A novel α-aminophosphonic acid-modified acrylamide-based hydrophobic associating copolymer with superb water solubility for enhanced oil recovery

Abstract

As a non-renewable resource, the rational exploitation of oil has attracted a large amount of attention. Among many methods for enhanced oil recovery, polymer flooding is the most suitable method of chemical flooding for non-marine reservoirs and therefore various modified acrylamide-based copolymers have been studied. In this study, a novel α-aminophosphonic acid-modified hydrophobic associating copolymer was successfully synthesized by copolymerization of acrylamide, acrylic acid, N-allyldodecanamide and 1-(dimethylamino)allylphosphonic acid. The copolymer was characterized by FT-IR, ¹H NMR and thermogravimetry and exhibited superior water solubility and thickening capability. Subsequently, the shear resistance, temperature resistance and salt tolerance of the copolymer solution were investigated. The value of apparent viscosity retention of a 2000 mg L⁻¹ copolymer solution was as high as 58.55 mPa s at a shear rate of 170 s⁻¹ and remained at 40.20 mPa s at 120 °C. The values of apparent viscosity retention of 55.41 mPa s, 59.95 mPa s and 52.97 mPa s were observed in solutions of 10 000 mg L⁻¹ NaCl, 1200 mg L⁻¹ MgCl₂, and 1200 mg L⁻¹ CaCl₂, respectively. These were better than those of partially hydrolyzed polyacrylamide under the same conditions. In addition, an increase of up to 14.52% in the oil recovery rate compared with that for water flooding could be achieved in a core flooding test using a 2000 mg L⁻¹ copolymer solution at 65 °C.

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Introduction

Petroleum is one of the most important non-renewable natural resources for humans and cannot be substituted completely so far. Therefore, we have to use it efficiently and exploit residual oil as much as possible from reservoirs.¹ As a traditional indispensable research field, enhanced oil recovery (EOR) technologies such as gas flooding, chemical flooding, thermal flooding and microbial flooding have received considerable attention from researchers around the world.^2–5 Among these, chemical flooding, which includes alkaline flooding, surfactant flooding, polymer flooding and alkaline–surfactant–polymer (ASP) flooding, has been extensively studied because of its promising performance and immediate effect.^6–9

Polymer flooding is the most suitable method of chemical flooding for the non-marine reservoirs that are mainly used in China, such as the Daqing and Shengli oilfields.^10,11 A polymer solution exhibits higher viscosity than water, so that it could enhance sweep efficiency by reducing water–oil mobility to attain the aim of enhancing oil recovery.¹² Among many polymers, polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) are widely used owing to their good capacity for EOR.¹³ However, the curling, hydrolysis, degradation and even fragmentation of the molecular chains at high temperature and high salinity reservoirs would lead to a sharp decline in viscosity.^14,15 Numerous modified methods based on acrylamide copolymers have been studied accordingly. Hydrophobic associating water-soluble polymers (HAWSP), which contain a small amount of hydrophobic groups, displayed favorable thickening properties, shear resistance, thermal stability and anti-polyelectrolyte behavior.^16–20 In aqueous solution, association was produced via hydrophobic groups in the polymer chains entangled with each other, and it could increase viscosity by means of increasing the hydrodynamic dimensions of the polymers.^21,22 Nevertheless, compared with conventional water-soluble polymers, the associating action of hydrophobic associating polymers would give rise to very poor solubility with an increase in hydrophobic content and hydrophobic chain length.^23–25 Several hours or even in excess of dozens of hours have been needed when dissolving these in water. Occasionally, the phenomenon of insolubility would occur, which has led to difficulties in operation and higher costs, especially in offshore oilfields.^26,27

To some extent, the solubility of HAWSP could be improved by introducing ionic groups. These groups, which include carboxyl groups, sulfonate groups, ammonium groups and others, when dissolved in water could form a layer of hydrate film to improve the hydrophilicity of the polymers.²⁸ So far, polymers containing phosphate groups have been widely studied and employed in tissue engineering,²⁹ scale inhibitors,³⁰ corrosion inhibitors,³¹ drilling fluid additives³² and other applications for oilfields^33,34 owing to their thermal stability, bioactivity and excellent water solubility,^35,36 but they have rarely been introduced into HAWSP for EOR. In fact, a phosphate-containing monomer could also easily be incorporated into a lipophilic chain to increase its water solubility by virtue of its increased ionic character.³⁷

Here, a novel α-aminophosphonic acid-modified acrylamide-based hydrophobic associating copolymer was synthesized. This copolymer, which was named P(AM-AA-NAD-DMAAPA), was composed of acrylamide (AM), acrylic acid (AA), N-allyldodecanamide (NAD), and 1-(dimethylamino)allylphosphonic acid (DMAAPA). Among these, AM and AA acted as the backbone to provide viscosity and NAD was a functional monomer to improve the shear resistance and thermal stability of the copolymer. DMAAPA was synthesized via the Mannich reaction and was for the first time introduced into HAWSP for EOR. In our previous work, we found that phosphate groups could reduce the salt sensitivity of polymers, and now they were also used to improve their water solubility.³⁸ The structure of DMAAPA is analogous to that of amino acids, and it is environmentally friendly with low toxicity.^39,40 Owing to the fact that only small amounts of a hydrophobic monomer and phosphate monomer can dramatically improve the performance of polymers, the cost and environmental damage of copolymers can be limited. A shorter dissolution time leads to higher production efficiency, so the copolymer has high economic value and the prospect of further applications. To evaluate the potential of this new copolymer for EOR applications, properties such as the shear resistance, temperature resistance, salt tolerance and oil displacement efficiency of the copolymer solution were investigated and compared with those of P(AM-AA-NAD) and HPAM, respectively.

Experimental

Materials

Acrylamide (AM), acrylic acid (AA), dodecanoic acid, thionyl chloride (SOCl₂), allylamine, dichloromethane, triethylamine (Et₃N), alkylphenol ethoxylates (OP-10), NaOH, (NH₄)₂S₂O₈, NaHSO₃, NaCl, MgCl₂·6H₂O, CaCl₂ and ethanol were of analytical purity and were used directly. The viscosity-average molecular weight of partially hydrolyzed polyacrylamide (HPAM) was 8.0 × 10⁶ and its degree of hydrolysis was 25%. All the chemicals were purchased from Kelong Chemical Reagent Co., Ltd., Chengdu, China. 1-(Dimethylamino)allylphosphonic acid (DMAAPA) was synthesized according to the method reported in the literature.†⁴¹

Synthesis of NAD

N-Allyldodecanamide was prepared by the following procedure: 8.00 g dodecanoic acid was dissolved in 30 mL dichloromethane in a 250 mL three-necked flask, and 4.84 g SOCl₂ was slowly dropped into the flask at room temperature. Then, the reaction temperature was raised to 50 °C, maintained at that level for about 1 h, and then cooled to room temperature. The solvent and excess SOCl₂ were removed by rotary evaporation and dodecanoyl chloride was obtained. The freshly prepared dodecanoyl chloride was added dropwise to a 250 mL round-bottom flask to which had been added 2.22 g allylamine, 3.14 g Et₃N and 50 mL dichloromethane at 0–5 °C, and the mixture was left to stand for 6 h. Then, the reaction mixture was washed with concentrated alkaline liquor and allowed to stand for a while. The solution separated into two layers, of which the upper organic phase was taken, dried over Na₂SO₄, and filtered. The filtrate was evaporated under reduced pressure, and a white powder with a yield of 85.3% was produced. ¹H NMR (400 MHz, CDCl₃, δ): 0.91 (t, 3H, CH₂CH₃), 1.27–1.31 (m, 16H, CH₂(CH₂)₈CH₃), 1.64–1.67 (m, 2H, CH₂CH₂CH₂CO), 2.20 (t, 2H, CH₂CH₂CO), 3.89 (d, 2H, NHCH₂CH), 5.16 (d, 2H, CH=CH₂), 5.57 (s, 1H, CONHCH₂), 5.83–5.88 (m, 1H, CH₂CH=CH₂).

Synthesis of the copolymers

P(AM-AA-NAD-DMAAPA) was synthesized by the following steps: AM, AA, NAD, DMAAPA and a moderate amount of OP-10 were first dissolved in deionized water in a certain proportion. The pH of the system was adjusted by NaOH. The mass of the monomers was maintained at 20% of the total quantity, and then the system was transferred into a three-necked flask, which was placed in a water bath. (NH₄)₂S₂O₈ and NaHSO₃ (in a molar ratio of 1 : 1) as an initiator were added to the solution when the temperature reached the set value under nitrogen flow, and the solution left to stand for 8 h. Then, the product was purified with ethanol repeatedly and dried in a vacuum oven for at least 24 h to obtain a white powder, namely, P(AM-AA-NAD-DMAAPA). ¹H NMR (400 MHz, D₂O, δ): 0.93 (t, 3H, (CH₂)₉CH₃), 1.27–1.64 (m, 26H, CH₃(CH₂)₉CH₂ and CHCH₂CH), 1.99–2.23 (m, 12H, COCHCH₂, CH₂CHCH₂, N(CH₃)₂ and COCH₂CH₂), 2.46 (d, 1H, –CHCHCO–), 3.39 (d, 2H, NHCH₂CH), 7.66 (t, 1H, CONHCH₂).

P(AA-AM-NAD) was synthesized using similar steps. The synthetic routes to NAD, P(AM-AA-NAD) and P(AM-AA-NAD-DMAAPA) are presented in Scheme 1.

Scheme 1 Synthetic routes to (a) NAD, (b) P(AM-AA-NAD-DMAAPA), and (c) P(AM-AA-NAD).

Characterization

IR. The Fourier transform infrared (FT-IR) spectra of NAD and P(AM-AA-NAD-DMAAPA) in KBr pellets were recorded in the wavelength range of 4000–500 cm⁻¹ by a WQF-520 Fourier transform infrared spectrometer (Beijing Rayleigh Analytical Instrument Corporation, China).

¹H NMR. The ¹H NMR spectra of NAD and P(AM-AA-NAD-DMAAPA) were recorded using a 400 MHz Bruker NMR spectrometer after the samples were dissolved in a deuterated solvent (CDCl₃ and D₂O).

Conversion rate determination. The conversion of each monomer into the copolymer was measured using high-performance liquid chromatography (HPLC; Shimadzu Co., Japan) with an ODS column and a UV detector (mobile phase: CH₃OH/H₂O = 9 : 1, wavelength: 210 nm, column temperature: 40 °C, flow rate: 1.0 mL min⁻¹). The sample used in the measurement was the ethanol that was used to purify the copolymer. The conversion was calculated by the following equation:
where α is the conversion rate (%); W is the total weight of the monomer in the reaction (g); A is the chromatographic peak area of the residual monomer in ethanol; C₀ (g L⁻¹) and A₀ are the concentration of a standard solution of the monomer and its chromatographic peak area, respectively; and V is the solution volume of ethanol.

Viscosity measurements. The intrinsic viscosity ([η]) of the copolymer was measured using an automatic Ubbelohde capillary viscometer (0.6 mm, NCY, Shanghai Sikeda Scientific Instruments, Inc., China) at 30 °C. The apparent viscosity of the copolymer solution was measured by a Brookfield DV-III programmable rheometer (Brookfield Co., America) with a 00# (6 rpm) or 61# (18.2 rpm) rotor.

Gel permeation chromatography. The weight-average molecular weight (M_w) and number-average molecular weight (M_n) were determined by gel permeation chromatography (GPC). The measurement was carried out on an Agilent 1100 apparatus with a refractive index detector at 35 °C. NaNO₃ aqueous solution (1.0 mol L⁻¹) as the eluent and polyethylene oxide as a standard were employed. The flow rate of the eluent was 1.0 mL min⁻¹, and the concentration of the copolymer solution was 1000 mg L⁻¹.

Water solubility

The copolymer (40–45 mesh, 200 mg) was added to 100 mL deionized water and stirred with a magnetic stirrer at 25 °C. The solution state was originally observed by the naked eye, and then the change in conductivity was measured using a conductivity meter (DSS-11A, Shanghai Rex Xinjing Instrument Co., Ltd.). The dissolution time was recorded when the conductivity remained unchanged. Each experiment was repeated three times.

Thermogravimetry (TG)

The thermal stability of 4.0452 mg P(AM-AA-NAD-DMAAPA) was tested by a TGA/SDTA851e thermogravimetric analyzer (Mettler Toledo Co., Switzerland) from 25 to 800 °C in a nitrogen atmosphere (flow rate: 50 mL min⁻¹). The heating rate was 10 °C min⁻¹. Then, TG and DTG curves were obtained to determine the thermal stability of the copolymer.

Pyrene fluorescence probe

To calculate the critical association concentration (CAC) of the copolymer, the relationship between the viscosity and concentration was first investigated. Then, a precise investigation was carried out using a pyrene fluorescence probe. The fluorescence intensities were measured by a spectrofluorophotometer (Shimadzu RF-5301PC, Japan). The emission spectra were in the range of 350–550 nm and the excitation wavelength was 335 nm. The slit widths for both excitation and emission were fixed at 5 nm. Different concentrations of the copolymer solution with pyrene were prepared with deionized water, and the concentration of pyrene was 5 × 10⁻⁶ mol L⁻¹. The ratios between the intensity of the first peak (I₁) and that of the third peak (I₃) were calculated.

Rheological experiments

The shear thinning behavior of a 2000 mg L⁻¹ copolymer solution was investigated at shear rates ranging from 7.34 s⁻¹ to 170 s⁻¹ at 25 °C. Shear recovery behavior of the 2000 mg L⁻¹ copolymer solution was observed when the shear rate was increased from 170 s⁻¹ to 510 s⁻¹ and then reduced again to 170 s⁻¹ at 25 °C. The temperature resistance of a 2000 mg L⁻¹ copolymer solution was studied from 30 °C to 120 °C using standard silicone oil as the heat transfer medium at a shear rate of 170 s⁻¹. All these properties were measured by a rotational rheometer (Haake RheoStress 6000).

Salt tolerance and aging test

The salt tolerance was determined by the change in apparent viscosity in different concentrations of NaCl, MgCl₂ or CaCl₂ solution at 25 °C. An aging test was carried out via the variation in apparent viscosity of the copolymer solution. The sample had been kept in a water bath for 240 h at 65 °C and was tested once every 24 h.

Core flooding test

The EOR ability of a 2000 mg L⁻¹ copolymer solution was investigated by a core flooding test. A stainless-steel cylinder with a length of 250 mm and an internal diameter of 25 mm was employed as the core assembly. It was packed with strongly water-wet quartz sand (80 mesh), which was obtained from the Shengli oilfield in China. The sand was washed with hydrochloric acid and then soaked in a large amount of water until the pH reached neutrality. The permeability (K_w) and porosity of the core assembly were determined by injecting saline water at a volume flow of 3.0 mL min⁻¹. Then, crude oil was injected into the core assembly, which was aged for 48 h at 65 °C. The density and apparent viscosity of the simulated crude oil (obtained from the Shengli oilfield) were, respectively, 0.90 g cm⁻³ and 70.3 mPa s at 65 °C. Water flooding was first carried out at 0.3 mL min⁻¹ until the water content reached 95%. Then, the copolymer, which was dissolved in mineralized water (the degree of mineralization was 5210 mg L⁻¹ ([Na⁺] = 1770 mg L⁻¹, [Ca²⁺] = 102 mg L⁻¹, [Mg²⁺] = 17 mg L⁻¹, [Cl⁻] = 2270 mg L⁻¹, [CO₃²⁻] = 71 mg L⁻¹, [HCO₃⁻] = 749 mg L⁻¹, [SO₄²⁻] = 231 mg L⁻¹)), was injected into the core assembly at 0.3 mL min⁻¹. Finally, water flooding was carried out again until the water content reached 95%. The EOR value was calculated by formula (1):

EOR = E₁ − E₂ (1)

where EOR is the enhanced oil recovery (%), E₁ is the polymer flooding recovery (%), and E₂ is the water flooding recovery (%).

The residual resistance factor (RRF) was calculated by the following formula:

RRF = K_wb/K_wa (2)

where K_wa is the permeability of the aqueous phase in the subsequent water injection process (md) and K_wb is the permeability of the aqueous phase in the water injection process before polymer flooding (md).

Results and discussion

Synthesis and characterization

The effects of the reaction conditions on the apparent viscosity of P(AM-AA-NAD-DMAAPA) were investigated via a single-variable method (for details, see Tables S1 and S2 in the ESI material†), and the optimal synthesis conditions are listed in Table 1. The structures of NAD and P(AM-AA-NAD-DMAAPA) were determined by ¹H NMR spectra (Fig. 2). The values of [η], M_w, M_n and the apparent viscosity of P(AM-AA-NAD-DMAAPA) are listed in Table 2.

Table 1 Optimum synthesis conditions for P(AM-AA-NAD-DMAAPA)

Mass ratio of monomer (%)				Mass ratio of initiator (%)	pH	Temperature (°C)
AM	AA	NAD	DMAAPA
60.0	39.0	0.8	0.2	0.3	7	45

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Table 2 Values of [η], M_w and M_n of each copolymer

Copolymer	Mw (×10^6)	Mn (×10^6)	Mw/Mn	[η] (mL g^−1)	Apparent viscosity^a (mPa s)
a The concentration of each copolymer solution was 2000 mg L^−1.
P(AM-AA-NAD-DMAAPA)	5.2	2.9	1.8	1073.23	534.4
P(AM-AA-NAD)	5.0	2.8	1.8	1006.80	506.5

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Composition of copolymer. In order to determine the content of the soluble fraction, the proportion of each component was calculated from the conversion rate. Both the feed ratio and the calculated final percentages by mass of the monomers in the copolymer are listed in Table 3. The calibration curves of the monomers are A_AM = 1 638 700 + 4.58547 × 10⁷ × C, A_AA = 924 454.32386 + 1.36313 × 10⁷ × C, and A_NAD = 241 867.58691 + 2.24428 × 10⁶ × C, respectively. It follows that the conversion rates of AM, AA, and NAD are 99.43%, 98.10% and 83.96%, respectively.

Table 3 Composition of P(AM-AA-NAD-DMAAPA)

Copolymer	Feed ratio/wt%				Final composition^a/wt%
	AM	AA	NAD	DMAAPA	AM	AA	NAD	DMAAPA
a The mass of the purified copolymer was 9.8750 g.
P(AM-AA-NAD-DMAAPA)	60.0	39.0	0.8	0.2	60.41	38.74	0.68	0.17

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IR. The FT-IR spectra of NAD and P(AM-AA-NAD-DMAAPA) are shown in Fig. 1. For NAD, there were strong absorption peaks at 3298 cm⁻¹ and 1645 cm⁻¹ because of the stretching vibrations of N–H and C=O. The characteristic absorption of C=C was present at 1557 cm⁻¹. The peaks at 2916 cm⁻¹ and 2841 cm⁻¹ were assigned to the vibrations of –CH₂ in the allyl group and –CH₃, and the peak at 716 cm⁻¹ was assigned to the absorption of –(CH₂)_n–. For the spectrum of P(AM-AA-NAD-DMAAPA), the stretching absorption vibrations of O–H, N–H, and C=O, respectively, appeared at 3417 cm⁻¹, 3198 cm⁻¹ and 1678 cm⁻¹, which indicated that the structures of AA and AM were already present in the copolymer. The peak for P=O, which was present at 1112 cm⁻¹, showed that the DMAAPA structure was present in the molecular chain. This has confirmed the successful copolymerization of AM, AA, NAD and DMAAPA.

Fig. 1 IR spectra of NAD and P(AM-AA-NAD-DMAAPA).

Fig. 2 ¹H NMR spectra of (a) NAD and (b) P(AM-AA-NAD-DMAAPA).

Dissolution time

The content of hydrophobic groups can significantly affect the water solubility of copolymers. The dissolution times of copolymers containing different lengths of hydrophobic chain, which had previously been measured, were therefore compared to determine their water solubility. The results are shown in Fig. 3. (The data are listed in Table S3 in the ESI material.†) It could be concluded that these hydrophobic associating polymers required a lot of time to dissolve. The dissolution time of these copolymers generally increased with an increase in hydrophobic chain content and the introduction of a cyclic structure. P(AM-AA-NAD-DMAAPA) only required 0.75 h to dissolve, which was remarkably less time than other copolymers, owing to the introduction of the hydrophilic group. A possible dissolution process is shown in Fig. 4. An interaction between phosphate groups and water molecules was generated via hydrogen bonds and ionic bonds in II; then, a layer of hydrate film was formed on the surface of the phosphate groups and gradually expanded in III; finally, the molecular chains dispersed evenly in water in IV. The results proved that the phosphate group greatly improved the water solubility of the hydrophobic associating polymer.

Fig. 3 Dissolution time of copolymers. (a) NIMA is the abbreviation for 3-(2-(2-heptadec-8-enyl-4,5-dihydroimidazol-1-yl)ethylcarbamoyl)acrylic acid.⁴² (b) AP-P4, which has been used in oilfields, is a copolymer of AM, AA and N-(2-(acryloyloxy)ethyl)-N,N-dimethylhexadecan-1-aminium bromide. (c) NAE is the abbreviation for N-allyloctadec-9-enamide.⁴³ (d) DBDAP is the abbreviation for N,N-diallyl-2-dodecylbenzenesulfonamide.⁴⁴ (e) and (f) were synthetized in this work.

Fig. 4 Solubility behavior of P(AM-AA-NAD-DMAAPA).

Thermal stability

The thermogravimetric curve of P(AM-AA-NAD-DMAAPA) was shown in Fig. 5. It could be divided into three stages of weight loss. The first stage occurred in the range of 25–199 °C with a mass loss of 16.67%, which was due to intramolecular and intermolecular evaporation of moisture. The copolymer contained a number of strongly hydrophilic groups, which could cause the sample to take in water.²⁰ The second stage occurred at 199–603 °C, where the mass loss was 50.38%. Three peaks accordingly appeared at 280 °C, 331 °C and 434 °C in the DTG curve. The breaking of carbon–hydrogen, carbon–oxygen and carbon–nitrogen bonds and the decomposition of phosphate groups occurred during this process. The third stage took place at temperatures above 603 °C, where there was a mass loss of 16.13%, which could be attributed to carbonization of the copolymer. The result indicated that P(AM-AA-NAD-DMAAPA) has outstanding thermal stability and is not easy to degrade at higher temperatures.

Fig. 5 Thermogravimetric and derivative thermogravimetric curves of P(AM-AA-NAD-DMAAPA).

Critical association concentration

Hydrophobic association is the most important characteristic of hydrophobic associating polymers for increasing the viscosity of the polymer solution. Therefore, it is essential to study the critical association concentration (CAC) of P(AM-AA-NAD-DMAAPA) using a pyrene fluorescence probe. This is shown in Fig. 6a. There are five absorption peaks located, respectively, at 373, 379, 385, 395 and 410 nm that need to be studied. The ratio between the intensity of the first peak (I₁) and the intensity of the third peak (I₃) at different concentrations of the copolymer solution could be determined to estimate the break point in Fig. 6b. The values of I₁/I₃ fell slowly when the concentration of the copolymer solution was less than 400 mg L⁻¹ and began to decrease sharply when the concentration increased from 400 mg L⁻¹ to 2000 mg L⁻¹. The obvious decline in the value of I₁/I₃ meant that a strongly hydrophobic association was formed and the pyrene molecules were transferred from water to the hydrophobic microdomains.⁴⁵

Fig. 6 Hydrophobic associating behavior of P(AM-AA-NAD-DMAAPA) solution: (a) fluorescence emission spectra at different concentrations of the copolymer solution at 25 °C, (b) values of I₁/I₃ for the emission of pyrene at different concentrations of the copolymer solution, and (c) apparent viscosity of various concentrations of the copolymer solution at a shear rate of 7.34 s⁻¹ in deionized water and the inset is an enlarged drawing.

It can be concluded that hydrophobic interaction would visibly occur when the concentration of P(AM-AA-NAD-DMAAPA) was above 370 mg L⁻¹. This result is close to the change in tendency of the viscosity–concentration curve in Fig. 6c. There is a critical point on this curve, which is the so-called critical association concentration. The reason for this phenomenon is a combination of intramolecular and intermolecular association. The molecular chains will form a supramolecular network primarily based on intermolecular association when the solution reaches a certain concentration. The structure of the network can increase the viscosity of the copolymer solution via increasing the hydrodynamic volume.

Effect of shear rate on apparent viscosity

The shear thinning behavior of an aqueous solution of the copolymers (2000 mg L⁻¹) was investigated, as illustrated in Fig. 7a. It could be found that the copolymers displayed non-Newtonian and shear-thinning behavior. The viscosity of the copolymer solutions fell markedly with an increase in the shear rate. P(AM-AA-NAD-DMAAPA) and P(AM-AA-NAD) had better values of viscosity retention (58.55 and 52.43 mPa s) than that of HPAM (37.86 mPa s). This could be attributed to the long hydrophobic chain that was introduced into the macromolecule and the associated intramolecular or intermolecular interactions of the aliphatic chain. Shear thinning, in a manner, can contribute to the injectivity of the copolymer solution, but it is very important to maintain a certain viscosity after the polymer solution is injected into the formation. Therefore, shear recovery behavior is another significant property for polymer flooding.

Fig. 7 (a) Shear shinning behavior of the copolymers, (b) shear recovery of the copolymers.

Shear recovery behavior is also called thixotropic, which means that the viscosity of the polymer solution will decrease under the influence of shear stress and will return to its original value when the shear force is removed.⁴⁶ The shear recovery behavior of P(AM-AA-NAD-DMAAPA) was detected because the molecular structure of the polymer was readily destroyed by a long period of shearing action when the displacing fluid was injected into the ground. As shown in Fig. 7b, the apparent viscosity of P(AM-AA-NAD-DMAAPA) and P(AM-AA-NAD) did not decline greatly when the shear rate increased from 170 s⁻¹ to 510 s⁻¹ and then returned to 170 s⁻¹. However, there was an obvious decline in the viscosity of HPAM (viscosity loss of 11.9%) at the end of a round. The low viscosity loss of P(AM-AA-NAD-DMAAPA) and P(AM-AA-NAD) could be explained by the introduction of NAD, which could regain reversible association via intramolecular or intermolecular hydrogen bonding. The results indicated that P(AM-AA-NAD-DMAAPA) displayed preferable performance in shear recovery ability, which was beneficial for enhancing the efficiency of recovery.

Effect of temperature on apparent viscosity

The influence of temperature on the viscosity of the copolymers was investigated from 30 °C to 120 °C at a shear rate of 170 s⁻¹. The change in the trend is shown in Fig. 8a. It can be observed that the apparent viscosity of all the copolymers decreased with an increase in temperature. In the beginning, each curve decreased sharply, but the rate of decrease in the apparent viscosity of HPAM was faster. Then, the downward tendency in apparent viscosity gradually became smooth when the temperature was over 90 °C. The value of the viscosity retention of P(AM-AA-NAD-DMAAPA) reached 40.20 mPa s at 120 °C, which was superior to those of P(AM-AA-NAD) (36.37 mPa s) and HPAM (7.2 mPa s). This phenomenon was ascribed to many associated groups twisted together to form reversible physical supermolecular structures by means of a certain intensity of van der Waals interactions and the intertwisting of molecular chains via hydrogen bonding interactions in solution.⁴⁷

Fig. 8 (a) Temperature resistance and (b) aging resistance of the copolymers.

Aging resistance is another important property for copolymer solutions, which is indicated by the change in apparent viscosity of the copolymer solution at the temperature of the reservoir after a certain time. Good anti-aging performance makes the copolymer solution remain effective for a long time in strata. As shown in Fig. 8b, the apparent viscosity of P(AM-AA-NAD-DMAAPA) dramatically decreased when it was kept in a water bath for 24 h. Then, the rate of decrease in apparent viscosity gradually became slower. Compared with HPAM, the value of the viscosity retention of P(AM-AA-NAD-DMAAPA) remained at 173.4 mPa s after storage for 240 h, which is much higher than that of HPAM. This could be attributed to the presence of the hydrophobic chain, which could prevent the molecular chains from hydrolyzing.

Effect of salt solution on apparent viscosity

Salt tolerance is another crucial property for copolymers as chemical flooding agents. In an aqueous solution, the apparent viscosity of a copolymer is increased mainly via the electrostatic repulsive force, and this force is greatly weakened in mineralized water owing to screening effects. However, the associating effect of a hydrophobic associating polymer can be strengthened by an increase in the polarity of the solution, thus maintaining a higher viscosity.^48,49 Here, the salt tolerance of a 2000 mg L⁻¹ copolymer solution was studied in various concentrations of salt solutions, and the results are shown in Fig. 9. The values of the viscosity retention of P(AM-AA-NAD-DMAAPA), P(AM-AA-NAD) and HPAM with a 10 000 mg L⁻¹ NaCl solution were 55.41 mPa s, 42.33 mPa s, and 16.86 mPa s, respectively, as shown in Fig. 9a. In Fig. 9b, the copolymers with a 1200 mg L⁻¹ MgCl₂ solution exhibited a lower apparent viscosity (P(AM-AA-NAD-DMAAPA): 59.95 mPa s, P(AM-AA-NAD): 45.11 mPa s, HPAM: 17.4 mPa s) than in a NaCl solution of the same concentration because the divalent metal ions had a greater screening effect. A similar situation occurred for the copolymers with a 1200 mg L⁻¹ CaCl₂ solution, as shown in Fig. 9c. The values of the viscosity retention of P(AM-AA-NAD-DMAAPA), P(AM-AA-NAD) and HPAM, respectively, were 52.97 mPa s, 40.81 mPa s, and 15.43 mPa s.

Fig. 9 Tolerance of (a) NaCl, (b) MgCl₂ and (c) CaCl₂ of P(AM-AA-NAD-DMAAPA), P(AM-AA-NAD), and HPAM.

Overall, after a period of rapid decline, the apparent viscosity of the mixture remained nearly constant. However, P(AM-AA-NAD-DMAAPA) had a higher value of viscosity retention in contrast to P(AM-AA-NAD) and HPAM. This could be attributed to intermolecular or intramolecular association, which enhanced the rigidity of the macromolecules and could prevent the chains from curling in certain conditions of salinity. Besides, the presence of a phosphate group in the side chain might act like a sulfonate group to reduce the salt sensitivity of the copolymer. Thus, it can be inferred that the salt tolerance of P(AM-AA-NAD-DMAAPA) was improved compared with that of HPAM.

EOR ability

Core flooding tests were conducted to evaluate flooding capabilities. Mineralized water blended with 2000 mg L⁻¹ P(AM-AA-NAD) and HPAM, respectively, was used in the core flooding tests. The injection sequence is displayed in Fig. 10, and 0.4 PV (pore volume) of the polymer solution was injected. It can be found that the flooding capacities of the copolymer solutions were better than that of HPAM in Fig. 10a. P(AM-AA-NAD-DMAAPA) solution increased the oil recovery ratio by 14.52% compared with water flooding and 8.0% compared with that of HPAM solution. The injection pressure in polymer flooding rose significantly but reached different levels, as shown in Fig. 10b. P(AM-AA-NAD-DMAAPA) solution reached a higher value than the other solutions owing to its higher viscosity. Besides, P(AM-AA-NAD-DMAAPA) solution exhibited the highest residual resistance factor (2.32). All the results are summarized in Table 4. They show that P(AM-AA-NAD-DMAAPA) had outstanding ability for EOR because of its excellent performance (Fig. 10b).

Fig. 10 (a) Recovery ratio of different solution systems and (b) injection pressure curves.

Table 4 Core flooding conditions and the results of EOR

Entry	Porosity (%)	Kw (md)	Sample	E1^a (%)	E2^b (%)	EOR (%)	RRF^c
a E1 is the oil recovery ratio by water flooding.b E2 is the total oil recovery ratio.c RRF is the residual resistance factor.
1	26.01	954.65	2000 mg L^−1 HPAM	45.03	51.55	6.52	1.22
2	24.95	933.69	2000 mg L^−1 P(AM-AA-NAD)	46.07	59.68	13.61	1.89
3	25.34	940.88	2000 mg L^−1 P(AM-AA-NAD-DMAAPA)	45.53	60.05	14.52	2.32

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Conclusions

An α-aminophosphonic acid-modified, rapidly dissolving hydrophobic associating copolymer, namely, P(AM-AA-NAD-DMAAPA), was successfully synthesized by redox free-radical copolymerization of AM, AA, NAD, and DMAAPA. The copolymer exhibited superior water solubility (its dissolution time was 0.75 h) and thickening capability. The value of apparent viscosity retention of a 2000 mg L⁻¹ copolymer solution was 58.55 mPa s at a shear rate of 170 s⁻¹, and it lost only 6.3% of its apparent viscosity as the shear rate increased from 170 s⁻¹ to 510 s⁻¹ and then returned to 170 s⁻¹. When the temperature reached 120 °C, the value of apparent viscosity retention of the copolymer solution remained at 40.20 mPa s. When the solution was kept in water for 240 h at 65 °C, its apparent viscosity was 173.4 mPa s. It also displayed high salt tolerance: the value of apparent viscosity retention remained at 55.41 mPa s, 59.95 mPa s, and 52.97 mPa s in solutions with concentrations of 10 000 mg L⁻¹ NaCl, 1200 mg L⁻¹ MgCl₂, and 1200 mg L⁻¹ CaCl₂, respectively. Moreover, the EOR rate of a 2000 mg L⁻¹ copolymer solution was higher than that of water flooding by 14.52%. This suggests that this copolymer could be an excellent candidate for potential applications in enhanced oil recovery, especially for high-temperature and high-mineralization oilfields.

Acknowledgements

This work was supported by the Foundation of Youth Science and Technology Innovation Team of Sichuan Province (contract grant number 2015TD0007), the Support Program of Science and Technology of Sichuan Province (contract grant number 2016GZ0274), the Opening Project of Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province (YQKF201404) and the Open Extracurricular Experiment of Southwest Petroleum University (KSZ15066).

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Footnote

† Electronic supplementary information (ESI) available: Optimum copolymerization conditions; dissolution time of the copolymers. See DOI: 10.1039/c6ra11952b

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