficiency, the IFT is lowest, suggesting that the ion pairs exhibit the same affinity for both phases. A PVM confirmed that 500 ppm C12TAB produced significant coalescence shortly after it was added to the emulsions. The cationic surfactant decreased the anionic charge on the surface of the oil droplets, thereby facilitating the approach and coalescence of oil droplets. This is in agreement with an increase of the oil droplet size in the presence of the demulsifier. The WOR of the SP emulsion has a significant affect on the emulsion breaking rate for the untreated sample; however, when treated with 200 ppm C8TAB, the emulsion breaking rate is quick and essentially the same for a variety of WORs. This suggests that C8TAB is quite robust. Increasing the temperature and decreasing the salinity enhance the emulsion stability.
Figure 15. Effect of the chain length on equilibrium IFTs of ASP emulsions treated with alkyltrimethylammonium bromide.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Nalco for permission to publish this work and express sincere gratitude to Professors George Hirasaki and Clarence Miller at Rice University for valuable discussion.
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Figure 16. Effect of the demulsifier dosage (C8TAB) on ζ potential.
decreased the anionic charge on the surface of oil droplets, lowered the film elasticity, and decreased the IFT reduction rate, making phase separation faster.
REFERENCES
(1) Thomas, S. Chemical EOR: The pastDoes it have a future? SPE Distinguished Lecture; Society of Petroleum Engineers (SPE): Richardson, TX, 2005.
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(2) Krawczy, M.; Wasan, D.; Shetty, C. Chemical demulsification of petroleum emulsions using oil-soluble demulsifiers. Ind. Eng. Chem. Res. 1991, 30, 367−375. (3) Hirasaki, G.; Miller, C.; Raney, O.; Poindexter, M.; Nguyen, D.; Hera, J. Separation of produced emulsions from surfactant enhanced oil recovery processes. Energy Fuels 2010, 25, 555−561. (4) Dukhin, A.; Goetz, P. Acoustic and electroacoustic spectroscopy for characterizing concentrated dispersions and emulsions. Adv. Colloid Interface Sci. 2001, 92, 73. (5) Griffin, W. C. Classification of surface-active agents by HLB. J. Soc. Cosmet. Chem. 1949, 1, 311−326. (6) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworth: London, U.K., 1954. (7) Goldszal, A; Bourrel, M. Demulsification of crude oil emulsions: Correlation to microemulsion phase behavior. Ind. Eng. Chem. Res. 2000, 39, 2746. (8) Pena, A. A.; Hirasaki, G.; Miller, C. Chemically induced destabilization of water-in-crude-oil. Ind. Eng. Chem. Res. 2005, 44, 1139−1149. (9) Salager, J. L.; Minana-Perez, M.; Perez-Sanchez, M.; RamirezGouveia, M.; Rojas, C. Surfactant-oil-water systems near the affinity inversion. 3. The two kinds of emulsion inversion. J. Dispersion Sci. Technol. 1983, 4, 313. (10) Salager, J. L. Emulsion properties and related know-how to attain them. In Pharmaceutical Emulsions and Suspensions; Nielloud, F., Marti-Mestres, G., Eds.; Marcel Dekker: New York, 1998. (11) Shinoda, K.; Arai, H. The correlation between phase inversion temperature in emulsion and cloud point in solution of nonionic emulsifier. J. Phys. Chem. 1964, 68, 3485. (12) Salager, J. L.; Marquez, N.; Graciaa, A.; Lachaise, J. Partitioning of ethoxylated octylphenol surfactants in microemulsion−oil−water systems: Influence of temperature and relation between partitioning coefficient and physicochemical formulation. Langmuir 2000, 16, 5534. (13) Salager, J. L. The fundamental basis for the action of a chemical dehydrant. Influence of the physical and chemical formulation on the stability of an emulsion. Int. Chem. Eng. 1990, 30, 103−116. (14) Rondon, M.; Bouriat, P.; Lachaise, J. Breaking of water-in-crude oil emulsion. 1. Physicochemical phenomenology of demulsifier action. Energy Fuels 2006, 20, 1600−1604. (15) Bourrel, M.; Graciaa, A.; Schechter, R.; Wade, W. H. The relation of emulsion stability to phase behavior and interfacial tension of surfactant systems. J. Colloid Interface Sci. 1979, 72, 161−163. (16) Salager, J. L.; Morgan, J. C.; Schechter, R. S.; Wade, W. H.; Vasquez, E. Optimum formulation of surfactant/water/oil systems for minimum interfacial tension or phase behavior. Soc. Pet. Eng. J. 1979, 19, 107−115. (17) Kiran, S. K.; Acosta, E. J.; Malhotra, V. K. Microemulsion modeling of anionic−cationic mixtures for use in detergency. Proceedings of the 101st American Oil Chemists’ Society (AOCS) Annual Meeting and Expo; Phoenix, AZ, May 16−19, 2010. (18) Nguyen, D.; Sadeghi, N. The selection of the right demulsifier for chemical enhanced oil recovery. Proceedings of the Society of Petroleum Engineers (SPE) International Symposium on Oilfield Chemistry; The Woodlands, TX, April 11−13, 2011; SPE 140860. (19) Salager, J. L.; Anton, R.; Forgiarini, A.; Marquez, L. Formulation of Microemulsion in Microemulsions: Background, New Concepts, Applications, Perspectives; Stubenrauch, C., Ed.; Blackwell Publishing, Ltd.: Oxford, U.K., 2009; Chapter 3. (20) Rondon, M.; Pereira, J. C.; Bouriat, P.; Graciaa, A.; Lachaise, J.; Salager, J. L. Breaking of water-in-crude oil emulsions. 2. Influence of asphaltene concentration and diluent nature on demulsifier action. Energy Fuels 2008, 22, 702−707. (21) Borges, B.; Rondon, M.; Sereno, O.; Asuaje, J. Breaking of water-in-crude oil emulsion. 3. Influence of salinity and water−oil ratio on demulsifier action. Energy Fuels 2009, 23, 1568−1574. (22) Salager, J. The fundamental basis for the action of a chemical dehydrant. Influence of the physical and chemical formulation on the stability of an emulsion. Int. Chem. Eng. 1990, 30, 103−116.
(23) Salager, J. L.; Minana-Perez, M.; Perez-Sanchez, M.; RamirezGouveia, M.; Rojas, C. Sorfactant−oil−water systems near the affinity inversion. Part III: The two kinds of emulsion inversion. J. Dispersion Sci. Technol. 1983, 4, 313. (24) Goldszal, A.; Bourrel, M. Demulsification of crude oil emulsions: Correlation to microemulsion phase behavior. Ind. Eng. Chem. Res. 2000, 39, 2746−2752. (25) Breen, P. J. Adsorption kinetics of demulsifiers to an expanded oil−water interface. In Surfactant Adsorption and Surface Solubilization; American Chemical Society (ACS): Washington, D.C., 1995; ACS Symposium Series, Vol. 615, Chapter 18, pp 268−279. (26) Salager, J. L. Guidelines for the formulation, composition and stirring to attain desired emulsion properties. In Surfactants in Solution; Chattopadhyay, A. K., Mittal, K. L., Eds.; Marcel Dekker: New York, 1996; Surfactant Science Series 64.
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NOTE ADDED AFTER ASAP PUBLICATION This paper published February 16, 2012 with an incorrect Figure 7. The correct version published March 22, 2012.
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dx.doi.org/10.1021/ef201800b | Energy Fuels 2012, 26, 2742−2750
