Study on the Stability of Produced Water from Alkali/Surfactant/Polymer Flooding Under the Synergetic Eﬀect of Quartz Sand Particles and Oil Displacement Agents

processes

Article Study on the Stability of Produced Water from Alkali/Surfactant/Polymer Flooding under the Synergetic Eﬀect of Quartz Sand Particles and Oil Displacement Agents

Bin Huang 1,2,*, Chen Wang 1, Weisen Zhang 1, Cheng Fu 1,2,*, Haibo Liu 3 and Hongwei Wang 3

1 The Key Laboratory of Enhanced Oil and Gas Recovery of Educational Ministry, Northeast Petroleum University, Daqing 163318, China; [email protected] (C.W.); [email protected] (W.Z.) 2 Post-Doctoral Research Station of Daqing Oilﬁeld, Daqing 163458, China 3 Exploration and Development Research Institute of Daqing oilﬁeld, Daqing 163712, China; [email protected] (H.L.); [email protected] (H.W.) * Correspondence: [email protected] or [email protected] (B.H.); [email protected] or [email protected] (C.F.)

Received: 6 February 2020; Accepted: 6 March 2020; Published: 9 March 2020

Abstract: With the wide application of ASP (alkali/surfactant/polymer) flooding oil recovery technology, the produced water from ASP flooding has increased greatly. The clay particles carried by crude oil in the process of flow have a synergetic effect with oil displacement agents in the produced water, which increases the treatment difficulty of produced water. The stability of produced water is decided by the stability of oil droplets in the ASP-flooding-produced water system. The oil content, Zeta potential, interfacial tension and oil droplet size are important parameters to characterize the stability of produced water. In this paper, the changes of the oil content, Zeta potential, interfacial tension and oil droplet size of ASP flooding oily wastewater under the synergetic effect of different concentrations of quartz sand particles and oil displacement agents were studied by laboratory experiments. The experimental results show that the negatively charged quartz sand particles can absorb active substances in crude oil and surfactant molecules in the water phase and migrate to the oil–water interface, which increases the repulsion between quartz sand particles, decreasing the oil–water interfacial tension. Thus, the stability of oil droplets is enhanced, and the aggregation difficulty between oil droplets and quartz sand particles is increased. With the continually increasing quartz sand concentration, quartz sand particles combine with surfactant molecules adsorbed on the oil–water interface to form an aggregate. Meanwhile, the polymer molecules crimp from the stretching state, and the number of them surrounding the surface of the flocculation structure is close to saturation, which makes the oil droplets and quartz sand particles prone to aggregation, and the carried active substances desorb from the interface, resulting in the instability of the produced water system. The research on the synergetic effect between quartz sand particles and oil displacement agents is of great significance for deepening the treatment of ASP-produced water.

Keywords: quartz sand particle; produced water system; ASP oil displacement agent; oil–water interface; oil droplet stability

1. Introduction At present, the water content of produced liquid in most Chinese oil fields is up to more than 90%. With the continuous decline of water production and the scattered distribution of remaining oil, it is increasingly difficult to develop oilfields, and the recovery ratio is generally 30%–40% [1].

Processes 2020, 8, 315; doi:10.3390/pr8030315 www.mdpi.com/journal/processes Processes 2020, 8, 315 2 of 20

Alkali–surfactant–polymer (ASP) flooding oil recovery technology is built on alkali–polymer flooding and surfactant–polymer flooding. ASP flooding is a method to greatly improve oil recovery [2]. It can increase the recovery ratio by more than 20% under the condition that the water cut of oil fields reaches 98%, which has become an economic and effective method to enhance oil recovery [3]. The ASP-flooding-produced water is an oily wastewater containing oil-displacing agents after the oil and water separation from the ASP-flooding-produced liquid [4]. Due to the synergy between the alkali, polymer and surfactant, ASP-flooding-produced water is difficult to treat, and some conventional oil–water separation treatment technologies become invalid in ASP-flooding-produced water [5,6]. China needs to treat 500 million m3 of produced water from oil fields every year, and oil fields spend a lot of money on produced water treatment every year. The effect of treatment is related to the injection quality of produced water in oil fields, as well as to environmental problems [7]. In recent years, a large number of research works have been studied on the treatment of ASP-flooding-produced water at home and abroad, mainly focusing on the effects of oil displacement agents on the properties of the oil–water interface [5,6,8], the screening of chemical agents [9,10] and the improvement of treatment equipment [11,12]. Deng Shubo et al. studied the effect of oil displacement agents on the stability of ASP-flooding-produced water and found that the surfactant had the greatest influence on the stability of oil droplets, which hindered the coalescence of oil droplets and increased the difficulty of oil–water separation [6]. The Daqing oilfield institute hydration chamber has studied the oil–water separation characteristics, microstructure, interfacial properties and chemical agents of ASP-flooding-produced water and achieved good results [13,14]. Ye Qing et al. found that, in addition to the type of oil displacement agents, the pH value and salinity of the produced water also affect the oil–water separation [15]. Di et al. [16] designed a new type of ASP-flooding oil–water separator, and the experimental results showed that the oil–water separation rate could reach above 98%. Although some progress has been made in these studies, the existence of clay particles in produced water has not been considered. When the oil displacement agent flows through the formation, it will absorb the mineral substances and clay particles in the formation, carry the clay particles and combine with crude oil in the formation. The crude-oil-produced liquid is dehydrated to obtain crude oil and produced water; some clay particles can enter the produced water, making the produced water contain clay particles from formation. Clay particles have a synergy with oil displacement agents, which affects the stability of ASP-flooding-produced water, increases the treatment difficulty of ASP-flooding-produced water [17] and thus limits the popularization of ASP flooding oil recovery technology. Based on the analysis of the soil geology of Daqing oil field, it is found that quartz sand is the most common type of solid particle of the clay [18]. In this paper, simulated ASP-flooding-produced water containing quartz sand particles was prepared by a laboratory experiment. The effects of oil displacement agent concentration and quartz sand particle concentration on the stability of the produced water system were systematically studied, summarizing the influence rule of the quartz sand particles and oil displacement agent on the stability of oil droplets in ASP-flooding-produced oily water and analyzing the active mechanism of quartz sand particles and oil displacement agent on oil droplets in the produced water system.

2. Experimental Materials and Methods

2.1. Experimental Materials Crude oil with a density of 860 kg/m3, viscosity of 60 mPa s and water content of less than 0.5% · at 45 °C was provided by Daqing oilfield No.1 oil recovery plant (Daqing, China). The polymer was analytically pure hydrolyzed polyacrylamide (HPAM) with a molecular weight of 3 million and hydrolysis degree of 25%–30%, which was provided by Daqing oilfield oil recovery engineering (Daqing, China) research institute. The surfactant was alkyl benzene sulfonate (ORS-41), mass fraction was 50%, provided by Daqing oilfield No. 1 oil recovery plant (Daqing, China). The alkali was analytically pure NaOH with a mass fraction of 30% and was provided by Daqing oilfield oil recovery Processes 2020, 8, 315 3 of 20 engineering research institute (Daqing, China). Analytically pure quartz sand particles were provided by Daqing oilfield No. 1 oil recovery plant (Daqing, China). Analytically pure sodium chloride was provided by the Tianjin Damao Chemical Reagent Plant (Tianjin, China). Analytically pure sodium bicarbonate, anhydrous sodium sulfate, and anhydrous sodium carbonate were provided by the Tianjin Yaohua Chemical Plant (Tianjin, China). Analytically pure anhydrous calcium chloride and magnesium chloride were provided by the Harbin Xinda Chemical Plant (Harbin, China). Petroleum ether was provided by Shenyang Elepkes Chemical Co. Ltd (Shenyang, China). The experimental water was distilled water.

2.2. Instruments A temperature-controlled bath box, type S501-2, was used to keep the emulsion temperature stable, from the Liaoyang Huaguang Instrument Factory (Liaoyang, China). A digital display dispersing machine, type IKAT25, was used for the preparation of diﬀerent types of emulsion by uniform stirring, from the IKA company, (Staufen, Germany). An ultraviolet spectrophotometer, type TU-1901, was used for the determination of the oil content in emulsion, from Shanghai Mapda Instrument Equipment Co. Ltd (Shanghai, China). A laser particle size analyzer, type BT-9300H, was used to measure the particle size distribution of oil droplets in emulsion, from Suzhou Hinos Industrial Co. Ltd (Suzhou, China). A Zeta potentiometer, type JS94H, was used to measure the electrode potential between oil droplets, from Shanghai Zhongchen Digital Technology Instrument Co. LTD (Shanghai, China). A rotary drop ultra-low interfacial tensiometer, type XZD-5, was used for the determination of interfacial tension of the produced water system, from Beijing Hake Experimental Instrument factory (Beijing, China). The electronic balance, type BS210S, was from Shanghai Sadris Trading Co. Ltd (Shanghai, China). The micropipette, type Eppendorf, was from Eppendorf China Co. Ltd (Shanghai, China).

2.3. Preparation of Simulated ASP-Flooding-Produced Water Containing Quartz Sand Particles According to the water quality characteristics of ASP-flooding-produced water in Daqing oilfield, the specific steps of simulated ASP-flooding-produced water containing quartz sand particles preparation were as follows: (1) Preparation of mineralized water: refer to the salinity content of ASP-flooding-produced water in Daqing oilfield, the salinity of the mineralized water was determined; that is, the contents of NaCl, NaHCO3, Na2CO3, Na2SO4, CaCl2, MgCl2, and other main components were, respectively, 1523, 2820, 168.7, 10.5, 56.9 and 35.5 mg/L. (2) Preparation of oil droplet mother liquor: 0.4 g surfactant with a mass fraction of 50% and 199.6 g mineralized water were added to a 500 mL beaker, heated in a water bath to 45 °C, and then added to 200 g crude oil at 45 °C. Next, the high-speed shearing dispersing emulsifier was used to emulsify the emulsion at 20000 rpm for 10 min to produce the oil droplet mother liquor with an oil content of 50%. (3) Next, 99.6 mL of mineralized water with different contents of oil displacement agents (alkali, polymer and surfactant) and quartz sand particles were added into a 250 mL beaker, then 0.4 g oil droplet mother liquor with a mass fraction of 50% was added. After shaking well, the simulated ASP-flooding-produced water containing quartz sand particles with an oil content of 2000 mg/L was prepared. The main indexes were an oil content < 3000 mg/L, alkali < 1500 mg/L, polymer < 800 mg/L, surfactant < 600 mg/L. In order to introduce the processes between each measurement parameter more intuitively, we made a figure to explain the experimental set-up in the lab, as shown in Figure1. Processes 2019, 7, x FOR PEER REVIEW 4 of 21 flooding-produced water containing quartz sand particles with an oil content of 2000 mg/L was prepared. The main indexes were an oil content < 3000 mg/L, alkali < 1500 mg/L, polymer < 800 mg/L, surfactant < 600 mg/L. Processes 2020, 8, 315 4 of 20 In order to introduce the processes between each measurement parameter more intuitively, we made a figure to explain the experimental set-up in the lab, as shown in Figure 1.

Figure 1. TheThe experimental set-up in the laboratory. 2.4. Determination of Oil–Water Separation Characteristics 2.4. Determination of Oil–Water Separation Characteristics The characteristics of oil–water separation can be characterized by oil content, which is an The characteristics of oil–water separation can be characterized by oil content, which is an important index to measure the stability of produced water emulsion [6]. With the increase of oil important index to measure the stability of produced water emulsion [6]. With the increase of oil content, the stability of emulsion increases. The oil content in water is determined by ultraviolet content, the stability of emulsion increases. The oil content in water is determined by ultraviolet spectrophotometry [19]. As the standard oil is crude oil, 254 nm is used as the absorption wavelength. spectrophotometry [19]. As the standard oil is crude oil, 254 nm is used as the absorption wavelength. First, the standard curve of oil content in water was drawn: 0.1 g crude oil was accurately weighed, First, the standard curve of oil content in water was drawn: 0.1 g crude oil was accurately weighed, dissolved in a 100 mL capacity bottle with petroleum ether and diluted to the scale. The oil content of dissolved in a 100 mL capacity bottle with petroleum ether and diluted to the scale. The oil content the standard oil solution was 1000 mg/L. Then, 2 mL, 3 mL, 4 mL, 6 mL, 8 mL and 10 mL of the standard of the standard oil solution was 1000 mg/L. Then, 2 mL, 3 mL, 4 mL, 6 mL, 8 mL and 10 mL of the oil solution were transferred into the 10 mL colorimetrical cylinder by pipette; the volume was fixed standard oil solution were transferred into the 10 mL colorimetrical cylinder by pipette; the volume with petroleum ether and shaken well. A series of samples with a known oil content was configured. was fixed with petroleum ether and shaken well. A series of samples with a known oil content was We took petroleum ether as a blank, compared the colors on the ultraviolet spectrophotometer, drew configured. We took petroleum ether as a blank, compared the colors on the ultraviolet the standard curve according to the measured luminosity value and corresponding oil content and spectrophotometer, drew the standard curve according to the measured luminosity value and took the oil content (mg/L) as the abscissa and absorbance (A) as the ordinate, as shown in Figure2. corresponding oil content and took the oil content (mg/L) as the abscissa and absorbance (A) as the The linear relationship in the figure is as follows: ordinate, as shown in Figure 2. The linear relationship in the figure is as follows: 2 y = 0.00069x 0.01619; R =2 0.99992. yx=−0.00069− 0.01619； R = 0.99992.

With petroleum ether as the reference liquid and simulated produced water containing different concentrations of quartz sand particles and oil displacement agents as the sample to be tested, the

Processes 2019, 7, x FOR PEER REVIEW 5 of 21

Processesmeasured2020 absorbance, 8, 315 was substituted into the standard equation to calculate the oil content in5 ofthe 20 produced water.

Figure 2. The relationship between absorbance and oil content. Figure 2. The relationship between absorbance and oil content. With petroleum ether as the reference liquid and simulated produced water containing different concentrations2.5. Determination of of quartz Zeta Potential sand particles and oil displacement agents as the sample to be tested, the measuredAccording absorbance to the wastheory substituted of interfacial into thecharge standard [20], the equation oil droplets to calculate in ASP-flooding-produced the oil content in the producedwater are charged. water. When two oil droplets approach each other, they generate electrostatic repulsion. The electrostatic repulsion must be overcome for the droplets to coalesce. Therefore, the greater the 2.5. Determination of Zeta Potential absolute value of the Zeta potential, the more difficult it is for oil droplets to coalesce and the more stableAccording the emulsion to the is. theoryConversely, of interfacial the smaller charge the [absolute20], the oilvalue droplets of Zeta in po ASP-flooding-producedtential, the weaker the waterrepulsion are charged.between oil When droplets, two oil and droplets the more approach unstable each the other, emulsion they is. generate electrostatic repulsion. The electrostaticAfter the simulated repulsion ASP-flooding-produced must be overcome for thewater droplets containing to coalesce. quartz sand Therefore, particles the of greater different the absoluteconcentrations value ofsettled the Zeta for 2 potential, h, 5–10 mL the water more disampfficultles itwere is for taken oil droplets out of the to coalescebottom of and the the reagent more stablebottle and the emulsionput into the is. Conversely,colorimetrical the cylinder smaller as the th absolutee samples value to be of tested. Zeta potential,We repeated the this weaker 5 times the repulsionfor each sample, between removed oil droplets, the maxi andmum the more and minimum unstable the values, emulsion and the is. average Zeta potential values of theAfter remaining the simulated 3 times ASP-flooding-produced were calculated. The waterrelationship containing between quartz syst sandem particles stability of and diff erentZeta concentrationspotential is shown settled in Table for 2 1. h, 5–10 mL water samples were taken out of the bottom of the reagent bottle and put into the colorimetrical cylinder as the samples to be tested. We repeated this 5 times for each sample, removedTable the 1. maximumThe relationship and minimumbetween system values, stability and theand averageZeta potential. Zeta potential values of the remaining 3 times were calculated. The relationship between system stability and Zeta potential is shown in Table1Zeta. potential/mV Colloidal stability

Tablebelow-70 1. The relationship between system stabilityExcellent and Zeta stability potential.

Zeta-70 to Potential -50 /mV Colloidal Good stability Stability below-70 Excellent stability -50 to -30 General stability -70 to -50 Good stability -30 to-50 -10 to -30 Begins toGeneral become stability unstable

-10–-30 0 to -10 Begins Rapid to aggregation become unstable -10– 0 Rapid aggregation 2.6. Determination of interfacial tension In the stability system of emulsion, the oil–water interfacial tension is an important factor affecting its stability. The smaller the oil–water interfacial tension is, the more stable the emulsion

Processes 2020, 8, 315 6 of 20

2.6. Determination of interfacial tension In the stability system of emulsion, the oil–water interfacial tension is an important factor affecting its stability. The smaller the oil–water interfacial tension is, the more stable the emulsion system is. In terms of ASP-flooding-produced water, in addition to the interfacial active substances in the crude oil, the residual oil displacement agent components and the existence of clay particles will cause the change of interfacial tension [21]. First, we set the test temperature at 45 °C and the speed at 8500 r/min. Then, we injected the crude oil into the middle of the centrifuge tube by a microinjector, the centrifuge tube was put into the rotation axis of the instrument, the compression cap was tightened, and the rotation speed control was started to make the tube rotate at the set speed; the observation and reading were carried out after the tube was stabilized. Finally, the following formula was used for the calculation: 3 3 2 γ = 1.233 10 ∆ρ (Kd) (6000/S)− . In the formula, γ represents interfacial tension, mN/m; ∆ρ × × × represents the oil–water phase density difference, g/cm3; K represents amplification factor; d represents the width of the oil droplets, mm; and S represents the rotation speed, r/min.

2.7. Determination of Oil Droplet Size The oil droplets particle size is determined by a type BT-9300H laser particle size distribution instrument. We loaded 600 mL deionized distilled water into a beaker, heated it to 45 °C and then poured it into the sample pool of the laser particle size distribution instrument, turned on the water circulation system and computer operating system and measured the background value at this time. Then, 50 mL of simulated ASP ﬂooding oily sewage containing quartz sand particles at 45 °C was shaken well and held up for 4 hours, and an appropriate sample amount was extracted from the lower layer with a syringe, measuring the particle size distribution of oil droplets.

3. ExperimentalProcesses 2019 Results, 7, x FOR andPEER REVIEW Discussion 7 of 21

3.1. The Synergetic3. Experimental Effect Results of Quartz and SandDiscussion Particles and Oil Displacement Agents on Oil–Water Separation Characteristics 3.1. The Synergetic Effect of Quartz Sand Particles and Oil Displacement Agents on Oil–Water Separation The simulatedCharacteristics ASP-flooding-produced water containing quartz sand particles was prepared in the laboratory,The and simulated the effects ASP-floodin of differentg-produced concentrations water containing of quartz quartz sand particles particles was and prepared oil displacement in agents onthe the laboratory, oil–water and separation the effects characteristics of different conc wereentrations investigated; of quartz sand the resultsparticles areand shown oil in the displacement agents on the oil–water separation characteristics were investigated; the results are Figures3–5shown below. in the figures 3–5 below.

Figure 3. TheFigure synergetic 3. The synergetic eﬀect effect of alkali of alkali and and quartz quartz sa sandnd particles particles on oil–water on oil–water separation separation (polymer (polymer concentrationconcentration 500 mg /500L, surfactantmg/L, surfactant concentration concentration 200200 mg/L). mg/L).

Figure 4. The synergetic effect of surfactant and quartz sand particles on oil–water separation (polymer concentration 500 mg/L, alkali concentration 600 mg/L).

Processes 2019, 7, x FOR PEER REVIEW 7 of 21

3. Experimental Results and Discussion

3.1. The Synergetic Effect of Quartz Sand Particles and Oil Displacement Agents on Oil–Water Separation Characteristics The simulated ASP-flooding-produced water containing quartz sand particles was prepared in the laboratory, and the effects of different concentrations of quartz sand particles and oil displacement agents on the oil–water separation characteristics were investigated; the results are shown in the figures 3–5 below.

Figure 3. The synergetic effect of alkali and quartz sand particles on oil–water separation (polymer Processesconcentration2020, 8, 315 500 mg/L, surfactant concentration 200 mg/L). 7 of 20

Processes 2019, 7, x FOR PEER REVIEW 8 of 21 Figure 4. The synergetic eﬀect of surfactant and quartz sand particles on oil–water separation (polymer Figure 4. The synergetic effect of surfactant and quartz sand particles on oil–water separation concentration 500 mg/L, alkali concentration 600 mg/L). (polymer concentration 500 mg/L, alkali concentration 600 mg/L).

Figure 5. The synergetic effect of polymer and quartz sand particles on oil–water separation (alkali Figureconcentration 5. The 600synergetic mg/L, surfactanteffect of polymer concentration and quartz 200 mg sand/L). particles on oil–water separation (alkali concentration 600 mg/L, surfactant concentration 200 mg/L). 3.1.1. The Synergetic Effect of Quartz Sand Particles and Alkali 3.1.1.When The Synergetic the NaOH Effect concentration of Quartz increasesSand Particles from and 0 to 400Alkali mg /L, the oil content increases with the increaseWhen of the NaOH NaOH concentration. concentration As increases the NaOH from concentration 0 to 400 mg/L, continues the oil tocontent increase, increases the oil with content the increasegradually ofdecreases; NaOH concentration. that is, the stabilityAs the NaOH of oil dropletsconcentration increases continues first and to increase, then weakens the oil with content the graduallyincrease of decreases; NaOH concentration. that is, the stability Because of there oil droplets are acidic increases components first and in crude then weakens oil, they canwith react the withincrease NaOH of NaOH to form concentration. a surfactant, and Because the newly there generated are acidic surfactant components and thein crude original oil, surfactant they can ofreact the withsystem NaOH will reduceto form the a surfactant, interfacial tensionand the betweennewly gene oil andrated water, surfactant enhance and the the stability original of surfactant oil droplets. of Thethe system higher thewill concentrationreduce the interfacial of surfactant tension adsorbed between on oil the and oil–water water, interface,enhance the the stability higherthe of oil droplets.content. WhenThe higher the NaOH the concentrat concentrationion of surfactant is more than adsorbed 400 mg on/L, the NaOH oil–water has neutralized interface, the the higher acidic + + thecomponents oil content. in crudeWhen oil, the there NaOH is a concentration large amount ofis excessmore than Na in400 the mg/L, water NaOH phase. has Na neutralizedcan neutralize the acidicthe oil components droplets’ surface in crude negative oil, th charge,ere is reducea large theamount oil droplets’ of excess surface Na+ in negative the water charge phase. density Na+ andcan neutralize the oil droplets’ surface negative charge, reduce the oil droplets’ surface negative charge density and weaken the stability of the oil droplets [22]. At the same time, Na+ can compress the structure of the double layer, weaken the acting force between oil droplets and facilitate the coalescence of oil droplets [23]. At first, the oil content increases with the increase of quartz sand concentration; then, the oil content decreases with the increase of quartz sand concentration because quartz sand particles are negatively charged and can be adsorbed on the oil droplets’ oil–water interface, increasing the negative charge density of oil droplets’ surface and enhancing the stability of oil droplets. The stability of the produced water system is the strongest when the quartz sand particle concentration is 200 mg/L. Since then, with the continuous addition of quartz sand, its concentration in the liquid phase increases continuously. Due to the action of gravity, quartz sand particles tend to aggregate, and polymer has the ability of flocculation. The aggregation of quartz sand and oil droplets can effectively promote the oil–water separation, making the produced water system unstable.

3.1.2. The Synergetic Effect of Quartz Sand Particles and Surfactant As shown in Figure 4, the oil content increases with the increase of surfactant concentration, indicating that the surfactant can enhance the stability of oil droplets and make it difficult for small oil droplets to aggregate; this is because the surfactant can be adsorbed on the surface of oil droplets in the form of nonpolar groups sticking into oil droplets and polar groups sticking into the water, changing the surface properties of the oil droplets and making the surface of the oil droplets from

Processes 2020, 8, 315 8 of 20 weaken the stability of the oil droplets [22]. At the same time, Na+ can compress the structure of the double layer, weaken the acting force between oil droplets and facilitate the coalescence of oil droplets [23]. At first, the oil content increases with the increase of quartz sand concentration; then, the oil content decreases with the increase of quartz sand concentration because quartz sand particles are negatively charged and can be adsorbed on the oil droplets’ oil–water interface, increasing the negative charge density of oil droplets’ surface and enhancing the stability of oil droplets. The stability of the produced water system is the strongest when the quartz sand particle concentration is 200 mg/L. Since then, with the continuous addition of quartz sand, its concentration in the liquid phase increases continuously. Due to the action of gravity, quartz sand particles tend to aggregate, and polymer has the ability of flocculation. The aggregation of quartz sand and oil droplets can effectively promote the oil–water separation, making the produced water system unstable.

3.1.2. The Synergetic Effect of Quartz Sand Particles and Surfactant As shown in Figure4, the oil content increases with the increase of surfactant concentration, indicating that the surfactant can enhance the stability of oil droplets and make it difficult for small oil droplets to aggregate; this is because the surfactant can be adsorbed on the surface of oil droplets in the form of nonpolar groups sticking into oil droplets and polar groups sticking into the water, changing the surface properties of the oil droplets and making the surface of the oil droplets from hydrophobicity to hydrophilicity, thus making it difficult for oil droplets to approach each other, resulting in the increasing stability of oil droplets. At the beginning, the oil content in the produced water increases with the increase of quartz sand concentration. When the quartz sand particle concentration is 200 mg/L, the oil content in the produced water reaches the maximum. Then, with the increase of quartz sand concentration, the oil content decreases, but the oil content is higher than the initial oil content because surfactant molecules can gather on the oil–water interface, forming a resistant film around the oil droplets that prevents them from coalescing [24]. When the quartz sand and surfactant exist in produced water at the same time, the surfactant will be adsorbed on the quartz sand particles in an aqueous phase, increasing the repulsion between the particles, hindering the coalescence of quartz sand particles and oil droplets and leading to the enhanced stability of produced water system. As the quartz sand concentration increases, the number of quartz sand particles adsorbed with surfactant molecules gradually reaches saturation and the excess quartz sand particles tend to aggregate with oil droplets, thus promoting the oil–water separation process.

3.1.3. The Synergetic Effect of Quartz Sand Particles and Polymer It can be seen in Figure5 that the oil content decreases first and then increases with the increase of polymer concentration; that is, the stability of oil drops decreases first and then increases. Because the polymer used in the experiment has a high molecular weight of 3 million, when the polymer concentration is low, though the addition of polymer increases the fluid film strength of oil droplets, the polymer molecules have bridging and flocculating effects, which can be adsorbed on the oil droplets’ surface, causing the flocculation of dispersed oil droplets. Meanwhile, the polymer can compress the double electrode layer on the surface of oil droplets, weaken the acting force between oil droplets and reduce the stability of oil droplets. When the cover degree of polymer on the oil droplets’ surface reaches 50% [25], the flocculation reaches its maximum. Then, as the polymer concentration continues to increase, on the one hand, the polymer molecules will produce a steric hindrance effect, which is not conducive to the coalescence of oil droplets [8]; on the other hand, with the increase of polymer concentration, the viscosity of the aqueous phase increases, which slows down the aggregation rate of oil droplets and enhances the stability of oil droplets. When the quartz sand concentration is less than 200 mg/L, the oil content increases with the increase of quartz sand concentration. When the quartz sand concentration is more than 200 mg/L, Processes 2020, 8, 315 9 of 20 the oil content decreases with the increase of quartz sand concentration because the system salinity content is high. When the concentration of quartz sand particles is high, the polymer molecular morphology changes; it crimps from the stretching state [26]. The coverage on the surface of quartz sand particles decreases, and quartz sand particles tend to aggregate with oil droplets, thus facilitating the oil–water separation.

3.2. The Synergetic Eﬀect of Quartz Sand Particles and Oil Displacement Agents on Zeta Potential

3.2.1. The Synergetic Eﬀect of Quartz Sand Particles and Alkali It can be seen in Figure6 that when the alkali concentration increases from 0 to 400 mg /L, with the raising of alkali concentration, the Zeta potential decreases. With the further raising of alkali concentration, the Zeta potential increases, because when the alkali concentration is low, NaOH will react with acidic components to form a surface-active substance. The newly generated surface-active substance is adsorbed on the interface between oil and water, increasing the negative charge density on the oil droplets’ surface and reducing the Zeta potential on the oil droplets’ surface. When the alkali concentration further increases, the acidic components of crude oil are neutralized. The excess Na+ in aqueous phase can neutralize the negative charge on the oil droplets’ surface, thus reducing the + negativeProcesses 2019 charge, 7, x FOR density PEER REVIEW of the oil droplets’ surface. At the same time, Na can compress the di10ﬀ ofuse 21 electric double layer of the oil droplets’ surface [22]. Under this joint action, the Zeta potential increases.

Figure 6. The synergetic eﬀect of alkali and quartz sand particles on Zeta potential (polymer concentrationFigure 6. The 500 synergetic mg/L, surfactant effect of concentration alkali and quartz 200 mg sand/L). particles on Zeta potential (polymer concentration 500 mg/L, surfactant concentration 200 mg/L). When the quartz sand concentration is lower than 100 mg/L, the Zeta potential decreases gradually withWhen the addition the quartz of quartz sand sand concentration particles. With is lower the further than 100 raising mg/L, ofquartz the Zeta sand potential concentration, decreases the Zetagradually potential with remains the addition almost of unchanged quartz sand and part tendsicles. to beWith stable. the Thefurther quartz raising sand of particles quartz carrysand negativeconcentration, charges the and Zeta can potential adhere toremains the oil almost droplets’ unchanged surface, increasing and tends the to negativebe stable. charge The quartz density sand on theparticles oil droplets’ carry negative surface andcharges increasing and can the adhere electronegativity to the oil droplets’ on the oil surface, droplets’ increasing surface, andthe negative the Zeta potentialcharge density decreases. on the When oil droplets’ the quartz surface sand concentration and increasing is high, the electronegativity its adsorption reaches on the saturation, oil droplets’ and thesurface, Zeta and potential the Zeta no longerpotential changes. decreases. When the quartz sand concentration is high, its adsorption reaches saturation, and the Zeta potential no longer changes.

3.2.2. The Synergetic Effect of Quartz Sand Particles and Surfactant It can be seen in Figure 7 that when the concentration of alkali and polymer are constant and the concentration of surfactant is lower than 400 mg/L, the Zeta potential significantly decreases with the raising of surfactant concentration. When the surfactant concentration increases further, the Zeta potential decreases slowly. Because the surfactant can raise the negative charge density, the Zeta potential can be significantly reduced. As the surfactant concentration continues to rise, the number of surfactant molecules adsorbed on the surface of oil droplets reaches saturation, and the Zeta potential decreases at a slower rate [27].

Processes 2020, 8, 315 10 of 20

3.2.2. The Synergetic Effect of Quartz Sand Particles and Surfactant It can be seen in Figure7 that when the concentration of alkali and polymer are constant and the concentration of surfactant is lower than 400 mg/L, the Zeta potential significantly decreases with the raising of surfactant concentration. When the surfactant concentration increases further, the Zeta potential decreases slowly. Because the surfactant can raise the negative charge density, the Zeta potential can be significantly reduced. As the surfactant concentration continues to rise, the number of surfactant molecules adsorbed on the surface of oil droplets reaches saturation, and the Zeta potential Processes 2019, 7, x FOR PEER REVIEW 11 of 21 decreases at a slower rate [27].

Figure 7. The synergetic effect of surfactant and quartz sand particles on Zeta potential (polymer Figure 7. The synergetic effect of surfactant and quartz sand particles on Zeta potential (polymer concentration 500 mg/L, alkali concentration 600 mg/L). concentration 500 mg/L, alkali concentration 600 mg/L). When the quartz sand concentration is between 0 to 100 mg/L, the Zeta potential decreases When the quartz sand concentration is between 0 to 100 mg/L, the Zeta potential decreases significantly with the rise of quartz sand concentration; when the quartz sand concentration increases significantly with the rise of quartz sand concentration; when the quartz sand concentration increases further, the Zeta potential remains almost unchanged. This is because, when the quartz sand further, the Zeta potential remains almost unchanged. This is because, when the quartz sand concentration is low, quartz sand particles can form a flocculated structure with the surfactant, oil concentration is low, quartz sand particles can form a flocculated structure with the surfactant, oil droplets and polymer. Quartz sand particles carry negative charges, which increases the surface droplets and polymer. Quartz sand particles carry negative charges, which increases the surface negative charge density of oil droplets and decreases the Zeta potential by surrounding the surface negative charge density of oil droplets and decreases the Zeta potential by surrounding the surface of the structure. With the increase of the quartz sand concentration, the amount of quartz sand of the structure. With the increase of the quartz sand concentration, the amount of quartz sand surrounding the surface of the structure is close to saturation, the electrostatic repulsion increases, the surrounding the surface of the structure is close to saturation, the electrostatic repulsion increases, negative charge density increment slows down and the Zeta potential decreases slowly and tends to the negative charge density increment slows down and the Zeta potential decreases slowly and tends be stable. to be stable. 3.2.3. The Synergetic Effect of Quartz Sand Particles and Polymer 3.2.3. The Synergetic Effect of Quartz Sand Particles and Polymer It can be seen in Figure8 that the Zeta potential of oil droplets’ surface decreases gradually with the increaseIt can be ofseen polymer in Figure concentration, 8 that the Zeta because potentia polymerl of oil droplets’ molecules surface also adsorb decreases on thegradually oil droplets’ with thesurface, increase thus of increasing polymer theconcentration, negative charge because density polymer on the molecules oil droplets’ also surface. adsorb However,on the oil duedroplets’ to the surface,weak surface thus increasing activity of the polymers, negative thecharge number density of polymeron the oil molecules droplets’ surface. adsorbed However, on the oil due droplets’ to the weaksurface surface is relatively activity low of [polymers,28]; as a result, the number the Zeta of potential polymer decreases molecules slowly. adsorbed on the oil droplets’ surface is relatively low [28]; as a result, the Zeta potential decreases slowly.

Processes 20202019,, 87,, 315x FOR PEER REVIEW 1112 of 2021

Figure 8. The synergetic eﬀect of polymer and quartz sand particles on Zeta potential (alkali Figure 8. The synergetic effect of polymer and quartz sand particles on Zeta potential (alkali concentration 600 mg/L, surfactant concentration 200 mg/L). concentration 600 mg/L, surfactant concentration 200 mg/L). At the beginning, the Zeta potential gradually decreases as the quartz sand concentration increases. At the beginning, the Zeta potential gradually decreases as the quartz sand concentration When the quartz sand concentration reaches 150 mg/L, no matter how much the concentration of increases. When the quartz sand concentration reaches 150 mg/L, no matter how much the polymer increases, the Zeta potential basically remains unchanged. The reason is that the polymer and concentration of polymer increases, the Zeta potential basically remains unchanged. The reason is quartz sand particles themselves are negatively charged, which adsorb on the interface between oil that the polymer and quartz sand particles themselves are negatively charged, which adsorb on the and water, enhancing the electronegativity of oil droplets and reducing the Zeta potential. When the interface between oil and water, enhancing the electronegativity of oil droplets and reducing the Zeta quartz sand concentration in the liquid phase is high, on the one hand, the adsorption and desorption potential. When the quartz sand concentration in the liquid phase is high, on the one hand, the processes of quartz sand particles on the oil droplets’ surface reach dynamic equilibrium; on the other adsorption and desorption processes of quartz sand particles on the oil droplets’ surface reach hand, the electrostatic repulsion between quartz sand particles and oil droplets increases, and the dynamic equilibrium; on the other hand, the electrostatic repulsion between quartz sand particles generated electrostatic repulsion enhances the stability of the emulsion system. and oil droplets increases, and the generated electrostatic repulsion enhances the stability of the 3.3.emulsion The Synergetic system. Eﬀect of Quartz Sand Particles and Oil Displacement Agents on Interfacial Tension

3.3.1.3.3. The The Synergetic Synergetic Effect Eff ectof Quar of Quartztz Sand Sand Particles Particles and Oil and Displaceme Alkali nt Agents on Interfacial Tension It can be seen in Figure9 that when the alkali concentration is in the range of 0 to 400 mg /L, the 3.3.1. The Synergetic Effect of Quartz Sand Particles and Alkali interfacial tension decreases with the rise of alkali concentration because the surfactant generated by the reactionIt can be of seen NaOH in Figure with acidic 9 that components when the alkali of crude concentration oil has the is property in the range of reducing of 0 to the400 interfacialmg/L, the tension.interfacial When tension the decreases alkali concentration with the rise is of more alkali than concentration 400 mg/L, NaOHbecause has the basically surfactant neutralized generated theby acidicthe reaction components, of NaOH and with a large acidic amount components of excess of cr Naude+ will oil freelyhas the di propertyffuse to the of reducing oil–water the interface interfacial and compresstension. When the di theffuse alkali electric concentration double layer is onmore the th surfacean 400 of mg/L, oil droplets, NaOH has thus basically increasing neutralized the oil–water the interfacialacidic components, tension. and a large amount of excess Na+ will freely diffuse to the oil–water interface and compress the diffuse electric double layer on the surface of oil droplets, thus increasing the oil–water interfacial tension.

Processes 2020, 8, 315 12 of 20 Processes 2019, 7, x FOR PEER REVIEW 13 of 21

Figure 9.9. The synergetic effect effect of quartz sand particles and alkali on interfacialinterfacial tensiontension (polymer(polymer concentration 500500 mgmg/L,/L, surfactantsurfactant concentrationconcentration 200200 mgmg/L)./L). When the concentration of alkali is no more than 600 mg/L and the quartz sand concentration is in When the concentration of alkali is no more than 600 mg/L and the quartz sand concentration is the range of 0–100 mg/L, the interfacial tension decreases with the increase of quartz sand concentration. in the range of 0–100 mg/L, the interfacial tension decreases with the increase of quartz sand When the quartz sand concentration is more than 100 mg/L, the interfacial tension increases with the concentration. When the quartz sand concentration is more than 100 mg/L, the interfacial tension increase of quartz sand concentration. Because quartz sand particles are negatively charged, they increases with the increase of quartz sand concentration. Because quartz sand particles are negatively can carry the active substances in crude oil and adsorb on the interface between oil and water of oil charged, they can carry the active substances in crude oil and adsorb on the interface between oil and droplets, reducing the oil–water interface tension. When the quartz sand concentration is too high, water of oil droplets, reducing the oil–water interface tension. When the quartz sand concentration quartz sand particles tend to aggregate, and the polymer itself has the ability of flocculation, which is too high, quartz sand particles tend to aggregate, and the polymer itself has the ability of reduces the number of active substances adsorbed on the interface between oil and water, resulting in flocculation, which reduces the number of active substances adsorbed on the interface between oil the increase of interfacial tension [29]. When the alkali concentration exceeds 600 mg/L, the interfacial and water, resulting in the increase of interfacial tension [29]. When the alkali concentration exceeds tension increases with the increase of quartz sand concentration. Because the system’s pH value is high 600 mg/L, the interfacial tension increases with the increase of quartz sand concentration. Because at this time, the alkali hydrolyzes the polymer, causing a lot of polymer molecules to curl, reducing the system’s pH value is high at this time, the alkali hydrolyzes the polymer, causing a lot of polymer the adsorption capacity on the oil–water interface, increasing the flocculation of quartz sand particles, molecules to curl, reducing the adsorption capacity on the oil–water interface, increasing the resulting in the desorption of the active substances from the interface, and the interfacial tension flocculation of quartz sand particles, resulting in the desorption of the active substances from the gradually increases. interface, and the interfacial tension gradually increases. 3.3.2. The Synergetic Effect of Quartz Sand Particles and Surfactant 3.3.2. The Synergetic Effect of Quartz Sand Particles and Surfactant It can be seen in Figure 10 that the concentration of alkali and polymer is constant and the It can be seen in Figure 10 that the concentration of alkali and polymer is constant and the concentration of surfactant is between 0–400 mg/L, the interfacial tension decreases significantly with concentration of surfactant is between 0–400 mg/L, the interfacial tension decreases significantly with the increase of surfactant concentration. When the surfactant concentration is more than 400 mg/L, the increase of surfactant concentration. When the surfactant concentration is more than 400 mg/L, with the rise of surfactant concentration, the interfacial tension continues to decrease, but the decreased with the rise of surfactant concentration, the interfacial tension continues to decrease, but the degree is smaller. This indicates that the surfactant has a great influence on the system’s interfacial decreased degree is smaller. This indicates that the surfactant has a great influence on the system’s tension. The surfactant used in the experiment is alkyl benzene sulfonate, which has good surface interfacial tension. The surfactant used in the experiment is alkyl benzene sulfonate, which has good activity and can be adsorbed on the oil–water interface, and the interfacial tension can be significantly surface activity and can be adsorbed on the oil–water interface, and the interfacial tension can be reduced through the strong interaction with the oil droplets. The more surfactant molecules there are significantly reduced through the strong interaction with the oil droplets. The more surfactant on the interface between oil and water, the lower the interfacial tension value. molecules there are on the interface between oil and water, the lower the interfacial tension value.

ProcessesProcesses 20192020, ,78, ,x 315 FOR PEER REVIEW 1413 of of 21 20

Figure 10. The synergetic eﬀect of quartz sand particles and surfactant on interfacial tension (polymer Figure 10. The synergetic effect of quartz sand particles and surfactant on interfacial tension (polymer concentration 500 mg/L, alkali concentration 600 mg/L). concentration 500 mg/L, alkali concentration 600 mg/L). At the beginning, the interfacial tension decreases with the increase of quartz sand concentration. At the beginning, the interfacial tension decreases with the increase of quartz sand When the quartz sand concentration exceeds 150 mg/L, the interfacial tension slightly increases with concentration. When the quartz sand concentration exceeds 150 mg/L, the interfacial tension slightly the rise of quartz sand concentration. The negatively charged quartz sand particles can carry active increases with the rise of quartz sand concentration. The negatively charged quartz sand particles substances in crude oil and adsorb on the interface between oil and water of oil droplets, thus reducing can carry active substances in crude oil and adsorb on the interface between oil and water of oil the oil–water interface tension. As increasing numbers of quartz sand particles are adsorbed on the droplets, thus reducing the oil–water interface tension. As increasing numbers of quartz sand oil–water interface, a composite membrane will be formed, thus reducing the adsorption amount of particles are adsorbed on the oil–water interface, a composite membrane will be formed, thus surfactant molecules on the interface between oil and water. At the same time, quartz sand particles reducing the adsorption amount of surfactant molecules on the interface between oil and water. At may combine with surfactant molecules to form an aggregate, which further reduces the adsorption the same time, quartz sand particles may combine with surfactant molecules to form an aggregate, amount of surfactant molecules and increases the interfacial tension. which further reduces the adsorption amount of surfactant molecules and increases the interfacial tension.3.3.3. The Synergetic Eﬀect of Quartz Sand Particles and Polymer

3.3.3. WhenThe Synergetic the concentration Effect of ofQuartz alkali Sand and surfactant Particles and is constant Polymer and the concentration of quartz sand is 0 mg/L, as can be seen in Figure 11, with the change of polymer concentration, the oil–water interfacial tensionWhen is the basically concentration unchanged. of alkali In otherand surfactant words, the is polymerconstant and has the little concentration inﬂuence on of the quartz interfacial sand istension. 0 mg/L, However, as can be when seen quartzin Figure sand 11, particles with th exist,e change the interfacialof polymer tension concentration, decreases the with oil–water the rise interfacialof polymer tension concentration. is basically Because unchanged. the polymer In other used words, in the the experiment polymer has is high-molecular-weightlittle influence on the interfacialpolyacrylamide, tension. which However, has goodwhen waterquartz solubility sand part andicles canexist, be the di ﬀinterfacialused from tension the aqueous decreases phase with to thethe rise interface of polymer between concentration. oil and water Because and then the po adsorbedlymer used with in quartz the experiment sand particles is high-molecular- and the active weightsubstances polyacrylamide, carried by them, which the has interaction good water between solubility quartz and can sand be particles diffused andfrom polymer the aqueous reduces phase the tointerfacial the interface tension. between oil and water and then adsorbed with quartz sand particles and the active substances carried by them, the interaction between quartz sand particles and polymer reduces the interfacial tension.

Processes 20202019,, 87,, 315x FOR PEER REVIEW 1514 of 21 20

Figure 11. The synergetic effect of quartz sand particles and polymer on interfacial tension (alkali Figure 11. The synergetic effect of quartz sand particles and polymer on interfacial tension (alkali concentration 600 mg/L, surfactant concentration 200 mg/L) concentration 600 mg/L, surfactant concentration 200 mg/L) For polymers of different concentrations, when quartz sand concentration is less than 150 mg/L, the interfacialFor polymers tension of different decreases concentrations, with the increase when of quartzquartz sand concentration.concentration is Because less than quartz 150 mg/L, sand particlesthe interfacial can absorb tension the decreases active substances with the in increase crude oil of and quartz migrate sand to concentration. the interface between Because oil quartz and water, sand theseparticles active can substancesabsorb the significantlyactive substances reduce in thecrud oil–watere oil and interfacemigrate to tension. the interface When between the quartz oil sand and concentrationwater, these active is between substances 100–150 significantly mg/L, the reduce interfacial the tensionoil–water reaches interface the minimizedtension. When level, the and quartz then thesand interfacial concentration tension is increasesbetween 100–150 with the mg/L, rise of the quartz interfacial sand concentration; tension reaches because the minimized the area of level, oil–water and interfacethen the interfacial is limited, tension when theincreases number with of quartzthe rise sand of quartz particles sand adsorbed concentration; on the because interface the between area of oiloil–water and water interface reaches is limited, saturation, when the the excess number quartz of quartz sand particles sand particles will adsorb adsorbed some on surface-active the interface substances,between oil andand accumulationwater reaches andsaturation, settlement the occurexcess [ 30quartz], leading sand toparticles the increase will adsorb of interfacial some surface- tension. active substances, and accumulation and settlement occur [30], leading to the increase of interfacial 3.4.tension. The Synergetic Effect of Quartz Sand Particles and Oil Displacement Agents on Oil Droplet size The measurement results are represented by the median diameter. In other words, the median 3.4. The Synergetic Effect of Quartz Sand Particles and Oil Displacement Agents on Oil Droplet size diameter is used to represent the oil droplet size under different conditions. When there are no ASP oil displacementThe measurement agents in theresults system, are therepresented initial particle by the size median of oil dropletsdiameter. is In 3.93 otherµm. words, After settling the median for 2 hours,diameter the is oil used droplets’ to represent particle the size oil isdroplet 4.32 µ sizem; compared under different with theconditions. initial particle When size,there the areincrease no ASP inoil particledisplacement size is agents very small, in the indicatingsystem, the that initial only part aicle slight size coalescence of oil droplets has is occurred 3.93 µm. in After oil droplets. settling Whenfor 2 hours, the polymer the oil concentration droplets’ particle in the size system is 4.32 is 500 µm; mg /comparedL and the alkaliwith concentrationthe initial particle and surfactant size, the concentrationincrease in particle is 0 mg size/L, theis very particle small, size indicating of the oil dropletsthat only is a 8.17 slightµm—much coalescence larger has than occurred when in there oil aredroplets. no ASP When oil displacement the polymer agentsconcentration in the system—indicating in the system is 500 that mg/L the and existence the alkali of HPAM concentration is conducive and tosurfactant the aggregation concentration of oil droplets.is 0 mg/L, When the particle the NaOH size of concentration the oil droplets in the is 8.17 system µm—much is 600 mg larger/L and than the polymerwhen there concentration are no ASP oil and displacement surfactant concentration agents in the system—indicating is 0 mg/L, the oil droplets that the particleexistence size of HPAM is 4.10 µism—smaller conducive to than the when aggregation there are of no oil ASP droplets. oil displacement When the NaOH agents concentration in the system—indicating in the system that is 600 the existencemg/L and of the NaOH polymer is not concentration conducive to and the surfactant aggregation concentration of oil droplets, is 0 but mg/L, the hindrancethe oil droplets effect isparticle weak. Whensize is the4.10 surfactant µm—smaller concentration than when in the there system are is no 200 ASP mg /Loil and displacement the alkali concentration agents in the and system— polymer concentrationindicating that is the 0 mg existence/L, the oil of droplets NaOH particleis not cond sizeucive is 3.81 toµ m—muchthe aggregation smaller of than oil whendroplets, there but are the no ASPhindrance oil displacement effect is weak. agents When in the the system— surfactant which concentration indicates thatin the the system existence is 200 of mg/L the surfactant and the alkali is not onlyconcentration unfavorable and to polymer the aggregation concentration of oil is droplets 0 mg/L, but the also oil droplets can enhance particle the stabilitysize is 3.81 of oilµm—much droplets. Thesmaller effects than of when different there concentrations are no ASP oil of displacement quartz sand particlesagents inand the system— oil displacement which indicates agents on that the the oil dropletexistence size of werethe surfactant investigated; is not the only results unfavorable are shown to in the Figuresaggregation 12–14 of below. oil droplets but also can enhance the stability of oil droplets. The effects of different concentrations of quartz sand particles

Processes 2019, 7, x FOR PEER REVIEW 16 of 21 and oil displacement agents on the oil droplet size were investigated; the results are shown in the figures 12–14 below. Processes 2020, 8, 315 15 of 20 3.4.1. The Synergetic Effect of Quartz Sand Particles and Alkali

ProcessesFigure 2019, 7 12., x FORThe PEER synergetic REVIEW e ﬀect of quartz sand particles and alkali on oil droplets size (polymer17 of 21 Figure 12. The synergetic effect of quartz sand particles and alkali on oil droplets size (polymer concentration 500 mg/L, surfactant concentration 200 mg/L). concentration 500 mg/L, surfactant concentration 200 mg/L).

When the NaOH concentration is low, the oil droplet size gradually decreases with the rise of NaOH concentration, indicating that NaOH hinders the coalescence of oil droplets at this time, because the surface-active substance generated by the reaction of NaOH and acidic components of crude oil can adsorb on the oil droplets’ surface, reduce the oil–water interfacial tension and enhance the stability of oil droplets, and the oil droplets size decreases. When the NaOH concentration is more than 800 mg/L, the acidic components in crude oil are completely reacted. At this time, NaOH acts as an electrolyte, and Na+ can compress the double electrode layer, weakening the stability of oil droplets—which is conducive to the coalescence of oil droplets—and increasing the oil droplet size. At the beginning, the oil droplets’ particle size decreases with the increase of quartz sand concentration; then, the oil droplets’ particle size increases with the rise of quartz sand concentration. Negatively charged quartz sand particles adhere to the oil droplets’ surface, enhancing the electronegativity on the oil droplets’ surface; thus, the oil droplets’ stability is enhanced, which hinders the coalescence of oil droplets. Quartz sand particles tend to aggregate with the further rise of the quartz sand concentration. The aggregation of quartz sand and oil droplets can promote the oil–water separation, and the polymer itself has the ability of flocculation, which is conducive to the increase of the oil droplet size.

3.4.2. The Synergetic Effect of Quartz Sand Particles and Surfactant Figure 13. The synergetic eﬀect of the quartz sand particles and surfactant on the oil droplet size Figure(polymer 13. concentrationThe synergetic 500 effect mg/ L,of alkalithe quartz concentration sand particles 600 mg and/L). surfactant on the oil droplet size (polymer concentration 500 mg/L, alkali concentration 600 mg/L).

When the surfactant concentration in the system is 0, the particle size of the oil droplets is 6.73 µm—much larger than when there are no ASP oil displacement agents in the system, but smaller than that, there are only polymer oil displacement agents in the system—indicating that the flocculation effect of polymer is much stronger than the hindrance effect of NaOH on the coalescence of oil droplets. At first, the oil droplet size decreases sharply with the rise of surfactant concentration; then, the oil droplet size begins to decrease. The existence of the surfactant makes the oil droplets disperse more evenly, which weakens the flocculation effect of the polymer on the oil droplets. Meanwhile, the surfactant reduces the Zeta potential on the oil droplets’ surface. Moreover the electrostatic repulsion between the oil droplets increases, thus reducing the probability of a collision and hindering the coalescence of oil droplets. When the quartz sand concentration is low, the oil droplet size decreases with the rise of quartz sand concentration. When the quartz sand concentration is in the range of 150–200 mg/L, the oil droplet size reaches its smallest value. With the further increase of quartz sand concentration, the oil droplet size gradually increases due to the quartz sand particles and their surface-active substances adsorbing on the interface between oil and water, which is equivalent to forming a certain structured interfacial film, enhancing the stability of produced water, increasing the coalescence difficulty of oil droplet coalescence and decreasing the oil droplet size [31]. With the further increase of quartz sand concentration, the oil droplets and quartz sand particles combine to form a larger aggregated structure. The number of surface-active substances on the interface between oil and water decreases, the strength of the interfacial film weakens, and thus quartz sand particles carry part of the oil droplets, accumulate and settle, leading to the stability of produced water decreasing, promoting the oil–water separation, and increasing the oil droplet size.

3.4.3. The Synergetic Effect of Quartz Sand Particles and Polymer

Processes 2019, 7, x FOR PEER REVIEW 18 of 21 Processes 2020, 8, 315 16 of 20

Figure 14. The synergetic effect of quartz sand particles and polymer on oil droplet size (alkali Figureconcentration 14. The 600 synergetic mg/L, surfactant effect of concentration quartz sand 200particles mg/L). and polymer on oil droplet size (alkali concentration 600 mg/L, surfactant concentration 200 mg/L). 3.4.1. The Synergetic Effect of Quartz Sand Particles and Alkali TheWhen particle the NaOH size concentrationof oil droplets is increases low, the oilfirst droplet and then size decreases gradually decreaseswith the rise with of the polymer rise of concentration.NaOH concentration, When the indicating polymer that concentration NaOH hinders is 700 the mg/L, coalescence the particle of oil size droplets of oil atdroplets this time, decreases, because butthe surface-activethe oil droplets substance size at this generated time is still by the larger reaction than ofwhen NaOH there and are acidic no ASP components oil displacement of crude agents oil can inadsorb the system. on the oilDue droplets’ to the flocculation surface, reduce effects the of oil–water the polymer, interfacial when tension the polymer and enhance concentration the stability is low, of theoil droplets,polymer andmolecules the oil dropletscan adsorb size on decreases. the oil droplets’ When the surface, NaOH generate concentration the bridge is more connection than 800 effect, mg/L, combinethe acidic small components oil droplets in crude together oil are and completely construct reacted. the floc Atunit. this However, time, NaOH the actscoalescence as an electrolyte, between oiland droplets Na+ can does compress not occur, the and double the electrodehigher the layer, polymer weakening concentration, the stability the larger of oil the droplets—which flocs unit volume is formed,conducive and to the coalescencemore oil droplets of oil droplets—and are contained increasingin the floc the unit. oil dropletWith the size. continuous increase of polymerAt theconcentration, beginning, the oilviscosity droplets’ of the particle aqueous size phase decreases also increases with the [32], increase and ofthe quartz movement sand velocityconcentration; of oil droplets then, the slows oil droplets’ down, thus particle hindering size increases the coalescence with the riseof oil of droplets. quartz sand At the concentration. same time, theNegatively polymer chargedmolecules quartz cover on sand the particlesoil droplets’ adhere surface, to forming the oil a droplets’ steric hindrance surface, effect. enhancing This effect the iselectronegativity not conducive to on the the coalescence oil droplets’ of oil surface; droplets, thus, resulting the oil in droplets’ a decrease stability of oil droplet is enhanced, size. Because which thehinders flocculation the coalescence effect of of the oil droplets.polymer Quartzweakens sand but particles still exists, tend and to aggregatethe flocculation with the effect further of risethe polymerof the quartz is still sand stronger concentration. than the effect The of aggregation hindering the of quartz coalescence sand andof oil oil droplets, droplets so can the promoteparticle size the ofoil–water oil droplets separation, at this time and theis still polymer larger itselfthan haswhen the there ability were of flocculation,no ASP oil displacement which is conducive agents in to the system.increase of the oil droplet size. When the quartz sand concentration is low, the oil droplet size decreases with the rise of the quartz3.4.2. The sand Synergetic concentration. Effect When of Quartz the quartz Sand Particles sand concentration and Surfactant is more than 200 mg/L, the oil droplet size increases gradually because quartz sand particles and their surface-active substances can adsorb When the surfactant concentration in the system is 0, the particle size of the oil droplets is 6.73 on the interface between oil and water, increasing the intensity of oil–water interfacial film, and the µm—much larger than when there are no ASP oil displacement agents in the system, but smaller than stability of oil droplets are enhanced, which hinders the coalescence of oil droplets. Along with the that, there are only polymer oil displacement agents in the system—indicating that the flocculation continuous increase of quartz sand concentration, the aggregated structure formed by quartz sand effect of polymer is much stronger than the hindrance effect of NaOH on the coalescence of oil droplets. particles and oil droplets increases, and the aggregates tend to coalescence. At the same time, the At first, the oil droplet size decreases sharply with the rise of surfactant concentration; then, the oil surface-active substances in crude oil are limited. When the quartz sand concentration is high, the droplet size begins to decrease. The existence of the surfactant makes the oil droplets disperse more adsorption of quartz sand particles on the surface-active substances results in a reduction of surface- evenly, which weakens the flocculation effect of the polymer on the oil droplets. Meanwhile, the active substances concentration on the interface. The formed interfacial film is weak and not sufficient surfactant reduces the Zeta potential on the oil droplets’ surface. Moreover the electrostatic repulsion to stabilize the emulsion [33]. Under the combined action of the two, the stability of the produced between the oil droplets increases, thus reducing the probability of a collision and hindering the water system weakens and the oil droplet size increases. coalescence of oil droplets.

Processes 2020, 8, 315 17 of 20

When the quartz sand concentration is low, the oil droplet size decreases with the rise of quartz sand concentration. When the quartz sand concentration is in the range of 150–200 mg/L, the oil droplet size reaches its smallest value. With the further increase of quartz sand concentration, the oil droplet size gradually increases due to the quartz sand particles and their surface-active substances adsorbing on the interface between oil and water, which is equivalent to forming a certain structured interfacial film, enhancing the stability of produced water, increasing the coalescence difficulty of oil droplet coalescence and decreasing the oil droplet size [31]. With the further increase of quartz sand concentration, the oil droplets and quartz sand particles combine to form a larger aggregated structure. The number of surface-active substances on the interface between oil and water decreases, the strength of the interfacial film weakens, and thus quartz sand particles carry part of the oil droplets, accumulate and settle, leading to the stability of produced water decreasing, promoting the oil–water separation, and increasing the oil droplet size.

3.4.3. The Synergetic Effect of Quartz Sand Particles and Polymer The particle size of oil droplets increases first and then decreases with the rise of polymer concentration. When the polymer concentration is 700 mg/L, the particle size of oil droplets decreases, but the oil droplets size at this time is still larger than when there are no ASP oil displacement agents in the system. Due to the flocculation effects of the polymer, when the polymer concentration is low, the polymer molecules can adsorb on the oil droplets’ surface, generate the bridge connection effect, combine small oil droplets together and construct the floc unit. However, the coalescence between oil droplets does not occur, and the higher the polymer concentration, the larger the flocs unit volume formed, and the more oil droplets are contained in the floc unit. With the continuous increase of polymer concentration, the viscosity of the aqueous phase also increases [32], and the movement velocity of oil droplets slows down, thus hindering the coalescence of oil droplets. At the same time, the polymer molecules cover on the oil droplets’ surface, forming a steric hindrance effect. This effect is not conducive to the coalescence of oil droplets, resulting in a decrease of oil droplet size. Because the flocculation effect of the polymer weakens but still exists, and the flocculation effect of the polymer is still stronger than the effect of hindering the coalescence of oil droplets, so the particle size of oil droplets at this time is still larger than when there were no ASP oil displacement agents in the system. When the quartz sand concentration is low, the oil droplet size decreases with the rise of the quartz sand concentration. When the quartz sand concentration is more than 200 mg/L, the oil droplet size increases gradually because quartz sand particles and their surface-active substances can adsorb on the interface between oil and water, increasing the intensity of oil–water interfacial film, and the stability of oil droplets are enhanced, which hinders the coalescence of oil droplets. Along with the continuous increase of quartz sand concentration, the aggregated structure formed by quartz sand particles and oil droplets increases, and the aggregates tend to coalescence. At the same time, the surface-active substances in crude oil are limited. When the quartz sand concentration is high, the adsorption of quartz sand particles on the surface-active substances results in a reduction of surface-active substances concentration on the interface. The formed interfacial film is weak and not sufficient to stabilize the emulsion [33]. Under the combined action of the two, the stability of the produced water system weakens and the oil droplet size increases.

4. Conclusions In this paper, simulated ASP-flooding-produced water containing different concentrations of quartz sand particles and oil displacement agents was prepared by laboratory experiment. The oil–water separation characteristic of the produced water system was characterized by the oil content. By measuring the Zeta potential, oil–water interfacial tension and oil droplets size, the effects and mechanism of quartz sand particles and oil displacement agents on the oil droplets’ stability were summarized and analyzed, as shown in Figure 15. The content described below is shown in Figure 15 intuitively from the initial state to the coalescence occurrence and coalescence finishes. Processes 2019, 7, x FOR PEER REVIEW 19 of 21

4. Conclusions In this paper, simulated ASP-flooding-produced water containing different concentrations of quartz sand particles and oil displacement agents was prepared by laboratory experiment. The oil– water separation characteristic of the produced water system was characterized by the oil content. By measuring the Zeta potential, oil–water interfacial tension and oil droplets size, the effects and mechanism of quartz sand particles and oil displacement agents on the oil droplets’ stability were summarized and analyzed, as shown in Figure 15. The content described below is shown in Figure 15 intuitively from the initial state to the coalescence occurrence and coalescence finishes. When the NaOH concentration is less than 400 mg/L, with the increase of its concentration, the surface-active substances generated by the reaction of NaOH and acidic components of crude oil can adsorb on the oil–water interface; the surfactant can adsorb on the interface between oil and water in the form of nonpolar groups sticking into oil phase and polar groups sticking into water phase, changing the surface properties of the oil droplets from hydrophobicity to hydrophilicity, making it more difficult for oil droplets to coalesce. When the NaOH concentration is more than 600 mg/L, the acidic components in crude oil are all neutralized; the excess Na+ in aqueous phase neutralizes the negative charges on the oil droplets’ surface and compresses the diffuse electric double layer. Thus, the stability of the system is gradually weakened. Polymer molecules have a flocculation effect, which can adsorb on the oil droplets’ surface; when the coverage degree of the polymer on the surface of oil droplets reaches 50%, the flocculation reaches its maximum. With the increase of polymer concentration, the viscosity of the aqueous phase increases. At the same time, a steric hindrance effect is generated between polymer molecules. Thus, the stability of the system is gradually enhanced. When the oil displacement agent concentration is constant (the polymer concentration is not zero), with the increase of the quartz sand concentration, the oil content first increases and then decreases, the Zeta potential first decreases and then basically remains stable, and the interfacial tension and oil droplet size first decrease and then increase. The quartz sand particles are negatively charged, which can carry the active substances in crude oil and surfactant molecules in the aqueous phase and adsorb on the oil–water interface of oil droplets, increasing the negative charge density on the oil droplets’ surface, decreasing the Zeta potential and interfacial tension, enhancing the stability of the oil droplets and increasing the difficulty of coalescence. When the quartz sand particle concentration is in the range of 150–250 mg/L, the produced water is the most difficult to deal with. When the quartz sand particle concentration is high, they adsorb or combine with surfactant molecules adsorbed on the oil–water interface to form aggregates. Thus, the adsorption capacity of surfactant molecules on the oil–water interface is decreased. The polymer molecules crimp from the stretching state. The number of polymer molecules surrounding the surface of the flocculation structure is close to saturation. Quartz sand particles tend to aggregate, and the active substances Processescarried 2020by ,them8, 315 are desorbed from the interface, resulting in the produced water system becoming18 of 20 unstable.

Figure 15. The mechanism of quartz sand particles and oi oill displacement agents on the stability of oil droplets in the alkalialkali/surfactant/polyme/surfactant/polymerr (ASP)-ﬂooding-produced(ASP)-flooding-produced waterwater system.system.

When the NaOH concentration is less than 400 mg/L, with the increase of its concentration, the surface-active substances generated by the reaction of NaOH and acidic components of crude oil can adsorb on the oil–water interface; the surfactant can adsorb on the interface between oil and water in the form of nonpolar groups sticking into oil phase and polar groups sticking into water phase, changing the surface properties of the oil droplets from hydrophobicity to hydrophilicity, making it more difficult for oil droplets to coalesce. When the NaOH concentration is more than 600 mg/L, the acidic components in crude oil are all neutralized; the excess Na+ in aqueous phase neutralizes the negative charges on the oil droplets’ surface and compresses the diffuse electric double layer. Thus, the stability of the system is gradually weakened. Polymer molecules have a flocculation effect, which can adsorb on the oil droplets’ surface; when the coverage degree of the polymer on the surface of oil droplets reaches 50%, the flocculation reaches its maximum. With the increase of polymer concentration, the viscosity of the aqueous phase increases. At the same time, a steric hindrance effect is generated between polymer molecules. Thus, the stability of the system is gradually enhanced. When the oil displacement agent concentration is constant (the polymer concentration is not zero), with the increase of the quartz sand concentration, the oil content first increases and then decreases, the Zeta potential first decreases and then basically remains stable, and the interfacial tension and oil droplet size first decrease and then increase. The quartz sand particles are negatively charged, which can carry the active substances in crude oil and surfactant molecules in the aqueous phase and adsorb on the oil–water interface of oil droplets, increasing the negative charge density on the oil droplets’ surface, decreasing the Zeta potential and interfacial tension, enhancing the stability of the oil droplets and increasing the difficulty of coalescence. When the quartz sand particle concentration is in the range of 150–250 mg/L, the produced water is the most difficult to deal with. When the quartz sand particle concentration is high, they adsorb or combine with surfactant molecules adsorbed on the oil–water interface to form aggregates. Thus, the adsorption capacity of surfactant molecules on the oil–water interface is decreased. The polymer molecules crimp from the stretching state. The number of polymer molecules surrounding the surface of the flocculation structure is close to saturation. Quartz sand particles tend to aggregate, and the active substances carried by them are desorbed from the interface, resulting in the produced water system becoming unstable.

Author Contributions: Conceptualization, B.H., C.W.; methodology, formal analysis, investigation and writing—original draft preparation, B.H., C.W.; writing—Review and editing, supervision and project administration, W.Z., C.F.; writing—Review and Editing, H.L., H.W. All authors have read and agreed to the published version of the manuscript. Funding: The authors gratefully acknowledge the financial support by National Natural Science Foundation of China (51974088). Conflicts of Interest: The authors declare no conflict of interest. Processes 2020, 8, 315 19 of 20

References

1. Cheng, J.; Wu, J.; Hu, J. Key theories and technologies for enhanced oil recovery of alkaline/surfactant/polymer flooding. Acta Pet. Sin. 2014, 35, 310–318. 2. Wang, D.; Cheng, J. Summary of alkali/surfactant/polymer flooding in Daqing oilfield. In Proceedings of the SPE Arctic and Extreme Environments Technical Conference and Exhibition, Houston, TX, USA, 3–6 October 1999. 3. Vladimir, A.; Eduardo, M. Enhanced Oil Recovery; Gulf Professional Publishing: Houston, TX, USA, 2010. 4. Li, F.; He, W.; Sun, D.; Wu, T.; Li, Y. Effect of sodium-montmorillonite particles on the stability of oil droplets in produced water from alkali/surfactant/polymer flooding. J. Clean. Prod. 2015, 104, 468–474. [CrossRef] 5. Deng, S.; Yu, G.; Jiang, Z.; Zhang, R.; Ting, Y.P. Destabilization of oil droplets in produced water from ASP flooding. Colloids Surfaces A: Physicochem. Eng. Asp. 2005, 252, 113–119. [CrossRef] 6. Deng, S.; Bai, R.; Chen, J.P.; Yu, G.; Jiang, Z.; Zhou, F. Effects of alkaline/surfactant/polymer on stability of oil droplets in produced water from ASP flooding. Colloids Surfaces A Physicochem. Eng. Asp. 2002, 211, 275–284. [CrossRef] 7. Yang, L.; Zhihua, W.; Xianglong, Z.; Hongqi, Z.; Xinpeng, L. An environmentally-friendly method for disposal of the ASP flooding produced water. In Proceedings of the SPE Arctic and Extreme Environments Technical Conference and Exhibition, Moscow, Russia, 15–17 October 2013. 8. Wang, F. Research on Oil/Water Separation Specialities and Disposal Technologies in ASP Flooding Produced Water. Ph.D. Thesis, Shanghai Jiaotong University, Shanghai, China, 2007. 9. Huang, B.; Li, X.; Zhang, W.; Fu, C.; Wang, Y.; Fu, S. Study on Demulsification-Flocculation Mechanism of Oil-Water Emulsion in Produced Water from Alkali/Surfactant/Polymer Flooding. Polymers 2019, 11, 395. [CrossRef] 10. Huang, B.; Wang, J.; Zhang, W.; Fu, C.; Wang, Y.; Liu, X. Screening and Optimization of Demulsifiers and Flocculants Based on ASP Flooding-Produced Water. Processes 2019, 7, 239. [CrossRef] 11. Etchepare, R.; Oliveira, H.; Azevedo, A.; Rubio, J. Separation of emulsified crude oil in saline water by dissolved air flotation with micro and nanobubbles. Sep. Purif. Technol. 2017, 186, 326–332. [CrossRef] 12. Al-Maamari, R.S.; Sueyoshi, M.; Tasaki, M.; Okamura, K.; Al-Lawati, Y.; Nabulsi, R.; Al-Battashi, M. Flotation, Filtration, and Adsorption: Pilot Trials for Oilfield Produced-Water Treatment. Oil Gas Facil. 2014, 3, 56–66. [CrossRef] 13. Deng, S.B.; Zhou, F.S.; Yu, G. Speciality of oil-field produced water and its treatment technology. Industrial Water Treatment 2000, 20, 10–12. 14. Zhao, L.H.; Li, B.L.; Chen, Z.X.; Ji, G.S.; Shu, Z.M. Character of Oilfield Produced Water and Factor infLuencing on its Treatment; Liaoning Chemical Industry: Liaoning, China, 2007; pp. 69–72. 15. Ye, Q. Research on Oil/Water Separation Characteristic and Treatment in ASP Produced Water. Ph.D. Thesis, Shanghai Jiaotong University, Shanghai, China, 2008. 16. Di, M.; Xv, C.; Zhao, S. Air floating /hydrolysis acidification /biological contact oxidation /filtration process for treatment of ASP flooding produced water. China Water Wastewater 2016, 32, 69–70. 17. Yan, N.; Masliyah, J.H. Characterization and demulsification of solids-stabilized oil-in-water emulsions Part 1. Partitioning of clay particles and preparation of emulsions. Colloids Surfaces A Physicochem. Eng. Asp. 1995, 96, 229–242. [CrossRef] 18. Guo, Q. Experimental Study on the Effect of the Wetting Property of Support Agent on the Fracturing Operation. Ph.D. Thesis, Northeast Petroleum University, Daqing, China, 2016. 19. SY/T 5329-94, Water quality standard and practice for analysis of oilfield injecting waters in clastic reservoirs. In Proceedings of the MATEC Web of Conferences, Cape Town, South Africa, 24–26 September 2018. 20. Gan, Z.; Xing, X.; Xu, Z. Effects of image charges, interfacial charge discreteness, and surface roughness on the zeta potential of spherical electric double layers. J. Chem. Phys. 2012, 137, 34708. [CrossRef][PubMed] 21. Hong, J.S.; Fischer, P. Bulk and interfacial rheology of emulsions stabilized with clay particles. Colloids Surfaces A Physicochem. Eng. Asp. 2016, 508, 316–326. [CrossRef] 22. Huang, B.; Zhang, W.; Wang, J.; Fu, C. Effect of oil displacement agent on oil droplet stability in alkali/surfactant/polymer flooding oily wastewater. Chem. Ind. Eng. Prog. 2019, 38, 1053–1061. 23. Lakatos-Szabó, J.; Lakatos, I. Effect of alkaline materials on interfacial rheological properties of oil-water systems. Colloid Polym. Sci. 1999, 277, 41–47. [CrossRef] Processes 2020, 8, 315 20 of 20

24. Yarranton, W.; Urrutia, P.; Sztukowski, D.M. Effect of interfacial rheology on model emulsion coalescence II. Emulsion coalescence. J. Colloid Interface Sci. 2007, 310, 253–259. [CrossRef] 25. Nguyen, D.T.; Sadeghi, N. Stable Emulsion and Demulsification in Chemical EOR Flooding: Challenges and Best Practices. In Proceedings of the SPE EOR Conference at Oil and Gas West Asia; Society of Petroleum Engineers (SPE), Muscat, China, 16–18 April 2012; pp. 16–18. 26. Nedjhioui, M.; Moulai-Mostefa, N.; Morsli, A.; Bensmaili, A. Combined effects of polymer/surfactant/oil/alkali on physical chemical properties. Desalination 2005, 185, 543–550. [CrossRef] 27. Taylor, K.C.; Nasr-El-Din, H.A. The effect of synthetic surfactants on the interfacial behaviour of crude oil/alkali/polymer systems. Colloids Surfaces A Physicochem. Eng. Asp. 1996, 108, 49–72. [CrossRef] 28. Ling, S.; Wei-Dong, L.; Zhan-Xiang, T. Rheologic Behavior of Aqueous Solutions of Surface Active Polymer SLH-I for EOR. Oilfield Chem. 2008. 29. Schorling, P.-C.; Kessel, D.; Rahimian, I. Influence of the crude oil resin/asphaltene ratio on the stability of oil/water emulsions. Colloids Surfaces A Physicochem. Eng. Asp. 1999, 152, 95–102. [CrossRef] 30. Khelifa, A.; Stoffyn-Egli, P.; Hill, P.S.; Lee, K. Effects of salinity and clay type on oil-mineral aggregation. Mar. Environ. Res. 2005, 59, 235–254. [CrossRef][PubMed] 31. Jeon, T.Y.; Hong, J.S. Stabilization of O/W emulsion with hydrophilic/hydrophobic clay particles. Colloid Polym. Sci. 2014, 292, 2939–2947. [CrossRef] 32. Huang, B.; Hu, X.; Fu, C.; Cheng, H.; Li, Y. Experimental and numerical simulation study on the flow characteristics and optimization of structural parameters of displacement agent in partial pressure tools. Energy Sci. Eng. 2019, 1–21. [CrossRef] 33. Tarimala, S.; Dai, L.L. Structure of Microparticles in Solid-Stabilized Emulsions. Langmuir 2004, 20, 3492–3494. [CrossRef][PubMed]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).