Long Branched-Chain Amphiphilic Copolymers - American Chemical

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Long Branched-Chain Amphiphilic Copolymers: Synthesis, Properties and Application in Heavy Oil Recovery Juan Li, Qiuxia Wang, Yigang Liu, Minggang Wang, and Yebang Tan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00973 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Long Branched-Chain Amphiphilic Copolymers: Synthesis, Properties and Application in Heavy Oil Recovery Juan Li, † Qiuxia Wang, ‡ Yigang Liu, ‡ Minggang Wang, *, † Yebang Tan *, †

†School of Chemistry and Chemical Engineering Shandong University, Jinan 250100, China; Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Jinan 250100, China ‡CNOOC, Ltd, Tianjin Branch, Bohai Oilfield Research Institute, Tanggu Tianjin 300452, China

* Corresponding author. E-mail: [email protected]

KEYWORDS: Long branched-chain; amphiphilic copolymer; enhanced oil recovery; heavy oil

ABSTRACT: A series of water-soluble long branched-chain amphiphilic copolymers, AAGASs, were synthesized and applied in the enhanced oil recovery (EOR) of heavy oil. Hydrophobically associating water-soluble copolymers (AAGs) with an epoxy group were first synthesized by free-radical copolymerization of acrylamide (AM) and sodium 2-acrylamido-2-methylpropanesulfonic acid (AMPS) with glycidyl methacrylate (GMA). 1

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An amino-terminated amphiphilic copolymer (AS-N) was also prepared with AM and sodium 4-styrenesulfonate (SSS) via chain transfer polymerization. AAGASs were then obtained by the chain-extending reaction between the epoxy groups of the AAG and amino group of the AS-N. FT-IR, 1H NMR spectroscopy, static light scattering (SLS), and TGA measurements, were performed to determine the polymer structures and properties. Shear viscosity, surface tension and interfacial tension were also investigated to understand the oil-displacement mechanism of AAGAS solutions. The results showed that AAGASs have a unique associative property in solution and possess good surface and interfacial activity, allowing AAGAS solutions to both thicken water and convert highly viscous heavy oil into low-viscosity oil-in-water (O/W) emulsions. Measurement of the apparent viscosities showed that AAGAS-3 achieves optimal performance with a degree of viscosity reduction (DVR) of heavy oil up to 96.8% at 1000 mg/L. Therefore, our work showed a two-pronged approach for enhancing the recovery of heavy oil, namely, using a polymer incorporating water-thickening properties with heavy oil viscosity reducing ability.

1. INTRODUCTION Only a small dosage of polymers can significantly thicken water, thereby improving the water-oil mobility ratio, decreasing water permeability and enhancing sweep efficiency, which is expected to result in a much higher oil recovery factor. 1, 2 On this basis, polymer flooding has been well studied and successfully applied since it was first proposed in the 2


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1960s.

3-5

Various kinds of polymers have been developed, including polyacrylamide

6

partially hydrolyzed polyacrylamide (PHAM), hydrophobically associating

(PAM),

water-soluble polymers (HAWPs), xanthan gum, guar gum, and topological polymers.7-12 Among them, HAWPs are most important types both academically and industrially. On the one hand, the unique solution properties resulting from reversible association between hydrophobic groups make HAWPs well suited for polymer flooding.

13-15

On the other

hand, these solution properties can be easily varied by changing the type or amount of hydrophobic groups to make them more adaptive to diverse application conditions. 16, 17 Although various HAWPs have enabled excellent enhanced oil recovery (EOR), a worrying fact that must be noted is the oil reservoirs we dealt with have been changed from low-salt, low-temperature light-oil to high-salt, high-temperature heavy-oil. 18 In fact, heavy oil accounts for ca. 70% of total world oil reserves.19 The most challenging problem with heavy oil is its high viscosity, i.e., from 1000 to greater than 100 000 mPa s, whereas the viscosity of conventional crude oil is always 1~100 mPa s, and the heavy oil viscosity desired for flow through a pipeline is less than 200 mPa s. 20, 21 Higher viscosity leads to higher difficulty in improving the affinity of common HAWPs for oil merely by adjusting the type and amount of hydrophobic groups, resulting in lower sweep efficiency and oil recovery. Therefore, polymers with not only high apparent viscosity but also good viscosity reducing properties are urgently needed for EOR in heavy oil reservoirs. Herein, we report a long branched-chain amphiphilic copolymer derived from HAWPs aimed at the EOR of heavy oil. First, a novel HAWP (AAG) was prepared by free-radical 3


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copolymerization

of

acrylamide

(AM)

and

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sodium

2-acrylamido-2-methyl

propanesulfonic acid (AMPS) with glycidyl methacrylate (GMA) and then was grafted with an amino-terminated amphiphilic copolymer (AS-N) obtained from chain transfer copolymerization of AM and sodium 4-styrenesulfonate (SSS) to afford a long branched-chain amphiphilic copolymer (AAGAS). This AAGAS inherits the associative property of common HAWPs in solution owing to the hydrophobic groups of the remaining GMA units and the amphiphilic long branched-chains. Moreover, the long branched-chain structures endow the polymer solution with sufficient viscosity and enhanced environmental resistance. Furthermore, AAGAS solutions show good surface and interfacial activities, which contribute to the viscosity reduction of heavy oil based on emulsification and increase the oil recovery efficiency.

2. EXPERIMENTAL SECTION 2.1. Materials. AM, AMPS, SSS and small molecule emulsifier sodium dodecyl benzene sulfonate (SDBS) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Aminoethanethiol hydrochloride (AET.HCl) was obtained from Sigma-Aldrich (Shanghai, China). GMA, dimethyl sulfoxide (DMSO), azobisisobutyronitrile (AIBN) and azobis(2-methylpropionamide) dihydrochloride (AIBA) were purchased from J&K Chemical, Ltd. (Beijing, China). GMA was freshly redistilled to remove inhibitor before use, whereas the other chemical reagents mentioned were used as received. Heavy crude oil and commercial polymer emulsifier CPE were 4


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supplied by the Bohai Oilfield of China. The properties of the heavy oil are presented in Table 1 as determined with SARA (saturates, aromatics, resins, and asphaltenes) analysis. 22

Table 1. Physico-chemical Properties of Crude Oil. Parameter

Bohai oil sample

density at 25 ℃ (g/cm)

0.977

API gravity

13.3

viscosity at 50 ℃ (m Pa s)

4300

wax content (wt %)

10.10

water and sediment (wt %)

6.82

saturates (wt%)

48.69

aromatics (wt%)

20.61

resins (wt%)

10.93

asphaltenes (wt%)

2.35

2.2. Synthesis. 2.2.1 Synthesis of the AAG Copolymers. AAG samples were obtained through free-radical copolymerization in DMSO according to the procedure described by Asad et al. 23 Certain amounts of AM, AMPS and GMA (Table 2) were added to a 100 mL round-bottomed flask equipped with a mechanical stirrer and a N2 inlet/outlet, dissolved in 45 mL of DMSO, stirred and purged with N2 for 30 min. Then, 0.025 g of AIBN (0.5 wt% of the total monomers) was added as an initiator. The system was heated to 65 ℃ and maintained for 10 h. After cooling down to room temperature, the resulting solution was precipitated in an excess of ethanol. The copolymer precipitate was collected by filtration, re-dissolved in water, purified by dialysis against water with MD55-14 dialysis 5


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tubing (molecular weight cutoff = 10000 g/mol) for 1 week and freeze-dried. The final products were ground into white powder. In all cases, the initial concentration of total monomer was 10 wt%, and the amount of AMPS was maintained at 20 mol% relative to the total monomer feed. Table 2. Synthesis and Composition of AAG Copolymers. AM/AMPS/GMA Sample

a

AIBA (g)

Yield (%)

79.0:20.0:1.0

0.0250

83.6

79.7:20.0:0.3

79.0:19.9:1.1

0.0242

88.3

AAG-3

79.5:20.0:0.5

78.9:20.0:1.1

0.0242

89.5

AAG-4

79.3:20.0:0.7

78.9:19.9:1.2

0.0237

85.7

AAG-5

79.0:20.0:1.0

78.8:20.0:1.2

0.0240

81.7

Feed mole ratio

Calculated mole ratio a

AAG-1

79.9:20.0:0.1

AAG-2

Calculated via 1H NMR spectra. 2.2.2 Synthesis of the Amino-terminated Copolymer AS-N. The AS-N copolymer was

synthesized by chain transfer copolymerization in water according to the literature. 24 AM (5.796 g), SSS (4.204 g) and AET.HCl (0.100 g) were dissolved in 50 mL of water. After fully mixing and degassing, AIBA (0.100 g) was injected at 55 ℃ to initiate copolymerization of the system for 12 h. The resulting solution was neutralized by 1.0 mol/L NaOH solution and then treated the same as the AAG system. The white powder obtained was defined as the AS-N sample. 2.2.3. Synthesis of the Long Branched-Chain Amphiphilic Copolymer AAGAS. AAG (0.278 g) and AN-S (1.390 g) were dissolved in 15 mL of water, and the reaction mixture 6


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was allowed to proceed at room temperature for 24 h. The resulting solution was directly dialyzed against water with MD44 dialysis tubing (molecular weight cutoff = 30000 g/mol). After further treatment with freeze-drying and grinding, the long branched-chain polymer samples composed of an AAG backbone and AS-N long branched- chains were finally obtained as white powders and were denoted AAGAS.

2.3. Polymer Characterization. FT-IR spectroscopy was performed on a Bruker Tensor 27 spectrophotometer with samples prepared as KBr pellets. 1H NMR spectroscopy was performed on a Bruker AVANCE400 NMR spectrometer with D2O as the solvent. Light scattering (LS) was carried out on a DAWN HELEOS 18 angle (from 15o to 165o) light scattering detector using a Ga-As laser (658 nm and 40 mW). The weight-average molecular weight (Mw) from static light scattering (SLS) was obtained using Astra software. All samples for light scattering had been filtered through 0.8-µm Millipore filters and kept standing overnight before measurement. The thermal properties of the samples were analyzed with a Mettler Toledo TGA/DSC1 thermogravimetric analyzer from 50 to 800 °C at a heating rate of 10 °C min −1 under a dynamic nitrogen atmosphere with a flow rate of 100 mL min −1.

2.4. Solution Property Analysis. The solution properties of the AAGAS copolymers were carefully investigated to understand the oil recovery mechanism and provide reference for their application. The hydrodynamic diameter (R h) of AAGAS copolymer solutions from dynamic light scattering (DLS) was also obtained using Astra software. 7


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The shear viscosity measurements were carried out on a HAAKE MARS III rheometer with a coaxial cylinder sensor system (CC 26 Ti, CC 27 Ti). The temperature during the viscosity measurements was set to 25.0±0.1 ℃. Surface tension was determined by a PROCESSOR TENSIOMETER K-12. The interfacial tension between the copolymer solution and heavy oil was determined by a TX-500C rotary drop interface tensiometer.

2.5. Heavy Oil Viscosity Reduction Performance. The apparent viscosity of the oil emulsion was chosen as a standard for the heavy oil viscosity reducing ability of the AAGAS copolymers. The data were collected with a DV-II + PRO viscometer (Brookfield) at 50 ℃ (the application temperature of the Bohai Oilfield). According to the application requirement of the oilfield, all test samples were made of heavy oil and AAGAS copolymer solutions at a volume ratio of 7:3, and each mixture was heated at 50 ℃ for 1 h and stirred for 5 min to emulsify before testing. The degree of viscosity reduction (DVR) of AAGAS could be obtained from the apparent viscosity as shown in eq 1: 25 DVR% = (ŋ r - ŋ c) × 100 / ŋ r

(1)

where ŋ r is the reference viscosity of heavy oil before emulsification, which is 4300 mPa s, and ŋ c is the corresponding viscosity of heavy oil after emulsification. Micrographs of the emulsion morphology were taken by a Scope A1 microscope.

3. RESULTS AND DISCUSSION 3.1. Synthesis. The AAGAS was synthesized in three steps as outlined in Fig. 1. First, 8


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epoxy

reaction

sites

were

introduced

into

the

polymer

backbone

through

copolymerization of AM and AMPS with a hydrophobic block comprising GMA. Second, the activated amino-terminated amphiphilic copolymer AS-N was obtained by chain transfer copolymerization. It is known that more and longer hydrophobic blocks lead to increased thickening efficiency and surface and interfacial activity but decreased water solubility. Amphiphilic long branched-chains are an ideal substitute to balance this contradiction. Whereas amphiphilic molecules maintain the good dispersion and interfacial activity, 22 long branched-chains strengthen the thickening efficiency. Finally, the high efficiency of the reaction between epoxy groups and NH2 groups made it feasible to attach the long-chain amphiphilic AS-N to the AAG backbone and produce the long branched-chain AAGAS copolymer.

9


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Figure 1. Synthesis of long branched-chain amphiphilic copolymers.

3.2. Polymer Characterization. 3.2.1. FT-IR spectra. FT-IR spectra of AAG, AS-N and AAGAS samples are shown in Fig. 2. All samples exhibited absorption peaks around 3420 cm-1 assigned to the N-H stretching vibration and 1665 cm-1 assigned to the C=O stretching vibration and the characteristic absorption peaks of the sulfonic acid groups at about 1250 and 1040 cm-1. Signals corresponding to the benzene rings can be seen in the spectra of both the AS-N and AAGAS samples at 1500 and 850 cm-1. The characteristic absorption peak of the epoxy groups at 910 cm-1 was clear in the AAG spectrum and 10


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could also be found in the AAGAS spectrum, but not in the AS-N spectrum, indicating that the epoxy group in the AAG participated in the chain-extending reaction used to prepare the AAGAS.

Figure 2. FT-IR spectra of the obtained copolymers. 3.2.2. 1H NMR spectra. The 1H NMR spectra shown in Fig. 3 demonstrate the successful synthesis of AAGAS. For copolymer AAG-1, the peaks near 1.2 ppm can be ascribed to -CH3 protons, the peaks around 1.6 and 2.2 ppm are contributed by the -CH2and -CH- protons in the polymer backbone and the signals appearing at 3.5 ppm belong to the epoxy group and -CH2- protons of -CH2SO3- in the AMPS unit. For the copolymer AS-N, signals of the -CH2 and -CH- protons in the polymer backbone appear at the same position as those of AAG-1, and signals of the benzene protons appear as two peaks at around 7.0 ppm. For the long branched-chain copolymer AAGAS-1, the peaks located at 3.5 ppm demonstrate the incorporation of the epoxy and -CH2SO3- groups from AAG into AAGAS. Furthermore, the presence of the peaks assigned to the benzene ring protons also reflects that the chain-extending reaction effectively produced the long 11


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branched-chain copolymer AAGAS.

Figure 3. 1H NMR spectra of the obtained copolymers. 3.2.3. Static Light Scattering. SLS measurement was used to determine the Mw of the copolymer samples in 0.1 mol/L NaCl solutions. The second virial coefficient A2 and refractive index increment dn/dc were also collected. Significant changes could be seen from AAG and AS-N to AAGAS, as shown by the data listed in Table 3. The Mw of the AAG copolymer increased as the amount of the GMA monomer was raised, and then the molar amount of the AAG copolymer decreased at constant mass ratio to the AS-N copolymer, which led to a much larger Mw of the AAGAS copolymer. Therefore, AAGAS-5, the copolymer with the most branched-chains, also possessed the largest Mw and could be expected to have the highest viscosity in solution, which was confirmed by the shear viscosity measurements as well. The graft ratio first increased for the same reason but then decreased slightly because the increasing viscosity of the polymer made 12


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further grafting difficult. There was no general trend in the evolution of A2 as a function of the GMA content, but the fact that all the data were positive indicated that 0.1 mol/L NaCl solution is a good solvent for all samples. Table 3. SLS Results of the Copolymers. dn/dc Sample

Mw (g/mol)

A2 (mol.mL/g2)

(mL/g)

a

Graft ratio (%) a

AAG-1

(3.018±0.303)×105

0.157

(1.908±0.919)×10-3

--

AAG-2

(3.279±0.186)×105

0.150

(2.118±0.349)×10-3

--

AAG-3

(3.651±0.597)×105

0.148

(2.181±0.656)×10-3

--

AAG-4

(4.075±0.041)×105

0.145

(1.900±0.128)×10-3

--

AAG-5

(4.816±0.990)×105

0.143

(1.328±0.100)×10-3

--

AS-N

(1.225±0.118)×104

0.256

(1.942±1.526)×10-3

--

AAGAS-1

(6.430±0.965)×105

0.178

(1.515±0.784)×10-3

78.8

AAGAS-2

(7.223±0.484)×105

0.178

(2.085±0.962)×10-3

84.2

AAGAS-3

(8.119±0.325)×105

0.168

(1.783±0.126)×10-3

85.9

AAGAS-4

(8.967±0.811)×105

0.142

(1.085±0.713)×10-3

84.5

AAGAS-5

(1.024±0.763)×106

0.128

(3.535±2.307)×10-3

79.7

Graft ratio was calculated from the Mw of the copolymers. 3.2.4. Thermal Stability Measurement. The thermal behavior of the AAGAS

copolymers are shown as TGA curves in Fig. 4. The curves displayed two weight loss steps: the first weight loss (150-350 ℃) of 20 wt% is due to the cleavage of amide bonds and the second one (350-500 ℃) of approximately 50 wt% is attributed to the decomposition of the polymer chain. AAGAS samples with fewer branched-chains showed more and faster weight loss. This is because incorporating more branched chains 13


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introduced more benzene rings, which require more heat to degrade, thereby resulting in less weight loss. In addition, the tight entanglement of additional branched-chains around the backbone also increased the degradation temperature of the copolymer. Therefore, the AAGAS-5 copolymer with most branched-chains exhibited the highest thermal stability, making it more suitable for oilfield applications.

Figure 4. TGA curves of AAGAS copolymers.

3.3. Investigation of Solution Properties. 3.3.1. Dynamic Light Scattering. DLS measurement was used to investigate the aggregation behavior of AAGAS samples in solution in order to provide reference for application conditions. As shown in Fig. 5a and 5b, the average hydrodynamic radius (R h) underwent a slight decrease with increasing concentration both at room temperature and higher temperature but increased monotonically as the long branched-chain content or temperature increased. These phenomena can be explained by the hydrophobic interactions and association behavior of AAGAS in solution (Fig. 5c). First, when the copolymer concentration was below 100 14


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mg/L, intramolecular hydrophobic association dominated, and the coiled polymer conformation gradually stretched as the concentration increased, which corresponded to the relatively large increase in R h shown in Fig. 5a and 5b. Second, the R

h

reached an

inflexion point when the copolymer concentration was 100 mg/L because the polymer chain was not aggregated. Third, when the polymer concentration increased from 100 mg/L to 300 mg/L, intramolecular association of the amphiphilic long branched-chains began to occur, and the R h showed a slight decrease in response to the shrinkage. Finally, during the last stage, copolymer concentrations greater than 300 mg/L resulted in intermolecular association and the formation of much larger aggregates, inevitably leading to the sharp increase in R

h

observed. Taking AAGAS-2 solutions at room

temperature as an example, the R h at 1000 mg/L was 115.9, almost three times greater than that of 39.7 at 300 mg/L. The R h continued to increase at higher concentrations during the last stage (i.e., the R h of AAGAS-2 at room temperature increased to 201.0 at 2000 mg/L), possibly because more copolymers participated in the formation of larger aggregates and because according to the critical packing parameter theory proposed by Israelchvili, 26 the aggregate conformation within a large concentration range will settle as spherical micelles. For different AAGAS samples, those with a greater number of long branched-chains could present relatively larger conformations, the R h of which would also be larger. Such changes between different AAGAS samples became much more obvious with the increase in the number of long branched-chains in the copolymers. For AAGAS-2 solutions at different temperatures (Fig. 5b), the R h at certain concentration 15


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increased as the temperature increased, which reflected the stretching and aggregation of the copolymer chains at high temperature. This result indicated that AAGAS samples might show more desirable properties at high temperature. From this point of view, AAGAS solutions at concentrations of 100, 500, 1000, and 2000 mg/L should be expected to have relatively high viscosities, surface activities and interfacial activities, making them more likely to be applied in the EOR of heavy oil at the high temperatures required.

Figure 5. (a) R h variation of AAGAS solutions at room temperature, (b) R h variation of AAGAS-2 solutions at different temperatures, and (c) schematic illustration of the association behavior of AAGAS copolymers in solution. 3.3.2. Viscosity Measurement. Fig. 6b displays the viscosity variation in AAGAS samples at different concentrations in water (at a shear rate of 7.34 s -1) as a primary 16


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reference for oil-displacing polymers. The viscosity variation in AAG samples under the same conditions was also studied for comparison (Fig. 6a). On the one hand, the apparent viscosities of AAGAS solutions were much larger than those of AAG solutions, which is due to the larger numbers of long branched-chains in the AAGAS samples. This could be attributed

to

the

chain-extending

reaction

affording

polymers

with

optimal

water-thickening properties. On the other hand, based on the variation in the apparent viscosity of AAGAS copolymer solutions at different concentrations, we can find that (1) the viscosity of AAGAS copolymers increased as the AAGAS solution concentration increased; (2) the first apparent increase occurred at 100 mg/L, then slow growth was observed up to 300 mg/L, and finally a significant increase was found from 300 mg/L to 2000 mg/L with an inflexion point appearing at about 1000 mg/L, indicating the beginning of the formation of large aggregates; and (3) such changes in the growth rate of viscosity at different concentrations were related to the aggregation behavior of AAGAS introduced before and confirmed the previous estimate of suitable application concentrations in the context of viscosity. What’s more, copolymers AAGAS with more long branched-chains show smaller inflexion concentrations than those of the AAGAS copolymers with less long branched-chains, indicating that the long branched-chains could accelerate the aggregation of the copolymers, thus copolymers with more long branched-chains could expect better oil-displacing performance at relatively lower concentration.

17


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Figure 6. Viscosity variation in (a) AAG copolymer solutions and (b) AAGAS copolymer solutions. 3.3.3. Surface Tension. Fig. 7a revealed the concentration dependence of the surface tension of AAGAS samples in solution at room temperature. There were two inflexion points as the surface tension decreased sharply from 300 mg/L to 2000 mg/L and then remained almost constant as the concentration increased further to 5000 mg/L. The first flexion point can be attributed to the initiation of intermolecular association at about 300 mg/L, which agrees with the DLS results, and the second flexion point which could also be called as CAC (critical association concentration) is related to the formation of large aggregates starting from approximately 1000 mg/L, which agrees with the shear viscosity results. These two agreements validated the association behavior of the AAGAS in solution proposed in Fig. 5c. The flexion points in the surface tension curves of the AAGAS copolymers decreased as the amount of the amphiphilic long branched-chains increased, because that the amphiphilic long branched-chains affect the aggregation behavior of the copolymers and the aggregation behavior determines the surface activity. 18


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All samples showed favorable surface activity: the surface tension decreased as the grafted long branched-chain increased, and the more the grafted long branched-chain the lower the surface tension. The lowest surface tension was 31.2 mN/m for AAGAS-5 at a concentration of 5000 mg/L, which was almost the same as that of AAGAS-5 at 2000 mg/L but was much lower than that of AAGAS-1 at the same concentration. The surface activities at concentrations below 500 mg/L were not very good for AAGAS-1 and AAGAS-2 but were fairly good for the other AAGAS samples. It could be concluded that the amphiphilic long branched-chains are beneficial to surface activity.

Figure 7. Surface tension variation in (a) AAGAS samples at room temperature and (b) AAGAS-2 samples at different temperatures. The relationship between surface tension and temperature was also examined using AAGAS-2 as an example (Fig. 7b). The surface tension of all solutions decreased with an increase in temperature, especially when the temperature reached 40 ℃, indicating the good temperature tolerance of the surface activity. As explained regarding the relationship between R

h

and temperature, the polymer chain configuration at high 19


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temperatures tended to be stretched, which made it much easier for the surface active sites to function. The surface tension of AAGAS-2 solution at a concentration of 1000 mg/L was 7.1 mN/m lower at 60 ℃ than at room temperature. The results from the surface tension measurement further verified the previous estimate of suitable application concentrations. 3.3.4. Interfacial Tension. The promising results from surface tension testing encouraged us to further study the interfacial activities of AAGAS used on heavy oil at the required temperature. Based on the theory of spontaneous emulsification, interfacial tension is one of the most important parameters for the EOR of heavy oil. Low interfacial tension allows aqueous solutions to break up the bulk viscous oil into fine droplets dispersed in water, 27 wherein the high internal-fluid friction between viscous oil phases is substituted with the low internal-fluid friction between aqueous phases, thereby reducing the viscosity and improving the flowability of heavy oil. Fig. 8a shows the interfacial tension between AAGAS solutions at various copolymer concentrations and crude oil at 50 ℃. All samples were found to have good interfacial activities, allowing them to significantly decrease the interfacial tension between water and crude oil to below 5.510 mN/m at a copolymer concentration of only 100 mg/L. As the concentration increased, the interfacial activities of the AAGAS samples improved rapidly with the interfacial tension decreasing to less than 0.001 mN/m at a concentration of 1000 mg/L, which could be called ultralow interfacial tension.

28

Then, the interfacial

tension slowly decreased as the concentration increased to 5000 mg/L. The differences 20


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between the interfacial tensions for different AAGAS samples could be observed up to 2 000 mg/L but became negligible when the concentrations increased to 5000 mg/L because they were all extremely low. We considered that increasing the AAGAS concentration above 2000 mg/L had little significance and was not economical for EOR applications. The variation of CAC among different AAGAS copolymers was obvious in the interfacial tension curves: the CAC of AAGAS-5 with the most amphiphilic long branched-chains was about 500 mg/L, while the CAC of AAGAS-1 with the least long amphiphilic long branched-chains was around 1000 mg/L. The effect of the amphiphilic long branched-chains on the aggregation behavior was greater on the interface between heavy oil and copolymer solutions, and these aggregation behaviors help to diminish the interfacial tension. The above analysis could be visualized by the corresponding test images shown with Fig. 8b, which records the changes of oil-water interface under different interfacial tensions. From the upper left corner to the lower right corner, the oil droplets are gradually drawn from ellipse to line as the interfacial tension decreases. The changes in the shape of oil droplets reflect the changes of interfacial tension. Therefore, the concentrations used for the heavy oil viscosity reduction test were finalized based on our previously chosen values.

Combining the above experimental data with the theory of spontaneous emulsification, we analyzed the oil-displacement mechanism of the AAGAS. As depicted in Fig. 8c, heavy oil and water should be two incompatible phases because of the large interfacial 21


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tension between them, but the introduction of the AAGAS and its aggregates into the oil-water interface led to low interfacial tension, allowing the conversion of the two phases into O/W (oil-in-water) emulsions. Emulsification would significantly reduce the apparent viscosity of the heavy oil and enhance the oil carrying capacity of the water, thereby improving the displacement efficiency of heavy oil enormously. Both the copolymer composition and solution concentration were linked to the performance since both of them affected the aggregation of AAGAS in two aspects. With regard to the interfacial tension, a low number of long branched-chains and low concentration could not afford low enough interfacial tension because of incompact aggregation, leading to large emulsion droplets and low DVRs and displacement efficiencies. However, a high number of long branched-chains and high concentration produced ultralow interfacial tension and small emulsion droplets with compact aggregation; thus, the performance might plateau, but addition of excess copolymer did not provide any benefits. With regard to the water phase viscosity, incompact aggregation with a low number of long branched-chains and low concentration produced low viscosity of the water phase, which allowed rapid adsorption of the copolymer to the interface and low apparent viscosity of the emulsion. By contrast, compact aggregation with a large number of long branched-chains and high concentration produced high viscosity of water phase, which obstructed the distribution of copolymer but helped strengthen the interfacial film and oil carrying capacity. Our next task is to determine the optimal performance conditions.

22


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Figure 8. (a) Interfacial tension of AAGAS solutions at different concentrations with their corresponding spinning drop pictures, (b) images of interfacial tension tests and (c) theoretical model of AAGAS used on heavy oil.

3.4. Heavy Oil Viscosity Reduction Performance. The heavy oil viscosity reduction performance of the AAGAS polymer is shown in Fig. 9a. Samples with higher amounts of GMA and more long branched-chains exhibited much better viscosity reducing ability at lower concentrations. The DVRs of AAGAS-3, AAGAS-4 and AAGAS-5 at a concentration of 500 mg/L were all above 90% according to the formula described previously. Such regularity was broken as the concentration increased: the viscosity of those copolymers with more long branched-chains increased faster, which destroyed their 23


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heavy oil viscosity reducing ability. By contrast, the viscosity of AAGAS samples with fewer long branched-chains was not affected. There seems to be a balance between the water-thickening properties and heavy oil viscosity reducing ability of AAGAS, which dovetailed with the mechanistic analysis. All samples reached a high viscosity reduction ratio above 95.0% at 1000 mg/L, and the highest viscosity reduction ratio was 96.9% for AAGAS-2 at 2000 mg/L. The optimal effect of 96.8% was obtained with AAGAS-3 at 1000 mg/L, which corresponded to a decrease in the viscosity of heavy oil to 134.9 m Pa s. The heavy oil viscosity reduction performance of polymer emulsifier CPE used in Bohai Oilfield and small molecule emulsifier SDBS were also evaluated as a comparison with the obtained copolymer AAGAS-2. As shown in Fig. 9b, the DVR values of the AAGAS-2 copolymer are apparently higher than those of CPE and SDBS. The lowest DVR value of AAGAS-2 at a concentration of 500 mg/L is 87.8%, while the lowest DVR values of CPE and SDBS at the same concentration are 78.6% and 55.9%, respectively. These results demonstrate that the AAGAS copolymers are more suitable for the viscosity reduction of the tested heavy oil.

24


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Figure 9. Heavy oil viscosity reduction behavior of AAGAS copolymers: (a) Apparent viscosity of crude oil after reduction, (b) DVR comparison among AAGAS-2, CPE and SDS and (c) morphology of crude oil after reduction (the copolymer concentration was 1000 mg/L). Micrographs of the samples revealed that the AAGAS copolymer solutions emulsified the crude oil into O/W emulsions (Fig. 9c). The emulsion droplet size ranged from 20 µm to 100 µm at a copolymer concentration of 1000 mg/L and shared the same relationship with the copolymer solution composition as the viscosity reducing ability. These results were also consistent with the mechanism we considered previously.

4. CONCLUSION In conclusion, we have reported a new route for the EOR of heavy oil through the 25


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combination of traditional polymer flooding with oil viscosity reduction via emulsification. The key element of our approach is the synthesis of long branched-chain amphiphilic copolymers, AAGASs. The synthetic method developed here is easy to achieve in industrial production, and the materials used are cheap. Measurement of the solution properties showed that the AAGAS copolymers have unique association behavior, which in turns leads to excellent surface and interfacial activity. The mixture of AAGAS solutions and heavy oil can easily form O/W emulsions and reduce the apparent viscosity of the heavy oil, and in this work, the corresponding mechanism and theoretical model of this spontaneous emulsification were developed. We also provided the optimal conditions for applying the AAGAS in the EOR of heavy oil. This work not only offers a feasible approach to the synthesis of a highly efficient oil-displacing agent but also provides novel water-soluble polymers to apply in the EOR of heavy oil.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID：：0000-0003-1804-5592 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 26


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This study was supported by the Major Research of Science and Technology, China (Grant No. 2016ZX05058-003-012). The authors would like to thank Shiyanjia lab for the support of TG analysis.

REFERENCES (1) Wever, D. A. Z.; Picchioni, F.; Broekhuis, A. A. Polymers for enhanced oil recovery: A paradigm for structure-property relationship in aqueous solution. Prog. Polym. Sci. 2011, 36, 1558-1628. (2) Olajire, A. A. Review of ASP EOR (alkaline surfactant polymer enhanced oil recovery) technology in the petroleum industry: Prospects and challenges. Energy 2014, 77, 963-982. (3) Daripa, P.; Dutta, S. Modeling and simulation of surfactant-polymer flooding using a new hybrid method. J. Comput. Phys. 2017, 335, 249-282. (4) Jiang, F.; Pu, W. F.; Li, Y. B.; Du, D. J. A double-tailed acrylamide hydrophobically associating polymer: Synthesis, characterization, and solution properties. J. Appl. Polym. Sci. 2015, 132, 42569-42578. (5) Gao, C.; Shi, J.; Zhao, F. Successful polymer flooding and surfactant–polymer flooding projects at Shengli oilfield from 1992 to 2012. J. Petr. Explor. Prod. Technol. 2014, 4 (1), 1-8. (6) Kulicke, W. M.; Kniewske, R.; Klein, J. Preparation, characterization, solution properties and rheological behavior of polyacrylamide. Prog. Polym. Sci. 1982, 8 (4), 27


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373-468. (7) Sukpisan, J.; Kanatharana, J.; Sirivat, A.; Wang, Y. The specific viscosity of partially hydrolyzed polyacrylamide solutions: effects of degree of hydrolysis, molecular weight, solvent quality and temperature. J. Polym. Sci. Pol. Phys. 1998, 36, 743-753. (8) Zou, Ch. J.; Zhao, P. W.; Hu, X. Z.; Yan, X. L.; Zhang, Y. Y.; Wang, X. J.; Song, R. T.; Luo, P. Y. β-Cyclodextrin- Functionalized hydrophobically Associating Acrylamide Copolymer for Enhanced Oil Recovery. Energy Fuels. 2013, 27 (5), 2827-2834. (9) Taylor, K. C.; Nasr-EI-Din, H. A. Water-soluble hydropobically associating polymers for improved oil recovery: A literature review. J. Petrol. Sci. Eng. 1998, 19 (3-4), 265-280. (10) Garcia-Ochoa, F.; Santos, V. E.; Casas, J.A.; Gomez, E. Xanthan gum: Production, recovery, and properties. Biotechnol. Adv. 2000, 18, 549-579. (11) Heyne, E.; Whistler, R. L. Chemical composition and properties of guar polysaccharides. J. Am. Chem. Soc. 1948, 70, 2249-2252. (12) Lai, N.; Zhang, Y.; Xu, Q.; Zhou, N.; Wang, H. J.; Ye, Zh. B. A water-soluble hyperbranched copolymer based on a dendritic structure for low-to-moderate permeability reservoirs. RSC Adv. 2016, 6 (39): 32586-32597. (13) Kronberg, B. The hydrophobic effect. Curr. Opin. Colloid In. 2016, 22, 14-22. (14) Wei, B.; Romero-Zeron, L.; Rodrigue, D. Novel self-assembling polymeric system based on a hydrophobic modified copolymer: formulation, rheological characterization, and performance in enhanced heavy oil recovery. Polym. Adv. Technol. 2014, 25, 28


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732-741. (15) Wu, G.; Jiang, X. H.; Yu, L. M.; Yan. X. F. Hydrophobically associating polyacrylamide derivatives with double bond for enhanced solution properties. Ploym. Eng. Sci. 2016, 1203-1211. (16) Guo, Y. J.; Liang, Y.; Yan, X. Sh.; Feng, R. Sh.; Song, R. T.; Zhou, J. D.; Gao, F. L. Hydrophobic micro block length effect on the interaction strength and binding capacity between a partially hydrolyzed micro block hydrophobically associating polyacrylamide terpolymer and surfactant. J. Appl. Polym. Sci. 2014,131, 40633-40644. (17) Ali, S.A.; Umar, Y.; Abu-Sharkh, B.F.; Al-Muallem, H. A. Synthesis and comparative solution properties of single-, twin-, and triple-tailed associating ionic polymers based on diallylammonium salts. J. Polyml. Sci. Pol. Chem. 2006, 44(19), 5480-5494. (18) Mao, J. Ch.; Liu, J. W.; Wang, H. B.; Yang, X. J.; Zhang, Zh. Y.; Yang, B.; Zhao, J. Zh. Novel terpolymers as viscosity reducing agent for Tahe super heavy oil. RSC Adv. 2017, 7, 19257- 19261. (19) Guo, K.; Li, H. L.; Yu, Zh. X. In-situ heavy and extra-heavy oil recovery: A review. Fuel 2016, 185, 886-902. (20) Saboorian-Jooybari, H.; Dejam, M.; Chen, Zh. X. Heavy oil polymer flooding from laboratory core floods to pilot tests and field applications: Half-century studies. J. Petrol. Sci. Eng. 2016, 142, 85-100. (21) Kumar, S.; Mahto, V. Use of a novel surfactant to prepare oil-in-water emulsion of 29


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an indian heavy crude oil for pipeline transportation. Energy Fuels 2017, 31, 12010-12020. (22) Mao, J. Ch.; Liu, J. W.; Peng, Y. K.; Zhang, Zh. Y.; Zhao, J. Zh. Quadripolymers as viscosity reducers for heavy oil. Energy Fuels 2018, 32 (1), 119-124. (23) Asadujjaman, A.; Kent, B.; Bertin, A. Phase transition and aggregation behavior of an UCST-type copolymer poly (acrylamide-co-acrylonitrile) in water: effect of acrylonitrile content, concentration in solution, copolymer chain length and presence of electrolyte. Soft Matter 2017, 13, 658-669. (24) Sun, F. L.; Wang, Y. X.; Wei, Y.; Cheng, G.; Ma, G. H. Thermo-triggered drug delivery from polymeric micelles of poly (N-isopropylacrylamide-co-acrylamide)-b-poly (n-butylmethacrylate) for tumor targeting. J. Bioact. Compat. Pol. 2014, 29(4), 301-317. (25) Hasan, S. W.; Ghannam, M. T.; Esmail, N. Heavy crude oil viscosity reduction and theology for pipeline transportation. Fuel 2010, 1095-1100. (26) By J. N.; Israelachvli, D.; Mitchell, J.; Ninham, B.W. Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers. J. Chem. Soc., Faraday Trans. 1976, 72, 1525-1568. (27) Dong, M. Z.; Ma, Sh. Zh.; Liu, Q. Enhanced heavy oil recovery through interfacial instability: A study of chemical flooding for Brintnell heavy oil. Fuel 2009, 88 (6), 1049-1056. (28) Nowbahar, A. N.; Whitaker, K. A.; Schmitt, A. K.; Kuo, T. Ch. Mechanistic study of water droplet coalescence and flocculation in diluted bitumen emulsions with additives 30


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using microfluidics. Energy Fuels 2017, 31 (10), 10555-10565.

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Table 1. Physico-chemical Properties of Crude Oil. Parameter

Bohai oil sample

density at 25 ℃ (g/cm)

0.977

API gravity

13.3

viscosity at 50 ℃ (m Pa s)

4300

wax content (wt %)

10.10

water and sediment (wt %)

6.82

saturates (wt%)

48.69

aromatics (wt%)

20.61

resins (wt%)

10.93

asphaltenes (wt%)

2.35

32


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Table 2. Synthesis and Composition of AAG Copolymers. AM/AMPS/GMA Sample

a

AIBA (g)

Yield (%)

79.0:20.0:1.0

0.0250

83.6

79.7:20.0:0.3

79.0:19.9:1.1

0.0242

88.3

AAG-3

79.5:20.0:0.5

78.9:20.0:1.1

0.0242

89.5

AAG-4

79.3:20.0:0.7

78.9:19.9:1.2

0.0237

85.7

AAG-5

79.0:20.0:1.0

78.8:20.0:1.2

0.0240

81.7

Feed mole ratio

Calculated mole ratio a

AAG-1

79.9:20.0:0.1

AAG-2

Calculated via 1H NMR spectra.

33


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Table 3. SLS Results of the Copolymers. dn/dc Sample

Mw (g/mol)

A2 (mol.mL/g2)

(mL/g)

a

Graft ratio (%) a

AAG-1

(3.018±0.303)×105

0.157

(1.908±0.919)×10-3

--

AAG-2

(3.279±0.186)×105

0.150

(2.118±0.349)×10-3

--

AAG-3

(3.651±0.597)×105

0.148

(2.181±0.656)×10-3

--

AAG-4

(4.075±0.041)×105

0.145

(1.900±0.128)×10-3

--

AAG-5

(4.816±0.990)×105

0.143

(1.328±0.100)×10-3

--

AS-N

(1.225±0.118)×104

0.256

(1.942±1.526)×10-3

--

AAGAS-1

(6.430±0.965)×105

0.178

(1.515±0.784)×10-3

78.8

AAGAS-2

(7.223±0.484)×105

0.178

(2.085±0.962)×10-3

84.2

AAGAS-3

(8.119±0.325)×105

0.168

(1.783±0.126)×10-3

85.9

AAGAS-4

(8.967±0.811)×105

0.142

(1.085±0.713)×10-3

84.5

AAGAS-5

(1.024±0.763)×106

0.128

(3.535±2.307)×10-3

79.7

Graft ratio was calculated from the Mw of the copolymers.

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Figure Captions Figure 1. Synthesis of long branched-chain amphiphilic copolymers. Figure 2. FT-IR spectra of the obtained copolymers. Figure 3. 1H NMR spectra of the obtained copolymers. Figure 4. TGA curves of AAGAS copolymers. Figure 5. (a) R h variation of AAGAS solutions at room temperature, (b) R h variation of AAGAS-2 solutions at different temperatures and (c) schematic illustration of the association behavior of AAGAS copolymers in solution. Figure 6. Viscosity variation in (a) AAG copolymer solutions and (b) AAGAS copolymer solutions. Figure 7. Surface tension variation in (a) AAGAS samples at room temperature and (b) AAGAS-2 samples at different temperatures. Figure 8. (a) Interfacial tension of AAGAS solutions at different concentrations with their corresponding spinning drop pictures, (b) images of interfacial tension tests and (c) theoretical model of AAGAS used on heavy oil. Figure 9. Heavy oil viscosity reduction behavior of AAGAS copolymers: (a) Apparent viscosity of crude oil after reduction, (b) DVR comparison among AAGAS-2, CPE and SDBS and (c) morphology of crude oil after reduction (the copolymer concentration was 1000 mg/L). Figure 10

Graphic abstract.

35


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Figure 1. Synthesis of long branched-chain amphiphilic copolymers. 109x89mm (600 x 600 DPI)


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Figure 2. FT-IR spectra of the obtained copolymers. 63x50mm (600 x 600 DPI)


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Figure 3. 1H NMR spectra of the obtained copolymers. 84x89mm (600 x 600 DPI)


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Figure 4. TGA curves of AAGAS copolymers. 61x47mm (600 x 600 DPI)


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Figure 5. (a) R h variation of AAGAS solutions at room temperature, (b) R h variation of AAGAS-2 solutions at different temperatures and (c) schematic illustration of the association behavior of AAGAS copolymers in solution. 89x53mm (600 x 600 DPI)


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Figure 6. Viscosity variation in (a) AAG copolymer solutions and (b) AAGAS copolymer solutions. 57x21mm (600 x 600 DPI)


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Figure 7. Surface tension variation in (a) AAGAS samples at room temperature and (b) AAGAS-2 samples at different temperatures. 58x22mm (600 x 600 DPI)


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Figure 8. (a) Interfacial tension of AAGAS solutions at different concentrations with their corresponding spinning drop pictures, (b) images of interfacial tension tests and (c) theoretical model of AAGAS used on heavy oil. 99x65mm (300 x 300 DPI)


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Figure 9. Heavy oil viscosity reduction behavior of AAGAS copolymers: (a) Apparent viscosity of crude oil after reduction, (b) DVR comparison among AAGAS-2, CPE and SDBS and (c) morphology of crude oil after reduction (the copolymer concentration was 1000 mg/L). 90x54mm (300 x 300 DPI)


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Graphic abstract 57x40mm (600 x 600 DPI)


Long Branched-Chain Amphiphilic Copolymers - American Chemical

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