Water-Soluble CoreâShell Hyperbranched Polymers for Enhanced Oil

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Water Soluble Core-Shell Hyperbranched Polymers for Enhanced Oil Recovery Wan-Fen Pu, Rui Liu, Keyu Wang, Kexing Li, Zhaopeng Yan, Bin Li, and Lei Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5039693 • Publication Date (Web): 06 Jan 2015 Downloaded from http://pubs.acs.org on January 8, 2015

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Industrial & Engineering Chemistry Research

Water Soluble Core-Shell Hyperbranched Polymers for Enhanced Oil Recovery Wan-Fen Pu,†,‡ Rui Liu,*,†,‡ Ke-Yu Wang,§ Ke-Xing Li,†,‡ Zhao-Peng Yan,‡ Bin Li,‡ and Lei Zhao‡ †

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, PR China ‡

School of Petroleum & Gas Engineering, Southwest Petroleum University, Chengdu 610500, PR China §

Research Institute, Shaanxi Yan Chang Petroleum (Group) Co., Ltd. Xian 710075, PR China ABSTRACT: Novel water soluble core-shell hyperbranched polymers (HBPAMs), consisting of nano-silica as core, hyperbranched polyamidoamide (PAMAM) as subshell and linear hydrophilic chains as outermost layer, were synthesized through in-situ free radical polymerization strategy. The PAMAM hybrid nano-silica preferentially modified by 3-aminopropyltriethoxysilane, was prepared by successively repeating Michael addition of methyl acrylate and amidation reaction of ethylendiamine. By varying the numbers of functionalized branch-cell units, the numbers of outmost linear hydrophilic chains can be tuned with the mean diameter being 462 nm for HBPAM-1 and being 573nm for HBPAM-2. Rheological measurements demonstrated that HBPAMs were classical power law fluid and the critical shear rate shifts towards the higher region as increasing linear hydrophilic chains number of the outermost shell. Intersection modulus and relaxation time were elaborately calculated. Static experiments convincingly proved that the 3D morphology endowed HBPAMs excellent shear degradation resistance, desirable salt resistance and temperature tolerance. Core flooding experiment further confirmed that this unique type of core-shell polymers may have robust applications for EOR. Key words: core-shell hyperbanched polymer, hybrid, rheological properties, displacement performance

INTRODUCTION With the world’s attention on enhancing fossil-fuel production to satisfy national promotion and daily lives. Enhanced oil recovery (EOR) is of considerable academic and technological interest. Serving as mobility control agent, polymers have been employed to increase the viscosity of injected water in chemical EOR application over several decades.1-3 Commercial polymers utilized as thickening agent are partially hydrolyzed polyacrylamide (HPAM) and biological polymers. HPAM suffers great deal at elevated temperature in the presence of salinity due to its linear flexible hydrophilic chain.4 The irreversible shear degradation of this flexible chain impairs its viscosifying power.5 Biological polymers, including xanthan gum, cellulose and chitosan, behaving like rigid rods, are indifferent to temperature, salinity and shear degradation.1 But poor water soluble shortcoming, together with less thickening efficiency limits such eco-friendly polymers popular application. Numerous efforts have been made to improve linear flexible chain more rigid, referred to as polyacrylamide derivates2, 6-8 that are somewhat stable in harsh formation conditions. Given the unique aqueous and rheological characters of hyperbranched polymers whose structures are similar to three-dimensional (3D) sphere,9-11 the notion of replacing polyacrylamide derivates with water soluble hyperbranched polymers seems reasonable. More recently, polymer hybrid nano-sized silica has shown robust thermal stability,12,13 toughness14,15 and responsive property.16-18 These novel hyperbanched polymers may be promising for chemical EOR applications. However, for naked nano-sized silica, its agglomeration and incompatibility with hydrophilic monomers,19-21 such as acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2 methyl propyl sulfonic acid (AMPS), are impeding issues. Surface modification by hybridizing hyperbranched polymer like polyamidoamine (PAMAM) onto nano-sized silica is an effective method to enhance its hydrophilic performance and compatible with other monomers.22-26 Obviously, PAMAM hybrid nanoparticle used to prepare 1

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water soluble polymer with excellent viscosifying efficiency, thermal stability, salt tolerance and anti-shear degradation should contain three architectural components: (1) a nano-sized core, (2) an interior of hydrophilic shell consisting of repeating branch-cell units, (3) most importantly, terminal functional groups (double bonds) which can copolymerize with other monomers by simple in-situ free radical polymerization. Therefore, a full generation of PAMAM hybrid nano-sized silica should be encapsulated to manipulate terminal functional groups, namely PAMAM hybrid nano-sized silica monomer. In this article, one and two generations (G1.0, G2.0)of PAMAM hybrid nano-sized silica (PAMAMSG) were synthesized according to the method of Liu et al22 and Gartmann et al27 (Scheme 1a). The terminated amino groups of PAMAMSG were functionalized by encapsulation with maleic anhydride producing PAMAM hybrid nano-sized silica monomer (PAMAMSGF, Scheme 1b). A simple in-situ free radical copolymerization strategy was employed to synthesize novel water soluble core-shell hyperbranched polymers (HBPAMs) (Scheme 1c) by adjusting the hydrophilic numbers of PAMAMSGF. The morphologies for PAMAMSGF and for HBPAMs were characterized by IR, TGA, DLS and SEM, respectively. In light of their 3D topologies in aqueous solution, we have further investigated the displacement properties for enhanced oil recovery, such as rheological performance, anti-shear degradation, salt resistance, temperature tolerance, resistance factor (fr) and residual resistance factor (frr).

EXPERIMENTAL SECTION Reagents and materials. Methyl acrylate (MA), ethylenediamine (EDA), Acrylic acid (AA), Maleic anhydride (MAH), and ethanol (C2H5OH) purchased from Chengdu KeLong Chemical Reagent Co., Ltd (China) were of analytical grade and used without further purification. V-50 (2, 2’-azobis (2-amidinopropane) dihydrochloride, Alfa Aesar 99.99%) was used as received. The following reagents were used after purification: acrylamide (AM, KeLong), 2-acrylamide-2 methyl propyl sulfonic acid (AMPS, Alfa Aesar), 3-aminopropyltriethoxysilane (APTES, Aladdin), nano-sized silica (10nm, 1.162mmol/g of surface -OH number, Aladdin). All supplementary materials (Kelong) were of analytical grade and used as received. Water was double deionized with a Millipore Milli-Q system to produce the 18 MΩ deionized water. Synthesis of water soluble hyperbranched polymers (HBPAMs). PAMAM hybrid nano-sized silica (PAMAMSG) was prepared in consistent with Tomalia’s28 well known divergent approach by gradually repeating Michael addition and amidation reaction. The detialled procedure of synthesis of G1.0 and G2.0 PAMAM hybrid nano-silica monomers, and of water soluble hyperbranched polymers (HBPAMs) based on these monomers were documented in the Supporting Information. poly(AM/AA/AMPS)/SiO2 containing nano-sized SiO2 and hydrophilic chains, and poly(AM/AA/AMPS) were synthesized under identical experimental condition and purification method mentioned above (Scheme 1S in the Supporting Information). The IR spectrum (which was carried out using Shimadzu-1800S spectrometer on KBr pellets in the range of 4400-400cm-1.) PAMAMSG, PAMAMSGF and polymers were characterized by IR (Figure S1 in the Supporting Information). For PAMAMSGF (2.0) (Scheme 1b), the very strong absorptions at 3422cm-1 was attributed to stretching vibration for N-H of polyamidoamine. The strong absorptions at 3281cm-1 was attributed to synergistically stretching vibration for O-H of nano-silica core and for O-H of –COOH. The strong absorptions at 3281cm-1 was attributed to O-H stretching vibration for C=C. The absorptions at 1651cm-1 was attributed to stretching vibration for C=O. The absorptions at 1253cm-1 was attributed to stretching vibration for COO-, and the very strong absorptions at 1100cm-1 was attributed to stretching vibration for Si-O of nano-silica core. For HBPAM-2 (Scheme 1c), the very strong absorptions at 3422cm-1 was assigined to synergistically stretching vibration for NH2 of acylamido groups (CONH2) and for N-H of polyamidoamine, respectively. The strong absorptions at 1637cm-1 was assigined to stretching vibration for C=O. The strong absorptions at 1562cm-1 was assigined to stretching vibration for C-N. The absorptions at 1169cm-1 and 1120 cm-1 were confirmed groups of stretching vibration for S=O and S-O of -SO3-. Additionally, the absorptions at 1108cm-1 was attributed to stretching vibration for Si-O for nano-silica core. All of these revealed the existence of MAH functionalized PAMAM hybrid nano-SiO2 and the expected polymers HBPAMs with hyperbranched structure. 2


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Characterization.The degree of PAMAM hybrid SiO2 and its functionalized monomer can be analyzed by scaling terminal -NH2 and -COOH with diluted hydrochloric acid solution and dilute sodium hydroxide solution. 0.5g PAMAMSG was added into 100ml hydrochloric acid solution (cHCl=0.01mol/L) and stirred for 4 hours by a magnetic stirrer, then the degree of modification was measured by back titration with dilute sodium hydroxide solution (cNaOH=0.01mol/L).29 This method can be effectively used to measure the degree of MAH modified PAMAMSG. The slight difference was that functional monomer with -COOH as terminal groups should be firstly dissolved in 100ml dilute sodium hydroxide solution and scaled by back titration with dilute hydrochloric acid solution.

Theoretical value − Experimantal value

Hybrid degree (%) =

× 100% (1) Theoretical value The hydrodynamic diameter (Hd) of the each sample was obtained by using a BI-200SM wide angle dynamic laser light scattering (DLS) instrument. The weight loss of each sample versus temperatures was determined with a Mettler Toledo Stare System (Switzerland) (TGA/SDTA851e). The results were acquired with an elevating rate of 10℃/min from room temperature to 1000℃ under nitrogen gas at constant flow rate of 20ml/min. The weight-average molecular weight (Mw) of each polymer was determined in water/methanol mixture (3/7 v/v) containing 0.1M NaCl by classical light scattering using a multiangle spectrometer (AMTEC Model MM1) at 633 nm. The intrinsic viscosity [η] of the polymers was determined by an automatic capillary viscometer (Ubbelohds type) in 0.1M NaCl aqueous solution at room temperature. The morphologies of samples were observed by Environment Scanning Electron Microscope (SEM XL 30). Core flooding experiment. Six Berea sandstone cores were prepared and the basic parameters are listed in Table 1. The flooding rate for both water flooding and polymer flooding was set as 0.5ml/min and kept unchanged. The mobility control ability of polymer can be characterized by resistance factor (fr) and residual resistance factor (frr). The fr and the frr were calculated by the following equations: 30 Kw fr =

µw

(2)

Kp

µp f rr =

K wb

(3)

K wa

Where, K w is the aqueous phase permeability, mD； K p is the polymer phase permeability, mD； µ w is the aqueous viscosity, mPa·s; µ p is the polymer phase viscosity, mPa.s;

K wb is the aqueous phase permeability

before polymer flooding, K wb = K wa , mD； K wa is the aqueous phase permeability after polymer flooding, mD. The enhanced oil recovery (EOR) for polymer flooding was calculated by the equation: 31

EOR = Et − Ew

(4)

Where, EOR, %; Et , is the total oil recovery in the whole flooding process, %; Ew , is the initial water flooding recovery prior to polymer flooding, %. Table 1.Basic Parameters of Berea Sandstone Cores in the Core Flooding Experiment core samples

diameter

length

Volume 3

dry weight

wet

porosity

permeabiliity

oil

cm

cm

cm

g

weight g

%

mD

saturation %

150-47

3.79

6.982

78.77

145.86

160.07

17.18

157.3

/

150-50

3.794

7.06

79.82

152.76

164.17

13.61

141.6

/

150-36

3.792

7.054

79.62

152.38

163.97

14.56

158.4

/

3



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150-65

3.793

7.021

79.66

152.46

163.96

14.43

156.2

65.4

150-58

3.795

7.018

79.75

152.62

164.31

14.65

160.1

67.6

150-21

3.692

7.021

79.62

152.37

164.09

14.72

168.5

68.2

RESULTS AND DISCUSSION

(a)

(b)

(c)

(d)

Scheme 1. Synthesis procedure of water soluble core-shell hyperbranched polymer HBPAM-2.

Characterization of PAMAM hybrid nano-sized silica and of core-shell hyperbranched polymers. TGA analysis. As shown in Figure 1, the indication of the coating film on nano-SiO2 surface was measured from TGA measurement. Since the mean diameter of the nano-SiO2 particles is 10nm and there is no mass loss for untreated these nanoparticles. Organic coating onto nano particles surface trended to thermal decomposition (around 200-900℃), whereas the inorganic silica remains. The APTES modified nano-SiO2 particles (SG) were 4


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chemically bonded onto the surfaces of these particles and the percentage loss of weight organic coating was 6.2%. The number of APTES molecules was calculated by the following formula:32

4 3 , = (1 − ω ) M APTES

ω N a ρ π R3

N APTES

Where, N APTES is the APTES number on each nanoparticle, R is the mean radium of nanoparticles, the density of nano-SiO2, N a is the Avogadro’s number, M APTES is the relative weight of APTES, and

(5)

ρ is

ω

is

the weight loss (6.2% from the TGA curve of SG ). As a result, there are approximate 67 APTES molecules on each nanoparticle surface to form a layer, i.e., 67 hyperbranched PAMAM arms on each nanoparticle. Similarly, the weight losses of PAMAMSG and PAMAMSGF were theoretically calculated by the formula:32,33

67 M W =

4

Na

×100% ,

(6)

ρ π R 3 + 67 M N 3 a Here, W is the theoretic weight loss of dendrimer hybrid nanoparticle or maleic anhydride functionalized dendrimer hybrid nanoparticle, M is the relative molecular weight of each PAMAM arm calculated from their chemical structure. The theoretical values and experimental values of weight losses are shown in Table 2, the weight losses are respective 8.19% and 18.36% for PAMAMSG (1.0) and for PAMAMSG (2.0). The weight loss for PAMAMSGF (2.0) was 25.13% higher than that of 12.49% for PAMAMSGF (1.0). The experimental values are consistent with theoretical values indicating that hyperbranced structure onto the surface of SG through chemically bonded modification was manipulated. Moreover, both the amino (-NH2) content for PAMAM and the carboxyl (-COOH) content for maleic anhydride functionalized PAMAM can be theoretically measured by the formulas:20

nM NH 2

Na ×100% nM NH 2 4 3 ρ πR + Na 3 nM COOH Na COOH = ×100% 4 ρ π R 3 + nM COOH N 3 a NH 2 =

(7)

(8)

Here, NH2 and COOH are respective amino content and carboxyl content of PAMAM hybrid SiO2;

M NH 2 and M COOH are respective molecular weight of amino group and carboxyl group, n is the number of amino groups or carboxyl groups on each nanoparticle, which is calculated from the chemical structure. The theoretical values through the formulas (7) and (8), and their experimental values by back titration method mentioned above are given in Table 2. It is interesting to figure out that the experimental values from TGA curves analysis are agreement with theoretical values, implying maleic anhydride well functionalized PAMAM.

5



100

98.02%

95

93.72% 91.80%

Weight (%)

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90 85 80

SiO2

87.51%

SG PAMAMSG (1.0) PAMAMSG (2.0) PAMAMSGF (1.0) PAMAMSGF (2.0)

81.58%

74.87%

75 100

200

300

400 500 600 700 Temperature (℃)

800

900 1000

Figure 1. TGA curves of SiO2, SG, PAMAMSGs and PAMAMSGFs.

Table 2. Weight loss, surface amino group numbers of PAMAM hybrid nano-silica and carboxyl group numbers of PAMAM hybrid nano-silica monomer treminal group content on

-NH2 nmuber on each

-COOH nmuber on each

each nanoparticle(wt%)

nanoparticle

nanoparticle

weight loss (wt%) sample theoretical

experimental

theoretical

experimental

theoretical

experimental

theoretical

experimental

value

value

value

value

value

value

value

value

SG

/

6.2

0.41

0.39

/

67

/

/

PAMAMSG (1.0)

8.43

8.19

0.82

0.83

134

131

/

/

PAMAMSG (2.0)

18.47

18.36

1.62

1.58

268

263

/

/

PAMAMSGF (1.0)

12.75

12.49

2.58

2.55

/

/

134

132

PAMAMSGF (2.0)

25.49

25.13

5.03

5.12

/

/

268

265

Diameter distribution and morphology.The basic structural parameters of the synthesized polymers are listed in Table 3. DLS studies (Figure 2) were carried out on (a) nano-SiO2, PAMAMSGs, PAMAMSGF (2.0) and (b) polymers. And the typical morphologies for nano-SiO2, PAMSAMSGs and HBPAM-2 are shown in Figure 3. One observes that the experimental diameter of nano-SiO2 is 100nm, larger than the veritable diameter. The discrepancy between the experimental diameter and the veritable value is originating from the agglomeration of untreated nano-silica particles (Figure 3a). However, the functionalized silica particle exhibited smaller experimental diameter in comparison to naked silica particle, which is consistent with the results reported by Müller et al33 that hyperbranced structure film on the surface of nano-particle could somewhat prevent the coalescence of nanoparticles (Figure 3b). Some isolated form is also observed, directly indicating that the hybrid PAMAM to form polymer shell to hinder coalescence of the silica particles. According to Tomalia et al,9 the shell thickness is estimated to be approximately 2.6nm and 3nm for PAMAM (1.0) and PAMAM (2.0). Notably, the shell thickness of functionalized PAMAM hybrid nano-silica is higher.

6


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14 20

12 10

15 SiO2

Intensity (%)

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PAMAMSG (1.0) PAMAMSG (2.0) PAMAMSGF (2.0)

10

5

0 10

100

1000

8 6 4 2

HBPAM-2 HBPAM-1 poly(AM/AA/AMPS)/SiO2

0

poly(AM/AA/AMPS)

10000

1

a Diameter (nm)

10

100

1000

10000

b Diameter (nm)

Figure 2. Diameter distribution for (a) naked SiO2, PAMAMSGs and PAMAMSGF (2.0) in deionized water and (b) HBPAMs, poly(AA/AA/AMPS)/SiO2 and poly(AA/AA/AMPS) with diluted concentration in deionized water.

Table 3. Structural Parameters of the synthesized polymers polymer characterization sample

10-6×Mw(g/mol)

[η] ml/g

[η]calc ml/ga

solubility in waterb

poly(AM/AA/AMPS)

9.85

1565

1640

++

poly(AM/AA/AMPS)/SiO2

9.08

1464

/

+

HBPAM-1

9.12

1436

/

+

HBPAM-2

10.26

1497

/

+

a

Intrinsic viscosity established for PAM.

32 b

++,easily soluble; +, soluble.

The mean diameter of poly(AM/AA/AMPS), poly(AM/AA/AMPS)/SiO2, HBPAM-1 and HBPAM-2, is 404nm, 428 nm, 462 nm and 573nm respectively (Figure 2b). The significant difference of hydrodynamic diameter for linear poly(AM/AA/AMPS) and hyperbranched polymers indicating that PAMAM hybrid nano-silca monomer could radical copolymerize with other water soluble monomers to form 3D structure. Moreover, increasing the terminal functional groups (double bond numbers) markedly increases the hydrodynamic volume.

Figure 3. SEM morphologies of (a) nano-SiO2, (b) PAMSAMSG (2.0), (c) poly (AM/AA/AMPS), (d) poly(AM/AA/AMPS)/SiO2 and (e) HBPAM-2 in deionized water (polymer concentration, 5000mg/L). 7



The morphology of poly(AM/AA/AMPS) is irregular and rough due to the disordered aggregation of macromolecular chains (Figure 3c). However, many nano particles reinforce the topology of coiled hydrophilic chains leading to the relatively regular microstructure for poly(AM/AA/AMPS)/SiO2 (Figure 3d). Most significantly, regular microstructure for HBPAM-2 (Figure 3e) with uniform cavity is observed. This unique morphology is considered to be derived from that the hyperbranched macromolecular chains of HBPAM-2 orderly arrange to form 3D structure. The surface of microstructure for HBPAM-2 is much smooth in comparison to that for poly(AM/AA/AMPS), suggesting the stretching hydrophilic long chains as shell around the nano-silica core. All the experiments revealed that PAMAMSGF containing nano-SiO2 as core, functionalized PAMAM as branch-shell units was synthesized. Furthermore, hyperbranched polymers, based on this functional monomer, comprising of SiO2 as core, PAMAM as subshell, and hydrophilic long chains as outmost shell were successfully prepared. Displacement properties. After water flooding, the unrecovered oil is left as microscopic droplets of residual oil, and the advancing water front bypasses a significant portion of the reservoir resulting from poor mobility ration of injected fluid to displaced crude oil.3 Polymers employed in EOR applications should tolerate the high shear forces present during the flooding of a reservoir.35 The structure of porous medium can be seen as variation openings and confined throats through which the polymer morphology have to navigate in terms of orientation and deformation.36 It is reversible to produce the successful mobility control during the lifetime of polymers in the reservoir. Thus, the rheological performance of polymer solution is the primary necessity of a polymer used in EOR applications. Viscometric behavior for polymers in dilute aqueous solution presented in Figure 4 prove that the viscosity of all the polymers increases with increasing polymer concentration. To acquire detailed information, a fitted curve of corresponding apparent viscosity is scaled. As expected, these polymers exhibit classical behavior of power law fluid. The power law index for HBPAM-1 is 2.3, greater than that for poly(AM/AA/AMPS) (1.9) and for poly(AM/AA/AMPS)/SiO2 (2.1), suggesting greater power in thickening efficiency. the power law index of HBPAM-2 (2.2) is slightly lower than that of HBPAM-1, but the coefficient of power law equation for HBPAM-2 is as much three times as that for HBPAM-1. It not only greatly makes a compromise on such tiny discrepancy of power law index of HBPAM-2, but directly reflects the greatest enhancement efficiency among all the polymers. This can be interpreted that the hyperbranched polymer structure makes it easier to spontaneously connect and integrated through non covalent interactions (van der Waals interaction, electrostatic interaction, especially, entangled efficiency)37 in comparison to linear polymer chain above a critical polymer concentration. Noted that poly(AM/AA/AMPS)/SiO2 has the similar thickening efficiency in comparison to HBPAM-1, due to they bearing the same numbers of hydrophilic chains. Additionally, the number of hydrophilic chain for HBPAM-2 is twice than that of HBPAM-1 and of poly(AM/AA/AMPS)/SiO2, which makes much more opportunities into intermolecular entanglement to construct 3D dynamic physical network. 4000 sample

3500

HAPAM-2

HAPAM-1 poly(AM/AA/AMPS)

Equation

2500

poly(AM/AA/AMPS)/SiO2

y = a*(1 + x)^b

Reduced Chi-Sqr Adj. R-Square Value a b Standard Error a b

3000

η app (mPa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

817146 0.991 2.94E-5 2.185 2.926E-5 0.118

7508 0.988 6.645E-6 2.312 8.274E-6 0.148

2000

HBPAM-2 HBPAM-1 poly(AM/AA/AMPS)/SiO2

1500

poly(AM/AA/AMPS)

1985 0.993 1.151E-4 1.895 1.151E-4 0.091

4796 0.991 1.633E-5 2.107 1.651E-5 0.12

Fitted curve

1000 500 0 0

1000

2000 3000 4000 polymer concentration (mg/L)

5000

Figure 4. Apparent viscosity as a function of polymer concentration for polymers HBPAM-2, HBPAM-1, poly(AM/AA/AMPS)/SiO2 and poly(AM/AA/AMPS) samples (shear rate 7.34s-1,T=25℃). 8


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Viscoelasticity. We speculate that poly(AM/AA/AMPS)/SiO2 would have close characters to HBPAM-1 on other diplacement properties. This is beacuse they have the same numbers of terminal functional groups (double bonds) leading to bear the same numbers of hydrophilic chains. Therefore, we selected HBPAM-1 for the further ''

'

study. The variation of the elastic ( G ) and viscous ( G ) modulus was curved dependence on frequency( f ) (Figure 5). For HBPAMs, at low frequencies, the behavior of the shear modulus is Maxwellian behavior. The '

''

2

variations of G ( f ) and G ( f ) that scaled as f and f . The curves obtained for HBPAMs have the same profile as that for linear polymer solutions whose viscoelasticity is governed by chain entanglement. They are ''

'

''

'

characterized at low frequency by a slope of G and G , where the values of G are below that of G . Above '

''

the frequency at which curves G ( f ) and G ( f ) cross each other ( Gc ), indicating that elastic modulus plays a dominating role.38 The dynamics of the system in the terminal zone was scaled by the slope of G and '

''

G , Gc and characteristic time tc (1 / f ) corresponding to the crossing point Gc . According to the sticky reputation theory of Leibler et al,39 the value of tc indicated the disentanglement time of polymer chain limited outside the tube. Both Gc and tc quantificationally character viscoelastic properties. We note that hyperbranched polymer chains make more contributions to enhance elastic efficiency and relaxation time in the range of large-amplitude oscillatory. The results are documented in Table 4. 100 poly(AM/AA/AMPS) G' poly(AM/AA/AMPS) G" HBPAM-2 G' HBPAM-2 G" HBPAM-1 G' HBPAM-1 G"

10 G', G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.1

0.01 0.01 '

0.1

1 frequency/Hz

10

100

''

Figure 5. Storage ( G ) and loss ( G ) modulus as a function of frequency for polymers HBPAM-1, HBPAM-2 and poly(AM/AA/AMPS) ( 2000mg/L polymer concentration, T=25℃). '

''

Table 4. Parameters of the slopes of G and G , and values of Gc and tc for polymers. slope

samples

Gc (Pa)

tc (s)

0.043

0.813

0.46

5.11

1.22

5.5

4.49

1.36

1.46

G ' (Pa)

G" (Pa)

poly(AM/AA/AMPS)

0.053

HBPAM-2

7.88

HBPAM-1

7.02

Anti-shearing performance. Apparent viscosity related to shear rate of polymer samples are curved in Figure 6. In the whole shear rate range (from 7.34s-1 to 500s-1), all the polymers exhibit shear-thinning behavior. Core-shell hyperbranched structure induced viscosity of polymer solution hysteretic properties. At the onset of higher shear rate region, the entangled chains remain morphology persistent. With further increasing shear time, the entangled hyperbranched chains begin to disentangle, resulting in the dropped viscosity.

9



600

shear rate apparent viscosity

500

500

400

400

350

500

250

400

200

100

100

150 100

100

300

150

200 100 100

50 0

0

-1

200

Shear rate (s )

200

-1

-1

200

200

50 0

0

0 0

100

200

300 400 a Time (s)

500

600

0

700

100

200

300

400

500

ηapp (mPa.s)

300

300

ηapp (mPa.s)

300

Shear rate (s )

250

400

Shear rate (s )

500

300

ηapp (mPa.s)

600

0

100

b Time (s)

200

300

400

0 500

c Time (s)

Figure 6. Apparent viscosity of 2000mg/L copolymer samples as a function of shear rate and shear time (T=25℃), (a) HBPAM-2, (b) HBPAM-1 , (c) poly(AM/AA/AMPS).

The compromise apparent viscosity is observed when entanglement and disentanglement of hydrophilic chains strike a balance. Increasing shear rate from 7.34s-1 to 167.6s-1, it found that respective 50s and 43s would be taken for HBPAM-2 and for HBPAM-1 to obtain balanced viscosity in comparison to 19s for poly(AM/AA/AMPS). Further increasing shear rate to 500s-1, 36s and 24s for HBPAM-2 and HBPAM-1 samples were taken compared to 10s for poly(AM/AA/AMPS). The same lag properties of apparent viscosity recovery as a function of time at the decreased shear rate region were observed. The multiple hydrophilic chains of hyperbranched polymer enhance the strength of chain entanglement.40,41 Moreover, the shear rate is as high as 500s-1, the 3D PAMAM hybrid nano-silica core provides enough roughness to protect polymer chains from shear scission. Representative scheme of morphologies for hyperbranched polymer and linear hydrophilic polymer under shear force (Figures S2, S3 in the Supporting Information). Salt resistance. Increasing salt concentration, the viscosity of all the polymer samples decreased significantly and then kept a plateau region.34 Indeed, both HBPAMs and linear poly(AM/AA/AMPS) bear negative charges (carboxyl groups and sulfonic acid group). The thickening capability mainly lies in their large hydrodynamic chain entanglement and electrostatic repulsion.1 The shielding effect of the charges results in a reduction in electrostatic repulsion when added into salt. This is accompanied by relatively lower hydrodynamic volume, which is synonymous with a reduction in viscosity. Obviously, sulfonic acid groups into chain backbone can partly enhance polyelectrolytes slat resistance.3 Compared to amide group and carboxyl group, the longer side chain and larger hydraulics volume of AMPS provides stronger steric hindrance, rendering stretched polymer chain more stable in the presence of monovalent and divalent ions. Moreover, both the larger hydrodynamic dimension and more regular microstructure of hyperbranched macromolecule prevent the multiple hydrophilic chains from coiling.42 We noted that even at the highest concentration of salt, no precipitation and phase separation behavior is displayed. This is suggesting that all the polymers have excellent compatible with wide range slat concentration. The results are listed in Figure 7.

1000

1000

1000

poly(AM/AA/AMPS) HBPAM-1 HBPAM-2

100

10

η app (mPa.s)



η app (mPa.s)

ηapp (mPa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 14

100

10

10 0

20000

40000 60000 80000 a NaCl concentration (mg/L)

100000

100

0

200

400

600

800

1000

b CaCl2 concentration (mg/L)

0

200

400

600

800

1000

1200

1400

1600

c MgCl2 concentration (mg/L)

Figure 7. Apparent viscosity of polymer samples as a function of (a) NaCl and (b) CaCl2 and (c) MgCl2 concentration (2000mg/L polymer concentration, shear rate 7.34s-1, T=25℃).

Temperature tolerance. It has long been accepted that increasing the temperature of the polymer solution resulted in a reduction in viscosity.4 poly(AM/AA/AMPS) remains high temperature tolerance due to introduction of large sulfonic acid group. From the perspective of macromolecular morphology, the multiple 10


Page 11 of 14

chains of HBPAMs exhibiting 3D conformation which provides steric hinderance for the macromolecular that protects the hydrophilic subchain from coiling at the increased temperature (Figure 8). 700

250 225

600

400 300


175 η app (mPa.s)

500 η app (mPa.s)

200


150 125 100 75

200

50 25

100 30

40

50

60

70

80

90

30

40

50

a Temperature (℃)

60

70

80

90

b Temperature (℃)

Figure 8. Apparent viscosity polymer samples versus temperature (a) deionized water, (b) 20000mg/L NaCl concentration (2250mg/L polymer concentration, shear rate 7.34s-1).

Core flooding experiment. The fr and the frr constructed by polymer flooding are shown in Table 5. One observes that HBPAMs more effectively control flooding phase mobility, reside in the porous medium and adhere on some part of core walls to establish residual resistance. 70

80

subsequent water flooding

water flooding

30

0.3

20

0.2

10

0.1

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.0 5.0

60

8

2.5 oil recovery pressure drop

Polymer flooding

40

2.0 1.5 1.0

20

0.5 0 0.0

a Cumulative injection volume (PV)

0.5

1.0 1.5 2.0 2.5 b Cumulative injection volume (PV)

3.0

Cumulative oil recovery (%)

oil recovery pressure drop

Pressure drop (MPa)

0.4

polymer flooding


50

0 0.0

9 80

3.0

0.5

40

3.5

0.6

7 60

oil recovery pressure drop

polymer flooding

6 5

40

4 3

20

0.0 3.5

Pressure drop (MPa)


60

Pressure drop (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1

0 0.0

0.5

c

0 3.5

1.0 1.5 2.0 2.5 3.0 Cumulative injection volume (PV)

Figure 9. Core flooding experiments of polymer flooding (a) poly(AM/AA/AMPS), (b) HBPAM-1 and (c) HBPAM-2 for enhanced oil recovery at 70℃.

Table 5. The displacement parameters of polymer core flooding. salt concentration (TDS, 11439mg/L) core samples

polymer flooding system oil

(1750mg/L)

viscosity Na++K+

Ca2++Mg2+

Cl-+SO42-+HCO3-

(mPa.s)

150-47

polymer system poly(AM/AA/AMPS)

viscosity

fr

frr

53

5.4

Ew

E

Et

%

%

%

/

/

/

(mPa.s) 23.8

150-50

HBPAM-1

29.3

157

14.9

/

/

/

150-36

HBPAM-2

37.9

245

25.2

/

/

/

150-65

poly(AM/AA/AMPS)

23.8

/

/

51.3

9.9

61.2

150-58

HBPAM-1

29.7

/

/

60.2

13.6

73.8

150-21

HBPAM-2

37.9

/

/

60.8

16.3

77.1

4008.8

63

7367.2

20.6

As shown in Figure 9, HBPAMs display excellent injective performance. Because the injection pressure gradually increases during the injection of polymer solution, and gradually decreases after injection of some volume of water. More convincingly, HBPAMs (Figure 9b, c) flooding exhibits stronger reducing water cut and improving sweep efficiency. Following water cut to 98% for water flooding, 0.25PV polymer flooding and subsequent water flooding can further increases 13.6 % (HBPAM-1) and 16.3% (HBPAM-2) oil recovery 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

compared with 9.9% oil recovery for linear polymer (Figure 9a). The results further suggesting that core-shell hyperbranched polymers have robust application for enhanced oil recovery.

CONCLSIONS A new type of core shell hyperbranched polymer (HBPAM) consisting of core, subshell and outermost flexible shell by simply free water radical polymerization are successfully synthesized. IR, TGA, back titration, SEM and DLS measurements confirmed that both the approach and the preparation of HBPAM are scientific and practicable. Intersection modulus (Gc) and relaxation time (tc) were 1.36Pa and 1.46s for HBPAM-1 and 1.22Pa and 5.5s for HBPAM-2 in comparison to 0.813Pa of Gc and 0.46s of tc for poly(AM/AA/AMPS). Static experiments convincingly proved that the hyperbranched topological structure provides HBPAMs excellent shear resistance, desirable salt resistance and temperature tolerance. Core flooding experiment directly demonstrated that HBPAMs have more effective on tuning flooding phase mobility and improving sweep efficiency by constructing fr and frr in the porous medium. To updating its comprehensive characters, positively responsive water soluble hyperbranched polymer upon temperature and salinity by introducing hydrophobic groups and thermo-sensitive pendants into hydrophilic backbone of outermost shell should be deeply and systematically investigated.

AUTHOR INFORMATION Corresponding Author *Tel: +86 15928967990. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful to the Open Fund (PLN1417) by State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University) and to the Major Special Project of China (Grant No. 2011ZX05049-004-004-008) for financial support of this work. We express our sincere appreciation to the reviewers for their constructive comments.

Supporting Information Detailled experimental part, typical FTIR spetrum and Three additional figures. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Water-Soluble CoreâShell Hyperbranched Polymers for Enhanced Oil

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