Adamantane Modified Copolymer in

Article pubs.acs.org/IECR

Potential of a β‑Cyclodextrin/Adamantane Modified Copolymer in Enhancing Oil Recovery through Host−Guest Interactions Wan-Fen Pu,†,‡ Yang Yang,*,†,‡ Bing Wei,*,†,‡ and Cheng-Dong Yuan†,‡ †

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation and ‡School of Oil & Natural Gas Engineering, Southwest Petroleum University, Chengdu 610500, People’s Republic of China

ABSTRACT: In this work, a novel enhanced oil recovery (EOR) copolymer, which is expected to be an alternative to currently used polymers, was successfully synthesized through free radical micellar copolymerization of acrylamide, acrylic acid, 6acrylamide-β-cyclodextrin (N-β-CD), and acrylamide-adamantane (N-ADA). The copolymer was characterized by Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, thermal gravimetric analysis, and scanning electron microscopy. The results demonstrated that because of the incorporation of ADA and β-CD groups, host−guest interactions occurred in the aqueous solution, which subsequently improved the salt tolerance, temperature resistance, shear resistance, and viscoelastic properties of the synthesized polymer. Regarding the EOR performance, it was observed that 8.58% incremental oil was produced using this polymer after water flooding was exhausted. The superior properties make this novel polymer promising in enhancing oil recovery, particularly for reservoirs which have discarded polymer flooding techniques because of harsh conditions.

1. INTRODUCTION Ever-increasing world demand for energy requires the increase in crude oil production. Therefore, enhanced oil recovery (EOR) methods have attracted more and more attention.1−5 As an effective technology for EOR, polymer flooding has been widely used.6−8 Water-soluble polymers can correct the water/ oil mobility ratio by thickening the aqueous solution, resulting in a higher volumetric sweep efficiency. Because of the low cost and good thickening ability, polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) are two main polymers for use in mobility control.9,10 However, the thickening ability of PAM and HPAM decreases sharply under harsh conditions such as high temperature or high mineralization, which limits the applicable reservoir environments of PAM and HPAM. As the easy-to-develop type I reservoirs are depleted, type II and III reservoirs, which are characterized by high temperature and salinity, become the next target; therefore, developing a chemically robust polymer for EOR is needed. Acrylamide (AM) copolymerized with proper functional monomers can yield an acrylamide-based copolymer having better temperature resistance, salt tolerance, and antishearing performance.11−15 Hydrophobically associating polymers (HAP) is a kind of modified acrylamide copolymer © 2016 American Chemical Society

that can be obtained by introducing a small amount of hydrophobic groups (generally less than 2 mol %) into acrylamide-based copolymer backbone.16−18 The reversible three-dimensional network structures can be formed by the intermolecular association of the hydrophobic groups, which thus results in a significant increase in polymer solution viscosity and an excellent antishearing property.19,20 Meanwhile, the appropriate increasing of temperature and mineralization can somewhat enhance the hydrophobic association and further increase the viscosity of the polymer solution. Therefore, in terms of thickening ability, HAP significantly outperforms HPAM and PAM as a result of hydrophobic associations. In other words, the introduction of intermolecular noncovalent interactions is a promising way to develop chemically robust EOR polymers. β-Cyclodextrin (β-CD) is a torus-shaped cyclic oligomer consisting of seven glucose units linked by 1,4-α-glucosidic bonds.21 The hydrophobic interior cavity of β-CD can Received: Revised: Accepted: Published: 8679

May 10, 2016 July 5, 2016 July 18, 2016 July 18, 2016 DOI: 10.1021/acs.iecr.6b01793 Ind. Eng. Chem. Res. 2016, 55, 8679−8689

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Industrial & Engineering Chemistry Research

Figure 1. Schematic representation of synthesis of AM/AA/N-ADA/N-β-CD.

2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals, acrylamide (AM), β-CD, acrylic acid (AA), 4-toluene sulfonyl chloride (TsCl), sodium hydroxide (NaOH), acetone, N,N-dimethylformamide (DMF), tetrahydrofuran (THF), chloroform, acetonitrile, ammonium hydroxide (25 wt %), triethylamine (TEA), ammonium persulfate ((NH 4 ) 2 S 2 O 8 ), sodium hydrogen sulfite (NaHSO3), and sodium dodecyl sulfate (SDS) were of analytical reagent grade and all purchased from Kelong chemical reagent factory (Chengdu, China). Acryloyl chloride and 1-adamantanamine were of analytical reagent grade and purchased from Aladdin Bio-Chem Technology Co., LTD (Shanghai, China). HPAM (hydrolysis degree, 25%; Mw= 1.6 × 107) was provided by Hengju Chemical (Beijing, China). All the chemicals were used without further purification. Water was double deionized with a Millipore Milli-Q system to produce the 18 MΩ deionized water. Crude oil was obtained from Dagang Oilfield (China). The viscosity and density of crude oil were 5.2 mPa·s (90 °C) and 0.84 g/cm3. 2.2. Synthesis of N-β-CD and N-ADA. N-β-CD was synthesized according to previous reports.26,31 A 60 g sample of β-CD (52.9 mmol) was suspended in 500 mL of water, and 6.57 g of NaOH in 20 mL of water was added dropwise. After β-CD was totally dissolved, 10.1 g of TsCl (53.0 mmol) in 30 mL of acetonitrile was added dropwise. After the mixture was stirred at room temperature for 2 h, the precipitate was removed by suction filtration and the filtrate refrigerated overnight at 4 °C. The resulting white precipitate was recovered by suction filtration and dried under vacuum oven at 60 °C for 12 h. After that, the white precipitate was dissolved in excess ammonium hydroxide (50 mL, 0.32 mol) and stirring at 60 °C for 48 h. Then the solution was poured into a large amount of acetone, and a white powder was obtained. The white powder (6-deoxy-6-amino-β-CD) was recovered by suction filtration and dried for 12 h. A 2 g sample of 6-deoxy-6-amino-β-CD (1.76 mmol) was completely dissolved by the addition of an appropriate amount of DMF in ice bath, and 0.25 mL of TEA (1.8 mmol) was added as a catalyst; then, 0.15 mL of acryloyl chloride (1.85 mmol) in 5 mL of DMF was added dropwise. The solution was stirred for another 2 h in the cold and subsequently at room temperature for 12 h. The precipitate was filtered, and the solution was poured into a large amount of acetone; then, a faint yellow powder was obtained. The faint yellow powder was recovered by suction filtration and washed with acetone several times. The target product (N-β-CD), a faint yellow powder, was obtained and dried under vacuum oven at 40 °C for 24 h.

selectively incorporate the hydrophobic molecules of appropriate size to generate host−guest inclusion complexes.22−24 The host−guest interactions involve various noncovalent interactions covering hydrophobicity, van der Waals forces, dispersive forces, dipole−dipole interactions, electrostatic forces, and hydrogen bonding;25 thus, the polymer network can be constructed by host−guest interactions. Previous studies25,26 also indicated that introducing the β-CD group to HAP can generate host−guest interactions and thus improves the temperature resistance, salt tolerance, and antishearing performance of the polymer. Nevertheless, it is worth noting that as a host β-CD has different binding ability with different guest molecules, which is related to the size and polarity of guest molecules.22 The host−guest inclusion complexes can be formed effectively and are more stable when the binding constant is higher. The common hydrophobic groups of HAP are aliphatic chain (C12 ∼ C18) and benzene, and the binding constants for β-CD/hexadecyl and β-CD/benzyl are about 8.86 × 104 M−1 and 2.63 × 104 M−1, respectively.27−29 So using a group that has a higher binding constant with β-CD as the hydrophobic group of HAP can produce stronger host−guest interactions, which can further improve the thickening ability of the polymer under harsh conditions. Adamantane (ADA) is a polycyclic hydrocarbon with a cage-like symmetric structure and high stability. As one of the most investigated pairs, ADA and β-CD have strong binding ability with a binding constant around 1 × 105 M−1 in water.22 Besides, unlike common hydrophobic groups of HAP, ADA has a three-dimensional rigid structure, which can increase the rigidity of the polymer chain and hinder thermal degradation;30 therefore, introducing ADA to a polymer contributes to improving the stability of the polymer. Although introducing ADA and β-CD simultaneously to acrylamide polymers seems to be a potential way to produce a chemically robust polymer, reports on synthesizing an ADA and β-CD modified copolymer for EOR are rare. In this work, an ADA and β-CD modified acrylamide copolymer AM/AA/N-ADA/N-β-CD was prepared by a free radical micellar polymerization method. The structure of the copolymer was determined by Fourier transfrom infrared (FTIR) spectroscopy, 1H NMR, and scanning elecron microscopy (SEM). The thermal property of the copolymer was characterized by TGA. The solution properties of AM/AA/ N-ADA/N-β-CD including temperature resistance, salt tolerance, antishearing performance, and viscoelasticity were evaluated. In addition, core flooding experiments were employed to evaluate the displacement performance of AM/ AA/N-ADA/N-β-CD. 8680

DOI: 10.1021/acs.iecr.6b01793 Ind. Eng. Chem. Res. 2016, 55, 8679−8689

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the intercept is not the same point, then the average of the intercept is taken. 2.6. Evaluation of Solution Properties. The solution properties of copolymer AM/AA/N-ADA/N-β-CD including salt tolerance and temperature resistance were evaluated by viscosity measurement method. The shear degradation of polymer samples was evaluated by the Waring blender method. The shear degradation could be quantified by the viscosity loss (percentage), which is given in eq 5: μ −μ × 100% Shear Degradation = o μo (5)

N-ADA was also synthesized according to a previously reported method.32 Briefly, 0.76 g (5.0 mmol) of 1-adamantylamine and 0.77 mL (5.5 mmol) of TEA were dissolved in 50 mL of THF in ice bath. After that, 0.45 mL (5.5 mmol) of acryloyl chloride in 10 mL of THF were added dropwise. The mixture was stirred for another 2 h in the cold and subsequently at room temperature for 12 h. The precipitate was filtered, and the solvent was removed by rotary evaporation. The white solid was purified by flash chromatography in chloroform/acetone v/ v 25/1 and dried under vacuum oven at 40 °C. Then the target product (N-ADA) was obtained. 2.3. Synthesis of AM/AA/N-ADA/N-β-CD. An 8 g sample of AM, 2 g of AA, 0.05 g of N-ADA, 0.06 g of N-β-CD, and 0.4 g of SDS were dissolved in 30 g of distilled water and placed in the flask. NaOH was used to adjust the pH value of the reaction solution around 7. Then the initiator APS/NaHSO3 (molar ratio 1:1) was added with the concentration 0.08 wt % relative to the total monomer feed. After being stirred under nitrogen atmosphere for 30 min, the solution was heated to 40 °C for 4 h. Ultimately, the polymeric colloid was obtained and purified with ethanol several times. The solid was further dried under vacuum oven at 50 °C for 48 h. A copolymer AM/AA/N-ADA used for comparison was also synthesized under the experimental conditions and purification methods identical to those mentioned above. The synthetic process of AM/AA/NADA/N-β-CD and AM/AA/N-ADA are shown in Figure 1. 2.4. Characterization. FT-IR spectra of N-β-CD, N-ADA, AM/AA/N-ADA, and AM/AA/N-ADA/N-β-CD were obtained using a Nicolet Nexus 170SX Fourier transform infrared spectrophotometer on a KBr tablet. The 1H NMR spectrum of AM/AA/N-ADA/N-β-CD was recorded by a Bruker ASCEND-400 NMR spectrometer. The thermal properties of AM/AA/N-ADA/N-β-CD were investigated by TGA, which was carried out on an STA 449F3 synchronal thermal analyzer at a heating rate of 10 °C/min under N2 atmosphere over a temperature range from 40 °C up to 700 °C at a constant flow rate of 60 mL/min. The morphologies of polymer samples were observed by Quanta 450 SEM. 2.5. Measurement of the Intrinsic Viscosity. The intrinsic viscosity (η) of the polymers was determined by Ubbelohde capillary viscometer in 1 M NaCl aqueous solution at room temperature. Every solution was repeatedly measured three times, and the difference among each time was not more than 0.2 s. The final flow times were determined by the arithmetic mean values. The viscosity-average molecular weight of polymer samples was calculated by the Mark−Houwink equation,33 which is given in the following equations: ηsp = (t − t0)/t0

(1)

ηr = t /t0

(2)

[η] =

H C

[η] = 4.75 × 10−3 M η 0.80

where μo is the original viscosity of the fresh polymer solution and μ is the viscosity of the polymer solution after shearing. All of the viscosities were measured with a Brookfield DV-III viscometer at 7.34 s−1. Rheology and viscoelasticity of polymer samples were determined by Anton Paar GmbH MCR302 rheometer. As one of the most widely used polymers in polymer flooding, HPAM was used for comparison with AM/ AA/N-ADA/N-β-CD. The copolymer AM/AA/N-ADA, which was synthesized under the same experimental conditions and purification methods as AM/AA/N-ADA/N-β-CD, was also used for comparison. All the polymer solution concentrations in this section kept the constant of 2000 mg/L. 2.7. Core Flooding Experiment. Core flooding experiments were carried out to investigate the mobility control ability and the EOR ability of the polymer. Four artificial sandstone cores were employed for the experiments, and the basic parameters are listed in Table 1. Table 2 presents the Table 1. Basic Parameters of Sandstone Cores parameters of sandstone core core no.

diameter (cm)

length (cm)

porosity (%)

permeability (mD)

1 2 3 4

3.804 3.801 3.806 3.810

7.692 7.712 7.664 7.684

18.93 19.16 19.23 19.42

884 917 842 826

Table 2. Synthetic Brine Composition inorganic salts concentrations (mg/L) NaCl

CaCl2

MgCl2

Na2SO4

total concentration

60000

500

500

100

61100

composition of synthetic brine used in the experiments. Resistance factor (RF) and residual resistance factor (RRF) were used to characterize the polymer mobility control ability. RF and RRF were calculated using eqs 6 and 7, respectively. RF =

K w /μw

(3) (4)

RRF =

where ηsp is the specific viscosity; ηr is the relative viscosity; t is the flow time for polymer solution, s; t0 is the flow time for 1 M NaCl solution, s; C is the concentration of polymer solution, g/ mL; [η] is the intrinsic viscosity, mL/g; and Mη is the viscosityaverage molecular weight, g/mol. By plotting ηsp/C and ln ηr/C of polymer solution against concentration, extrapolating to infinite dilution, and taking the intercept, we determine H; if

K p/μp

(6)

K wb K wa

(7)

where Kw is the aqueous phase permeability, mD; Kp is the polymer phase permeability, mD; μw is the aqueous viscosity, mPa·s; μp is the polymer phase viscosity, mPa·s; Kwb is the aqueous phase permeability before polymer flooding, mD; and Kwa is the aqueous phase permeability after polymer flooding, mD. 8681


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Industrial & Engineering Chemistry Research The enhanced oil recovery for polymer flooding was calculated using eq 8: EOR = Et − Ew

were also observed, which indicated that N-ADA and N-β-CD had been successfully incorporated into AM/AA/N-ADA. As a result, a conclusion can be reached that the copolymer AM/ AA/N-ADA/N-β-CD was synthesized successfully. The 1H NMR spectrum of AM/AA/N-ADA/N-β-CD is shown in Figure 3. The peak located at chemical shift of 4.98 ppm was associated with glucose unit protons of β-CD (O− CH−O), and the signals located at broad chemical shifts in the region of 3.68−3.79 ppm were attributed to the inner −CH− and −CH2− protons of β-CD. The signals at 1.59−1.62 ppm were due to the proton peaks of −CH2− consisting in ADA and the main chain of copolymer. The proton signals of −CH− in ADA and copolymer main chain appeared at 2.14 ppm. Consequently, the 1H NMR spectrum indicates that the target copolymer AM/AA/N-ADA/N-β-CD was synthesized successfully, which is in accordance with that revealed by the FT-IR spectra. The thermal gravimetric curve of AM/AA/N-ADA/N-β-CD is shown in Figure 4. When the TGA data is combined with DSC data, it could be found that AM/AA/N-ADA/N-β-CD displayed three stages for the weight loss. The first stage took place in the range of 40−214 °C with a mass loss of 5.91 wt %, which was ascribed to the evaporation of intramolecular and intermolecular moisture. The second stage occurred in the range of 214−392 °C with a mass loss of 40.49 wt %, corresponding to the decomposition of amide groups, β-CD groups, and ADA groups. The last one occurred beyond 392 °C with a mass loss of 35.52 wt %, which was attributed to the carbonization. Results of TGA show that AM/AA/N-ADA/Nβ-CD has favorable thermal stability. The morphology of AM/AA/N-ADA/N-β-CD, AM/AA/NADA, and HPAM were observed by SEM, as shown in Figure 5. The morphology of all the polymer samples presents network structures, containing connected skeletons and many cavities. It is evident that the connected skeletons of HPAM are markedly thinner than that of AM/AA/N-ADA/N-β-CD and AM/AA/ N-ADA. This may be because the hydrophobic association and host−guest interactions strengthens the connection among molecular chains, generating more robust connected skeletons. In addition, compared to the structure of AM/AA/N-ADA, AM/AA/N-ADA/N-β-CD has a more regular network structure and the shape of the cavities is more uniform. 3.2. Intrinsic Viscosity of Copolymers. As shown in Figure 6, the intrinsic viscosity (η) of AM/AA/N-ADA/N-βCD and AM/AA/N-ADA were determined as 986.64 and 1300.03 mL/g, respectively. Through the Mark−Houwink equation, the viscosity-average molecular weight of AM/AA/NADA/N-β-CD and AM/AA/N-ADA were determined as 4.43 × 106 and 6.26 × 106 g/mol, respectively. 3.3. Solution Properties of AM/AA/N-ADA/N-β-CD. 3.3.1. Viscosification Property of AM/AA/N-ADA/N-β-CD. The viscosification property of polymer samples in dilute aqueous solution was investigated at the temperature of 25 °C and the shear rate of 7.34 s−1. As shown in Figure 7, the viscosity of all polymer solutions increases as the polymer concentration increases. The viscosity of AM/AA/N-ADA solution increases slightly as the polymer concentration increases from 250 to 1250 mg/L, and then the viscosity increases sharply as the polymer concentration increases from 1250 to 2000 mg/L, which exhibits typical viscometric behavior of HAP. The critical association concentration (CAC) of AM/AA/N-ADA is 1250 mg/L. When the concentration is higher than the CAC, the

(8)

where Et is the total oil recovery in entire flooding process, %, and Ew is the initial water flooding recovery prior to polymer flooding, %. Core flooding experiments for investigating polymer EOR ability were carried out in accordance with the following steps: First, the core was saturated with brine and then crude oil was injected into the core until no water went out at the outlet. Second, water flooding was conducted until the water cut reached 98%. Finally, a 0.3 PV polymer slug was injected into the core followed by extended water flooding until the water cut reached 98%.

3. RESULTS AND DISCUSSION 3.1. Characterizations. Figure 2 shows the FT-IR spectra of N-β-CD, N-ADA, AM/AA/N-ADA, and AM/AA/N-ADA/

Figure 2. FT-IR spectra of (a) N-β-CD, (b) N-ADA, (c) AM/AA/NADA, and (d) AM/AA/N-ADA/N-β-CD.

N-β-CD. From the curve of N-β-CD, the bands observed at 3414, 2924, 1672, 1615, and 1155 cm−1 were attributed to the stretching vibration peaks of −OH and N−H, −CH2−, CO, CC, and C−O−C, respectively. The absorption peak at 1525 cm−1 was attributed to the bending vibration of N−H in the −CONH− group. The absorption peaks at 1030 and 586 cm−1 corresponded to the stretching vibration of C−O in the −C− OH group and the skeleton vibration of β-CD, respectively. In the curve of N-ADA, The bands observed at 3410, 2907, 1667, and 1612 cm−1 were assigned to the stretching vibration peaks of N−H, −CH2−, CO, and CC, respectively. The absorption peak at 1525 cm−1 was attributed to the bending vibration of N−H in the −CONH− group. In the curve of AM/AA/N-ADA, the characteristic absorption peaks of N-ADA at 3424, 2919, 1675, and 1558 cm−1 were present, which indicated that N-ADA had been successfully incorporated into AM/AA/N-ADA. The new absorption peak appeared at 1401 cm−1 was attributed to the stretching vibration of C−N in the −CONH2 group. From the curve of AM/AA/N-ADA/N-βCD, apart from the characteristic absorption peaks of N-ADA at 3424, 2918, 1677, 1546, and 1401 cm−1 were present, new characteristic absorption peaks at 1121, 1082, and 591 cm−1 8682


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Figure 3. 1H NMR spectrum of AM/AA/N-ADA/N-β-CD.

is very useful in practical application because polymer solution will be continuously diluted and absorbed after being injected underground. Hence, the polymer can maintain favorable thickening ability at relatively low concentration, which contributes to an effective mobility control role for a long period. 3.3.2. Antishearing Performance. Figure 8 plots the shear degradation of polymer samples as a function of shear time. The shear degradation of polymer samples increases sharply with the increase of shearing time at first and then reaches a plateau region. HPAM experienced approximately 84.8% of viscosity loss after shearing 10 min, while AM/AA/N-ADA and AM/AA/N-ADA/N-β-CD suffered about 59.5% and 44.5% viscosity loss, respectively. The shear degradation of HPAM mainly results from polymer chain breakage. The hydrodynamic volume of HPAM decreases as the result of polymer chain breakage, thus reducing the viscosity. The effect of shear on AM/AA/N-ADA and AM/AA/N-ADA/N-β-CD is not only the polymer chains breakage but also the dissociation of the interconnected hydrophobic groups (ADA groups) and the βCD/ADA inclusion complexes among polymer chains. The viscosity of AM/AA/N-ADA and AM/AA/N-ADA/N-β-CD can somewhat increase again after shearing as the result of reassociation of the hydrophobic groups and the β-CD/ADA complexes. Therefore, the viscosity loss of AM/AA/N-ADA and AM/AA/N-ADA/N-β-CD is lower than that of HPAM. In addition, the shear degradation of AM/AA/N-ADA/N-β-CD is lower than that of AM/AA/N-ADA, which may be related to

Figure 4. Thermal gravimetric curve of AM/AA/N-ADA/N-β-CD.

intermolecular hydrophobic association will be significantly improved and thus promotes the construction of reversible three-dimensional network structures, resulting in a higher hydrodynamic volume and higher viscosity. The AM/AA/NADA/N-β-CD, which simultaneously contains ADA groups and β-CD groups, also exhibited the same viscometric behavior as HAP. The CAC of AM/AA/N-ADA/N-β-CD is 1000 mg/L, which is lower than that of AM/AA/N-ADA evidently, indicating intermolecular host−guest interactions can be generated more easily than intermolecular hydrophobic association. This characteristic of AM/AA/N-ADA/N-β-CD 8683


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Figure 6. Relationship between ηsp/C (ln ηr/c) and polymer concentrations: (a) AM/AA/N-ADA and (b) AM/AA/N-ADA/N-βCD.

Figure 5. SEM morphologies of polymers in deionized water: (a) HPAM, magnified 2000 times; (b) HPAM, magnified 5000 times; (c) AM/AA/N-ADA, magnified 2000 times; (d) AM/AA/N-ADA, magnified 5000 times; (e) AM/AA/N-ADA/N-β-CD, magnified 2000 times; (f) AM/AA/N-ADA/N-β-CD, magnified 5000 times; (polymer concentration, 2000 mg/L).

the better shear resistance of polymer chains of AM/AA/NADA/N-β-CD. Figure 9 illustrates the relationship of AM/AA/N-ADA/N-βCD viscosity and shear stress versus shear rate. AM/AA/NADA/N-β-CD shows shear-thinning behavior, and the relationship of shear stress versus shear rate can be described using the power-law model: τ = K ·γ ṅ

where τ is the shear stress, Pa; K is the consistency factor; γ̇ is the shear rate, s−1; and n is the rheologic index. The rheologic index of AM/AA/N-ADA/N-β-CD is 0.43, indicating pseudoplastic fluid characteristic. The disentanglement of polymer chains at high shear rates is not the only reason for the pseudoplasticity of AM/AA/N-ADA/N-β-CD. The pseudoplasticity of AM/AA/N-ADA/N-β-CD may also result from the rapid “association” and “dissociation” of the ADA groups from and to the β-CD cavities under the effect of a flow field. At high shear rates, the nonbonding associations are unlocked (dissociated) causing viscosity decrease, which makes the AM/AA/N-ADA/N-β-CD easier to inject into the formation rock at the wellbore, while at low shear rates, for

Figure 7. Relationship between apparent viscosity and polymer concentrations.

example, far from the injection well, the ADA groups are relocked (reassociated) into the β-CD cavities and the higher viscosity is regained. This dynamic association characteristic of 8684


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Figure 8. Shear degradation of polymer solutions (polymer concentration, 2000 mg/L; Waring blender rotational speed, 6000 r/ min).

Figure 11. Effect of CaCl2 concentration on viscosity of polymer solutions (polymer concentration, 2000 mg/L).

Figure 9. Effect of shear rate on viscosity of AM/AA/N-ADA/N-βCD (polymer concentration, 2000 mg/L).

Figure 12. Effect of temperature on viscosity of polymer solutions (polymer concentration, 2000 mg/L).

Figure 10. Effect of NaCl concentration on viscosity of polymer solutions (polymer concentration, 2000 mg/L).

Figure 13. Viscoelasticity of the polymers (polymer concentration, 2000 mg/L).

AM/AA/N-ADA/N-β-CD is beneficial for well site applications.

3.3.3. Salt Tolerance of AM/AA/N-ADA/N-β-CD. Curves showing the relationship between the salinity and the viscosity of polymer samples are shown in Figures 10 and 11. When the 8685


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dominates the effect on polymer solution viscosity again. As a result, the viscosity of AM/AA/N-ADA/N-β-CD and AM/AA/ N-ADA solutions decreased again after the salt-thickening effect. The retention viscosity of AM/AA/N-ADA/N-β-CD is the highest of all polymer samples, indicating superior salt tolerance for NaCl and CaCl2. This may be because the ADA group and β-CD group both own great steric hindrance that enhances the rigidity of the polymer chains, resulting in the reduction of curling of polymer chains. 3.3.4. Temperature Resistance of AM/AA/N-ADA/N-β-CD. The relationship of polymer solution viscosity versus temperature is shown in Figure 12. The viscosity of AM/AA/N-ADA initially decreases, then increases, followed by a decrease. The increase of viscosity of AM/AA/N-ADA is related to the enhancement of intermolecular hydrophobic association. The hydrophobic association behavior is an endothermic process;34,35 thus, increasing temperature in a certain range is favorable for the hydrophobic association. However, molecules’ thermal motion will become faster with the increase of temperature. Therefore, the reversible three-dimensional network structure of AM/AA/N-ADA is destructed with further temperature increase, resulting in the decline of viscosity. The relationship of polymer viscosity versus temperature for AM/ AA/N-ADA/N-β-CD and HPAM are similar; the viscosity of polymer solution decreases as the temperature increases. However, interestingly, the viscosity of AM/AA/N-ADA/N-βCD decreases slowly at the initial stage followed by a sharp decrease, but the viscosity of HPAM decreases sharply at the initial stage followed by a slight decrease. This may be related to two aspects. On one hand, as an exothermic reaction, cyclodextrin encapsulating guest process is restrained and the inclusion complexes dissociates with the increase of temperature,36,37 causing the decrease of the viscosity. However, the hydrophobic association of the ADA groups without being encapsulated by β-CD is enhanced with the increase of temperature, which restrains the decline of viscosity. As a result, the viscosity of AM/AA/N-ADA/N-β-CD decreases slowly at initial temperature increasing stage. On the other hand, the dissociation of ADA groups hydrophobic aggregates happens with further temperature increase, which results in a sharp decline of viscosity. The retention viscosity of AM/AA/NADA/N-β-CD at 90 °C is higher than that of other polymer samples. This may be because β-CD and ADA groups provide steric hindrance for the macromolecules that hinders the coiling of polymer chains at the increased temperature. 3.3.5. Viscoelasticity of AM/AA/N-ADA/N-β-CD. The viscoelasticity plays an important role in the EOR property of the polymer.38 The polymer has a pulling effect on those small oil blocks in the dead angles of formation under the action of the viscoelasticity. Therefore, improving the viscoelasticity contributes to enhancing the displacement efficiency of the polymer. As indicated in Figure 13, elastic modulus (G′) and viscous modulus (G″) show an upward trend with the increase of the frequency. Meanwhile, AM/AA/N-ADA/N-β-CD and AM/AA/N-ADA exhibit elastic modulus (G′) and viscous modulus (G″) considerably higher than those of HPAM, which could be attributed to the intermolecular chain associations through nonbonding interactions in aqueous solutions. Besides, for all polymer samples, the G′ value and the G″ value are equal at the proper frequency ( f), and the characteristic time tc (1/f) can be obtained through the reciprocal of the frequency.39 A longer tc means better elastic efficiency. Therefore, the polymer with longer tc can exhibit pulling effect more easily in porous

Figure 14. Flow behavior of polymer solutions in porous media: (a) resistance factor, RF; (b) residual resistance factor, RRF; (polymer concentration, 2000 mg/L; 90 °C).

concentration of NaCl or CaCl2 is increased, the viscosity of HPAM solution decreases significantly and then reaches a plateau region. This is because the viscosity of polymers is closely related to the hydrodynamic volume, whereas, charge neutralization constantly occurs between Na+, Ca2+, and −COO− with the increase of salinity, so the electrostatic repulsion of polymer chain reduces and leads to a reduction of hydrodynamic volume. The charge shielding effect also influenced the thickening ability of AM/AA/N-ADA/N-β-CD and AM/AA/N-ADA, resulting in a sharp decline of polymer solution viscosity at first period. However, when the concentration of NaCl or CaCl2 increased further, the saltthickening effect was observed because solution polarity is enhanced as the salt concentration increases, which leads to the enhancement of intermolecular hydrophobic association. Nevertheless, remarkably, the salt-thickening effect of AM/ AA/N-ADA/N-β-CD is not as notable as that of AM/AA/NADA. This is because only a part of ADA groups that have not formed inclusion complexes are involved in the hydrophobic association, and the ADA groups that have already formed inclusion complexes do not participate in the formation of hydrophobic aggregation. When the concentration of NaCl or CaCl2 becomes higher and higher, the charge shielding effect 8686


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Figure 15. Core flooding experiments of polymer flooding: (a) HPAM and (b) AM/AA/N-ADA/N-β-CD for EOR at 90 °C.

N-ADA/N-β-CD. The larger retention of AM/AA/N-ADA/Nβ-CD may be attributed to the multiple-layer adsorption through host−guest interactions of β-CD and ADA groups, which increased the retention of polymer chains that were not directly in contact with the rock surfaces. Figure 15 shows the performance of polymer samples in displacing oil. The water cut significantly decreased after AM/ AA/N-ADA/N-β-CD injected into the core, while the water cut slightly decreased after HPAM injected into the core, which means AM/AA/N-ADA/N-β-CD exhibits better flooding effectiveness than HPAM. More convincingly, as shown in Table 3, AM/AA/N-ADA/N-β-CD could enhance 8.58% of oil recovery ratio, which was much higher than that of HPAM (1.76%). The results further suggest that AM/AA/N-ADA/Nβ-CD has superior performance in EOR under harsh conditions.

Table 3. Results of the Enhanced Oil Recovery of Polymer Flooding core no.

polymer

3 4

HPAM AM/AA/N-ADA/ N-β-CD

polymer concentration (mg/L)

Ew (%)

Et (%)

EOR (%)

2000 2000

47.88 49.45

49.64 58.03

1.76 8.58

media. The tc of AM/AA/N-ADA/N-β-CD, AM/AA/N-ADA, and HPAM are about 15.3, 9.2, and 2.1 s, respectively, suggesting that AM/AA/N-ADA/N-β-CD has the superior elastic efficiency of all the polymer samples. 3.4. Core Flooding Experiment. Figure 14 presents the flow behavior of AM/AA/N-ADA/N-β-CD and HPAM in porous media. RF is a measure of mobility control ability and provides information on the effective viscosity of the polymer solutions in porous media. The RF values gradually increased during the injection of polymers, and AM/AA/N-ADA/N-βCD generated RF values higher than those of HPAM, which means AM/AA/N-ADA/N-β-CD exhibits higher effective viscosity and better mobility control ability than HPAM in porous media. RRF gives an indication of the porous media permeability reduction caused by polymer retention. The RRF value of AM/AA/N-ADA/N-β-CD (13.1) was more than five times higher than that of HPAM (2.3) after 2 PV of brine was injected into the cores, suggesting larger retention of AM/AA/

4. CONCLUSIONS A novel polymer AM/AA/N-ADA/N-β-CD was synthesized through free radical micellar copolymerization. The copolymer was characterized by FT-IR, 1H NMR, TGA, and SEM. Solution properties evaluation experiments prove that AM/AA/ N-ADA/N-β-CD has favorable salt tolerance, temperature resistance, shear resistance, and viscoelastic properties, which are closely related to the existence of host−guest interactions. Core flooding experiments directly demonstrate that AM/AA/ N-ADA/N-β-CD has excellent mobility control ability and can 8687


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remarkably enhance 8.58% of the oil recovery ratio. These results suggest that AM/AA/N-ADA/N-β-CD has a potential application in harsh reservoir conditions for EOR. The adaptability of AM/AA/N-ADA/N-β-CD to various harsh reservoirs in comparison to that of other salt-tolerant and temperature-resistant polymers should be further investigated.

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AUTHOR INFORMATION

Corresponding Authors

*B.W.: e-mail, [email protected]. *Y.Y.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University for supporting this research work.

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REFERENCES

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