Heavy Oil System

ARTICLE pubs.acs.org/EF

Effect of Added Surfactants in an Enhanced Alkaline/Heavy Oil System Siwar Trabelsi,† Jean-Franc-ois Argillier,*,† Christine Dalmazzone,† Anthony Hutin,† Brigitte Bazin,† and Dominique Langevin‡ † ‡

IFP Energies nouvelles, 1-4 avenue de Bois Pr
eau, 92852 Rueil-Malmaison Cedex, France Laboratoire de Physique des Solides, Universit
e Paris-Sud, CNRS UMR8502, B^atiment 510, 91405 Orsay Cedex, France ABSTRACT: We report an experimental study of surfactant-enhanced alkaline/diluted heavy oil systems to understand the combined effect of different parameters, such as pH, salinity, surfactant type, and concentration, on diluted heavy oil/brine phase interfacial behavior. The interfacial tension (IFT) was measured using pendant drop and spinning drop tensiometers. Different surfactants [Triton X405, sodium dodecyl sulfonic acid (SDS), and sodium dodecylbenzene sulfonate (SDBS)] were tested. SDBS was by far the most efficient surfactant, reducing the IFT to ultra-low values of ∼4  10 4 mN/m at a concentration of only 0.05% and a pH of 11. This efficiency is due to a synergistic effect between the in situ surfactant (produced by saponification of the acid groups present in the crude oil) and the added surfactant (SDBS).

’ INTRODUCTION A large part of previously unrecoverable underground oil can be obtained by enhanced oil recovery (EOR) using a combination of alkaline chemicals and surfactants. Because of the high interfacial tension (IFT) between the crude oil and reservoir brine, oil remains trapped in the reservoir rock. If the IFT can be reduced to an ultra-low value (less than 0.01 mN/m),1 an important fraction of the residual oil trapped in the porous media can be mobilized. The investigation of alkaline EOR has been widely addressed.2 4 It has been shown that the added alkali will react with the acidic groups of the crude oil to generate interfacially active components called in situ surfactants that accumulate at the oil/water interface and reduce the IFT.5 15 For surfactant-enhanced alkaline/acidic oil, most studies deal with acidic crude oil16 18 or a model oil obtained by the addition of fatty acids to the oil phase, to simulate an acidic crude oil.19 21 The addition of alkaline chemicals is very beneficial for reducing the retention of anionic surfactants to the rock surface and lowering IFT. It has been shown that the addition of a properly chosen water-soluble surfactant to the alkaline additives raises the concentration of electrolyte required for minimum IFT (optimum salinity) for optimal oil recovery3 and that the optimal salinity decreases with increased surfactant molecular weight22 or increased surfactant concentration.23 Martin et al.16 investigated a surfactant-enhanced alkali/sand oil [36.7° American Petroleum Institute (API)] system. They observed that combining surfactant with various alkalis lowered the IFT of the system and that potassium hydroxide was the most efficient alkali in lowering optimal salinity. They also found that the IFT of their system is independent of the pH of the alkaline solution, probably because of the low amount of acid groups present in their crude oil. Rudin et al.19 attributed the large decrease of the IFT between an alkali containing petroleum sulfonate surfactants and an acidic model oil at low pH to a synergism between the added surfactant, ionized acids, and un-ionized acids in forming mixed micelles and a mixed adsorbed interfacial layer. At high pH, where all of the acid groups have been ionized, they found that the IFT with and r 2011 American Chemical Society

without surfactant is the same, meaning that ionized acids are the dominant species. Liu et al.24 investigated crude oil (28° API and 0.2 mg of KOH/g) phase behavior in the presence of low surfactant concentrations and alkaline additives. They attributed the low IFT values to the formation over time of a thin layer of colloid dispersion rich in soap at the interface between the microemulsion lower phase and excess oil. IFT measurements were also conducted for surfactant-enhanced alkali/heavy crude oil.25,26 It was found that the addition of a small amount of surfactant to alkaline solutions was very effective in reducing the IFT of the system, consequently emulsifying the heavy crude oil in brine. Most of the previous studies addressed only the influence of one parameter: pH, salinity, surfactant type, surfactant concentration, or type of oil (diluted crude oil or model crude oil). Therefore, there is still a need to study the effects of combining the different parameters on the interfacial behavior. The aim of this paper is to understand the combined effects of surfactants and alkalis on the IFT of a diluted heavy oil and to compare the interfacial behavior of model acidic oils (largely studied in the literature) to our diluted heavy oil. In the experiments reported here, pH, salinity, and surfactant concentration were varied to explore the influence of these parameters on the diluted heavy oil interfacial behavior. Different surfactants were tested to identify the surfactant that could enhance the interfacial behavior of our crude oil. Both pendant and spinning drop tensiometers were used to measure the IFT.

’ MATERIALS AND METHODS Samples were prepared from a 9° API Venezuelan heavy crude oil. The saturates, aromatics, resins, and asphaltenes (SARA) analysis gives 12% saturates, 37% aromatics, 33% resins, and 17% asphaltenes (obtained with pentane). This heavy oil is acidic, with a total acid Received: January 10, 2011 Revised: March 16, 2011 Published: March 21, 2011 1681

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Energy & Fuels number (TAN) of 4.2. Our diluted heavy oil has been prepared by dilution of the Venezuelan heavy oil in toluene (3.4 weight dilution, i.e., 2.4 of toluene per gram of heavy oil) to obtain a crude oil containing 5% asphaltenes. Asphaltene extraction from the heavy oil was performed using the American Society for Testing and Materials (ASTM) 863-69 standard. The crude oil was first stirred with an excess of n-pentane (1 g of oil and 30 mL of pentane). The precipitated fraction was filtered, dried, and ground. Rectapur toluene (99.9%) was used as a solvent, for both diluting the heavy oil and solubilizing the extracted asphaltenes. The extracted asphaltenes were simply solubilized with toluene to obtain the model oil. Ultrapure water from a Millipore Milli-Q UV system (resistivity of 18.2 MΩ/cm) was used as the aqueous phase. The pH of the aqueous phase was adjusted to the desired value by adding an aqueous solution of 30% sodium hydroxide. NaCl (Fisher Scientific) was added to increase the ionic strength of the aqueous phase. Except when noted, 5 g/L NaCl aqueous solutions were used. Three different surfactants, sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfonic acid (SDS), and Triton X405 (ethoxylated nonylphenol) were tested. SDS (98%) and Triton X405 (98%) were obtained from Sigma Aldrich. SDBS (>95%) was purchased from TCI Europe. SDBS solutions were heated for 10 min at 30 °C to facilitate solubilization. Before dilution, the heavy oil was also heated for 30 min at 60 °C before each experiment to ensure homogenization. Fresh surfactant and diluted heavy oil solutions were prepared daily. The IFT was measured with a commercial pendant drop tensiometer (Tracker from IT Concept, now Teclis) and a spinning drop tensiometer (Kr€uss). We used the pendant drop tensiometer for IFT larger than 2 mN/m and the spinning drop technique for IFT smaller than 2 mN/m. Aqueous solutions and diluted heavy oil were mutually saturated before each experiment, with oil and water, respectively, to minimize transfer across the crude oil/ aqueous phase interface. In both techniques, a drop of crude oil was contacted with a large volume of alkaline phase. Thus, very few hydroxide groups were consumed to produce in situ surfactant, and the pH did not change. For SDS and Triton X405, the measured IFT was observed to reach equilibrium values after a few minutes, independent of the pH and the surfactant concentrations. The same result was observed for SDBS at low pH as well as at high pH but only for surfactant concentrations lower than 0.02%. When the surfactant concentration was increased above 0.02%, the surface of the elongated drop became non-uniform and rough after a few minutes in the spinning drop machine, preventing IFT measurements at longer times. Such shape instabilities are probably due to the ultra-low IFT values. All of the IFT values presented in this paper are collected after 600 s and represent the average of at least six measurements. All of the measurements were performed at room temperature (23 ( 1 °C).

’ RESULTS Effect of Alkali. The influence of pH on the diluted heavy oil/ alkaline aqueous phase IFT is shown in Figure 1. At low pH (pH < 10), the effect of the added alkali is not significant and the IFT remains almost constant at ∼23 mN/m. Above pH 10, we were not able to use the pendant drop tensiometer to measure the IFT of the system because the drop immediately detaches from the capillary after formation. Instead, we used the spinning drop technique. Just after injection of the diluted heavy oil drop into the rotating capillary, the drop was observed to elongate spontaneously at low rotation velocity (∼1000 rpm) and fragment into smaller droplets (Figure 2a). This observation is due to an ultralow IFT between the diluted heavy oil and the alkaline solution. The acid groups present in the crude oil, such as asphaltenes and so-called naphthenic acids, react with the alkaline solution and

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Figure 1. IFT between diluted heavy oil and the alkaline phase versus pH.

produce surface-active molecules (in situ surfactant). These molecules accumulate easily at the diluted heavy oil/alkaline aqueous phase interface and reduce the IFT. After the fragmentation of the initial drop, the IFT of the droplets was measured as a function of time (Figure 2b). The IFT increases with time and, after 600 s, reaches a value of ∼1.2 mN/m. The IFT increase is probably due to a transfer of the diluted heavy oil molecules or aggregates across the oil/alkaline aqueous phase interface.27 This transfer phenomena is under investigation and will be addressed in an upcoming paper. Effect of Combining Alkali and Surfactant. Non-ionic Surfactant (Triton X405). Figure 3 shows the behavior of the IFT at different pH values when 0.3% Triton X405 was added. At low pH, the addition of the surfactant lowers the IFT from 22 to 5 mN/m, the same IFT value between the Triton X405 solution and toluene. This suggests that the IFT at low pH is controlled by the added surfactant. However, it is likely that oil components are co-adsorbed at the oil water interface. To check for this possibility, the behavior of oil droplets detached from the capillary and coming into contact with the air water interface was visually observed. In the presence of surfactant, the lifetime of diluted crude oil droplets is much longer than without surfactant and ever longer than with toluene droplets and surfactant. This is a clear indication that a mixed layer of surfactant and diluted heavy oil components is adsorbed at the surface of the drops, as demonstrated in earlier studies of similar systems.28 When the pH is increased above 10, the IFT drops and reaches the IFT of the diluted heavy oil without added surfactant, meaning that the ionized acids of the diluted heavy oil contribute to the IFT. Anionic Surfactants (SDBS and SDS). Figure 3 shows the evolution of IFT as a function of pH when 0.1% SDBS was added. At low pH, the addition of SDBS decreases the IFT from 23 to 0.08 mN/m. Upon a further increase of pH, the IFT decreases dramatically to ultra-low values of ∼2  10 4 mN/m. Independent of the pH value, the measured IFT is lower than the one measured for the toluene/surfactant solution and the brine (without added surfactant)/diluted heavy oil. This is probably due to a synergistic effect between the in situ surfactant and the added surfactant while forming an adsorbed mixed interfacial 1682

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Figure 2. (a) Video images of the elongation and fragmentation of a drop into smaller droplets (the elongation and fragmentation occur in a few seconds). (b) Dynamic IFT between the diluted heavy oil and an aqueous phase containing 5 g/L NaCl at pH 11. We were not able to measure the IFT above 600 s; it was impossible to deform the drop above this period of time (IFT > 2 mN/m).

layer. This behavior is different from the one observed by Rudin et al.19 for a sulfonate petroleum surfactant-enhanced alkaline/ acidic diluted heavy oil system. They found that the added surfactant was efficient in decreasing the IFT to ultra-low values only at low pH. However, at high pH, they showed that the ionized acids of the diluted heavy oil become dominant, resulting in the same IFT with and without added surfactant. This is the same as for Triton X405 in our case, but SDBS reduces the IFT at both low and high pH and the surfactant efficiency is also more pronounced at higher pH (Figure 3). Figure 4 shows the influence of NaCl with and without added alkali. At pH 7, we observe a small decrease of IFT with an increasing salt concentration for the three systems, slightly more pronounced for the diluted heavy oil in the presence of surfactant than for the diluted heavy oil or surfactant alone. At pH 11, this decrease is very important (around 3 decades of IFT) for the mixed system (diluted heavy oil in the presence of SDBS). This result suggests that the optimization of the pH along with the salt concentration is necessary to achieve very low IFT values. We were not able to determine the optimum salinity (lowest IFT) because of SDBS insolubility at higher salt concentrations, which prevented the extension of the IFT measurements. Note that the synergism between SDBS and the indigenous surfactants are predominantly seen at high pH. To have a better understanding of the synergic phenomena, the IFT of SDBS alone was measured using toluene and 60:40 toluene/dodecane (composition closer to the diluted heavy crude). For pure toluene, IFT of 0.1% SDBS at pH 7 with 5 g/L NaCl is close to 0.7 mN/m, whereas in a mixture of 60:40 toluene/dodecane, the IFT decreases to 0.2 mN/m, which is close to the value of 0.1 mN/m obtained with the system “diluted heavy oil with 0.1% SDBS”. IFT of the SDBS

Figure 3. IFT between diluted heavy oil and alkaline solutions as a function of pH: (9) without added surfactant, (1) with 0.3% Triton X405 solution, and (2) with 0.1% SDBS solution.

does not significantly depend upon pH for both oils. However, at pH 11 and for a sufficient amount of NaCl, the IFT of the diluted heavy oil with added SDBS significantly decreased (more than 3 decades). This is due to a very important synergism between the SDBS and the in situ surfactants. Another anionic surfactant (SDS) was tested (Figure 5). As with Triton X405, SDS is much less efficient than SDBS. This is possibly due to the fact that SDBS is more hydrophobic than SDS [its critical micellar concentration (cmc) is lower] or due to the presence of the aromatic cycle along SDBS molecules; 1683

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Figure 5. IFT between (2) toluene and SDBS solutions, (9) diluted heavy oil and SDBS solutions, and (1) diluted heavy oil and SDS solutions as a function of the surfactant concentration.

Figure 4. IFT between (O) toluene and 0.1% SDBS solutions, (2) diluted heavy oil and aqueous solutions, and (9) diluted heavy oil and 0.1% SDBS solutions as a function of NaCl concentrations at (a) pH 7 and (b) pH 11. We were not able to measure the IFT between the diluted heavy oil and 4, 5, and 6 g/L NaCl in the presence of 0.1% SDBS at pH 11 because of the fact that the elongated drop deformed before 600 s.

π electrons of the aromatic structure may link to the aromatic rings of diluted heavy oil components. With SDBS being the most efficient surfactant, it was used it in the rest of the study. The IFT between SDBS solutions and toluene is shown in Figure 5. One sees in the figure that the cmc is equal to 0.005% (the IFT levels off above this concentration). The IFT between the diluted heavy oil and the SDBS solutions was measured as a function of the SDBS concentration at pH 11 (Figure 5). A systematic decrease of the IFT was observed as the SDBS concentration is increased. The synergistic effect between SDBS and the different components of the diluted heavy oil occurs at a very low surfactant concentration of ∼0.001%, below the cmc of the pure surfactant. This is similar to the interfacial behavior of oppositely charged polyelectrolytes and surfactants where the coadsorption of the two species at the air/water interface occurs at a concentration much lower than the cmc of the pure surfactant.29 However, unlike the mixture of polyelectrolytes and surfactants

Figure 6. Comparison of the IFT between diluted heavy oil and asphaltene model oil in the presence of 0.1% SDBS solution and 10 g/L NaCl at pH 11.

where the interfacial interactions disappear above the cmc, for our diluted heavy oil, the surfactant was even more efficient in decreasing the IFT above the cmc. For a concentration of SDBS of only 0.05%, the IFT reaches an ultra-low value of ∼4  10 4 mN/m. To understand the interfacial role of the different components present in the heavy oil, we decided to change the dilution ratio of the heavy oil, to obtain different weight percentages of asphaltenes in the heavy oil (ranging from 0.1 to 5%). We measured the IFT of toluene-diluted heavy oil and the asphaltene model oil (pentane-extracted asphaltenes dissolved in toluene) in the presence of 0.1% SDBS and 10 g/L NaCl at a pH of 11. Figure 6 showed that the IFT measured for the diluted heavy oil is lower than the one measured for the asphaltene model oil. This result suggests that, at high pH, all of the acid groups present in the diluted heavy oil contribute to the IFT and that asphaltenes are not the only surface-active species responsible for lowering the IFT. Most of the results in this work are for toluene-diluted heavy 1684

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Energy & Fuels oil, because the original heavy crude was too highly viscous to allow for spinning drop measurement. However, such interactions are likely to be seen with reactive crude oils, i.e., crude oils containing indigenous surfactants that can be ionized by pH changes.

’ CONCLUSION The addition of alkaline chemicals to the aqueous phase was efficient in decreasing the IFT of our diluted heavy oil from 23 to 1 mN/m. The combination of any of the three surfactants (SDS, SDBS, and Triton X405) with the alkaline induced a further decrease of the IFT. IFT measurements revealed that SDBS was by far the most efficient surfactant used in this study for our diluted heavy oil. With 0.1% SDBS, we could obtain IFT around 10 4 mN/m at pH 11. We attributed the significant decrease of the IFT upon the addition of SDBS to a synergistic effect between the in-situ-formed surfactant and the SDBS. This work demonstrates that finding ultra-low IFT could be eased in the presence of indigenous surfactants contained in a reactive crude oil and implies not only a suitable choice of surfactant type and concentration but also optimization of salinity and pH.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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