Core Flooding of Complex Nanoscale Colloidal Dispersions for

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Core Flooding of Complex Nanoscale Colloidal Dispersions for Enhanced Oil Recovery by In-Situ Formation of Stable Oil-in-Water Pickering Emulsions Ki Youl Yoon, Han Am Son, Sang Koo Choi, Jin Woong Kim, Wonmo Sung, and Hyun Tae Kim Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02806 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 12, 2016

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Core Flooding of Complex Nanoscale Colloidal Dispersions for Enhanced Oil Recovery by In-Situ Formation of Stable Oil-in-Water Pickering Emulsions Ki Youl Yoon†,#, Han Am Son‡, §,#, Sang Koo Choi∥, Jin Woong Kim∥,*, Won Mo Sung‡, Hyun Tae Kim§,* †

Department of Chemical Engineering, The University of Texas at Austin, TX 78712, United

States ‡

Department of Natural Resources and Environmental Engineering, Hanyang University,

Seoul 133-791, Korea §

Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea

∥

Department of Bionano Technology and Department of Applied Chemistry, Hanyang Uni-

versity, Ansan 426-791, Korea #

Ki Youl Yoon and Han Am Son equally contributed to this work.

*Corresponding author. E-mail address: [email protected] Phone: +82-42-868-3216 Address: Gwahang-no 124, Yuseong-gu, Daejeon 305-350, Korea

*Cocorresponding author. E-mail address: [email protected] Phone : +82-31-400-5499 Address : 55 Hanyangdaehak-ro, Sangnok-gu, Ansan 426-791, Korea

ABSTRACT This study reports a Pickering emulsion flooding system, in which the oil-water interface is structurally stabilized by a complex colloidal layer consisting of silica nanoparticles, dodecyltrimethylammonium bromide (DTAB), and poly 4-styrenesulfonic acid-co-maleic acid sodium salt (PSS-co-MA). The colloidal layer was generated by adsorption of PSS-co-MA on 1


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the silica nanoparticles due to the van der Waals attraction and by adsorption of DTAB onto PSS-co-MA layer due to the electrostatic attraction, thus providing the mechanically robust, stable interface. To demonstrate a practical applicability to the enhanced oil recovery, the complex colloidal dispersion fluid was injected into the Berea sandstone for a core flooding experiment. The result revealed that the colloidal dispersion significantly increased the oil recovery by ~4%, compared with the case of flooding water. This means that the emulsion drops in-situ produced in the core could readily flow through the rock pores. We attribute this to the fact that the oil-water interface made with the complex colloidal phase not only increased the structural stability of the emulsion drops, but also provided them deformability without any drop break-up or coalescence. Key words: Pickering emulsion, complex colloidal layer, core flooding experiment.

1. INTRODUCTION Colloidal particles assemble at oil-water interfaces and form particles-stabilized emulsions, so called as the Pickering emulsions. The Pickering emulsion drops are very stable since their interface is made of a solid phase. Therefore, this Pickering emulsion system has been applied in a variety of industrial fields, such as foods, cosmetics, paints, pesticides, and so on. Many different types of colloidal particles, including polymer particles 1, 2, inorganic particles 3, 4

, and carbon materials

5-8

, have been utilized for development of the Pickering emulsion.

Unlike micro scale particles, when the Pickering emulsion is produced with nanoparticles, a more tunable interfacial structure can be obtained

9-12

. The emulsion stability is directly rele-

vant to their degree of packing 13, 14, size and shape 15, and wettability at the interface 16-18. When particles are either hybridized with surfactants 19, 20 or surface-modified with polymers 21-25, the Pickering emulsion displays more improved drop stability. In this process, any adsorption of surfactants on the surface of particles may change the wettability, which eventually affects their emulsifying efficiency 26. For instance, the electrostatic adsorption of cationic surfactants on silica nanoparticles makes the particle surface hydrophobic, thus lowering the oil/water interface tension

16

. Likewise, using particles coated with poly (acrylic acid)

leads to production of stable colloidal dispersions through creation of the hydrogen-bonding as well as the ionic complexation 27. Furthermore, the adsorption of poly (ethylene glycol) on the surface of silica particles can lower the interfacial tension of the oil-water interface 22. 2


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In oil and gas industries, recently, use of functional nanoparticles has gained a great deal of attention 28-32. In particular, many good examples can be found in oil recovery 13, 33-37. During travelling through reservoir rocks, well-designed nanoparticles are susceptible to adsorb at the oil-water interfaces in the rock pores and thereby decrease the interfacial tension

38, 39

,

which dramatically improve the oil recovery efficiency. Such adsorption of nanoparticles at the oil-water interface in the reservoirs can produce fine emulsion drops, which eventually leads to more favorable fluidity in the porous media

34, 40, 41

. However, to date, there have

been few reports on utilization of the Pickering emulsions whose interface is made with the mixture of nanoparticles/surfactant/polymer for oil reservoir applications. In this study, we fabricate oil-in-water Pickering emulsions stabilized by silica nanoparticles, cationic surfactant (dodecyltrimethylammonium bromide, DTAB), and anionic polymer (poly 4-styrenesulfonic acid-co-maleic acid sodium salt, PSS-co-MA). Basically this study characterized in detail the Pickering emulsion stabilized by a hybrid colloidal layer. More specifically, this study focused on how DTAB plays a role in altering the interfacial activity and long term stability of n-decane-water Pickering emulsions. Also, to demonstrate the practical applicability of our emulsion system, a core flooding experiment was carried out in Berea sandstone. Finally we evaluated its applicability for oil recovery by observing the oil recovery efficiency as well as injectability during core flooding.

2. EXPERIMENTAL 2.1. Materials Two silica solutions, Ludox CL-X and Ludox HS40 were purchased from Sigma Aldrich (45 wt% and 40 wt% suspension in H2O, > 99.8%). The average particle size was 22 nm (Ludox CL-X), 12nm (Ludox HS40) and the surface area determined with BET was 130 m2g-1 (Ludox CL-X) and 220 m2g-1 (Ludox HS40). Poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSS-co-MA, ~99%) and dodecyltrimethylammonium bromide (DTAB, ~99%) were purchased from Sigma Aldrich (USA) and used as the anionic polymer and cationic surfactant, respectively. The oil used for the emulsion fabrication was n-decane (> 99%, Junsei Chemical, Japan). De-ionized distilled (DI) water was used in all experiments.

2.2. Preparation of Pickering emulsions 3


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Two different aqueous solutions were prepared by homogenously mixing Ludox CL-X or Ludox HS40 (1.0 wt% silica content) with DI water. Then, 0.5 wt% of PSS-co-MA was introduced to each solution, and the mixture was gently mixed for 10 min with a magnetic stirrer. Next, DTAB was incorporated into the sample solution with varying the concentration form 0 to 0.1 wt%. The solution was mixed again for 1 h at room temperature. Then, NaOH solution (0.1 N) was added to adjust the pH to 7. Adding n-decane into the silica particle solution while applying a mechanical stress at room temperature produced microscale emulsions. Each sample was mixed for 2 min in 2 s/1 s bursts (on/off time) at 3000 rpm with an ultrasonic homogenizer (Digital Sonifier, Branson, USA). Table 1 lists the composition of the Pickering emulsions produced in this study.

Table 1. The composition of the Pickering emulsions stabilized with silica, PSS-co-MA polymer, and DTAB. Composition (wt%) Component Emulsion 1

Emulsion 2

Emulsion 3

Emulsion 4

1.00

1.00

1.00

1.00

PSS-co-MA polymer

0.50

0.50

0.50

0.50

DTAB surfactant

0.00

0.01

0.05

0.10

DI water

49.25

49.245

49.225

49.20

n-decane

49.25

49.245

49.225

49.20

Total

100.00

100.00

100.00

100.00

silica particles (Ludox CL-X or Ludox HS40)

2.3. Core flooding experiments The apparatus for the core flooding experiment was fabricated assembling an injection pump (500D Syringe Pump, Teledyne ISCO, USA), an accumulator (CFR-100-100, TEMCO, Inc., USA), a core holder (Young-sung Tech., Korea), and a measuring cylinder (Figure 1). The influent flow lines from the accumulator were connected to the core holder, packed with Be4


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rea sandstone. To prepare the initial state in Berea sandstone, water was injected into the sandstone, and then the n-decane was injected into the water saturated sandstone until no more water was produced. The initial water and oil volume fraction in sandstone were calculated from the amount of water produced. To core flooding experiments, the complex colloidal dispersion fluid (mixed with silica, PSS-co-MA polymer, and DTAB) was injected into the core holder through the accumulator with a constant flow rate of 1 cc min-1, corresponding to a Darcy velocity of 4.1 ft day-1. The fluid that flowed through the sandstone was collected in a measuring cylinder. Oil recovery was calculated by the amount of extracted oil from containing oil in sandstone (original oil in place). Pressure transducers (DXD, Heise, USA) were installed at the inlet and outlet of the core holder to monitor the pressure change. To avoid any effect coming from the thermal motion of nanoparticles, all the experiments were carried out at 25 °C.

Figure 1. Schematic diagram of core flooding apparatus.

2.4. Characterizations Interfacial tension was measured to figure out how nanoparticles and surfactants behavior at the interface. The measurement equipment used was a Theta tensionometer (Biolin Scientific, Sweden). Each measurement was conducted at 25°C. Transmission electron microscopy (TEM) was used to observe the shape and size of the nanoparticles. The measurement was performed with a JEN 2100F (JEOL, USA). The hydrodynamic size and size distribution of particles in the solution were analyzed with dynamic light scattering (DLS, Nano ZS90, Mal5


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vern, UK). The zeta potential was also measured over the pH range using the same analyzer. The steady-state viscosity of the aqueous solutions was measured at 25°C for a shear rate range of 0.1-100 s-1 using a Brookfield DV3. The Pickering emulsion drops were observed using a bright-field microscope (Leica DM 2500 P microscope). Emulsion stability was evaluated by observing the creaming kinetics for the four samples listed in Table 1. The interface generated by creaming was imaged at the set time interval at room temperature. By comparing the initial height of the emulsion against the separated height, the emulsion stability was determined.

3. RESULTS AND DISCUSSION 3.1. Dispersion property of silica nanoparticles in aqueous solutions To evaluate how adding cationic surfactants and polymers affects the suspension property of silica nanoparticles, we observed the size and electric potential property of two different silica nanoparticles (Ludox CL-X and Ludox HS40) in the presence of DTAB and PSS-co-MA. The bare silica nanoparticles showed a spherical shape and uniform size with average diameters of ~22 nm (Ludox CL-X) and ~12 nm (Ludox HS40), which was confirmed from the TEM analysis (Figure 2). They also formed a stable suspension in water with average hydrodynamic sizes of 20 nm and 10 nm, respectively (Figure 3). The bluish transparent appearance of the silica suspension indicates that the dispersion is thermodynamically stable and homogeneous. This stems from the negatively charged surface of silica nanoparticles 42. The zeta potential was -26.9 mV (Ludox CL-X) and -28.3 mV (Ludox HS40) at pH 6. Even after incorporating DTAB, the stability of the silica dispersion remained due to the presence of dense negative charges at the surface, thus resulting in little inter-particle attraction. The zeta potential slightly reduced to~ -38.6 mV (Ludox CL-X) and ~ -32.5 mV (Ludox HS40) at pH 6, meaning the sufficient particle-to-particle repulsion still presented.

6


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Figure 2. TEM image of (A) Ludox CL-X and (B) Ludox HS silica nanoparticles.

Figure 3. Dynamic light scattering (DLS) analysis for (A) Ludox CL-X and (B) Ludox HS40 silica nanoparticles dispersed in the aqueous solution. (a) line is 1 wt% silica particles. (b) line is 1 wt% silica particles with 0.5 wt% PSS-co-MA. (c) line is 1 wt% silica particles with 0.5 wt% PSS-co-MA and 0.1 wt% DTAB.

3.2. Control over the surface charges by adsorption of surfactants and polymers Basically, the zeta potential value of particles is varied depending on the pH of aqueous solutions. In our study, the zeta potential value of Ludox CL-X silica nanoparticles ranged from 15 to -42mV and Ludox HS40 showed the values of -12 to -50 mV over pH 4~10 (Figure 4). 7


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Regarding that the isoelectric point for typical silica nanoparticles is closed to pH ~2.0, it is reasonable to say that the silica nanoparticles are covered with negative charges for the given pH range 42. On adding PSS-co-MA up to 0.5wt%, the zeta potential values increased by 17% in Ludox CL-X and 32% in Ludox HS40 over the pH range. This means that PSS-co-MA chains effectively adsorbed onto the silica nanoparticles, while producing more negative charges on the particle surface by protonation of sulfonic and carboxylic groups

43

.This

would improve the suspension stability of the particles due to the enhanced electrostatic repulsion 44.

Figure 4. Zeta potential measurement of (A) Ludox CL-X and (B) Ludox HS40. Each dispersion contained only silica nanoparticles (), silica particles with 0.5 wt% PSS-co-MA (), silica particles with 0.5 wt% PSS-coMA and 0.1 wt% DTAB (), respectively. The concentration of silica particles was set to 1 wt%.

3.3. Proposed design of Pickering emulsion drops for emulsion flooding Even though DTAB was added in silica/PSS-co-MA dispersion, the zeta potential value was remained from -38 to -42 mV for Ludox CL-X (from -31 to -41 mV for Ludox HS) over the pH change. This indicates that the negatively charged groups of PSS-co-MA could still contribute to the particle-to-particle repulsion. The absorption of DTAB on the particles can deactivate the negative charges on the silica particles 45-47. This would rather improve the stabil8


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ity of the n-decane/water interface, as the hydrophobic alky chains of DTAB can orient into the n-decane drop. Under these conditions, the adsorption of PSS-co-MA likely occurs due to the electrostatic attraction with cationic DTAB on the surface of the silica nanoparticles. Meanwhile, DTAB alters the wettability of the particles surface, thus phyiscochemically stabilizing the n-decane/water interface, as schematically illustrated in Figure 5. Based on this understanding, this study proposes the importance of the electrostatic interaction between nanoparticle/surfactant/polymer dispersions as well as the interaction between emulsion drops and the surface of sandstone rock. The sandstone rock is negatively charged in the absence of saline solution or brine 48. Practical viewpoint of emulsion flooding, if an emulsion dispersion stabilized by a charged colloidal layer flows through a sandstone, any adsorption of emulsion drops onto the surface of sandstone may be reduced due to the electrostatic repulsion, which allows them to decrease any chance to retain in the sandstone reservoir.

Figure 5. A proposed complex colloidal layer with silica nanoparticles / cationic surfactant / anionic polymer.

3.4. Stability of the emulsions The Pickering emulsion produced in this study is stabilized by formation of a complex colloidal layer consisting of silica nanoparticles, DTAB, and PSS-co-MA. To obtain the improved emulsion stability, they should strongly adhere to the oil/water interface 49-52. In principle, once the particle size is set constant, the adhesion energy is influenced by variation of 9


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interfacial tension and wettability of nanoparticles 53, 54, as explained by E = πR2γow(1-cos θ)2, where, E is the adhesion energy of a particle, R is the particle radius, γow is the interfacial tension and θ is the contact angle of the particle at the oil/water interface. Therefore, to produce a structurally stable Pickering emulsion, the adhering particles should be tightly anchored at the oil/water interface. Use of DTAB surfactant in the Pickering emulsion can change the wettability of nanoparticles and make the contact angle closer to 90 degree on the oil droplets. Thus, although adding DTAB surfactant can reduce interfacial tension, the effect of wettability alteration is exponentially increased in equation. This should eventually lead to increased adhesion energy. At this point of view, our approach is indeed advantageous compared with the conventional Pickering emulsions solely made of particles that cannot provide the emulsion with a mechanically robust barrier. To experimentally demonstrate this, the Pickering emulsions were produced and their emulsion stabilities were investigated with varying the DTAB concentration. By observing the creaming rate, in this study, the emulsions stability was evaluated. The results are shown in Figure 6. Initially, all the emulsions produced were milky and stably suspended in the aqueous continuous phase (Figure 6A (a)). After 6 h on sample containing Ludox CL-X, phase separation started to occur between the emulsion phase and the continuous phase (Figure 6A (b)). The electrical conductivity in the lower phase was ~50 µs, meaning it is an aqueous phase. Within 6 h, phase separation of sample without DTAB was faster than other cases, showing the volume of emulsion having DTAB 0.1 wt% had 11% more than without DTAB in Figure 6A (b). This mean that use of DTAB can delay lead time for the phase separation in the early times. When comparing the degree of phase separation after 30 day storage, the emulsion containing high concentration of DTAB displayed the excellent stability, with no elution of n-decane oil on the top layer. After 30 day, emulsion stability on samples using small size particles (Ludox HS40; Figure 6B (d)) showed much variation according to change of DTAB concentration, comparing with large size particles (Ludox CL-X; Figure 6A (d)). The emulsion volume in case of DTAB 0.1 wt% is 5.5 times (Ludox HS40) and 2.2 times (Ludox CL-X) than absent of DTAB. We also observed any morphological changes of emulsion droplets via bright field microscope analysis (Figure 7). The emulsion produced without DTAB (0.74 μm in average size) showed larger droplets compared with that with 0.1 wt% DTAB (0.70 μm in average size) after 5 day storage. These results imply that the presence of DTAB indeed contributed to improving the interface property of the 10


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complex colloidal layer 27,45.

Figure 6. Phase separation of Pickering emulsions with varying concentrations of DTAB (a) on production and after storage for (b) 6 h, (c) 48 h, (d) 30 day at room temperature. The emulsions were stabilized by silica particles (1 wt%), PSS-co-MA (0.05 wt%), and DTAB (00.1wt%). (A) Ludox CL-X, and (B) Ludox HS40.

11


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Figure 7. Microscopic observation and size distribution of Pickering emulsion droplets. The emulsions stabilized by silica particles (Ludox CL-X, 1 wt%) and PSS-co-MA (0.05 wt%): (A) on preparation and (B) after 5 day storage. The emulsions stabilized by silica particles (1 wt%), PSS-co-MA (0.05 wt%), and DTAB (0-0.1wt%): (C) after 1 h storage and (D) after 5 day storage.

3.5. Confirmation of enhanced oil recovery by core flooding Core flooding experiments were conducted with water flood and injection of colloidal dispersions consisting of 1 wt% silica nanoparticles (Ludox CL-X, 22 nm or Ludox HS40, 12nm), 0.5 wt% PSS-co-MA, and 0.1 wt% DTAB in Berea sandstone. We could experimentally confirm that the oil recovery using the injection of the colloidal dispersions was greater than that of water only. The oil recovery of the original oil in place for the injection of colloidal dispersions was 63% in use of larger particle (Ludox CL-X) and 61.4% in use of smaller particle (Ludox HS40); by contrast, the oil recovery for water flood was 59% (Figure 8A). This showed that colloidal dispersions having larger particle (Ludox CL-X) can recovery more oil from sandstone, even though emulsion using smaller particle is more stable. The pressure drop, which typically corresponds to the pressure difference between the inlet and outlet, for the injection of colloidal dispersions was lower by ~1.3 psi for Ludox CL-X 12


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and ~ 1.5 psi for Ludox HS40 than that for water flood (Figure 8B). These indicate that injectivity of colloidal dispersions was higher than that of water. This could be confirmed by calculating the average injectivity, the injection rate per average pressure drop. The injectivityduring the injection of colloidal dispersions was 0.41 cc (min psi)-1 (Ludox CL-X) and0.48cc (min psi)-1 (Ludox HS40), which is higher by approximately 142% and 165% than water flooding, respectively.

Figure 8. (A) Oil recovery and (B) pressure drop during the injection of water and colloidal dispersions consisting of 1 wt% silica nanoparticles, 0.5 wt% PSS-co-MA, and 0.1 wt% DTAB in Berea sandstone. (a) The injection of Ludox CL-X colloidal dispersion. (b) The injection of Ludox HS40 colloidal dispersion. (c) The injection of water.

3.6. Interfacial fluid behaviors of complex colloidal dispersion during core flooding To figure out the reason to obtain such enhancement of oil recovery, we investigated the changes of the interfacial property of n-decane/water in the presence of colloidal dispersion. The interfacial tension at the n-decane/water interface was measured by 46.4 dyne/cm, similar to the previous report 22. On adding silica nanoparticles and PSS-co-MA, the tension value was lowered to 31 dyne/cm (Ludox CL-X) and 33.4 dyne/cm (Ludox HS40). This value further decreased to 10.9 dyne/cm (Ludox CL-X) and 8.5 dyne/cm (Ludox HS40), when the 13


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concentration of DTAB was increased in the colloidal dispersion (Figure 9). This result show well that silica particles and PSS-co-MA readily adsorbed to the n-decane/water interface. This adsorption behavior was even fortified with the increase of DTAB. Furthermore, to observe the fluidity of the colloidal dispersions, the wall shear rate in permeable media was calculated 55. The wall shear rate is expressed by γ= 4q/A(8kø)-0.5, where q is the flow rate, k is the permeability, and ø is the porosity. Based on the shear rate, the apparent viscosity of the colloidal dispersions is estimated in the porous medium. In our experiments, the shear rate was ~85 s-1. The estimated apparent viscosity ranged in 0.93–1.13 cp irrespective to the concentration of DTAB (Figure 10), thereby implying that the incorporation of DTAB didn't affect the apparent viscosity. These results highlight that the colloidal dispersions developed in this study would have a potential for an injection agent that can enhance oil recovery while displaying the higher injectivity with no specific apparent viscosity changes.

Figure 9. (A) Interfacial tension measurement of DI water and n-decane with varying the concentration of DTAB. The dotted line is the interfacial tension of DI water/n-decane. (B) Apparent viscosity of the colloidal dispersions with varying the concentrations of DTAB in the presence of silica particles (1 wt%) and PSS-co-MA (0.5 wt%): DTAB 0.01 wt% (),0.05wt% (), and 0.1wt% ().

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Figure 10. (A) Interfacial tension measurement of DI water and n-decane with varying the concentration of DTAB. The dotted line is the interfacial tension of DI water/n-decane. (B) Apparent viscosity of the colloidal dispersions with varying the concentrations of DTAB in the presence of silica particles (1 wt%) and PSS-co-MA (0.5 wt%): DTAB 0.01 wt% (),0.05wt% (), and 0.1wt% ().

4. CONCLUSIONS This study showed that a stable Pickering emulsion system could be successfully developed by incorporation of DTAB in the colloidal dispersion consisting of silica nanoparticles and PSS-co-MA. Unlike conventional colloidal dispersions in which nanoparticles are costabilized by simple association with surfactant molecules 27, we hybridized anionic polymer, PSS-co-MA, with cationic surfactant, DTAB, on the surface of silica nanoparticles. We have found that DTAB can help formation of stable colloidal dispersion with controlled adhesion ability to the n-decane/water interface, which eventually enabled production of stable Pickering emulsions. Using the colloidal dispersions, core flooding experiments were conducted to confirm the enhanced oil recovery in Berea sandstone. The results indicated that the colloidal dispersions increased the oil recovery by about 4% of the original oil in place, which was quite comparable with water flooding. This was attributed to the lowered interfacial tension as well as constant apparent viscosity. We expect that the nanoscale colloidal dispersion de15


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veloped in this study would find a practical applicability in the field of oil recovery.

■ACKNOWLEDGMENTS The work was carried out with financial support from the Korea Institute of Geosciences and Mineral Resources (No. 15-3312). This research was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 20080061891) and by the R&BD (Research & Business Development) program funded by the Ministry of Trade, Industry and Energy.

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