Study on Demulsification-Flocculation Mechanism of Oil-Water Emulsion

polymers

Article Study on Demulsiﬁcation-Flocculation Mechanism of Oil-Water Emulsion in Produced Water from Alkali/Surfactant/Polymer Flooding

Bin Huang 1, Xiaohui Li 1 , Wei Zhang 2, Cheng Fu 1,3,*, Ying Wang 4 and Siqiang Fu 5

1 The Key Laboratory of Enhanced Oil and Gas Recovery of Educational Ministry, Northeast Petroleum University, Daqing 163318, China; [email protected] (B.H.); [email protected] (X.L.) 2 Shandong Key Laboratory of Oil-Gas Storage and Transportation Safety, College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, China; [email protected] 3 Post-Doctoral Research Station of Daqing Oilﬁeld, Daqing 163458, China 4 Aramco Asia, Beijing 100102, China; [email protected] 5 No. 1 Oil Production Plant in Daqing Oilﬁeld, Daqing 163001, China; [email protected] * Correspondence: [email protected]; Tel.: +86-158-4616-2836

Received: 18 December 2018; Accepted: 22 February 2019; Published: 28 February 2019

Abstract: The issue of pipeline scaling and oil-water separation caused by treating produced water in Alkali/Surfactant/Polymer (ASP) flooding greatly limits the wide use of ASP flooding technology. Therefore, this study of the demulsification-flocculation mechanism of oil-water emulsion in ASP flooding produced water is of great importance for ASP produced water treatment and its application. In this paper, the demulsification-flocculation mechanism of produced water is studied by simulating the changes in oil-water interfacial tension, Zeta potential and the size of oil droplets of produced water with an added demulsifier or flocculent by laboratory experiments. The results show that the demulsifier molecules can be adsorbed onto the oil droplets and replace the surfactant absorbed on the surface of oil droplets, reducing interfacial tension and weakening interfacial film strength, resulting in decreased stability of the oil droplets. The demulsifier can also neutralize the negative charge on the surface of oil droplets and reduce the electrostatic repulsion between them which will be beneficial for the accumulation of oil droplets. The flocculent after demulsification of oil droplets by charge neutralization, adsorption bridging, and sweeping all functions together. Thus, the oil droplets form aggregates and the synthetic action by the demulsifier and the flocculent causes the oil drop film to break up and oil droplet coalescence occurs to separate oil water.

Keywords: Alkali/Surfactant/Polymer flooding; produced water; oil-water emulsion; demulsification; ﬂocculation

1. Introduction After years of development, most oil fields in China have now entered the late stage of high water cut development. Daqing Oilfield was the first in the world to commercialize the technology of Alkali/Surfactant/Polymer (ASP) flooding in 2014. ASP flooding has proved to be effective in improving oil recovery by 20% compared with water flooding. It has seen industrial application in the Daqing Oilfield [1]. Although ASP flooding technology can greatly increase field output and oil recovery compared with water flooding, it also brings severe challenges to oilfield surface engineering. Compared with the new methods of development with less pollution such as microbial flooding and nano-fluid flooding [2–4], pipeline scaling is a serious issue causing pipes to become easily blocked due to

Polymers 2019, 11, 395; doi:10.3390/polym11030395 www.mdpi.com/journal/polymers Polymers 2019, 11, 395 2 of 13 the existence of oil displacement agents. It has a serious influence on production and even safety accidents [5–7]. At the same time, the existence of residual chemical agents in the ASP flooding produced water will lead to complications in the produced water [8,9]. The produced water has a high salinity, high degree of emulsification, strong interfacial film strength and high content of small oil droplets [10–12]. As a result, it is difficult to separate oil and water which greatly limits commercialized ASP flooding technology. In order to form an ultra-low oil-water interfacial tension during the ASP flooding process, a surfactant was added. The flow with low interfacial tension can pass easily through the porous medium, resulting in an improvement in oil recovery [13]. However, the addition of a surfactant not only enhances oil recovery, but also inevitably causes the serious emulsification of the ASP flooding produced water, which increases the stability of the oil droplets, resulting in difficulty treating the produced water. Adequate mixing and the presence of a surface-active agent are two vital factors that lead to the emulsion formation when the oil and water phases are brought together [14]. The oil droplets rupture under the action of viscous shear stress. The alkali and surfactant reduce the interfacial tension greatly, which reduces the stability of the oil droplets. The polymers increase the emulsion stability by increasing water phase viscosity, decreasing the rate of raising the oil droplets, and higher polymer concentration results in higher interfacial film strength. The alkali enhances the stability of emulsion by reacting with acidic substances in crude oil to form a surfactant. The surfactant can adsorb onto the oil-water interface, which reduces the interfacial tension and increases the strength of the interface membrane. On the one hand, the capillary force that promotes the drainage of the liquid film and the fluidity at the oil-water interface is reduced. On the other hand, the electrostatic repulsion on both sides of the liquid film is reduced. The combined effect acts as a hindrance in the coalescence of oil beads. Under the interaction of the alkali, surfactant and polymer, the system is emulsified and forms the O/W emulsion [15,16]. The emulsion in ASP flooding produced water causes difficulties in its treatment. Many conventional processes have been used to treat produced water in oilfields, including gravity separation [17], floatation [18,19], demulsification [20,21], membrane separation [22], air flotation [23], adsorptive separation [24] and biotechnology [25]. However, only a few studies have reported the treatment of produced water from ASP flooding. Deng et al. studied the influence of an oil displacement agent on the stability of oil droplets in ASP flooding water and the influence of different demulsifiers and flocculants on the stability of oil droplets [26,27]. It was found that the effect of the oil-soluble demulsifier was better than that of water-soluble demulsifier. Oil-soluble demulsifiers can reduce the oil-water interfacial tension and Zeta potential on the surface of oil droplets. Therefore, it can increase the oil droplet size and facilitate the coalescence of oil droplets. Wang et al. studied the influence of alkali, polymer and surfactant on oil-water separation of simulated produced water from ASP flooding [28]. The results showed that the simulated produced water from ASP flooding can be successfully treated using a leaching solution of alkaline white mud. Due to the significant effect of the white mud on the treatment of produced water, Gao et al. [29] further studied this phenomenon. They evaluated the efficiency of removing emulsified oil by metallic hydroxides were in-situ generated (IGMHs) from simulated ASP flooding produced water and found the oil removal rate was up to 99%. They proved the economic viability of treatment process recovery of oil from oily wastewater. Zhang et al.[30] investigated the removal efficiency of PTFE film on the main pollutants of ASP produced water and proved that PTFE film also achieves a good treatment effect of produced water. Miao et al. [31] studied the effect of asphaltene on oil-water interface properties. The study showed that asphaltene aggregated to form film on the oil-water interface. The effluent velocity of the liquid film was reduced and the stability of produced water was improved. Although these methods have a good performance in the treatment of ASP flooding produced water, they are limited to laboratory experiments and cannot be commercialized. It is difficult to achieve the reinjection standard by only using a demulsifier or flocculant alone to treat the produced water Polymers 2019, 11, 395 3 of 13 from ASP flooding. Therefore, we consider using a demulsification-flocculation method to improve the treatment effect of produced water. The produced water is firstly demulsified and then flocculated. In order to verify the feasibility of treatment of produced water by the demulsification-flocculation method, the demulsification-flocculation mechanism of oil-water emulsion in ASP flooding produced water must be fully understood. In this study, the effect of the demulsification-flocculation mechanism of oil-water emulsion from ASP flooding produced water was investigated with regards to interfacial tension, Zeta potential, and median size and microscopic morphology of oil droplets.

2. Materials and Experimental Methods

2.1. Experimental Materials and Instruments Table1 shows the experimental materials and sources used.

Table 1. Experimental drugs and sources.

Drugs Sources NaCl Tianjin Damao Chemical Plant Na2HCO3 Tianjin Yaohua Chemical Plant Na2CO3 Tianjin Yaohua Chemical Plant Na2SO4 Tianjin Yaohua Chemical Plant CaCl2 Harbin Xinda Chemical Plant MgCl2 Tianjin Fuchen Chemical Plant Petroleum Ether Shenyang W&S Chemical Plant All of the drugs above are analytical reagents.

Distilled water was used in the experiments. The crude oil had a moisture content of less than 0.5%, a viscosity of 60 mPa·s, and a density of 860 kg/m3 at a temperature of 45 ◦C. The crude oil was provided by No. 1 Oil Production Plant in Daqing Oilfield. The polyacrylamide, hydrolyzed polyacrylamide (HPAM), with 300 × 104 molecular weight and 25–30% hydrolysis degree, was from the Daqing Oilfield Production Engineering Research Institute. The surfactant with 50% effective content was alkyl benzene sulfonate, ORS-41. This was provided by the No. 1 Oil Production Plant in Daqing Oilfield. The alkali was reagent grade NaOH. The oil-soluble demulsifiers DYRPR-1625 and DPR-1870 and the organic flocculents cationic polyacrylamide (CPAM) and D2N-1650 provided by the No. 1 Oil Production Plant in Daqing Oilfield were used in the experiments. Information regarding the experimental instruments is shown in Table2.

Table 2. Experimental instruments.

Purpose Instrument Name Types Manufacturer Preparation of emulsion Digital display disperser IKAT25 IKA Company Micro electrophoresis Shanghai Zhongchen Digital Zeta potential JS94H apparatus technology equipment Co. Ltd. Laser particle size Dandong Better Science and Oil droplet size distribution BT-9300H analyzer Technology Co. Ltd. Beijing Hake Experimental Interfacial tension Interface tension meter XZD-5 Instrument Factory Liaoyang Huaguang Instrument Temperature control Thermostatic water bath S501-2 Factory Shanghai Puhe Biotechnology Micrographs observation Biological microscope IX73 Co. Ltd. Sartorius scientiﬁc Instruments Weigh Electronic balance BS210S Co. Ltd. Quantitative transfer liquid Micropipette Eppendorf Eppendorf China Co. Ltd. Polymers 2019, 11, 395 4 of 13

2.2. Experimental Procedure Preparation of simulated mineralized water. According to the underground water and discharge water qualities in the Daqing oilfield, mineralized water was prepared, and the salts contained in the mineralized water were as follows (mg/L): NaCl 1523, NaHCO3 2820, Na2CO3 168.7, Na2SO4 10.5, CaCl2 56.9, MgCl2·6H2O 35.5. The degree of mineralization of the mineralized water was 4614.6 mg/L. Preparation of simulated ASP flooding produced water. The simulated ASP flooding produced water with a polymer (HPAM) concentration of 400 mg/L, surfactant (ORS-41) concentration of 200 mg/L, and alkali (NaOH) concentration of 800 mg/L. Treated with xylene, the 1% demulsifier was used as a solvent. The demulsifier solution was added into the produced water, stirred at a speed of 200 r/min for 2 min and settled in the 45 ◦C water bath for 4 h. The demulsifier, ZY, was a compound of DYRPR-1625 and DYRPR-1870 at a ratio of 3:1. The flocculent was added into the produced water, stirred for 2 min at 200 r/min and settled in the 45 ◦C water bath for 4 h. The flocculent, WS, was a compound of CPAM and D2N-1650 at a ratio of 3:1. Interfacial tension, Zeta potential and oil droplet size were measured after adding demulsifier and flocculent.

2.3. Experimental Methods The experimental methods for the preparation of simulated ASP flooding produced water, determination of the interfacial tension, Zeta potential, oil droplet size and observation of the microscopic morphology of oil droplets were as follows: Preparation of simulated ASP flooding produced water. The mineralized water was used to prepare simulated produced water from ASP flooding in the lab and the process is as follows: 200 g of mineralized water with 0.1% surfactant and 200 g of crude oil were added to a 500 mL jar. The mixture was then heated to 45 ◦C in a water bath for 60 min. Then, the mixture was emulsified for 5 min at 20,000 rpm with a T25 emulsifier (IKA Company, Guangzhou, China) to obtain a 50% oil-water mixture. Next, 0.4 g of the 50% oil-in-water emulsion was added to 99.4 mL mineralized water with different concentrations of oil displacement agents, and the mineralized water was shaken to produce simulated produced water with an oil concentration of 2000 mg/L. Determination of the oil-water interfacial tension. The oil-water interfacial tension was determined by a spinning drop method and measured by the interfacial tension meter after 60 min. The rotation rate was 8500 r/min. The temperature was 45 ◦C. The oil-water interfacial tension was obtained by Equation (1): − γ = 1.233 × 103 × ∆ρ × (Kdsv)3(6000/S) 2, (1) where the ∆ρ is density between phases, kg/m3; K is the amplification factor; dsv is droplet width, m; and S is speed, r/min. Determination of the Zeta potential. The Zeta potential of the produced water from the ASP flooding was conducted on a Micro electrophoresis apparatus. Firstly, 100 mL of the simulated produced water with the initial oil concentration of 1000 mg/L at different demulsifiers and flocculents were allowed to settle for 4 h at 45 ◦C. Then, a 5–10 mL sample was taken out of the jar with a syringe and added to the test utensil of the Micro electrophoresis apparatus for Zeta potential measurement. All measurements were repeated five times. The average values of these readings were reported. Determination of the oil droplet size. The size of the oil droplets was determined by a mastersizer. All experiments were repeated for 3~5 times. The average values of these readings were reported. Micrograph observations. A drop of produced water was placed on a glass holder held on the platform of a biological microscope and then observed and recorded on a camera. The magnification used was 10 × 40. Polymers 2019, 11, x FOR PEER REVIEW 5 of 13

Micrograph observations. A drop of produced water was placed on a glass holder held on the platform of a biological microscope and then observed and recorded on a camera. The magnificationPolymers 2019, 11, used 395 was 10 × 40. 5 of 13

3. Results 3. Results 3.1. The Effect of Demulsifier on Interfacial Tension 3.1. The Effect of Demulsifier on Interfacial Tension The simulated ASP flooding produced water with 400 mg/L of the polymer concentration, 200 The simulated ASP flooding produced water with 400 mg/L of the polymer concentration, mg/L of the surfactant concentration and 800 mg/L of the NaOH concentration was prepared and 200 mg/L of the surfactant concentration and 800 mg/L of the NaOH concentration was prepared settled in a 45 °C water bath for 4 h to study the effect on oil-water interfacial tension in Figure 1. and settled in a 45 ◦C water bath for 4 h to study the effect on oil-water interfacial tension in Figure1. For the emulsion of ASP flooding produced water without adding a demulsifier, the effect of NaOH For the emulsion of ASP flooding produced water without adding a demulsifier, the effect of NaOH on interfacial tension of the system was mainly achieved by the surface-active components formed on interfacial tension of the system was mainly achieved by the surface-active components formed by by the reaction with the acidic components and it had no obvious influence by itself on the the reaction with the acidic components and it had no obvious influence by itself on the interfacial interfacial tension. Surfactants had a great influence on the interfacial tension of the system, while tension. Surfactants had a great influence on the interfacial tension of the system, while the polymer the polymer had no effect on the interfacial tension. After adding the demulsifier, it can be observed had no effect on the interfacial tension. After adding the demulsifier, it can be observed that the that the demulsifier can reduce the oil-water interfacial tension. When the dosage of demulsifier demulsifier can reduce the oil-water interfacial tension. When the dosage of demulsifier increased increased from 0 to 100 mg/L, the interfacial tension of oil-water decreased from 4.425 mN/m to from 0 to 100 mg/L, the interfacial tension of oil-water decreased from 4.425 mN/m to 1.325 mN/m, 1.325 mN/m, and then stabilized even with further addition from 100 mg/L to 200 mg/L. When the and then stabilized even with further addition from 100 mg/L to 200 mg/L. When the dosage exceeded dosage exceeded 200 mg/L, the interfacial tension began to increase gradually. This is due to the 200 mg/L, the interfacial tension began to increase gradually. This is due to the fact that the very active fact that the very active demulsifier molecules can adsorb to the oil-water interface, thus reducing demulsifier molecules can adsorb to the oil-water interface, thus reducing the oil-water interfacial the oil-water interfacial tension. The adsorption of demulsifier molecules on the oil-water interface tension. The adsorption of demulsifier molecules on the oil-water interface increased gradually with increased gradually with the increasing dosage of demulsifier. When the concentration reached 100 the increasing dosage of demulsifier. When the concentration reached 100 mg/L, the adsorption mg/L, the adsorption of demulsifier molecules on the interface reached saturation. Therefore, the of demulsifier molecules on the interface reached saturation. Therefore, the oil-water interfacial oil-water interfacial tension decreased in concentration when the concentration of demulsifier tension decreased in concentration when the concentration of demulsifier further increased. Then, further increased. Then, when the concentration of the demulsifier was higher than its critical when the concentration of the demulsifier was higher than its critical micelle concentration (CMC), micelle concentration (CMC), the demulsifier molecules began to aggregate into clusters to form the demulsifier molecules began to aggregate into clusters to form micelles which caused the oil-water micelles which caused the oil-water interfacial tension. In this case, the effect of demulsification and interfacial tension. In this case, the effect of demulsification and stabilization was reduced. stabilization was reduced.

4.5

4.0

3.5

3.0

2.5

2.0

1.5 The oil-water interfacial tension(mN/m) interfacial The oil-water

1.0 0 50 100 150 200 250 The dosage of demulsifier (mg/L) Figure 1. TheThe effect effect of demulsifier demulsiﬁer on interfacial tension.

3.2. The Effect of Demulsifier Demulsifier on Zeta Potential The simulated ASP system with 400400 mg/Lmg/L of the polymer, 800800 mg/Lmg/L of NaOH concentration and 0~600 mg/L of of the the surfactant surfactant concentrations concentrations were prepared. Then, 100 mg/L of of demulsifier demulsifier ZY ◦ was added into the produced wate water.r. The The ASP ASP system system was was settled settled in in a a 45 °CC water water bath bath for for 4 4 h to study the effect of the demulsifier demulsifier on the Zeta of potential surfactants with different concentrations, as shownshown inin Figure Figure2. For2. theFor emulsion the emulsion of ASP of flooding ASP producedflooding produced water without water adding without demulsifier, adding demulsifier,the surfactants the weresurfactants absorbed were onto absorbed the oil dropletsonto the withoil droplets its non-polar with its head non-polar and polar head head and grouppolar headextending group in extending water. The in Zeta water. potential The onZeta the potent surfaceial of on the the oil dropletsurface decreasesof the oil quickly droplet because decreases the negative charges on the surface of the oil droplet increased. NaOH reacted with acidic components of crude oil and produced some surface-active components which adsorbed on the surface of oil droplets to reduce the Zeta potential. With increasing NaOH, the acid components were reacted completely. Polymers 2019, 11, x FOR PEER REVIEW 6 of 13 quickly because the negative charges on the surface of the oil droplet increased. NaOH reacted with acidic components of crude oil and produced some surface-active components which adsorbed on the surface of oil droplets to reduce the Zeta potential. With increasing NaOH, the acid components werePolymers reacted2019 completely., 11, 395 The electric double layer was compressed by the Na+ at the 6oil-water of 13 interface, resulting in the Zeta potential increasing on the surface of the oil droplet. The Zeta potentialThe electric increased double as a layer result was of compressed the above bytwo the co Nambined+ at the effects. oil-water The interface, polymer resulting had a in limited the Zeta effect on thepotential Zeta potential increasing and on thestability surface of of the the system oil droplet. because The Zeta of its potential low surface increased activity. as a result After of theadding demulsifier,above two the combined surfactant effects. can The polymersignificantly had a limitedimprove effect the on thestability Zeta potential of the and system. stability A of thehigher concentrationsystem because of the of surfactant its low surface results activity. in better After adding stability demulsifier, of the system the surfactant [32,33]. can Compared significantly to the systemimprove with theno stabilitydemulsifier, of the with system. 100 A mg/L higher of concentration demulsifier of the the absolute surfactant value results of in the better Zeta stability potential on theof thesurface system of [oil32, 33droplets]. Compared decreased to the significan system withtly no at demulsifier,the same surfactant with 100 mg/L concentration of demulsifier and the stabilitythe absoluteof the system value of was the change Zeta potentiald from on stable the surface to unstable. of oil droplets The Zeta decreased potential significantly of the oil at droplets the changedsame from surfactant −41.234 concentration mV to −15.547 and themV stability because of the the 100 system mg/L was of changed demulsifier from was stable added to unstable. when the − − surfactantThe Zeta concentration potential of the is oil0. The droplets Zeta changed potential from of the41.234 oil mVdroplet to 15.547 was 62.3%. mV because The Zeta the 100 potential mg/L of of demulsifier was added when the surfactant concentration is 0. The Zeta potential of the oil droplet the oil droplet was changed from −67.125 mV to −30.625 mV when 100 mg/L of demulsifier was was 62.3%. The Zeta potential of the oil droplet was changed from −67.125 mV to −30.625 mV when added to 200 mg/L of the surfactant solution. The absolute value of the Zeta potential of the oil 100 mg/L of demulsifier was added to 200 mg/L of the surfactant solution. The absolute value of the dropletZeta changed potential from of the 54.9% oil droplet to 51.8% changed when from the 54.9% concentration to 51.8% when of surfactant the concentration increased of surfactantto 400 mg/L, whileincreased the absolute to 400 value mg/L, of while Zeta thepotential absolute was value stab ofilized Zeta potentialwith increasing was stabilized surfactant with concentration. increasing The effectsurfactant of demulsifier concentration. on Thethe Zeta effect potential of demulsifier of oil on droplets the Zeta decreases potential ofgr oiladually droplets with decreases increasing surfactantgradually concentration. with increasing surfactant concentration.

-10

-20 100mg/L demulsifier No demulsifier -30

-40

-50

-60 Zeta potential(mV) Zeta potential(mV)

-70

-80

0 200 400 600 800 The concentration of surfactant (mg/L)

FigureFigure 2. 2.TheThe effect effect of of demulsifier demulsiﬁer onon Zeta Zeta potential. potential.

3.3. The Effect of Demulsifier on the Size of Oil Droplets 3.3. The Effect of Demulsifier on the Size of Oil Droplets The simulated ASP system with 400 mg/L of the polymer concentration, 800 mg/L of NaOH The simulated ASP system with 400 mg/L of the polymer concentration, 800 mg/L of NaOH and 200 mg/L of the surfactant were prepared. Then, 100 mg/L of demulsifier ZY was added into and 200the producedmg/L of the water. surfactant The ASP were system prepared. was settled Then, in a 100 45 ◦ mg/LC water of bathdemulsifier for 2 h to ZY study was the added effect into the producedof the demulsifier water. The on theASP size system of the was oil droplet,settled in as a shown 45 °C inwater Figure bath3. Forfor the2 h emulsionto study the of ASP effect of the demulsifierflooding produced on the water size withoutof the addingoil droplet, demulsifier, as shown NaOH in and Figure surfactant 3. For prevented the emulsion oil droplets of ASP floodingfrom produced coalescing water but the without polymer adding helped thedemulsifie oil dropletsr, NaOH to coalesce. and surfactant This is because prevented the NaOH oil droplets can fromreact coalescing with the but acidic the componentspolymer helped of crude the oil oil and drop producelets to some coalesce. surface-active This is because components. the NaOH Hence can reactthe with oil-water the acidic interfacial components properties of changed,crude oil which and produce can cause some the oil surface-active droplets to become components. more stable. Hence the oil-waterThe size ofinterfacial oil droplets properties decreased changed, with increasing which can surfactant cause the concentration, oil droplets which to become was due more to the stable. The sizesurfactant of oil increasing droplets thedecreased Zeta potential with onincreasing the surface surfactant of the oil droplets concentration, and the electrostatic which was repulsion due to the between oil droplets, resulting in a reduced probability of collision between oil droplets. The polymer surfactant increasing the Zeta potential on the surface of the oil droplets and the electrostatic had a dual function on the size of the oil droplet. On one hand, the polymer molecule had a bridging repulsion between oil droplets, resulting in a reduced probability of collision between oil droplets. and flocculation effect for low polymer concentrations and compressed the double electric layer on the The polymersurface of had the droplets,a dual function which is on helpful the size for theof th aggregatione oil droplet. and On coalescence one hand, of oilthe droplets. polymer On molecule the had aother bridging hand, theand polymer flocculation molecules effect covered for low the polymer surface of concentrations the oil droplets creatingand compressed a steric hindrance the double electric layer on the surface of the droplets, which is helpful for the aggregation and coalescence of

Polymers 2019, 11, x FOR PEER REVIEW 7 of 13 oil droplets. On the other hand, the polymer molecules covered the surface of the oil droplets creating a steric hindrance effect with a high concentration, which was not conducive to the coalescence of oil droplets. After adding demulsifier, the demulsifier molecules adsorbed onto the surface of the oil droplets, resulting in a weakening of the steric hindrance effect of the polymer molecules. Therefore, the polymer molecules in the produced water were beneficial in the separation of emulsions. The demulsifier can significantly increase the median diameter of oil droplets. The median diameter of oil droplets increased from 4.02 μm to 4.40 μm when settled for 4 h without a demulsifier. The small increase of oil droplets indicated that the ASP flooding produced water was very stable. However, when settled for 2 h with added demulsifier ZY, the median diameter of oil droplets increased from 4.02 μm to 30.54 μm. The significant increase in the median diameter of oil droplets showed that the oil droplets in the ASP flooding produced water had becomePolymers unstable2019, 11, 395after adding demulsifier. In order to further study the effect of demulsifier7 of on 13 the size of oil droplets, the shapes of oil droplets in the simulated produced water in different conditions were investigated with a microscope. The results are shown in Figure 4. The effect with a high concentration, which was not conducive to the coalescence of oil droplets. After magnificationadding demulsifier, used was the 10 demulsifier × 40. The molecules size of the adsorbed oil droplets onto the was surface basically of the stable oil droplets, without resulting demulsifier in and asettling weakening for 2 of h. the The steric size hindrance of the oil effect droplets of the polymerincreased molecules. significantly Therefore, after the adding polymer demulsifier molecules and settledin thefor produced60 min. By water increasing were beneficial the settling in the separationtime, the size of emulsions. of oil droplets The demulsifier increased can gradually. significantly increaseBy summarizing the median the diameter above of oilexperimental droplets. The result medians, the diameter interaction of oil dropletsof ASP increased with the from added demulsifiers4.02 µm tocan 4.40 beµ mfound. when settledThe effect for 4of h withoutNaOH aon demulsifier. the stability The of small produced increase water of oil dropletswas mainly achievedindicated by the that surface-active the ASP flooding components produced formed water was from very the stable. reaction However, with the when acidic settled components for 2 h of crudewith oil, added which demulsifier produces ZY,some the surface-active median diameter components of oil droplets which increased adsorbed from on 4.02 theµm surface to 30.54 ofµ m.the oil droplets.The significant During demulsification, increase in the median the demulsifier diameter of molecules oil droplets can showed be adsorbed that the oilonto droplets the oil in droplets, the ASP flooding produced water had become unstable after adding demulsifier. In order to further replacing the surfactant absorbed on the surface of oil droplets and resulting in a weakening of the study the effect of demulsifier on the size of oil droplets, the shapes of oil droplets in the simulated steric hindrance effect of the polymer molecules. The processes reduced interfacial tension and produced water in different conditions were investigated with a microscope. The results are shown weakenedin Figure interfacial4. The magnification film strength, used so was that 10 the× stability40. The sizeof oil of thedroplets oil droplets was decreased. was basically At stable the same time,without the demulsifier demulsifier can and also settling neutralize for 2 h. Thethe sizenega oftive the charge oil droplets on the increased surface significantly of the oil afterdroplet, reducingadding the demulsifier electrostatic and repulsion settled for 60between min. By the increasing oil droplets the settling and aiding time, the the size accumulation of oil droplets of oil droplets.increased gradually.

35 No demulsifier 30 100mg/L demulsifier

5 The median diameter of oil droplets(μm)

0 0 50 100 150 200 250 Settling time (min) FigureFigure 3. The 3. The effect effect of of demulsifier demulsiﬁer on the medianmedian diameter diameter of of oil oil droplets. droplets.

By summarizing the above experimental results, the interaction of ASP with the added demulsifiers can be found. The effect of NaOH on the stability of produced water was mainly achieved by the surface-active components formed from the reaction with the acidic components of crude oil, which produces some surface-active components which adsorbed on the surface of the oil droplets. During demulsification, the demulsifier molecules can be adsorbed onto the oil droplets, replacing the surfactant absorbed on the surface of oil droplets and resulting in a weakening of the steric hindrance effect of the polymer molecules. The processes reduced interfacial tension and weakened interfacial film strength, so that the stability of oil droplets was decreased. At the same time, the demulsifier can also neutralize the negative charge on the surface of the oil droplet, reducing the electrostatic repulsion between the oil droplets and aiding the accumulation of oil droplets.

Polymers 2019, 11, 395 8 of 13 Polymers 2019, 11, x FOR PEER REVIEW 8 of 13

(a) (b)

Figure 4. TheThe effect effect of of demulsifier demulsifier on on the the size size of oil of oildroplets. droplets. (a) Settlement (a) Settlement without without demulsifier demulsifier for 0 min,for 0 ( min,b) settlement (b) settlement without without demulsifier demulsifier for 120 for min, 120 min,(c) settlement (c) settlement with with demulsifier demulsifier for 0 for min, 0 min, (d) (settlementd) settlement with with demulsifier demulsifier for for120 120min. min.

3.4. The Effect of Floccula Flocculantnt on Interfacial Tension The simulated ASP floodingflooding producedproduced waterwater withwith 400400 mg/Lmg/L of polymer, 200200 mg/Lmg/L of surfactant concentration and 800 mg/L of of NaOH NaOH concentration concentration was was prepared prepared and and settled settled in in a water bath ◦ temperature of of 45 45 °CC for for 4 4h hto tostudy study the the effect effect of flocculant of flocculant WS WSon oil-water on oil-water interfacial interfacial tension. tension. The Theeffects effects of CPAM of CPAM and andD2N-1650 D2N-1650 on oil-water on oil-water interfacial interfacial tension tension were were studied studied respectively respectively under under the samethe same experimental experimental conditions, conditions, as asshown shown in inFigure Figure 5.5 .The The flocculant flocculant WS WS can can increase the oil-wateroil-water interfacial tension. When the concentration of flocculantflocculant increasedincreased fromfrom 00 toto 7070 mg/L,mg/L, the interfacial tension of oil-water increasedincreased fromfrom 4.4254.425 mN/mmN/m to 6.153 mN/m. The The fl flocculantocculant WS WS can can only only slightly increase oil-wateroil-water interfacialinterfacial tension, tension, and and its its ability ability to to do do so so is ais resulta result of theof the CPAM CPAM in the in flocculantthe flocculant WS. WithWS. With increasing increasing concentration concentration of the of flocculant the flocculant WS, the WS, concentration the concentration of polyacrylamide of polyacrylamide in the system in the systemincreased. increased. Therefore, Therefore, the viscosity the viscosity of the system of the increased system andincreased the transportation and the transportation and adsorption and adsorptionof surfactant of on surfactant the oil-water on the interface oil-water was interface influenced. was Thus,influenced. the amount Thus, ofthe surfactant amount of reaching surfactant the reachingoil-water the interface oil-water decreased, interface resulting decreased, in an resulting increase in in an oil-water increase interfacial in oil-water tension. interfacial However, tension. the However,hindrance wasthe hindrance weak, so the was increase weak, ofso oil-waterthe increase interfacial of oil-water tension interfacial was limited. tension The was flocculant limited. CPAM The flocculanthad no effect CPAM on the had oil-water no effect interfacial on the tension oil-water [34 ].interfacial Compared tension to flocculant [34]. Compared D2N-1650, theto flocculant flocculent D2N-1650,CPAM played the aflocculent larger role CPAM in reducing played the a oil-waterlarger role interfacial in reducing tension. the oil-wa Finally,ter the interfacial concentration tension. of Finally,flocculant the WS concentration can increase of the flocculant oil-water WS interfacial can increase tension. the oil-water interfacial tension. 3.5. The Effect of Flocculant on Zeta Potential The simulated ASP flooding produced water with 400 mg/L of polymer, 800 mg/L of NaOH and 200 mg/L of the surfactant was prepared. The flocculant WS was added into the produced water and both were settled in a water bath at a temperature of 45 ◦C for 4 h to study the effect of the flocculant on Zeta potential for the ASP flooding produced water. The effects of CPAM and D2N-1650 on Zeta potential were studied respectively under the same experimental conditions, as shown in

Polymers 2019, 11, x FOR PEER REVIEW 9 of 13

6.5 D2N-1650 WS CPAM 6.0

Polymers 2019, 11, 395 9 of 13 5.5

Figure6. The flocculant CPAM5.0 and D2N-1650 can reduce the absolute value of the Zeta potential individually, however the interaction of the two flocculants can reduce the absolute value of Zeta potential even further. A large4.5 number of positive charges on the molecular chain from the flocculant

CPAM neutralized the negative tension(mN/m) interfacial oil-water The charge on the surface of the oil droplet. Then the absolute value of

Zeta potential on the droplet4.0 surface decreased greatly. The flocculant D2N-1650 did not have positive charges. It can only bind the-10 oil 0 droplets 10 20 together 30 to 40 produce 50 60 precipitation 70 80 through “adsorption The dosage of flocculant (mg/L) Polymersbridging”. 2019, Therefore,11, x FOR PEER the REVIEW influence of the flocculant D2N-1650 on the Zeta potential was limited.9 of 13 Figure 5. The effect of flocculant on interfacial tension.

3.5. The Effect of Flocculant 6.5on Zeta Potential D2N-1650 WS The simulated ASP flooding produced CPAMwater with 400 mg/L of polymer, 800 mg/L of NaOH 6.0 and 200 mg/L of the surfactant was prepared. The flocculant WS was added into the produced water and both were settled in a water bath at a temperature of 45 °C for 4 h to study the effect of the flocculant on Zeta potential5.5 for the ASP flooding produced water. The effects of CPAM and D2N-1650 on Zeta potential were studied respectively under the same experimental conditions, as shown in Figure 6. The flocculant5.0 CPAM and D2N-1650 can reduce the absolute value of the Zeta potential individually, however the interaction of the two flocculants can reduce the absolute value of Zeta potential even further.4.5 A large number of positive charges on the molecular chain from the

flocculant CPAM neutralized tension(mN/m) interfacial oil-water The the negative charge on the surface of the oil droplet. Then the absolute value of Zeta potential4.0 on the droplet surface decreased greatly. The flocculant D2N-1650 did not have positive charges.-10 It 0can 10only 20bind 30the oil 40 droplets 50 60 together 70 to 80 produce precipitation The dosage of flocculant (mg/L) through “adsorption bridging”. Therefore, the influence of the flocculant D2N-1650 on the Zeta potential was limited. FigureFigure 5. 5. TheThe effect effect of of flocculant ﬂocculant on on interfacial interfacial tension.

3.5. The Effect of Flocculant on10 Zeta Potential D2N-1650 The simulated ASP flooding0 produced CPAMwater with 400 mg/L of polymer, 800 mg/L of NaOH WS and 200 mg/L of the surfactant-10 was prepared. The flocculant WS was added into the produced water and both were settled in a water bath at a temperature of 45 °C for 4 h to study the effect of -20 the flocculant on Zeta potential for the ASP flooding produced water. The effects of CPAM and D2N-1650 on Zeta potential-30 were studied respectively under the same experimental conditions, as shown in Figure 6. The flocculant-40 CPAM and D2N-1650 can reduce the absolute value of the Zeta

potential individually, howeverZeta potential (mV) the interaction of the two flocculants can reduce the absolute value -50 of Zeta potential even further. A large number of positive charges on the molecular chain from the flocculant CPAM neutralized-60 the negative charge on the surface of the oil droplet. Then the absolute value of Zeta potential-70 on the droplet surface decreased greatly. The flocculant D2N-1650 did not have positive charges.-10 It 0can 10only 20bind 30the oil 40 droplets 50 60 together 70 to 80 produce precipitation through “adsorption bridging”. Therefore,The dosa thege inflof flocculantuence of (m gthe/L) flocculant D2N-1650 on the Zeta potential was limited. FigureFigure 6. 6. TheThe effect effect of of flocculant ﬂocculant on on Zeta Zeta potential. potential.

10 3.6.3.6. The The Effect Effect of of Flocculant Flocculant on on the the Size Size of of Oil Oil Droplets Droplets D2N-1650 0 CPAM TheThe simulated simulated ASP ASP flooding flooding prod produceduced WSwater with 400 mg/Lmg/L of of polymer, 800 mg/L of of NaOH NaOH andand 200 200 mg/L of of the the surfactant surfactant-10 was was prepared. Th Then,en, 60 60 mg/L mg/L flocculant flocculant WS WS was was added added into into the the ◦ producedproduced water andand settledsettled-20 in in a 45a 45C °C water water bath bath for 2for h to2 h study to study the effect the effect of flocculant of flocculant on oil dropleton oil median diameter, as shown in Figures7 and8. The magnification used to produce Figure8 was droplet median diameter, -30as shown in Figures 7 and 8. The magnification used to produce Figure 8 10 × 40. There is an obvious effect of the flocculant increasing the median diameter of oil droplets. The median diameter of oil droplets-40 increased when settled for 2 h without flocculant. The limited increase

indicated that the ASP floodingZeta potential (mV) -50 produced water was very stable. However, the median diameter of oil droplets increased from 4.02 µm to 20.15 µm when settled for 2 h with added flocculant into the -60 system. The significant increase of the median diameter of oil droplets indicated that the oil droplets in the ASP flooding produced-70 water had become unstable after adding flocculant. However, when -10 0 10 20 30 40 50 60 70 80 The dosage of flocculant (mg/L) Figure 6. The effect of flocculant on Zeta potential.

3.6. The Effect of Flocculant on the Size of Oil Droplets The simulated ASP flooding produced water with 400 mg/L of polymer, 800 mg/L of NaOH and 200 mg/L of the surfactant was prepared. Then, 60 mg/L flocculant WS was added into the produced water and settled in a 45 °C water bath for 2 h to study the effect of flocculant on oil droplet median diameter, as shown in Figures 7 and 8. The magnification used to produce Figure 8

PolymersPolymers 20192019,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1313 waswas 1010 ×× 40.40. ThereThere isis anan obviousobvious effecteffect ofof thethe flflocculantocculant increasingincreasing thethe medianmedian diameterdiameter ofof oiloil droplets.droplets. TheThe medianmedian diameterdiameter ofof oiloil dropletsdroplets incrincreasedeased whenwhen settledsettled forfor 22 hh withoutwithout flocculant.flocculant. TheThe limitedlimited increaseincrease indicatedindicated thatthat thethe ASPASP floodingflooding producedproduced waterwater waswas veryvery stable.stable. However,However, thethe medianPolymersmedian2019 diameterdiameter, 11, 395 ofof oiloil dropdropletslets increasedincreased fromfrom 4.024.02 μμmm toto 20.1520.15 μμmm whenwhen settledsettled forfor 22 hh10 withwith of 13 addedadded flocculantflocculant intointo thethe system.system. TheThe significantsignificant increaseincrease ofof thethe medianmedian diameterdiameter ofof oiloil dropletsdroplets indicatedindicatedcompared thatthat with thethe adding oiloil dropletsdroplets demulsifier inin thethe ZY, ASPASP the effectfloofloodingding of flocculant producedproduced WS waterwater was had relativelyhad becomebecome small. unstableunstable In order afterafter to addingaddingfurther investigateflocculant.flocculant. However, theHowever, effect of whenwhen demulsifier comparedcompared on the withwith size addingadding of oil droplets,demulsifierdemulsifier the ZY, shapesZY, thethe of effecteffect oil droplets ofof flocculantflocculant in the WSWSsimulated waswas relativelyrelatively produced small.small. water InIn at order differentorder toto further conditionsfurther investigateinvestigate were studied thethe effecteffect with aofof microscope, demulsifierdemulsifier the onon results thethe sizesize of whichofof oiloil droplets,droplets,are shown thethe in shapesshapes Figure 8 ofof. The oiloil droplets sizedroplets of oil inin droplets thethe simulasimula in thetedted simulated producedproduced produced waterwater atat differentdifferent water without conditionsconditions flocculant werewere studiedwasstudied basically withwith aa unchanged microscope,microscope, after thethe settlement resultsresults ofof whichwhich for 2 h. areare The shownshown size of inin oil FigureFigure droplets 8.8. TheThe increased sizesize ofof oiloil significantly dropletsdroplets inin after thethe simulatedsimulatedadding flocculant producedproduced for waterwater settlement withoutwithout for flocculantflocculant 60 min. With waswas an basicallybasically increase unchangedunchanged in settling afterafter time, settlementsettlement the size of forfor oil 2 droplets2 h.h. TheThe sizesizeincreased ofof oiloil droplets gradually.droplets increasedincreased significantlysignificantly afterafter adaddingding flocculantflocculant forfor settlementsettlement forfor 6060 min.min. WithWith anan increaseincrease inin settlingsettling time,time, thethe sizesize ofof oiloil dropletsdroplets increasedincreased gradually.gradually. 2222 No flocculant 2020 No flocculant 60mg/L60mg/L flocculantflocculant 1818

1616

1414

1212

1010

4 The median diameter of oil droplets(μm) of oil diameter median The 4 The median diameter of oil droplets(μm) of oil diameter median The

22 00 50 50 100 100 150 150 200 200 250 250 SettlinSettlingg timetime ((minmin)) FigureFigure 7.7. TheTheThe effecteffect ofof flocculantflocculant ﬂocculant onon thethe medianmedian diameterdiameter of of oil oil droplets. droplets.

((aa)) ((bb))

((cc)) ((dd))

Figure 8. The effect of ﬂocculant on the size of oil droplet. (a) Settlement without ﬂocculant for 0 min,

(b) settlement without flocculant for 120 min, (c) settlement with flocculant for 0 min, (d) settlement with flocculant for 120 min. Polymers 2019, 11, x FOR PEER REVIEW 11 of 13

Figure 8. The effect of flocculant on the size of oil droplet. (a) Settlement without flocculant for 0 Polymersmin,2019 (b, 11) ,settlement 395 without flocculant for 120 min, (c) settlement with flocculant for 0 min, (d11) of 13 settlement with flocculant for 120 min.

4.4. Demulsification-FlocculationDemulsification-Flocculation MechanismMechanism ofof Oil-WaterOil-Water Emulsion Emulsion AfterAfter comparisoncomparison andand analysisanalysis ofof thethe theoreticaltheoretical andand experimentalexperimental results,results, thethe mechanismmechanism ofof demulsificationdemulsification andand flocculationflocculation waswas summarized,summarized, asas shownshown inin FigureFigure9 9.. The The demulsifier demulsifier molecules molecules cancan bebe adsorbed adsorbed onto onto the the oil oil droplets, droplets, replacing replacing the surfactantthe surfactant absorbed absorbed on the on surface the surface of oil droplet, of oil todroplet, reduce to the reduce interfacial the interfacial tension, and tension, weaken and interfacial weaken film interfacial strength, film so thatstrength, the stability so that of the oil dropletsstability decreased.of oil droplets At the decreased. same time, At thethe demulsifiersame time, canthe alsodem neutralizeulsifier can the also negative neutralize charge the onnegative the surface charge of theon the oil droplet,surface reducingof the oil thedroplet, electrostatic reducing repulsion the electrostatic between oilrepulsion droplets, between allowing oil the droplets, accumulation allowing of oilthe droplets. accumulation The flocculant of oil droplets. after demulsification The flocculant of oilafter droplets demulsification by charge neutralization,of oil droplets adsorptionby charge bridgingneutralization, and sweep adsorption function bridging together, and so sweep that the function oil droplets together, form aggregates,so that the thenoil droplets the synthetic form actionaggregates, of the then demulsifier the synthetic and theaction flocculant of the demulsi causesfier the and oil droplet the flocculant film to causes break upthe andoil droplet oil droplet film coalescenceto break up occurs,and oil sodroplet as to achievecoalescence the purposeoccurs, so of as oil-water to achieve separation. the purpose of oil-water separation.

FigureFigure 9.9. Demulsification–FlocculationDemulsification–Flocculation mechanism.mechanism. 5. Conclusion 5. Conclusion In this paper, we studied the demulsification-flocculation mechanism of produced water from In this paper, we studied the demulsification-flocculation mechanism of produced water from ASP flooding by considering oil-water interfacial tension, Zeta potential and the size of oil droplets of ASP flooding by considering oil-water interfacial tension, Zeta potential and the size of oil droplets produced water with added demulsifier and flocculent through laboratory experiments. of produced water with added demulsifier and flocculent through laboratory experiments. We demonstrated that the demulsification-flocculation mechanism is feasible and can be applied We demonstrated that the demulsification-flocculation mechanism is feasible and can be to the produced water from ASP flooding. The demulsifier molecules can be adsorbed onto the oil applied to the produced water from ASP flooding. The demulsifier molecules can be adsorbed onto droplets and replace the surfactant absorbed on the surface of oil droplets, thereby reducing the the oil droplets and replace the surfactant absorbed on the surface of oil droplets, thereby reducing interfacial tension and weakening the interfacial film strength. The demulsifier can also neutralize the interfacial tension and weakening the interfacial film strength. The demulsifier can also the negative charge on the surface of oil droplets and reduce the electrostatic repulsion between them, neutralize the negative charge on the surface of oil droplets and reduce the electrostatic repulsion which is beneficial in the accumulation of oil droplets. The flocculent after demulsification of oil between them, which is beneficial in the accumulation of oil droplets. The flocculent after droplets by charge neutralization, adsorption bridging, and sweep function together. Therefore, the oil demulsification of oil droplets by charge neutralization, adsorption bridging, and sweep function droplets form aggregates and the synthetic action of the demulsifier. The flocculent causes the oil drop together. Therefore, the oil droplets form aggregates and the synthetic action of the demulsifier. The film to break and then the oil droplets coalesce, resulting in oil water separation. flocculent causes the oil drop film to break and then the oil droplets coalesce, resulting in oil water Authorseparation. Contributions: Conceptualization, B.H.; Methodology, Formal Analysis, Investigation and Writing- Original Draft Preparation, B.H., X.L. and W.Z.; Writing-Review & Editing, Supervision and Project Administration, Author Contributions: Conceptualization, B.H.; Methodology, Formal Analysis, Investigation and C.F.; Writing-Review & Editing, Y.W. & S.F. Writing-Original Draft Preparation, B.H., X.L. and W.Z.; Writing-Review & Editing, Supervision and Project Acknowledgments:Administration, C.F.;The Writing-Review authors gratefully & Editing, acknowledge Y.W. & S.F. the financial support by National Natural Science Foundation of China (51804077); Excellent Scientific Research Talent Cultivation Fund of Northeast Petroleum UniversityAcknowledgments: (SJQHB201803). The authors gratefully acknowledge the financial support by National Natural Science ConflictsFoundation of Interest:of China The(51804077); authors Excellent declare no Scientific conflict ofResearch interest. Talent Cultivation Fund of Northeast Petroleum University (SJQHB201803).

ReferencesConflicts of Interest: The authors declare no conflict of interest.

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