Novel Biodegradable Graft-Modified Water-Soluble ... - ACS Publications

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Novel Biodegradable Graft-Modified Water-Soluble Copolymer Using Acrylamide and Konjac Glucomannan for Enhanced Oil Recovery Shaohua Gou,*,† Shiwei Li,† Mingming Feng,‡ Qin Zhang,† Qinglin Pan,† Jun Wen,§ Yuanpeng Wu,*,† and Qipeng Guo*,⊥ †

Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu 610500, China Xi’an Highway Research Institute, Xi’an, Shanxi 710065, China § Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China ⊥ Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia ‡

S Supporting Information *

ABSTRACT: Synthesis of a novel biodegradable hydrosoluble graft-copolymer named KGM-g-AM-g-AA-g-APEG was carried out using konjac glucomannan, acrylamide, acrylic acid, and allyl polyoxyethylene ether via free-radical copolymerization. Effective characterization methods , including XRD, TGA, FT-IR, 1H NMR, and SEM, were adopted to evaluate the graft-copolymer. The good biodegradability of the graft-copolymer was confirmed, with a degradation rate >60% in 28 days, by a biological degradation experiment. Subsequently, it was found that the graft-copolymer showed superior shear resistance, shear reversible performance, temperature resistance, viscoelasticity, and salt tolerance compared with partially hydrolyzed polyacrylamide under the same conditions through rheological experiments. What’s more, RF (14.1) and RRF (4.95) of the graft-copolymer provided a valuable proof of the capability of mobility control and the great flooding effect with enhancing oil recovery by 13.76% in the presence of 6700 mg/L salt solution at 65 °C according to the core flooding tests.

1. INTRODUCTION As a frequently used application, two chemicals in the oil gas field, polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM), which meet the needs of oil field mining, especially for enhanced oil recovery (EOR), have captured a lot of attention among domestic and overseas scientific research personnel.1−4 Nevertheless, a large number of Ca2+ and Mg2+ ions of formation water, high temperature, and high shearing action in the process of oil extraction make HPAM or acrylamide-based polymers unsuitable for the bad conditions, which obviously generates curling and thermal-degradation of molecular chains.5−7 Despite the fact that many considerable properties of different water-soluble acrylamide-based copolymers have been developed over the last three decades for deeper oil wells, such as the resistance to salt, heat, and shear, however, the fact that residual polymers in the formation cause damage to the environment cannot be neglected, such as makes oil−water separation difficult and endangers local ecosystems.8 Most studies showed that PAM was difficult to be used and degraded by microorganisms, and some early researchers ever put forward the nonbiodegradable properties of polyacrylamide.9,10 Although observations on copolymer biodegradation have been studied in recent decades, unsatisfactory exper© XXXX American Chemical Society

imental results show that microorganisms as the driving force of degradation are not conspicuous.11 Other scientific investigations suggested that soil microorganisms have no competence for the decomposition of carbon in the main chain or a sole N-source from the amide part of PAM.12,13 Biodegradation of PAM or HPAM isolated from an oil field after polymer flooding is still tremendously challenging. As for that, it is necessary to develop novel biodegradable types of water-soluble polymers as new EOR chemicals instead of conventional acrylamide polymers. Konjac Glucomannan (KGM), a natural macromolecular polysaccharide, is composed of β-D-glucose and-β-D-mannose following the mole ratio of 2:3 or 1.6:1 with β-1,4 pyranoside bonds and branched chain structures with β-1,3 bonds from the C3 position of the main chain mannose containing sugar residues in which there are acetyl groups (for its structure, see the Supporting Information).14−16 Due to the unique physical and chemical characteristics, KGM is a biocompatible and Received: Revised: Accepted: Published: A

November 30, 2016 January 5, 2017 January 5, 2017 January 5, 2017 DOI: 10.1021/acs.iecr.6b04649 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Synthetic route of KGM-g-AM-g-AA-g-APEG.

copolymer EOR chemical by graft modification with KGM, acrylic acid(AA), and allyl polyoxyethylene ether (APEG) to achieve structural diversity, aiming to get excellent temperature, salt resistance, and shear resistance.

biodegradable macromolecular polysaccharide, and the acetyl and alcoholic hydroxyl groups existing in the backbone of KGM develop its easily modified properties, such as etherification, esterification, and acetylation reaction.17−19 KGM can form a soft ribbon with modifying group−acetyl group that maintains its conformation, leading to providing a double helix structure with voids. These free and mobile sugar chains play a positive role in hydrogel formation through catching plenty of water, and the gel is of shear thinningthe characteristic of a nonNewtonian fluid.20,21 The hydrogels retain good performance under acidic conditions; however, they are prone to precipitation under alkaline conditions. Moreover, reversible hydrogels can be formed with the pH value being less than 12.2 or elastic hydrogels when the pH value is greater than 12.2, and being heated is rare among other polysaccharides.22 The higher the reaction temperature, the stronger the elasticity and toughness of the hydrogels obtained. Based on this behavior, KGM could be prepared as a filler, which is applied in oil drilling to increase drilling efficiency and achieve the hydrophilic−hydrophobic property.23 Maintaining the ability to bind and exchange cations, and adsorb and chelate organic molecules, KGM with molecular structure of the network is ideal for showing superadsorption of suspended particles and absorbent flocculation ability for the removal of organic compounds, such as methylbenzenes, long-chain organic acids, and heavy metal ions containing copper, nickel, and chromium ion from wastewaters, followed by the formation of large, fast sedimenting, nontoxic flocs.24,25 Recently, many research works on the graft modification of KGM greatly enhanced the water solubility, film forming ability, viscosity, thermal stability, rheological characteristic, excellent activity, and gelling characteristics, all of which play essential functions on the utilization ratio of KGM.26−33 Especially, graft-modified copolymers based on KGM being used as flocculant have made great progress in the process of sewage treatment to accelerate the trend that natural polysaccharides have gradually become the treatment material for environmental protection.34−36 To the best of our knowledge, there are a few examples of the graftcopolymers based on KGM focusing on enhanced oil recovery. Consequently, considering the performance advantages of KGM, it will show good applied prospects in polymer flooding. In previous works, we reported two acrylamide sulfonate copolymers as EOR chemicals based on N-phenylmaleimide (N-PMI) or nallylbenzamide (NABI), and excellent results have been obtained.37,38 Inspired by these works, we continued to search for a new highly efficient water-soluble acrylamide

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AM), acrylic acid (AA), allyl polyoxyethylene ether (APEG), ammonium ceric nitrate ((NH4)2Ce(NO3)6), HPAM (Mr > 5.0 × 106), acetone, and ethanol are all analytical reagents from Chengdu Kelong Chemical Reagent Co., Ltd., China. Konjac Glucomannan (>98%) was provided by Hefei Bomei Biotechnology Co., Ltd., China. 2.2. Preparation of Graft-copolymer KGM-g-AM-g-AAg-APEG. Quaternary graft-copolymer KGM-g-AM-g-AA-gAPEG was synthesized as shown in Figure 1. Certain amounts of KGM and deionized water were put in a 250 mL flask with three necks at constant stirring at 30 °C for about 0.5 h. Then monomers including AM, AA, and APEG were added sequentially to the previous mixture with adjusting pH to the indicated value with 25% NaOH solution. For the sake of removal of residual oxygen from the reaction system, a stream of nitrogen was bubbled for 20 min, and a given mass of (NH4)2Ce(NO3)6 was put into the reaction solution under the protection of nitrogen for 10 min. The final solution continued to maintain the reaction at constant temperature for 12 h. The graft-copolymer KGM-g-AM-g-AA-g-APEG was obtained by means of separation and purification (for details, see the Supporting Information). The copolymer AM-AA-APEG was also prepared under the same conditions and purified with ethanol to make a contrast with KGM-g-AM-g-AA-g-APEG. 2.3. Graft Degree of KGM-g-AM-g-AA-g-APEG. The graft degree of the graft-copolymer was calculated with eq 1: W − W1 GD (%) = 2 × 100% W1 (1) where W1 represents the weight of the original samples, and W2 is the weight of grafted samples. 2.4. Characterization. Fourier transform infrared (FT-IR) spectroscopy was individually performed with a PerkinElmer RX-1 spectrophtometer (Beijing Reili Analytical Instrument). All the specimens were homogeneously mixed with KBr powder to form tablets. X-ray diffraction of graft-copolymer and KGM was individually performed with a DX-2700 X-ray diffractometer (Dandong Hao Yuan Instrument, Liaoning). Under 30 mA and 40 kV, a Cu Kα target was executed with B

DOI: 10.1021/acs.iecr.6b04649 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research scattering angle from 10° to 80° for 30 min. 1H NMR spectra of the graft-copolymer (3 mg, D2O: 0.55 mL) were recorded with a Bruker AVANCE III 400 MHz NMR spectrometer (Bruker, Switzerland). A scanning electron microscope (SEM) (S-3000 N, Hitachi, Japan) was used to analyze the surface morphology of HPAM and the graft-copolymer on 20 kV of acceleration voltage. All the samples were frozen and dried with liquid nitrogen and sputter-coated with gold to increase conductivity. A thermal analyzer ((METTLER TOLEDO Co., Switzerland) was used to evaluate the thermogravimetric analysis (TGA) by changing the temperature from 40 to 800 °C following the rate of heat addition 10 °C/min under a nitrogen atmosphere of 0.67 mL/s. An efficient approach about monomer conversion was established by high performance liquid chromatography (HPLC; Shimadzu Co., Japan). The mobile phase (H2O/ CH3OH = 1:9) flowed through a C18 chromatographic column (40 °C) from a UV detector (wavelength: 210 nm) with a flow rate of 1.0 mL/min. The conversion of graft-copolymer was calculated by the residual monomers from ethanol which were used to purify the graft-copolymer with eq 2: W− D(%) =

AC0 A0

then was soaked in deionized water for 3 h to remove impurities with natural pH. At last, the cores were dried at 105 °C for 1 day to carry out the next displacement experiment. A chloride solution with 2.5 mL/min of volume flow was injected in the core by calculating the porosities of the core with Darcy’s law under constant pressure.42 The crude oil was injected into the core assembly and aged for 48 h at 65 °C. The apparent viscosity of the simulation of the crude oil was 72.26 mPa·s at 65 °C. Core flooding experiments were performed with the following steps: First, before the water ratio passed 95%, water flooding with NaCl solution was always performed at 0.3 mL/min, and then the copolymer was poured into the core assembly with 0.3 mL/ min. Finally, the extrapolated water flooding with 0.3 mL/min was conducted to receive the water cut 95% at the second time.43 All polymer solutions were prepared with the simulated formation water, and the composition and content of simulated formation water are listed in Table 1. Table 1. Content of Various Ions from Simulated Formation Water

×V

W

× 100%

(2)

where D is the rate of conversion for various monomers, W is the total quality of the feed, A is the peak area of various residual monomers in ethanol, C0 is the concentration of the corresponding monomer, A 0 is the peak area of the corresponding monomer, and V is the volume of ethanol of the purified graft-copolymer. 2.5. Biodegradation Test. The closed bottle (OECD 301 D) method was feasible to investigate the biodegradability of KGM-g-AM-g-AA-g-APEG, AM-AA-APEG, and HPAM.39−41 A certain amount of biochemical oxygen demand (BOD) bottles were used to hold a solution of microorganisms, with the secondary effluent and test specimen (3.0 mg/L) being regarded as the only source of carbon and nitrogen. It was ensured that BOD bottles with which the above solution was filled were hatched with no illumination at 20 °C for 4 weeks. The DO-200 dissolved oxygen analyzer (Nanjing Zhuoma electromechanical Co. Ltd., China) was used to measure dissolved oxygen from all the BOD bottles. The degradation rate (Dt) was obtained through BOD of the test specimen, and chemical oxygen demand (COD) was acquired by the reflux of potassium permanganate method with eq 3:

( D (%) = COD( BOD

t

O2 mg test O2 mg test

) × 100 substance mg)

Inorganic ions

Na+

Mg2+

Ca2+

Cl−

SO42−

Total con

Conc(mg/L)

5400

1000

500

8000

200

6700

3. RESULTS AND DISCUSSION 3.1. Optimum Conditions. The apparent viscosities of graft-copolymers with 0.1 wt % under diverse reaction conditions were determined with a Brookfield DV-III Pro viscometer at 7.34 s−1 at 30 °C. Polymerization conditions such as the ratio of AM and AA, content of APEG and KGM, reaction temperature, and pH were explored to get the best reaction conditions (for the details, see the Supporting Information).44 The optimum synthetic conditions for graftcopolymer are shown in Table 2. Table 2. Optimum Synthetic Conditions for GraftCopolymer Mass ratio of monomer (g) AM

AA

APEG

KGM

Initiator (g)

pH

Temp (°C)

6

4

0.03

0.04

0.03

7

40

3.2. Characterization. 3.2.1. IR Spectroscopy Analysis. The infrared spectra of APEG, graft-copolymer, and KGM are shown in Figure 2A. According to the IR spectrum of APEG, the remarkable absorption peaks at 3443 cm−1, 2884 cm−1, and

substance mg

(3)

2.6. Rheological Experiment. The rheological properties of graft-copolymer, AM-AA-APEG, and HPAM were further evaluated with a HAAKE MARS III rheometer (HAAKE, Germany). All solutions of the samples with 1000 mg L−1 as pseudoplastic fluids were carried out to demonstrate the shearthinning, viscoelasticity, temperature, and salt toleration under sorts of conditions adjusted by water pump and standard polydimethylsiloxane. 2.7. Core Flood Test. The type of sand pack specification was φ2.5 cm × 25 cm. The sand was quartz sand form pulverization of quartzite and was further purified with 99% ethanol for 24 h and 10% hydrochloric acid for 100 min and

Figure 2. (A) FT-IR spectra of APEG, KGM, and graft-copolymer; (B) XRD patterns of KGM and graft-copolymer. C


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Figure 3. 1H NMR spectrum of graft-copolymer.

1648 cm−1 correspond to −OH, −CH2−, and CC groups, and the characteristic peak at 1108 cm−1 was due to the C−O− C groups. In contrast to the IR spectrum of KGM, graftcopolymer displayed obvious peaks at 3188 cm−1, 1677 cm−1, and 1108 cm−1. The stretching vibrations of N−H and CO from the amide group belong to the characteristic peaks at 3188 and 1677 cm−1. And the weak peak at 1108 cm−1 was induced by the C−O−C group of the grafting APEG. The obvious difference with the stretching vibration between KGM and KGM-g-AM-g-AA-g-APEG indicated that APEG, AM, and AA were successfully grafted onto the KGM backbone. 3.2.2. X-ray Diffractometry Analysis. Crystalline structure analyses of KGM and KGM-g-AM-g-AA-g-APEG by XRD are shown in Figure 2B. The XRD patterns of the KGM suggested its obvious amorphous phase according to a wide characteristic peak appearing at 2θ = 20°.17 The peak of the graft-copolymer attenuated clearly at about 2θ = 20°, revealing that the copolymer graft-modified onto the KGM main chain reduced the degree of crystallinity of KGM. It was highly likely that the characteristic interactions of hydrogen bonding from the intraand intermolecule of KGM and the copolymer AM-AA-APEG contribute to the decrease of crystallinity, which had a great effect on the decrease of the molecular orientational state of KGM.45 From the molecular level, the chain of the copolymer mixed well with the backbone of KGM and also abated the crystallinity of KGM. 3.2.3. 1H NMR Analysis. The 1H NMR spectrum of KGM-gAM-g-AA-g-APEG was shown in Figure 3. It was obvious that the chemical shift value from 1.09 to 1.12 ppm was associated with −C-CH3 from KGM, and multiple proton peaks at 1.54− 1.67 ppm were due to the aliphatic group of copolymer −CHCH2-CH-CH2-. The methylene of −C-CH2−O-CH2-CH2−OH from APEG is assigned to distinct peaks around 3.57 ppm. The signal proton peak about -OH of KGM belongs to the chemical shift value of 3.71 ppm, and the dwarf characteristic peak at 6.85 ppm corresponds to −CONH2 of the copolymer backbone. As expected, the FTIR and 1H NMR spectra confirmed the successful grafting of the copolymer onto the KGM backbone. 3.2.4. Surface Morphology Analysis by SEM. The surface morphology of KGM-g-AM-g-AA-g-APEG and HPAM in aqueous solution for the concentration 0.1 wt % provided insight into the structural information (for the image, see the Supporting Information). The HPAM showed a porous

network structure composed of scattered coarse lines, which were invariably formed due to the entanglement of HPAM molecular chains in the aqueous solution. Compared with the HPAM, a distinct and dense surface with a three-dimensional porous network structure was exhibited clearly from KGM-gAM-g-AA-g-APEG, which could be a consequence of the three monomers branch on the main graft-polymer chain. In other words, interactions of hydrogen bonding between modified polymer chains were apparently more forceful. The three watersoluble monomers provided the carboxyl groups and hydroxyl groups, and the special chemical structure of many OH groups came from KGM molecules, which gave rise to strong hydrogen bonding between the functional groups of macromolecules.46 All of the reasons stabilized the great network structures of the graft-copolymer to bring about high viscosity of copolymer solutions. 3.2.5. Thermal Stability. The thermal gravimetric analysis of KGM and KGM-g-AM-g-AA-g-APEG was investigated in Figure 4. It could be easily found that the TGA profiles of

Figure 4. TGA curves of KGM and KGM-g-AM-g-AA-g-APEG.

graft-copolymer showed three weight loss stages, and the KGM only displayed two weight loss stages. Initially, the rising temperature in the range 40−163 °C caused the slight mass decrement of KGM, and the graft-copolymer occurred from 40 to 196 °C with the mass loss of 5.37%, which might account for the dehydration to the intramolecule or intermolecule of KGM and KGM-g-AM-g-AA-g-APEG. From the second process of the thermal decomposition, the mass loss of KGM started in the range 200−325 °C, and the residual weight sharply reduced to D


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Industrial & Engineering Chemistry Research Table 3. Composition of KGM-g-AM-g-AA-g-APEG Feed ratio (wt %)a

a

AM

AA

APEG

59.58

39.72

0.2979

Measured content (wt %)b KGM

AM

AA

0.3972

61.67

37.74

b

APEG

KGM

intrinsic viscosityc

0.3028

0.2872

1129.29 mL/g

c

The mass of original monomers is 10.07 g. The mass of purified copolymer is 9.6568 g. See Supporting Information

found that a shear-thinning behavior was described as follows: with the shear rate rising, the apparent viscosity of the measured copolymer fluids decreased rapidly, indicating the typical rheology of a pseudoplastic fluid. Furthermore, the viscosity and retention rate of viscosity (26%, 1000 s−1) of KGM-g-AM-g-AA-g-APEG were higher than that of HPAM (11%, 1000 s−1) and AM-AA-APEG (17%, 1000 s−1) under the same conditions. There was a strong possibility that the presence of long ether chain groups from APEG could be due to the shear resistance of the copolymer solution, with the strong hydrophilic force interaction among the surrounding chains making the molecular chain stretch. The almost stable chemical structure and strong viscosity-promoting of KGM also made the shear-resistance better than those of HAPM and AMAA-APEG. 3.4.2. Relationship of Shear Stress and Rate. Most copolymer solutions based on acrylamide and its derivatives generally presented a pseudoplastic fluid, with the increase of shear rate causing a smaller resistance to flow. Mathematically, the power law model suggests the relationship between shear stress and its rate by eq 4.47

40%, corresponding to the decomposition of KGM. Nevertheless, the curve of KGM-g-AM-g-AA-g-APEG revealed that the second stage of weight loss ended at 500 °C with the residual weight of 40%. The third one could be the breaking of the C−C bonds of KGM-g-AM-g-AA-g-APEG at 600−800 °C with a mass loss of 20%. The results indicated that KGM-g-AMg-AA-g-APEG had excellent thermal stability. 3.2.6. Analysis of Content of Graft-copolymer. The proportion of each monomer in the graft-copolymer was gauged by the conversion rate, which expresses the actual situation of graft copolymerization. The actual feed ratio and the ratio of the monomer of the graft-copolymer via the final calculation are listed in Table 3. The calculated conversion rates of AM, AA, and APEG are 99.25%, 91.11%, and 97.47%, respectively. 3.3. Biodegradation of Graft-copolymer. Biodegradation of KGM-g-AM-g-AA-g-APEG, AM-AA-APEG, KGM, and HPAM was carried on at the degradation rates shown in Figure 5. Comparing the four curves about the level of degradation, it

τ = Kγ n

(4)

The above formula could be translated into another form by taking logarithms as in eq 5: lg τ = lg K + n lg γ

(5)

Herein τ means shear stress, Pa; K means consistency coefficient, Pa s−n; n means non-Newtonian index; and γ means shear rate, s−1; lg τ and lg γ in the theory describe a linear relationship. As revealed by Figure 6B, the varying curves of KGM-g-AM-g-AA-g-APEG, AM-AA-APEG, and HPAM explain the power-law model for shear stress and its rate. Sequentially, n and K were calculated as 0.63 and 0.984 Pa s0.63 for KGM-g-AM-g-AA-g-APEG, 0.42 and 2.251 Pa s0.42 for AMAA-APEG and 0.43, and 2.457 Pa s0.43 for HPAM by means of linear fitting treatment, respectively. 3.4.3. Shear Recovery Behavior. As shown in Figure 6C, investigation of the shear recovery behavior of KGM-g-AM-gAA-g-APEG, AM-AA-APEG, and HPAM by circulating the two given shear rates 170 and 510 s−1 proposed that the graftcopolymer could be restored to the original viscosity with nearly 100% (110 mPa·s at second 170 s−1), in contrast to the observation that HPAM and AM-AA-APEG could only be restored to the original viscosity with 80% (101 mPa·s at second 170 s−1) and 87% (104.5 mPa·s at second 170 s−1), respectively. It is certain that the strong interaction of the inter-/intramolecular hydrogen bonding contributes to the reformation of the structures for KGM-g-AM-g-AA-g-APEG under different shearing actions. 3.5. Salt Tolerance. In Figure 6(D, E, F), the Na+, Ca2+, and Mg2+ tolerance of KGM-g-AM-g-AA-g-APEG, AM-AAAPEG, and HPAM of 0.1 wt % were measured with a Brookfield DV-III Pro viscometer at 7.34 s−1 and 30 °C. Similar currents for graft-copolymer, AM-AA-APEG, and HPAM with three different types of salt pointed out clearly that the apparent

Figure 5. Biodegradability of HPAM, AM-AA-APEG, KGM, and KGM-g-AM-g-AA-g-APEG.

was remarkably revealed that KGM and KGM-g-AM-g-AA-gAPEG exhibited a relatively high rate of degradation within an 18 d period compared to those of HPAM and AM-AA-APEG, until arrival at stable values of more than 60%. Ready biodegradability was defined as the value of degradation being more than 60% according to standard 301 from the Organization for Economic Co-operation and Development (OECD). The degradation rates of KGM and KGM-g-AM-gAA-g-APEG have been shown to be far more than 60%, indicating that the graft-copolymer is more easily degraded than HPAM and AM-AA-APEG in the formation to promote the idea of environmental protection without damage to the ecological environment. What’s more, it is prominent to avoid the long stay in the formation of chemicals such as HPAM in the oil exploitation, and improve the application of the polysaccharide polymer. 3.4. Effects of Shear on the Graft-copolymer. 3.4.1. Shear Thinning behavior. The 1000 mg L−1 KGM-gAM-g-AA-g-APEG, AM-AA-APEG, and HPAM were used to evaluate the rheology at 25 °C in Figure 6A. It can be clearly E


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Figure 6. (A) Shear thinning behavior of HPAM, AM-AA-APEG, and graft-copolymer; (B) Relationship between the shear stress and shear rate of HPAM, AM-AA-APEG, and graft-copolymer; (C): Shear recovery of HPAM, AM-AA-APEG, and graft-copolymer; (D−F) Effect of concentrations of Na+, Ca2+, and Mg2+ on the apparent viscosity, respectively.

3.6. Viscoelasticity. The viscoelastic property of the copolymer played a particular role in the process of conducting practical applications. According to the method provided for determining the viscoelastic ability of the copolymer, the copolymer solution was viscoelastic at a certain temperature, when the storage modulus (G′) was greater than the loss modulus (G″) and G′ > 10 −1 Pa at 0.1−10 s−1.48 Therefore, frequency scanning could be used to study the viscoelastic property of the polymer solution. The Maxwell model, which was one of the most classical linear viscoelastic models, was used to characterize the G′ and G″ of the viscoelastic fluid, respectively.49,50 The viscoelasticity was represented by eqs 6 and 7:

viscosity declined with the addition of a certain concentration of the salt until the remaining constant value, resulting from the polyelectrolyte effect of inorganic salts on copolymer solutions. With the increase of the concentration of chlorinated inorganic salt, there was a charge shielding effect on the molecular chain and electrolyte compressed the double electronic layer of the hydration membrane of the molecular chain following the thinning of the hydration film, which reduced the degree of the molecular chain’s extension and the hydrodynamic volume, resulting in a decrease in viscosity. The ability of salt tolerance for KGM-g-AM-g-AA-g-APEG (Na+: 33.8%, Ca2+: 41.7%, Mg2+: 44.2%) was extremely superior to those of HPAM (Na+: 10.3%, Ca2+: 6.3%, Mg2+: 9.2%) and AM-AA-APEG (Na+: 23.5%, Ca2+: 15.4%, Mg2+: 18.5%) through viscosity retention rate. The elegant results suggested that this graft-copolymer could be an excellent oil-displacing agent for potential applications for oil and gas reservoirs of high salinity.

G′(ω) = G

ω 2λ 2 1 + ω 2λ 2

(6)

ωλ 1 + ωλ

(7)

G″(ω) = G F


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Industrial & Engineering Chemistry Research where G is the stress relaxation modulus; λ is the relaxation time; and ω is the angular frequency. G′ can be said to be energy storage for the fluid; G″ reflects the size of the fluid viscosity, indicating the dissipation of energy of the fluid. The linear viscoelastic behavior of the copolymer could be reflected by a dynamic rheological test without destroying the copolymer fluid structure. Therefore, not only can the results demonstrate the interaction of inter-/intramolecular chains in copolymer solution, but indirectly analyze the relevant molecular structure of the copolymer. As shown in Figure 7A, the frequency scanning curves of G′ and G″ of HPAM, AM-AA-APEG, and 1000 mg/L graft-

interaction between inter- or intramolecular chains stronger than that of hydrolyzed polyacryamide. The better viscoelasticity of the modified copolymer solution can enhance the displacement efficiency by reducing the residual oil saturation and carrying out a larger amount of residual oil. 3.7. Temperature Tolerance. To gain insight into the temperature tolerance of the polymer, 0.1 wt % graftcopolymer, AM-AA-APEG, and HPAM were employed by HAAKERheoStress6000 with the temperature ranging from 50 to 120 °C in Figure 7B. It was found that the graft-copolymer, AM-AA-APEG, and HPAM established similar trends, with apparent viscosity decreasing with increasing temperature. For HPAM and AM-AA-APEG, the viscosity retention rates were only 25% and 34% at 100 °C, in stark contrast with that of the graft-copolymer, 46% at 100 °C with the extraordinary temperature tolerance. It is well-known that intermolecular hydrogen bonding contributes to the high viscosity of HPAM, the increasing temperature aggravates the thermal motion of the system, and weakened hydrogen bond leads to the decrease of viscosity. Nevertheless, graft-copolymer showed palpable temperature tolerance. This characteristic may be due to the KGM macromolecule having a special molecular morphology and high molecular weight and also to the graft-copolymer improving the original performance of HPAM. 3.8. Core Flood Test. The purpose of a core flood test is to appraise the performance characteristics of a graft-polymer in enhanced oil recovery and mobility control ability. Solutions of HPAM with 1000 mg/L and of KGM-g-AM-g-AA-g-APEG with 1000 mg/L or 1300 mg/L were all used to carry out core oildisplacement experiments. The value of EOR was measured following eq 8: EOR = H1 − H2

(8)

where H1 and H2 are the values of polymer and water flooding for oil recovery. The resistance factor (RF) and residual resistance factor (RRF), in accord with the mobility control ability, were obtained by eqs 9 and 10, respectively. RF =

Figure 7. (A) Viscoelastic behavior of HPAM, AM-AA-APEG, and graft-copolymer; (B) Temperature resistance of HPAM, AM-AAAPEG, and graft-copolymer.

P2/Q 2 P1/Q 1

RRF = copolymer were conducted with a HAAKE RheoStress 6000 from 0.01 to 10 s−1 at 30 °C. It could be certain that G″ occupied a predominant position before the first point of crossing under low frequency and then G′ was higher than G″ in a certain high frequency regime. Specifically, the frequencies ω1* and ω2* for HPAM were 0.15 rad·s−1 and 9.1 rad·s−1, and those for AM-AA-APEG were 0.09 rad·s−1 and 9.2 rad·s−1, respectively. However, the ω1* and ω2* of KGM-g-AM-g-AA-gAPEG were 0.06 and 9.5 rad·s−1; these results demonstrated the three polymers were excellent viscoelastic liquids, while the frequency at ω1* of the graft-copolymer was smaller than that of HPAM and AM-AA-APEG, indicating the average molecular weight of the graft-copolymer was higher than that of HPAM. Simultaneously, the modulus at ω1* for the graft-copolymer was higher than those of HPAM and AM-AA-APEG, pointing out that the molecular weight distribution of the graftcopolymer was narrower than that of HPAM. This meaningful performance may be supported by the fact that the long ether chain group and KGM macromolecular group on the KGM-gAM-g-AA-g-APEG worked, which made the hydrogen bonding

(9)

P3/Q 3 P1/Q 1

(10)

Herein P1, P2, or P3 is the value of constant pressure from primary water flooding, polymer flooding, or subsequent water flooding in MPa; and Q1, Q2, or Q3 is the rate of injection from primary water flooding, polymer flooding, or subsequent water flooding in mL/min. As shown in Figure 8A, a considerable amount of results demonstrated that the solution of 1000 mg/L KGM-g-AM-gAA-g-APEG can enhance oil recovery by 8.64% compared with water flooding of HPAM under the same concentration in the presence of 6700 mg/L simulated formation water at 65 °C. A higher oil displacement efficiency of 13.76% was obtained with 1300 mg/L KGM-g-AM-g-AA-g-APEG in the same experimental environment. As shown in Figure 8B, the injection pressure of KGM-g-AM-g-AA-g-APEG was obviously higher than that of HPAM during the polymer flooding. Moreover, the graft-copolymer revealed larger retention expressed by RF (14.1) and RRF (4.95), which were also higher than that of HPAM (RF: 6.9, RRF: 1.59). The more convincing results illustrated the advantage of KGM-g-AM-g-AA-g-APEG in G



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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04649. Structure of konjac glucomannan; optimum conditions; coarse product separation and purification; measurement of the intrinsic viscosity of graft-copolymer; intrinsic viscosities of graft-copolymer; SEM images; core flood test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.G.). *E-mail: [email protected] (Y.W.). *E-mail: [email protected] (Q.G.). ORCID

Yuanpeng Wu: 0000-0001-6106-1868 Qipeng Guo: 0000-0001-7113-651X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.G. and Y.W. received funding from the Key Project of Education Department of Sichuan Province (Grant NO. 15ZA0051). J.W. received funding from the Support Program of Science and Technology of Sichuan Province (Grant NO. 2016GZ0274). M.F. received funding from the Opening Project of Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province (Grant NO. YQKF201404). S.L., Q.Z., and Q.P. were financially supported by the Open Extracurricular Experiment of Southwest Petroleum University (Grant NO. KSZ15067 and KSP15118).

Figure 8. (A) Enhanced oil recovery ability of HPAM and graftcopolymer; (B) Mobility control ability of HPAM and graftcopolymer.

mobility control ability. It was an excellent performance that may greatly support that the copolymer introduced in the molecular chain of KGM made the graft-copolymer property improved tremendously, resulting in the mobility ratio of the injected copolymer solution to core oil being higher and the sweeping scale of the displacing fluid being broader.51 It is certain that the graft-copolymer was remarkable for enhanced oil recovery (for specific data tables, see the Supporting Information).

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REFERENCES

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4. CONCLUSION APEG, AM, and AA have been successfully graft-copolymerized onto the modular KGM backbone in aqueous media by the initiator ceric ammonium nitrate. The biodegradability test indicated good biological degradation property of the KGM-gAM-g-AA-g-APEG for environmental protection. TGA confirmed that KGM-g-AM-g-AA-g-APEG showed more favorable thermal stability. Moreover, the salt resistance, temperature tolerance, antishear ability, and viscoelastic properties of the KGM-g-AM-g-AA-g-APEG had been improved remarkably compared with partially hydrolyzed polyacrylamide. What’s more, the RF (14.1) and RRF (4.95) values of the KGM-g-AMg-AA-g-APEG provided a valuable proof for capability of mobility control and great flooding effect with enhancing oil recovery by 13.76%. These excellent performances indicated the KGM-g-AM-g-AA-g-APEG could be applied as a displacement agent superior to conventional copolymers based on acrylamide. In the follow-up work, after a statement of the application in EOR, different conditions involving the graftcopolymer are investigated. H


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