Synthesis and evaluation of β-cyclodextrin-functionalized hydrophobically associating polyacrylamide

Abstract

Modified β-cyclodextrin and N-phenethyl-methacrylamide were utilized to react with acrylamide and acrylic acid to synthesize hydrophobically associating polyacrylamide (HMPAM) via photoinitiated free-radical micellar copolymerization. HMPAM was characterized using Fourier transform infrared (FT-IR) spectroscopy, ¹H nuclear magnetic resonance (¹H NMR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and viscometry. Compared with partially hydrolyzed polyacrylamide (HPAM), HMPAM demonstrated superior properties on aspects of thickening ability, salt tolerance, temperature resistance and oil displacement efficiency. It was found that the apparent viscosity reached a maximum at 45 °C and the viscosity retention ratio reached 87.56% at 95 °C; at a certain range of salinity, HMPAM exhibited an evident salt-thickening phenomenon; the simulative enhanced oil recovery tests illustrated that HMPAM could remarkably enhance oil recovery by 16.4% while HPAM could enhance oil recovery by 10.8%. Moreover, the viscoelasticity and surfactant compatibility of HMPAM were investigated. The results indicated that HMPAM has potential application for enhanced oil recovery.

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1. Introduction

Polymer flooding has been used for large-scale industrial applications and excellent efficiency has been obtained in the Daqing oilfield (China), Shengli oilfield (China), North Brubank oilfield (American) and Sanand oilfield (India).^1,2 Although partially hydrolyzed polyacrylamide (HPAM) exhibits excellent thickening properties, viscoelasticity and rheological behaviours, its viscosity decreases sharply with the increase of temperature and salinity.^3,4 In addition, HPAM cannot resist high shear rates and its degradation is irreversible.^5–7 For practical application, increasing HPAM concentration is needed, but the production cost would increase steeply, which would limit its popularization and application in high-temperature and high-salinity reservoirs. Thus the research and development of novel heat-resistant and salt-tolerant polymers is an important task to promote the development of enhanced oil recovery technology.

Hydrophobic groups result in hydrophobically associating polymer (HAP) clusters because of the hydrophobic effect.^8,9 In dilute solution, macromolecules exist in the form of intramolecular association, resulting in macromolecular chain curls, and a decrease in the hydrodynamic volume and intrinsic viscosity; when the concentration of HAP reaches the critical association concentration (CAC), intermolecular association generates a physical cross-linking network structure and large hydrodynamic volume.^10–12 The addition of small-molecule electrolytes increases the polarity of the solution, leading to enhanced hydrophobic association, and embodied favourable salt resistance.^13,14 Under the condition of high shear rate, the physical cross-linking network structure reversibly breaks and the solution viscosity decreases; if the shearing is reduced or eliminated, the spacial network structure reforms, showing anti-shearing properties.^15–17

Consisting of six to eight glucose units, cyclodextrins (CDs) are cyclic oligosaccharides, linked by 1,4-α glycosidic bonds, with a shape similar to a torus-shaped ring structure.^18–20 β-cyclodextrin (β-CD) contains seven glucose units.²¹ β-CD molecules contain relatively high chemical activity hydroxyl groups, which provides a great advantage and is convenient for chemically modifying β-CD.²² Since all hydroxyl groups are located in the surface of the annular body, β-CD can dissolve in water to a certain extent, but due to β-CD’s rather rigid structure which contains intra-molecular hydrogen bonds, it has poor water solubility. Because of the existence of hydrogen atoms and glycosidic oxygen bridges, the central cavity of β-CD is hydrophobic.^23–25 This special structure of external surface hydrophilicity and internal cavity hydrophobicity gives β-CD the ability to form host–guest complexes with a wide range of guest species.^25–27

The present work intends to synthesize a novel hydrophobically associating polyacrylamide (HMPAM) via photoinitiated water free-radical copolymerization, based on functional β-CD as a functional monomer to enhance the recognition and assembly capacity, and N-phenethyl-methacrylamide as a hydrophobic monomer to improve the polymer solution properties, with the aim of developing an efficient oil displacing agent adapted to high-temperature and high-salinity oilfields. The investigation was focused on the synthesis and characterization of HMPAM, using Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), ¹H nuclear magnetic resonance (¹H NMR), scanning electron microscopy (SEM), viscosity-average molecular weight, and the evaluation of HMPAM solution properties, such as water solubility, temperature resistance, salt tolerance, shear resistance, viscoelasticity, surfactant compatibility, and oil displacement efficiency in contrast with partially hydrolyzed polyacrylamide (HPAM).

2. Experimental section

2.1. Materials

Acrylamide (AM), acrylic acid (AA), sodium hydroxide (NaOH), hydrochloric acid (HCl), β-cyclodextrin (β-CD), 4-toluene sulfonyl chloride (PTSC), acetonitrile, acetone, N-methyl pyrrolidone (NMP), 1,6-hexanediamine, dicyclohexyl-carbodiimide (DCC), N,N-dimethylformamide (DMF), dichloromethane, triethylamine, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (AIBA) and methacryloyl chloride were of analytical grade, purchased from Kelong Chemical Reagent Factory (Chengdou, China). Water was double-deionized with a Millipore Milli-Q system to produce the 18 MΩ deionized water.

2.2. Preparation of hydrophobically associating polyacrylamide (HMPAM)

The synthesis of HMPAM was composed of three parts: synthesis of the hydrophobic monomer N-phenethyl-methacrylamide (NPML), synthesis of the functional monomer β-CD (Fβ-CDhen) and copolymerization of the monomers based on AM, AA, NPML and Fβ-CDhen.

Synthesis of NPML. NPML was prepared using the following procedure. Firstly, 9.07 g (0.075 mol) of phenethylamine and 20 mL of dichloromethane were added into a 250 mL three-necked round-bottomed flask equipped with a magnetic stirrer, and then 7.58 g (0.075 mol) of triethylamine as an acid-capturer was dropped into the flask. After the mixture was cooled to 0–5 °C, 9.41 g (0.09 mol) of methacryloyl chloride was added drop-wise into the flask. Secondly, the reaction mixture was stirred for an additional 5 h, then the mixture was washed with deionized water and saturated salt water three times respectively, and then, the organic phase was concentrated on a rotary evaporator to yield rufous oil (yield 9 0.78%).

Synthesis of Fβ-CDhen. Firstly, 50.0 g (0.044 mol) of β-CD and 500 mL of deionized water were added into a 1 L round flask with a mechanical stirrer, to give β-CD suspended in water, then 16.7 mL of sodium hydroxide solution (8.5 mol L⁻¹) was slowly dropped into the flask at 0–5 °C in an ice-water bath, and suspension changed to be colourless slowly, after 2 h, 10.0 g (0.052 mol) of PTSC in 34 mL of acetonitrile was added and the mixture was stirred at 22 °C for 2 h, and then, unreacted PTSC solid was removed using a Buchner funnel and the pH value of the filtrate was regulated to 8 using HCl. Secondly, the filtrate was stored in a refrigerator for 12 h at 0–3 °C, and then filtered. The obtained crude product was recrystallized three times by dissolving in 150 mL of water at 80 °C and then cooling to room temperature, the product was dried in a vacuum oven at 40 °C for 24 h, and the first intermediate was obtained, which was labelled 6-OTs-β-CD (yield 18.3%). Thirdly, 10.0 g (0.00775 mol) of 6-OTs-β-CD and 20 mL of NMP were added into a 250 mL three-necked, round-bottomed flask, after the 6-OTs-β-CD was dissolved, 3.0 g (0.0258 mol) of 1,6-hexanediamine was added, and the mixture was heated to 70 °C, subsequently, after 12 h, the solution was cooled to room temperature, and the resulting materials were obtained by recrystallizing in acetone. The second intermediate was obtained which was called β-CDhen²⁸ (yield 53.1%). Fourthly, 5.0 g (0.004 mol) of β-CDhen and 0.36 g of DCC were dissolved in 20 mL of DMF, 0.34 g (0.004 mol) of AA was added slowly to the solution at 15 °C, after 1 h, the mixture was heated to room temperature, after 8 h, the resulting material was obtained by crystallizing in acetone, and refined by washing with deionized water/acetone three times repetitively and dried in a vacuum oven at 40 °C. Functional β-CD, namely Fβ-CDhen, was synthesized under an identical procedure to that mentioned above (yield 89.6%).

Synthesis of HMPAM. HMPAM was synthesized via photoinitiated water free-radical micellar copolymerization. 14.75 g of AM, 5.0 g of AA, 0.15 g of NPML, 0.5 g of SDS and 0.1 g of Fβ-CDhen were dissolved in 60.0 g of deionized water. Sodium hydroxide solution was used to control the pH value of the solution within 6–8. The dosage of AIBA was a constant 0.1 mol% of total monomer and the percent composition of total monomer in water was 20%. Then the solution was placed in a UV device for 6 h. HMPAM was washed and extracted with ethanol to remove water, residual monomers and initiator. Finally, HMPAM was further dried in a vacuous environment at 50 °C for 24 h. The synthesis routes are shown in Scheme 1.

Scheme 1 Synthesis routes of (a) NPML, (b) Fβ-CDhen and (c) HMPAM.

2.3. Copolymer characterizations

The FT-IR spectrum of HMPAM was investigated using a Nicolet Nexus IR spectrometer on KBr in the optical range of 400–4000 cm⁻¹; the ¹H NMR spectroscopy of NPML, 6-OTS-β-CD, Fβ-CDhen and HMPAM was carried out using a Bruker A400 nuclear magnetic resonance spectrometer; the aggregating morphology of HMPAM in deionized water and saline solution was observed by ESM (USA); the thermal stability study was carried out on an STA 449F3 synchronic thermal analyzer (Germany) at a heating rate of 10 °C min⁻¹ under an Ar₂ atmosphere over a heating range of 40 °C to 700 °C at a constant flow rate of 60 mL min⁻¹.

2.4. Viscosity-average molecular weight

The viscosity-average of HMPAM can be calculated from the limiting viscosity number by employing the Mark–Houwink equation. A Ubbelohde capillary viscometer was used to measure the limiting viscosity number and reduced viscosity of the HMPAM solution using the flow time at 30 °C. All solutions were prepared by dissolving a certain amount of HMPAM in 1.0 mol L⁻¹ of NaCl solution, and the polymer concentration was adjusted by adding NaCl solution into the flask. To obtain the viscosity-average molecular weight, the following equations were necessary:^29–31

η_sp = (t − t₀)/t₀ (1)

η_sp/C = [η] + K_H[η]²C (2)

[η] = 4.75 × 10⁻³M_η^0.80 (4)

where t is the flux time of the polymer brine solution, s; t₀ is flux time of the 1 mol L⁻¹ NaCl solution, s; C is the concentration of the polymer solution, g mL⁻¹; η_sp is the specific viscosity; [η] is the limiting viscosity number, mg L⁻¹; and M_η is the viscosity-average molecular weight, g mol⁻¹.

2.5. Water solubility test

The solution conductivity measured by a DDS-11A conductivity meter can be used to evaluate the water solubility of a polymer.³² A polymer particle sample was prepared and dissolved in deionized water at room temperature (about 25 °C). The water solubility curve illustrated the relationship of conductivity versus time from the initial time that the polymer was added, to the solution conductivity stabilization.

2.6. Rheological characterization

The rheological behaviours were measured by a Hakke RheoStress 6000 rotational rheometer (Germany); the viscosification property was investigated with a constant shear rate of 7.34 s⁻¹ and a constant temperature of 25 °C; shear tolerance was studied with shear rates ranging from 0.1 s⁻¹ to 100 s⁻¹ under the temperature of 25 °C; surfactant compatibility was studied by researching the surface tension and apparent viscosity of a mixture of HMPAM and surfactant through a ring-detachment method using a ZL-3000 automated tensiometer with a precision of ±0.1 mN m⁻¹ and a rheometer under the conditions of a temperature of 60 °C and a shear rate of 7.34 s⁻¹.

2.7. Enhanced oil recovery (EOR) test

The procedure of polymer flooding conducted in the artificial sandstone core at 45 °C was as follows. The first step was the measurement of sandstone core permeability (the basic parameters of the artificial sandstone core are listed in Table 1). The second step was the saturation of the sandstone core and establishment of initial water saturation (the composition of the inorganic ions are listed in Table 2). The third step was water flooding at a flow rate of 0.5 mL min⁻¹; water was injected to displace the crude oil (viscosity 7.2 mPa s) until the water cut reached 98%. The forth step was polymer flooding, 0.3 PV polymer slug was injected after water flooding and then subsequent water flooding was conducted until the water cut reached 98%.

Table 1 Basic parameters of artificial sandstone cores

Core sample	Diameter (cm)	Length (cm)	Porosity (%)	Permeability (mD)	Polymer viscosity (mPa s) (concentration 1250 mg L^−1)		Oil saturation (%)
500-1	7.004	3.802	13.50	153.4	HPAM	36.8	72.85
500-2	6.998	3.804	14.24	140.2	HMPAM	156.2	70.69

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Table 2 Composition of water for water flooding

Ion	Na^+ + K^+	Ca^2+	Mg^2+	Cl^−	HCO3^−	SO4^2−	CO3^2−	Total
Concentration (mg L^−1)	2187	11	13	2002	2104	54	141	6512

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The oil recovery ratio by water flooding was obtained from eqn (5), the total oil recovery ratio can be calculated using eqn (6), and the oil recovery by polymer flooding (EOR) was calculated using eqn (7).^33,34

E₁ = V₁/V × 100% (5)

E₂ = V₂/V × 100% (6)

EOR = E₂ − E₁ (7)

where E₁ is the total oil recovery by water flooding, %; E₂ is the total oil recovery, %; V₁ is the oil volume flooded by water flooding, mL; V₂ is the total oil volume flooded, mL; V is the oil reserve in sand, mL.

3. Results and discussion

3.1. Characterizations

Infrared spectroscopy analysis was carried out to confirm the chemical structure. Fig. 1 shows the FT-IR spectrum of HMPAM and verifies that the monomers NPML and Fβ-CDhen were successfully introduced into the backbone of HPAM molecular chains. From the curve of HMPAM, the bands observed at 3413 cm⁻¹ and 3226 cm⁻¹ were attributed to the stretching vibrations of N–H in the –CONH₂ groups. The absorption peak at 1622 cm⁻¹ was associated with the stretching vibrations of C=O in the –CONH₂ groups. The absorption peaks at 3552 cm⁻¹ and 3466 cm⁻¹ were associated with the stretching vibrations of O–H in β-CD. The absorption peaks at 3126 cm⁻¹ and 3016 cm⁻¹ were ascribed to the stretching vibrations of C–H in the benzene rings. The absorption peaks at 1456 cm⁻¹ and 1509 cm⁻¹ corresponded to the stretching vibrations of C=C in the benzene rings. The absorption peaks at 2922 cm⁻¹ and 2859 cm⁻¹ corresponded to the stretching vibrations of C–H in –CH₂–. The absorption peaks at 1540 cm⁻¹ and 1112 cm⁻¹ were assigned to the stretching vibrations of C=O in –COO– and C–N, respectively. The band observed at 614 cm⁻¹ belonged to the skeleton vibrations of β-CD.

Fig. 1 FT-IR spectrum of HMPAM.

The ¹H NMR spectra are shown in Fig. 2, and all the resonances are indicated by the corresponding chemical shifts. Fig. 2(a) shows the ¹H NMR spectrum of NPML in CDCl₃. The chemical shift value at 7.08–7.12 ppm (a–c) was assigned to H-Ar protons. The –CH₂–CH₂– protons appear at 3.57 ppm (e) and 2.87 ppm (d). The –CH₃ protons appear at 1.9 ppm (f). The chemical shift value at 5.6 ppm (g) was due to =CH₂ protons. Fig. 2(b) shows the ¹H NMR spectrum of 6-OTS-β-CD in D₂O. The chemical shift values at 5.03 ppm (a), 3.63 ppm (b), 3.49 ppm (c), 3.58 ppm (c), 3.89 ppm (d) and 3.81 ppm (f) were ascribed to β-CD protons. The Ar-H protons appear at 7.32 ppm and 7.64 ppm. The –CH₃ protons appear at 2.35 ppm. Compared with Fig. 2(b), we found that a difference occurred at 2.76 ppm (f), 2.35 ppm (g), 1.29 ppm (h) and 2.51 ppm (i) in Fig. 2(c), which was due to the introduction of 1,6-hexanediamine. Also, the difference between Fig. 2(c) and (d) was that peaks occurred at chemical shift values of 6.2 ppm (j) and 5.6 ppm (k) owing to the introduction of C=C. Fig. 2(e) shows the ¹H NMR spectrum of HMPAM, the chemical shift value observed at 6.84 ppm was assigned to the NH₂ protons of –CONH₂; the chemical shifts of 1.59 ppm, 3.4–4.0 ppm, 2.46 ppm and 2.78 ppm were attributed to –CH₂–(CH₂)₅–NH-β-CD; the chemical shift values at about 7.33 ppm and 7.25 ppm were attributed to Ar-H; the chemical shift value at 6.02–6.06 ppm was attributed to –NH₂ of CONH₂; the protons of –CH₃ appear at 1.09 ppm. The results verified that the synthesized polymer was HMPAM.

Fig. 2 ¹H NMR spectrum of NPML (a), 6-OTs-β-CD (b), β-CDhen (c), Fβ-CDhen (d), and HMPAM (e).

The thermal stability of HMPAM was investigated by TGA, and the gravimetric curve is depicted in Fig. 3. It was evident that HMPAM had three stages of mass loss. The first step of mass loss appeared in the range of 40–164 °C with a weight loss of 3.97%, which was owing to the evaporation of intra and intermolecular moisture. The second stage occurred in the range of 164–537 °C with a weight loss of 67.06%, which was ascribed to the imine reaction of the amide groups as well as the decomposition of hydrophobic side chains and cyclodextrin groups. The third stage occurred beyond 537 °C with a mass loss of 3.40%, which was due to carbonization. As seen from the DSC curve (heat flow/initial weight versus temperature) in Fig. 3, there was a peak of heat absorption at 312 °C, which was due to the decomposition of HMPAM. The results of TGA demonstrated that HMPAM had favourable thermal stability.

Fig. 3 Thermal gravimetric curve of HMPAM.

HMPAM solutions (HMPAM concentration, 2000 mg L⁻¹) were prepared by smearing a small amount of HMPAM solution on conducting resin. Then solutions were dried by Vacuum Freezing and Drying Technology before observation. Fig. 4 shows the micromorphology of HMPAM in deionized water and NaCl solution. Among these images, Fig. 4(a) and (b) are HMPAM in deionized water, and Fig. 4(c) and (d) are HMPAM in salt solution (NaCl concentration, 2000 mg L⁻¹). It can be seen clearly that the shape of HMPAM is an irregular space network structure which could be proof of the higher thickening ability of HMPAM. At high salt solution, a mesh structure could be found, meanwhile, NaCl crystallized at the surface of the backbone of HMPAM, which is evident in Fig. 4(d). The characteristics of HMPAM in aqueous solution verified the success of synthesizing HMPAM.

Fig. 4 The SEM images of HMPAM: (a) HMPAM solution of 50 μm in deionized water, 2000×; (b) HMPAM solution of 20 μm in deionized water, 5000×; (c) HMPAM solution of 50 μm in salt solution, 2000×; (d) HMPAM solution of 20 μm in salt solution, 5000×.

3.2. Viscosity-average molecular weight

The flow times of a 1.0 mol L⁻¹ NaCl solution and a HMPAM solution were measured three times repeatedly, and the difference between each flow time was not more than 0.2 s. The ultimate flow time was the average value. By plotting the reduced viscosity of the HMPAM solution against concentration, extrapolating to infinite dilution and taking the intercept, the limiting viscosity number was determined. The limiting viscosity number and viscosity-average molecular weight of HMPAM were 1113.85 mL g⁻¹ and 5.16 × 106 g mol⁻¹, respectively.

3.3. Water solubility

The existence of the hydrophobic monomer may decline the water solubility of the hydrophobically associating polyacrylamide, which would restrict its application in oilfields. Therefore the water solubility of HMPAM should be determined.

Fig. 5 shows the changing conductivity during the process of dissolution (polymer concentration 2000 mg L⁻¹). With increasing time, the conductivity increased due to the increased quantity of polymer dissolved in deionized water, and then tended to a constant value. HPAM required 25 min to dissolve completely while HMPAM required 40 min. The water solubility of HMPAM, functionalized on NPML with a benzene ring, declined.

Fig. 5 Curves of conductivity changes in the dissolution process of the polymer.

3.4. Thickening ability

Different concentrations of HMPAM solution were prepared with deionized water, and the apparent viscosity of each solution was measured at a temperature of 25 °C and shear rate of 7.34 s⁻¹. Fig. 6 shows the relationship between apparent viscosity and concentration.

Fig. 6 The apparent viscosity for different concentrations of HMPAM.

As shown in Fig. 6, there is an obvious turning point at the concentration of 1430 mg L⁻¹, which is called the critical association concentration (CAC). When the HMPAM concentration was lower than the CAC, the HMPAM chains existed in a curled configuration with a lower hydrodynamic volume, resulting in a slow increase of the apparent viscosity of HMPAM. When the HMPAM concentration exceeded the CAC, with the increase of concentration, intermolecular hydrophobic microdomains and inclusion complexes of β-CD and hydrophobic units, were generated, which led to the formation of a supramolecule with a spatial network, manifesting a sharp increase in the apparent viscosity of HMPAM.³⁵ Compared with HPAM, HMPAM showed a superior thickening ability indicating that it would be a better oil displacement agent for enhanced oil recovery.

3.5. Shear resistance

To investigate the anti-shear behaviour of HMPAM, the dependency of shear rate on apparent viscosity is depicted in Fig. 7. At low shear rates, there was a slight rise in the apparent viscosity initially, while shear thinning appeared at high shear rates. This phenomenon could be explained by how, at low shear rates, the balance of intramolecular and intermolecular association slightly increases the apparent viscosity. While with the increase of shear rate, the network structure is disrupted and the apparent viscosity decreases evidently.

Fig. 7 Shear resistance of HPAM and HMPAM.

3.6. Viscoelasticity

The viscoelastic properties of HMPAM were investigated using a Haake Rheostress 6000 rotational rheometer. The variations of elastic modulus (G′) and viscous modulus (G′′) were curved, indicting a dependency on frequency (see Fig. 8). As shown in Fig. 8, at low frequency regions, viscosity is obviously observed when G′ is less than G′′. Then, G′ and G′′ increase simultaneously with the increase of oscillation frequency. Once G′ surpasses G′′, elastic behaviour plays the major role, which shows great potential for enhanced oil recovery.

Fig. 8 Storage (G′) and loss (G′′) modulus as a function of frequency for HPAM and HMAPM.

3.7. Temperature resistance of HMPAM

The effects of temperature on HMPAM and HPAM were investigated for a polymer concentration of 2000 mg L⁻¹ in aqueous phase at a constant shear rate of 7.34 s⁻¹, the polymer solution was heated from 40 °C to 95 °C. The viscosity retention rate refers to the residual viscosity divided by the initial viscosity multiplied by 100%. As shown in Fig. 9, for HPAM, the apparent viscosity decreases with the increase of temperature, and has a viscosity retention of 56.67% at 95 °C. For HMPAM, at low temperature, an inconspicuous increase in the HMPAM solution viscosity was observed, which was imputed to hydrophobic association and β-CD encapsulating hydrophobic chains, showing the behaviour of an endothermic process.³⁶ When the temperature exceeded 50 °C, with the increase of temperature, firstly, intermolecular association was weakened due to the rapid movement of hydrophobic units and water molecules, resulting in the reduction of apparent molecular weight; secondly, cyclodextrin inclusion complexes dissociated due to inclusion association between β-CD and hydrophobic units, which was an exothermic reaction, causing a decrease in the apparent viscosity,^37,38 while HMPAM had a viscosity retention of 87.56% at 95 °C, demonstrating that HMPAM would be a favourable agent for EOR.

Fig. 9 Effect of temperature on apparent viscosity.

3.8. Salt tolerance of HMPAM

2000 mg L⁻¹ HPAM and HMPAM solutions were prepared with NaCl, CaCl₂ and MgCl₂, respectively. Then, the apparent viscosity of each solution with different types and different concentrations of salt was measured at a temperature of 25 °C and a shear rate of 7.34 s⁻¹. The relationship of polymer apparent viscosity versus salinity is illustrated in Fig. 10.

Fig. 10 Effect of NaCl solution (a), CaCl₂ solution (b) and MgCl₂ solution (c) on apparent viscosity.

Compared with HPAM, it could be concluded that the introduction of NPML and Fβ-CDhen significantly enhanced the salt tolerance of HMPAM. On the one hand, the addition of salt enhanced the polarity of the polymer solution, contributing to the enhancement of hydrophobic association;^35,39 the introduction of rigid β-CD led to a macromolecule electric double layer which was difficult to compress;¹⁸ inclusion association between β-CD and the hydrophobic monomer increased the apparent molecular weight of HMPAM. On the other hand, carboxylate ions (–COO–) were shielded resulting in the curl of molecular chains. These effects competed with each other, leading to the unique rheological behaviour.

3.9. Surfactant compatibility of HMPAM

Different molar masses of surfactant VES-05 were added to HMPAM solutions (HMPAM concentration was 1000 mg L⁻¹). The surface tensions of VES-05 and the mixtures of HMPAM and VES-05 were measured using a tensiometer at 60 °C, and the effect of VES-05 on HMPAM viscosity was investigated under the conditions of a temperature of 60 °C and a shear rate of 7.34 s⁻¹. From Fig. 11, it is clearly seen that the addition of HMPAM evidently affects the surface tension of the solution. Compared with the VES-05 solution, HMPAM increases the equilibrium interface tension of the mixture solution of HMPAM and VES-05, and then reached a higher value. The critical micelle concentrations (CMCs) of VES-05 and the mixture of HMPAM and VES-05 were 0.13 mmol L⁻¹ and 0.37 mmol L⁻¹, respectively. The ultimate surface tension value of the mixture of HMPAM and VES-05 was lower than VES-05 due to the introduction of β-CD. Cyclodextrin could selectively release and self-assemble the surfactant, controlling the formation of micelles. In addition, the apparent viscosity of the mixture of HMPAM and VES-05 firstly increased followed by a significant decrease, and the apparent viscosity reached maximum at the CMC. The association of the surfactant with the hydrophobic units competed with intermolecular hydrophobic association. At low concentration (below the CMC), mixed micelles of surfactant and hydrophobic groups formed, leading to the formation of cross-linked microdomains; at the CMC, the maximum synergistic interactions were reached, meanwhile the apparent viscosity reached its maximum; when the concentration was above the CMC, VES-05 made the hydrophobic units soluble and the cross-linked microdomains were disrupted, leading to the decrease of apparent viscosity.

Fig. 11 Effect of VES-05 on surface tension and apparent viscosity.

3.10. EOR test

The results of HMPAM and HPAM for the EOR test are shown in Fig. 12. Injection pressure, water cut and oil recovery gradually increased during the injection of a volume of water, when the water cut reached a certain value, the pressure and oil recovery maintained a steady value, after polymer flooding, the water cut dropped quickly and then the injection pressure raised quickly. Finally, the water cut and oil recovery reached a constant value. Compared with HPAM, HMPAM could evidently enhance 16.4% of oil recovery ratio while HPAM could enhance 10.8% of oil recovery ratio. The increased EOR was due to the excellent thickening ability of HMPAM, resulting in the synergistic effect of a supramolecular structure and hydrophobic association, which could effectively increase the molecule hydrodynamic volume. The results suggest that HMPAM has potential application for enhanced oil recovery.

Fig. 12 Core flooding experiments of polymer flooding (a) HPAM, (b) HMPAM.

4. Conclusion

In this work, modified β-cyclodextrin and N-phenethyl-methacrylamide were successfully synthesized and introduced into HMPAM via photoinitiated water free-radical micellar copolymerization, and HMPAM was characterized using FT-IR spectroscopy, ¹H NMR, TGA and SEM. Compared with HPAM, HMAPM exhibited superior properties on aspects of thickening ability, salt tolerance, temperature resistance and oil displacement efficiency. The introduction of functional β-CD and N-phenethyl-methacrylamide could significantly enhance acrylamide-based polymer properties, indicating that the acquired polymer has potential technological application in high-temperature and high salinity oil fields.

Acknowledgements

We are grateful to the Open Fund (PLN1417) by the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University) for financial support of this work.

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