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Foams stabilized by in situ modified nanoparticles and anionic surfactants for enhanced oil recovery Weipeng Yang, Tengfei Wang, Zexia Fan, Qiang Miao, Zhiyu Deng, and Yuanyuan Zhu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03217 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Foams stabilized by in situ modified nanoparticles and anionic surfactants for enhanced oil recovery Weipeng Yang,* Tengfei Wang, Zexia Fan,* Qiang Miao, Zhiyu Deng, Yuanyuan Zhu. School of Petroleum Engineering, China University of Petroleum, Qingdao 266580, Shandong, China ABSTRACT: Foams have been widely used in oilfields for effective profile control and displacement. However, foams stabilized by surfactants lack long-term stability, especially in an oil reservoir. Here, we studied the in situ modification of positively charged AlOOH nanoparticles via the adsorption of the anionic surfactant sodium dodecyl sulfate (SDS) and the characterization of foam stabilized by AlOOH nanoparticles in synergy with SDS under different conditions. Changes in the zeta potential and adsorption isotherm of the AlOOH nanoparticles confirmed their modification. The most stable foam was obtained with an SDS/AlOOH concentration ratio of 5:1; further increases of the SDS concentration led to a decrease and subsequent increase in foam stability. The relationships between the zeta potential, three-phase contact angle, nanoparticle aggregate size and foam stability were comprehensively analyzed, revealing that foam stability was affected by all of these factors. We concluded that nanoparticles with partial hydrophobicity, a positive or slightly negative charge and small aggregate size can be adsorbed tightly to foam surfaces and form compact networks in the foam’s film, thereby resulting in a

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stable foam. The SDS/AlOOH-stabilized foam also shows good stability under high temperatures and in the presence of oil. Sandpack flooding experiments showed that the SDS/AlOOH foam can increase and maintain the differential pressure more effectively than the SDS foam. This study provides additional options for using nanoparticles to stabilize foams for enhanced oil recovery.

1. INTRODUCTION Foam flooding as an enhanced oil recovery (EOR) method has gained increasing attention in China as a method for modifying the water injection profile and increasing the displacement efficiency. A foam is a dispersion system of gas in the liquid phase. Its high specific surface area and surface energy lead to thermodynamic instability. Thus, foams tend to rupture within a short time, especially under harsh conditions, such as in the presence of oil, high temperatures and high salinity.1-4 To satisfy the requirements for tertiary oil recovery, the development of foam systems with long-term stability is urgently needed. Over the past two decades, foams and emulsions stabilized by particles have been studied extensively, resulting in the development of foams and emulsions with outstanding stability compared with that of traditional surfactantstabilized foams.5-9 Nanoparticles have been used to stabilize foams and emulsions for various applications, including edible nanoparticles,10 pharmaceuticals11 and EOR.12,13 Nanoparticles with appropriate wettability can be adsorbed onto a gas-water interface similar to surfactants. Unlike surfactants, the detachment energy of nanoparticles is approximately on the order of thousands of kT (where k is the Boltzmann constant and T is the absolute temperature), making them almost impossible to remove from an air-water interface.14 Meanwhile, nanoparticles aggregate in the film between


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bubbles, forming network structures that can slow the processes of drainage and Ostwald ripening.14,15 Foams stabilized by surfactants usually collapse within minutes to hours, whereas foams stabilized by particles can be stable from days to years.5,9,16 Therefore, particle-stabilized foams appear to be a promising technique for use in EOR. Given the circumstance of an oil reservoir, the critical problem is how to set up suitable foam systems. Because of the natural hydrophilic surfaces of most metal oxides and clays, the detachment energy ∆G is too small for the particles to attach tightly to the gas-water interface; thus, hydrophilic particles contribute little to foam stability.6 Two main methods exist for hydrophobizing hydrophilic particles: chemical modification and in situ modification by adsorption. Many researchers have studied chemically modified nanoparticles and their ability to stabilize foams. Binks and Horozov6 found that aqueous foams can be stabilized by silica nanoparticles without the presence of surfactants. The most stable foams were obtained with particles that contained 32% SiOH. No foam was formed with particles that were hydrophilic or highly hydrophobic. Only particles with intermediate hydrophobicity could be adsorbed irreversibly onto air-water surfaces. Yu et al.17 investigated CO2-in-water foams stabilized by three kinds of silica nanoparticles with different hydrophobicities. The foam stabilized by particles with a contact angle of 59.5° exhibited the largest volume and apparent viscosity. Although stable foams can be obtained, chemically modified nanoparticles can be expensive and the volume of the resulting foams is relatively low compared to that of foams stabilized by surfactants. The other method, in situ modification by adsorption of amphiphiles, can be cheaper and more flexible. Foams and emulsions stabilized by hydrophilic nanoparticles in synergy with surfactants have been studied by many researchers. Binks et al.18 used a mixture of SiO2 nanoparticles and the cationic surfactant di-decyldimethylammonium bromide (di-C10DMAB) to


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generate foams, demonstrating the synergy between the nanoparticles and the surfactant molecules. With increasing surfactant concentration, foam stability first increased to a maximum value and then quickly decreased to a constant value. Cryo scanning electron microscopy (cryoSEM) images confirmed the adsorption of nanoparticles onto the foam surface. Liu et al.19 also reported the generation of a stable foam at a moderate surfactant concentration. Alkylammonium bromides with longer alkyl chains showed greater adsorption capacity because of the stronger hydrophobic interaction between the chains, resulting in a variation of the optimum surfactant concentration with varying alkyl chain length. Yoon et al.20 designed a complex emulsion system stabilized by colloidal silica, the cationic surfactant dodecyltrimethylammonium bromide (DTAB), and anionic polymer poly(4-styrenesulfonic acid-co-maleic acid) sodium salt (PSS-coMA). The presence of PSS-co-MA increased the zeta potential of the silica nanoparticles, leading to an enhanced suspension stability even in the presence of DTAB. Meanwhile, the adsorption of DTAB made nanoparticles sufficiently hydrophobic to attach to the water-oil interface. Foam stabilized by hydrophobic nanoparticles and surfactants have also been reported. Sun et al.21 applied sodium dodecyl sulfate (SDS) to change the wettability of SiO2 nanoparticles from strong hydrophobicity to partial hydrophobicity to form stable foam. Core flooding experiments under reservoir conditions corroborated that particle-stabilized foams was more stable than surfactant-stabilized foams. The mechanism of foam stabilization by nanoparticles and by short-chain amphiphiles is similar to that of foam stabilization by hydrophobic particles. Short-chain amphiphiles cannot stabilize foam, but they can attach to particle surfaces via physicochemical adsorption and render the particles hydrophobic. Gonzenbach et al.9 demonstrated that positively or negatively charged hydrophilic particles can be modified with appropriate short-chain amphiphiles. The short-chain


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amphiphiles were adsorbed through electrostatic interaction and a ligand-exchange reaction, which indicates that oppositely charged amphiphiles should be selected to modify the particles. Carl et al.22 studied foams stabilized by silica nanoparticles and alkylamines with different hydrocarbon chain lengths. Flocculation occurred when the concentration of alkylamine was greater than the critical micelle concentration (CMC), and alkylamines with longer chains flocculated the suspension at lower concentrations. A strong correlation between different length scales of particle-alkylamine systems indicated that the adsorption of alkylamine activated particles, enabling them to function as foam stabilizers. Thus, several viable approaches seem to be used to fabricate particle-stabilized foam systems for EOR. However, silica particles, which are the most widely used particles in other industries, have a negatively charged surface that prevents hydrophobization by anionic surfactants. Meanwhile, hydrophobization by cationic surfactants will cause surface charge reversal and cause strong adsorption in the negatively charged porous media of a reservoir.19,23,24 Moreover, silica nanoparticles interact more weakly with nonionic surfactants or zwitterionic surfactants than with cationic surfactants; they are therefore not an ideal choice for hydrophobizing the particles and generating a stable foam.8,25,26 Cui et al.27 investigated the synergy between CaCO3 and anionic surfactants and discovered a practicable way to form stable foams using positively charged particles and anionic surfactants. Alumina particle shows more efficiency in stabilizing foam than silica particle because of its higher Hamaker constant which will form a stronger network structure in foam film.9 The goal of this paper is to investigate foam stabilized by in situ modified AlOOH nanoparticles and the anionic surfactant SDS. The adsorption of SDS onto AlOOH nanoparticles was studied through zeta-potential measurements, adsorption isotherm measurements, Fourier transform


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infrared spectroscopy (FT-IR) analysis and three-phase contact angle measurements. Foam stability was evaluated under environmental conditions, in the presence of oil, and at different temperatures. Finally, the profile control ability was assessed in a sandpack. This study will provide another option for using nanoparticle to stabilize foams for EOR and the results are instructive for the utilizing of particle-stabilized foams in EOR.

2. METHODOLOGY 2.1. Materials. Alumina sol JR14W, composed of 20 wt% AlOOH nanoparticles in water, was purchased from Jingrui Co., Ltd. (China). Figure 1 shows distribution of the AlOOH nanoparticles size in water. SDS with a purity of 99% was purchased from Amresco, Inc. (USA); its critical micelle concentration is 8.1 mmol/L at 25°C. Cetyltrimethylammonium bromide (CTAB) (99% purity) and sodium chloride (99.5% purity) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Nitrogen (99.99% purity) was supplied by the Eastern Gas Co., Ltd. (China). Deionized water was used in all the experiments. Glass containers were cleaned with deionized water before use. All of the experiments were conducted at room temperature (22 ± 2°C) unless otherwise stated.


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Figure 1. Hydrodynamic diameters of the AlOOH nanoparticles used in the experiments. 2.2. Preparation of AlOOH/SDS Dispersions. An AlOOH dispersion was prepared by diluting the concentrated sol in deionized water under continuous magnetic stirring. Different concentrations of SDS solutions were prepared by dissolving SDS in deionized water. SDS solution was then added to the AlOOH dispersion dropwise under continuous magnetic stirring to make the dispersion homogenous. Dispersions were stirred continuously for 12 hours to ensure adsorption equilibrium and the total volume of dispersion was 100 mL. Based on our prior experience, the concentration of AlOOH nanoparticle was fixed at 1 wt% in all the experiments unless otherwise specified. The pH levels of the resulting mixtures were not adjusted. 2.3. Zeta Potential of AlOOH Nanoparticles. The zeta potentials of modified and unmodified nanoparticles were measured in a cell at room temperature using a Nano-ZS ZEN3600 (Malvern Instruments). Each dispersion was measured at least 3 times to minimize errors. The results were calculated automatically by the instrument on the basis of the Smolochowski equation.


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2.4. Adsorption Isotherm of SDS on AlOOH Nanoparticles. AlOOH dispersions in different SDS solutions with various concentrations were prepared under continuous stirring. After 12 hours, the dispersions were centrifuged at 6000 rpm for 30 min to separate the particles. The two-phase titration method was then used to determine the equilibrium SDS concentration after adsorption in the supernatant. The cationic surfactant CTAB was used to prepare a standard solution for titrating the SDS. 2.5. Three-phase Contact Angle. After the centrifugation step described in section 2.4, the sediments were washed with deionized water to remove unadsorbed SDS. The particles were then dried in an oven at 75°C for 6 hours, and 0.3 g of particles was compressed into tablets under 16 MPa. The contact angle was measured with sessile drop method using a contact angle meter (Harke Instruments). The results were the average of at least three measurements on each tablet. 2.6. Fourier Transform Infrared Spectroscopy. The AlOOH/SDS dispersions with a SDS concentration of 5 mmol/L were centrifuged at 6000 rpm for 10 min to separate the particles after the dispersions had reached adsorption equilibrium. The sediments were then dried at 75°C for 6 hours and ground into powder with KBr using an agate mortar. The FTIR spectra of modified and unmodified AlOOH nanoparticles were measured with a Vertex 70 spectrophotometer (Bruker Corp.). The resolution of spectrometer is 4 cm-1. 2.7. Characterization of Static Foams. Foams of AlOOH /SDS dispersions were generated using the Waring blender method. 27,28A 100 mL dispersion was transferred into the blender’s cup and was subsequently stirred at 6000 rpm for 3 min. The foam was then immediately diverted to a measuring cylinder for measurement of its volume. The cylinder was sealed to


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prevent evaporation and external interference. The time when the foam was generated was taken as the zero of time. The foam’s half-life, drainage half-life and water fraction were used to evaluate its stability. When the foam collapsed to half its initial volume, the time was recorded as the foam half-life. Foam apparent viscosity was measured using rotatory viscometer immediately after foam was generated. The drainage half-life was defined as the time when half of the water drained from the foam. This parameter was recorded as a measure of the foam’s stability at different temperatures. The change in the foam’s volume over time in the presence of oil was also recorded, and the relative foam volume was defined as V(t) VR = V(t=0)

(1)

where VR is the relative foam volume (mL) and V(t) is the foam volume at time t (mL).2,28 2.8. Sandpack Flooding Experiments. Foams were injected into sandpacks to test their properties under reservoir conditions. The differential pressure between the inlet and outlet of the sandpack was used to evaluate the foam’s blocking ability. The sandpacks used in these experiments had nearly the same permeability (2.7 ± 0.2 D) and porosity (35.5 ± 0.2%). The back pressure was set at 6.0 MPa. The SDS solution and SDS/AlOOH dispersion were coinjected with nitrogen to generate a foam. Initially, brine water (salinity = 10,000 mg/L) was injected into the sandpack at a rate of 0.5 mL/min to saturate it with water, and the permeability and porosity were measured during this process. Afterwards, brine water was injected at a rate of 1 mL/min for 1 pore volume (PV) to stabilize the differential pressure. The SDS solution and SDS/AlOOH dispersion were then co-injected with nitrogen at a rate of 0.5 mL/min with a liquid /gas ratio of 1 to generate foam for 1 PV. Finally, brine water was injected at a rate of 1 mL/min


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to test the foam blocking ability. The change in differential pressure over a certain time interval was recorded.

Pressure Sensor Foam generator

N2

AlOOH/SDS dispersion or SDS solution

PUMP

Pressure Sensor Sandpack

BPR

PUMP

Figure 2. Schematic of apparatus for sandpack flooding experiment.

3. Results and Discussion 3.1. Adsorption of SDS onto AlOOH Nanoparticles. The AlOOH nanoparticle is a slice-like particle with abundant surface hydroxyls (Al-OH) that adsorb H+ from water at environmental pH (pH = 4.25), making the particle surface positively charged (Al-OH2+). The anionic surfactant SDS can be adsorbed onto the particle surface because of the electrostatic interaction between its hydrophilic group, –OSO3-, and the positively charged hydroxyl groups.29,30 Figure 3 shows a schematic of the adsorption of SDS onto the AlOOH nanoparticle surface. To study the adsorption of SDS onto particle surfaces, the zeta potential was measured; the results are shown in Figure 4 (a). AlOOH nanoparticles were strongly positively charged (zeta potential = +55.4 mV) in the absence


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of SDS. As the concentration of SDS was increased, the zeta potential of the AlOOH nanoparticles gradually decreased to 0 mV at a concentration of less than 5 mmol/L and then reversed to a negative value. When the SDS concentration was 30 mmol/L, the zeta potential of the nanoparticles was -52.1 mV, indicating that the surface charge of the nanoparticles was completely reversed. Adsorption of SDS onto AlOOH nanoparticles is mainly driven by electrostatic and hydrophobic interactions.31 At low SDS concentrations, SDS molecules are adsorbed through ion exchange and ion pairing with their headgroups, resulting in a slow decrease in the zeta potential of the nanoparticles.19 Under these conditions, SDS adsorption is dominated by electrostatic interactions between nanoparticles and SDS. With increasing SDS concentration, the hydrophobic interaction between alkyl chains becomes considerable and SDS molecules tend to aggregate on the particle surface. The adsorption is driven by electrostatic and hydrophobic interactions, and the adsorption amount increases rapidly, leading to the formation of an adsorbed monolayer and to a rapid decrease of the zeta potential. Under these conditions, the hydrophobicity of the nanoparticle surface increases because of the formation of an SDS monolayer with alkyl chains directed toward water. The surface charge is simultaneously neutralized and finally reverses to a negative value. Electrostatic repulsion between the surface and SDS headgroups then prevents the continued formation of the monolayer. Thus, a second layer begins to form through hydrophobic interaction between alkyl chains with headgroups directed toward water, which renders the particles hydrophilic again.32 The formation of the second layer further decreases the particles’ zeta potential, and the particles become strongly negatively charged at an SDS concentration of 30 mmol/L. The adsorption of SDS in this region is dominated by hydrophobic interactions.


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Figure 3. Schematic of the adsorption of SDS onto the AlOOH nanoparticle surface.

Figure 4. (a) Zeta potential of SDS/AlOOH dispersions and the three-phase contact angle of the AlOOH nanoparticles with different SDS concentrations. (the AlOOH


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concentration are both 1.0 wt%) (b) The half-lives of foams with different relative SDS concentrations. The adsorption pattern of SDS on AlOOH nanoparticles was confirmed by the adsorption isotherm shown in Figure 5. The adsorption isotherm shows a pattern similar to that of the zeta potential. The adsorbed amount increased with increasing SDS concentration, especially in the hydrophobic-interaction-dominant region. When the equilibrium concentration of SDS is greater than its CMC (approximately 8.1 mmol/L), the second adsorbed layer is saturated; thus, the adsorbed amount remains unchanged at higher SDS concentrations.7,32 Therefore, the hydrophilic nanoparticles can be hydrophobic-modified only at a particular SDS concentration.

Figure 5. Adsorption isotherm of SDS on AlOOH nanoparticles at 22°C. The modification of AlOOH nanoparticles was confirmed by the infrared spectra shown in Figure 6. The spectra of the unmodified and modified nanoparticles are similar. The peaks of pure SDS at 2850.92 and 2918.30 cm-1 correspond to C-H symmetric and


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asymmetric stretching of -CH3 and -CH2, respectively. For the adsorbed SDS on AlOOH surface, these two peaks shift to 2853.22 and 2922.98 cm-1. The blue shift of these peaks indicates the adsorption makes the SDS molecule more disorder than before,33,34 thus it confirms the modification of AlOOH nanoparticles. The peak at 1468.12 cm-1 corresponds to a C-H asymmetric bending vibration of -CH3 and -CH2. The peak at 1384.40 cm-1 corresponds to -NO3, which was used to stabilize the sol, and the great change in the intensity of this peak is due to the washing of the modified nanoparticles.

Figure 6. Infrared spectra of unmodified AlOOH, modified AlOOH and SDS. 3.2. Foamability of SDS/AlOOH dispersions. Foam was generated from SDS/AlOOH dispersions with SDS concentrations of 0 to 15 mmol/L and nanoparticle concentrations of 0 to 1 wt%. Figure 7 shows the foamability of SDS/AlOOH dispersions.


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With increasing SDS concentration, the foam volume increased rapidly and then reached a plateau. The foam volume decreased to some extent with increasing AlOOH nanoparticle concentration, especially at low SDS concentrations. At an SDS concentration of 1 mmol/L, the foam volume of pure SDS was 622 mL, whereas the foam volume of the SDS/AlOOH dispersion with 1 wt% AlOOH nanoparticles was only 275 mL. With increasing SDS concentration, the difference between the foam volume of the dispersion with and without nanoparticles decreased, but foams with higher nanoparticle concentrations still exhibited lower foamability in the high SDS concentrations. The AlOOH nanoparticle concentration mainly affects two aspects of dispersion foamability: surfactant adsorption and dispersion viscosity. As evident in Figure 7(a), at low SDS concentrations, the change in SDS concentration strongly affects the foam volume. The adsorption of SDS onto AlOOH nanoparticles lowers the SDS concentration, resulting in a sharp decrease in foam volume. This result illustrates that the foamability is dominated by adsorption at low SDS concentrations and that the presence of nanoparticles adversely affects foamability. At high SDS concentrations, the foam volume of the dispersion with or without AlOOH nanoparticles generally remains unchanged; however, dispersions with higher nanoparticle concentrations exhibit slightly lower foamability. The nanoparticle concentration influences dispersion foamability by increasing the dispersion viscosity, which hampers bubble formation.10,12 Thus, an increase in nanoparticle concentration leads to a higher dispersion viscosity and to lower foamability. Nevertheless, dispersions with nanoparticle concentrations of 1 wt% still exhibit good foamability. According to previous studies,5,28 foams stabilized by particles alone or by particles and non-surface-active amphiphiles show low foamability. Thus, in


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a particle-surfactant-foam system, the surfactant is the main contributor to foamability and the reason is that the adsorption of surfactant to the interface is faster than that of particle.35 And the large size of nanoparticle aggregate also limit foamability which is difficult to be moved to interface.36 Figure 7(b) shows the change of particle size versus SDS concentration, and at all the concentration the diameter of particle aggregate is much larger than initial.

Figure 7. (a) Foamability of SDS/AlOOH dispersions. (b) The change of particle diameter versus SDS concentration.


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3.3. Stability of Foams Stabilized by SDS/AlOOH. To make a better comparison of foam stability between foams with different AlOOH nanoparticle concentrations, we defined the relative SDS concentration as the ratio of the SDS concentration (mmol/L) to the AlOOH nanoparticle (wt%) concentration in the dispersion. The AlOOH nanoparticle concentration was fixed at 0.5 wt% and 1.0 wt%, and the relative SDS concentration was varied from 0 to 15. The half-lives of foams with different relative SDS concentrations are shown in Figure 4(b). The half-life curves of foams with an AlOOH concentration of 0.5 wt% and 1.0 wt% show similar trends. In the case of relative SDS concentrations of less than 1, the foam half-life was 0 h for AlOOH concentrations of both 0.5% and 1.0% because a small amount of foam was generated at such low relative SDS concentrations. An increase of the relative SDS concentration from 1 to 5 increased foam stability substantially, and the most stable foam for both AlOOH nanoparticle concentrations was obtained at a relative SDS concentration of 5. The half-lives of foams with an AlOOH concentration of 0.5 wt% and 1.0 wt% were 41.3 h and 71.4 h, respectively. A further increase of the relative SDS concentration from 5 to 7.5 caused a rapid decrease of the foam half-life to a minimum, followed by another increase at even higher SDS concentrations. This secondary increase of foam stability has seldom been reported. To study the relationship between foam stability and nanoparticle wettability, the three-phase contact angle was measured and shown in Figure 4(a). The contact angle of unmodified AlOOH nanoparticles was 18.5°, indicating a hydrophilic surface that cannot attach tightly to the air-water interface.6,37 The contact angle increased rapidly to 55.5° when the SDS concentration was increased from 0 to 5 mmol/L. The contact angle then decreased with a further increase of the SDS concentration, which indicates the formation


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of a second adsorption layer. When the SDS concentration was increased from 0 to 5 mmol/L, the foam stability and the contact angle of the AlOOH nanoparticles increased. With an increase in hydrophobicity, nanoparticles have a stronger tendency to be adsorbed onto the air-water interface, which slows water drainage, disproportionation, and coalescence.6,38,39 The impact of particle wettability on foam stability can be assessed on the basis of the detachment energy,14 ∆G, which is the energy needed to remove a particle from the interface; it is given as

∆G=πR2γa/w(1±cosθ)2

(2)

where R is the particle radius, γa/w is the interfacial tension between air and water, and θ is the three-phase contact angle of the particle, the “-” and “+” are taken respectively when

θ ≤ 90° and θ ≥ 90°. Although the AlOOH nanoparticles in this study are slice-shaped rather than spherical, this principle is still meaningful. The foam half-life obeys this principle at a relative SDS concentration of less than 5. The contact angle of AlOOH nanoparticles is slightly lower when the relative SDS concentration is 7.5 compared to the angle when it is 2.5; however, the foam half-life is much lower compared to the contact angle. Per the detachment energy theory, a particle with stronger hydrophilicity contributes little to foam stability14,40; however, the foam half-life is increased at the higher relative SDS concentration. According to Abdel-Fattah and El-Genk,41 the adsorption of nanoparticles at the air-water interface needs to overcome an energy barrier between the nanoparticle and the interface. The zeta potentials of the AlOOH nanoparticles with SDS concentrations of 5 and 7.5 mmol/L are -3.16 and -29.43 mV, respectively. Therefore, at


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an SDS concentration of 5 mmol/L, the electrostatic repulsion between the interface and a negatively charged modified AlOOH particle is still sufficiently small to be overcome.42 Foam stability is enhanced by modified AlOOH nanoparticle synergy with SDS at this concentration. With an increase of the SDS concentration to 7.5 mmol/L, the zeta potential of the nanoparticles is reduced to -29.43 mV. At this concentration, the repulsion between the negatively charged interface and the nanoparticle is too large to overcome;37,41 thus, the adsorption of nanoparticles is nearly impossible without increasing the shear energy.43 According to Stocco44, the large aggregate size also play an important role in impeding the particle adsorption. Figure 7(b) shows the change of particle size with the increase of SDS concentration. The attachment energy increase with the increase of aggregate size indicates that more energy is needed to attach a larger particle to interface, and smaller particle performs better in stabilizing foams.36,45 Furthermore, the contact angle of the AlOOH nanoparticles at an SDS concentration of 7.5 mmol/L is still relatively high and the repulsion between nanoparticles is insufficient against the van der Waals force; the nanoparticles therefore still aggregate, mostly in foam film. However, these unadsorbed particles have little effect on foam stability. This phenomenon is confirmed by Figure 8, which shows that the foam surface at an SDS concentration of 5 mmol/L was still covered by nanoparticles after drainage. When SDS concentration was increased from 7.5 to 15 mmol/L, the unadsorbed nanoparticle aggregates on the foam film were reduced and the water phase became opaque, indicating that the nanoparticles were dispersed.8,19 At an SDS concentration of 15 mmol/L a few particles are found in foams, overall foam stability is dominated by the SDS concentration. In figure 4(b) can find at relative SDS concentrations ≥ 7.5 foam stability


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is mainly determined by residual SDS concentration after adsorption, the increase of the relative SDS concentration leads to the secondary increase of the foam half-life. Figure 9 shows a schematic of the influence of the relative SDS concentration on AlOOH nanoparticle aggregation and distribution. At very low relative SDS concentration, the influence of adsorption of SDS on nanoparticle surface is negligible and colloid system is still stable. With the relative SDS concentration increase to about 5 the strong aggregation happens and nanoparticle surface becomes partial hydrophobic because of the single layer adsorption of SDS. The further increase of relative SDS concentration to 7.5 brings about the begin of second layer adsorption which decrease the hydrophobicity and increase the zeta potential to some extent making the aggregate size reduces. At relative SDS concentration higher than 15, the nanoparticle surface becomes hydrophilic again and the zeta potential is high enough to disperse the nanoparticle that the further reduction of nanoparticle aggregate size happens.


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Figure 8. Photographs of foams stabilized by SDS/AlOOH, after drainage.

Figure 9. Schematic of the influence of the relative SDS concentration on AlOOH nanoparticle aggregation and distribution. Unlike other studies,8,10,46 we observed no direct correlation between foam stability and the reserve amount of nanoparticles in the foam film for foams stabilized by SDS/AlOOH mixtures. Only the nanoparticles adsorbed at the air-water interface, not just reserved in the foam film, can enhance foam stability. Whether a nanoparticle can contribute to foam stability is mainly controlled by the surface charge, hydrophobicity and aggregate size.47


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Therefore, a nanoparticle with a positive or slightly negative surface, a partial hydrophobicity and a small aggregate size is superior in stabilizing foams.2,36,44 Figure 10 shows the optical micrographs of a foam stabilized solely by SDS and a foam stabilized by AlOOH nanoparticles in synergy with SDS. The bubble size of the foam stabilized solely by SDS increased quickly. A comparison of the graphs for 5 min and 30 min shows that small bubbles almost disappeared because of gas diffusion from small bubbles to large bubbles.48 Additionally, the irregular foam morphology enhanced the drainage pressure, leading to rapid thinning of the foam film and then coalescence.49 These two factors contribute to the enlargement of bubbles and to greater instability. The foam stabilized by SDS/AlOOH exhibited much greater stability because of the thick, compact, nanoparticle-enhanced bubble film that significantly reduced drainage, gas diffusion and coalescence.50,51 A comparison of the graphs between these two different foam systems shows that the presence of AlOOH nanoparticles maintained the spherical morphology of bubbles, which reduced drainage pressure, and that the network structure composed of nanoparticle aggregates effectively decreased the rate of coalescence.52 Apparent viscosity of SDS/AlOOH foam and SDS foam was measured at the spin speed of 30 RPM shown in Figure 11. It can be seen that viscosity of SDS/AlOOH foam increased to the maximum with the increase of SDS concentration then decreased with the further increase of SDS concentration while the viscosity of SDS foam almost constant at all the concentration. For surfactant foam, viscosity of foam is strongly related with solution viscosity.53 Because the increase of SDS concentration from 2.5 to 15 mmol/L cannot lead to an apparent increase of solution viscosity, the viscosity of SDS foam almost remained unchanged. Few study about the viscosity of particle stabilized


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foam has been done so far. It can be found out that the most viscous foam can be obtained at SDS concentration of 5 mmol/L which the most stable foam can also be obtained. At other SDS concentration the SDS/AlOOH foam viscosity was even lower than SDS foam. It can be inferred that nanoparticle aggregation status is related to foam viscosity, because nanoparticle strongly aggregates at the SDS concentration of 5 mmol/L, and at other SDS concentration the aggregate size is relatively small. Because nanoparticle can attach to the air-water interface at both SDS concentration of 2.5 mmol/L and 5 mmol/L but the foam viscosity is lowest at SDS concentration of 2.5 mmol/L, maybe we can exclude the adsorption of nanoparticle as the major influence of foam viscosity.

Figure 10. Time-varying optical micrographs of foam stabilized solely by SDS and foam stabilized by AlOOH nanoparticles in synergy with SDS.


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Figure 11. Viscosity of AlOOH/SDS foam and SDS foam versus SDS concentration. Figure 4(b) shows that the foam half-life at SDS concentrations of 12.5 and 15 mmol/L exceeds 50 h, which indicates good stability. However, this surfactant-dominated dry foam had a very thin bubble film that any small disturbance could collapse.18,54 Figure 12 shows the water fraction of foam stabilized by SDS and SDS/AlOOH mixtures after a certain time. Foam stabilized solely by SDS showed poor water retention ability. With the increase of SDS concentration from 1.5 to 5 mmol/L, the water fraction at 10 min increased and remained unchanged at higher concentrations. At 20 min, only 7% or less of the water remained in the foams for all SDS concentrations, which indicates that the foams were unstable and fragile. By contrast, the foam stabilized by SDS/AlOOH with an AlOOH nanoparticle concentration of 1 wt% exhibited a much greater ability to hold water. The water fraction of foam reached a maximum when the SDS concentration was increased from 2.5 to 5 mmol/L and then decreased to a low value as the SDS concentration was increased further. Foam stabilized by SDS/AlOOH with an SDS concentration of 5 mmol/L exhibited good water retention ability, with a water fraction of


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47% and 37% at 3 h and 6 h, respectively. The adsorption of hydrophobic nanoparticles at the air-water interface and the complex aggregate structure effectively keep water in the foam film.18,49 Even after the half-life period, the foam was still wet and much stronger than foams stabilized by SDS. At SDS concentrations greater than 10 mmol/L, the foam was dominated by SDS and the presence of hydrophilic nanoparticles slightly slowed drainage by inducing congestion in the node.55

Figure 12. The water fraction of foam stabilized by SDS and by SDS/AlOOH mixtures. 3.4. Influence of Temperature and Oil on Foam Stability. Considering the harsh conditions in a reservoir, evaluating foam stability under high temperatures and in the presence of oil is necessary to ensure good foam performance. In these experiments, the concentrations of SDS and the AlOOH nanoparticles were 5 mmol/L and 1 wt%, respectively. To quantify the influence of temperature on foam stability precisely and conveniently, the foam drainage half-life was measured. As shown in Figure 13, when the temperature


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was increased from 22.5°C to 80°C, the foam drainage half-life of SDS foam and SDS/AlOOH foam decreased steadily. The drainage half-life of the foam stabilized by SDS was 8.5 min at a temperature of 22.5°C, indicating poor stability. With an increase of temperature, the thermal motion of molecules is accelerated, which leads to a reduction of dispersion viscosity, an increase of the gas diffusion rate and enhanced SDS molecule and AlOOH nanoparticle mobility; all of these factors accelerate the drainage rate. At high temperatures, a smaller amount of SDS molecules is adsorbed at the air-water interface, which reduces the electrostatic repulsion between the bubble film and therefore leads to coalescence.49 A similar influence is observed in the case of the AlOOH nanoparticles. These results indicate that an increase of temperature adversely affects foam stability. However, the network formed by nanoparticle aggregates in foam film still reduces drainage and coalescence effectively at high temperatures.56

Figure 13. The influence of temperature on the foam drainage half-life. Kerosene was used as the oil to evaluate the influence of oil on foam stability. After a foam was generated, 10 mL of kerosene was added gently to the front of the foam and the


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change in the foam volume with time was recorded. Figure 14 shows the influence of oil on foam stability. The foam half-life was abruptly shortened by the presence of oil, and the half-lives of foams stabilized by SDS and SDS/AlOOH were only approximately 39 min and 273 min, respectively. Foam collapse was immediate upon the addition of oil, and foam stabilized by SDS disappeared completely after 195 min. Generally, foams show poor stability when in contact with oil, and oil of a shorter hydrocarbon chain is more detrimental to foam stability.2 The mechanism of foam destruction in the presence of oil can be summarized as follows: an oil droplet enters the bubble film, spreads on the bubble surface and forms an unstable bridge between bubbles.2,3 Kerosene is a light petroleum product with a shorter hydrocarbon chain length than crude oil and is more capable of entering and spreading over the bubble film.57 After kerosene contacted foam stabilized by SDS, the entry and spread processes were rapid, and kerosene was found in the upper water phase just minutes later. Foam stabilized by an SDS/AlOOH mixture showed comparatively better oil resistance. The destruction mechanism of oil on foam generated from particle/surfactant dispersion is not fully understood. We inferred that the complex nanoparticle network in a foam film slows oil spread and that some of the oil can be adsorbed onto the alkyl-chain-covered surface of the hydrophobic modified nanoparticles through hydrophobic interaction. Additionally, the adsorption of nanoparticles onto the foam surface reduces the SDS-covered area, which slows the oil entry process.


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Figure 14. The influence of oil on foam stability. 3.5. Sandpack Flooding Experiments. An SDS solution with a concentration of 5 mmol/L and an SDS/AlOOH dispersion with an AlOOH nanoparticle concentration of 1 wt% and the same SDS concentration were used to generate foams. Figure 15 shows the changes in differential pressure during the flooding process. In the brine injection process, the differential pressures were stable at 0.02 MPa. After the injection of 1 PV SDS foam, the differential pressure increased and reached 0.23 MPa, indicating that the foam blocked the porous media to some extent. However, when the subsequent water flooding was conducted, the differential pressure decreased rapidly and remained almost constant after 0.9 PV water was injected. This result means that the SDS foam lacks long-term stability under reservoir conditions; thus, it cannot plug large pores effectively and is easily destroyed by water. In the case of the SDS/AlOOH foam, the differential pressure reached 1.05 MPa after 1 PV of foam was injected and the differential pressure decreased more slowly than that of the SDS foam. Even after the injection of 4.2 PV of water, the differential pressure remained higher than that during the water flooding process,


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indicating effective pore blocking. The high stability and viscosity of SDS/AlOOH foam enables it to withstand greater pressures and a water flush. After the foam collapses, some of the nanoparticles retained in the pore throat can also block the pores to some extent.

Figure 15. The changes in differential pressure during flooding.

4. Conclusions (1) The foamability of SDS/AlOOH dispersions showed that, in the case of low-SDSconcentration foams, the foam volume was greatly reduced because of the adsorption of SDS on the AlOOH nanoparticle surface. The foam volume remained nearly unchanged at high SDS concentrations and the presence of AlOOH nanoparticles increased dispersion viscosity, leading to a slight decrease of foamability. Overall, foam generated from SDS/AlOOH dispersions with higher nanoparticle concentrations exhibited lower foamability because of SDS adsorption and viscosity increase. (2) Foam stability was enhanced at an appropriate SDS/AlOOH concentration ratio (SDS/AlOOH concentration ratio = 5). The adsorption of hydrophobic-modified AlOOH


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nanoparticles at the air-water interface and the formation of a nanoparticle network in the foam film slows drainage, gas diffusion and coalescence, which effectively increases foam stability. At an inappropriate SDS/AlOOH concentration ratio (SDS/AlOOH concentration ratio ≥ 7.5) particle little affected foam stability. (3) The SDS/AlOOH foam exhibited much better stability than the SDS foam at high temperatures and in the presence of oil. The adsorption of nanoparticles at the air-water interface presumably hindered the oil entry process, and the nanoparticle aggregates in the foam film slowed the spread of the oil and adsorbed a portion of the oil. (4) The sandpack flooding experiments showed that SDS/AlOOH foam had favorable blocking ability. SDS/AlOOH foam could endure a water flush and maintained stability. We expect the results of this study to expand the available choices of nanoparticles for stabilizing foams during EOR. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The financial support received from the “Research Development Fund of China University of Petroleum” is acknowledged gratefully.


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Foams Stabilized by - American Chemical Society

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