Synthesis and Evaluation of a Water-Soluble Hyperbranched Polymer as Enhanced Oil Recovery Chemical

A novel hyperbranched polymer was synthesized using acrylamide (AM), acrylic acid (AA), N-vinyl-2-pyrrolidone (NVP), and dendrite functional monomer as raw materials by redox initiation system in an aqueous medium. The hyperbranched polymer was characterized by infrared (IR) spectroscopy, H NMR spectroscopy, C NMR spectroscopy, elemental analysis, and scanning electron microscope (SEM).The viscosity retention rate of the hyperbranched polymer was 22.89% higher than that of the AM/AA copolymer (HPAM) at 95C, and the viscosity retention ratewas 8.17%, 12.49%, and 13.68%higher than that ofHPAM in 18000mg/L NaCl, 1800mg/L CaCl 2 , and 1800mg/L MgCl 2 ⋅6H 2 O brine, respectively. The hyperbranched polymer exhibited higher apparent viscosity (25.2mPa⋅s versus 8.1mPa⋅s) under 500 s shear rate at 80C. Furthermore, the enhanced oil recovery (EOR) of 1500mg/L hyperbranched polymer solutions was up to 23.51% by the core flooding test at 80C.

Keeping in mind all the above points, herein, a novel hyperbranched polymer was synthesized by free radical polymerization based on AM, AA, NVP, and dendrite functional monomer aiming to obtain satisfying temperature-resistance, salt-resistance, and shear-resistance.

Synthesis of Dendrite Functional
Monomer.Synthesis of 0.5 generation dendritic macromolecule (DM 0.5 ): 113.5 g methyl acrylate was added into a three-necked flask with methanol as solvent, and then 9.0 g ethylenediamine was dripped into the stirred solution in the three-necked flask.The reaction time was 24 h at 25 ∘ C.After reaction, the product was purified by vacuum distillation and silica gel column.Then the DM 0.5 was obtained [18,20].
Synthesis of 1.0 generation dendritic macromolecule (DM 1.0 ): 40.0 g ethylenediamine was added into a threenecked flask with methanol as solvent, then 20.2 g DM 0.5 was dripped into the stirred solution in the three-necked flask.
The reaction time was 48 h at 25 ∘ C, and the product was the DM 1.0 , which was purified by vacuum distillation and silica gel column [18,20].
Modification of DM 1.0 : 4.4 g maleic anhydride was added into a round-bottom flask with N,N-dimethylformamide as solvent, and 8.0 g DM 1.0 was dripped into the round-bottom flask.The reaction time was 8 h at 70 ∘ C, and the dendrite functional monomer was obtained by vacuum filtration.

Synthesis of Hyperbranched
Polymer and HPAM.Firstly, 7.00 g AM, 2.95 g AA, 0.01 g dendrite functional monomer, 0.04 g NVP, and 1.65 g sodium hydroxide were added into a 100 mL three-necked flask with 38.35 mL distilled water as solvent.Secondly, 0.04 g NaHSO 3 -(NH 4 ) 2 S 2 O 8 initiator (mol ratio = 1 : 1) was taken along with distilled water in the three-necked flask.And then, the copolymerization was carried out for 4 h at 50 ∘ C under nitrogen atmosphere.Finally, the hyperbranched polymer was obtained by ethanol washing, drying, and pulverizing.The synthesis route of the hyperbranched polymer is shown in Scheme 1.The AM/AA copolymer (HPAM) was synthesized using 7.00 g AM and 2.95 g AA as raw materials through the same synthesis method.

Weight-Average Molecular Weight.
The weight-average molecular weight (  ) of the hyperbranched polymer and HPAM was determined by a BI-200SM wide angle dynamic/static laser light scattering apparatus at 25 ∘ C. The laser wavelength was 532 nm.The   of the hyperbranched polymer and HPAM can be obtained by the following equation [26,27]: where  is the concentration of polymer solution, g/mL;  is a constant; ⟨  ⟩ is the average radius of gyration, nm;  VV () is the Rayleigh ratio; and  = (4/  ) sin(/2) with ,   , and  being the solvent refractive index, the wavelength of laser in vacuo, and the scattering angle, respectively.
2.6.Temperature-Resistance and Salt-Resistance.Hyperbranched polymer and HPAM solutions (5000 mg/L) were prepared with distilled water.The apparent viscosity of these polymers solutions was tested using Brookfiled DV-III viscometer at different temperatures.The salt-resistance performance was studied by increasing salt (NaCl, CaCl 2 , or MgCl 2 ⋅6H 2 O) concentration, and then the apparent viscosity of these polymers solutions was measured via Brookfield DV-III viscometer at 20 ∘ C.
2.8.Core Flooding Experiments.Two Berea sandstone cores were used to study the EOR ability of these copolymer solutions (1500 mg/L) prepared with brine.Total dissolved solids (TDS) and chemical composition of the brine are listed in Table 1.The core was placed into Hassler core holder with 1.0 MPa backpressure and 3.0 MPa confining pressure.
It was saturated with the brine, and then it was saturated with crude oil (62.2 mPa⋅s at 80 ∘ C) at different injection rate (0.1-0.2 mL/min) until irreducible water saturation ( wi ) was established.After 96 h of aging, the brine was injected at 0.2 mL/min to displace the crude oil until water cut reached 95%, and then the polymer solution was injected at 0.2 mL/min to obtain water cut 95% once more.The EOR of polymer solutions is calculated with the following equation: where EOR is enhanced oil recovery of polymer solution, %;  is the oil recovery of water flooding and polymer flooding process, %;  w is the oil recovery of water flooding process, %.
All core flooding experiments were conducted at 80 ∘ C. The maximum work pressure of the ISCO pump is 50 MPa, and its maximum and minimum displacement rates are 50.000and 0.001 mL/min, respectively.The pressure drop was recorded by a pressure sensor with a precision of ±0.0001 MPa.And flow chart of the core flooding tests is shown in Figure 1.

Results and Discussion
3.1.IR Spectra Analysis.The structures of the dendrite functional monomer and hyperbranched polymer were confirmed by IR spectra as illustrated in Figure 2. The dendrite functional monomer, which was prepared using EDA, methyl acrylate, and maleic anhydride, was confirmed by strong absorptions at 3383.78 cm −1 (-NH stretching vibration), 2949.63 cm −1 (-CH 2 stretching vibration), 1646.91 cm −1 (C=O stretching vibration and carbon double-bond stretching vibration), and 1554.98 cm −1 (C-N stretching vibration and -NH bending vibration) in the IR spectroscopy of the dendrite functional monomer.Pure NVP exhibited a very strong absorption at 1703.01 cm −1 , which reflected the carbonyl stretching; at 1629.32 cm −1 , which was a carbon doublebond stretching vibration; and at 1445.11 cm −1 , which was the characteristic absorption peak of NVP.The characteristic absorptions of the dendrite functional monomer and NVP were clearly presented, and the carbon double-bond was not detected in the IR spectroscopy of the hyperbranched polymer.As expected, the IR spectra demonstrated that the hyperbranched polymer was successfully synthesized.The characteristic peak at 48.88 ppm belongs to C 3 .The characteristic peak at 36.90 ppm is due to C 2 .The chemical shift value at 31.01 ppm is assigned to C 1 .The results of 1 H NMR spectrum and 13 C NMR spectrum showed that the dendrite functional monomer was synthesized.
The 1 H NMR spectrum and 13 C NMR spectrum of the hyperbranched polymer are shown in Figures 4(a The 1 H NMR spectrum and 13 C NMR spectrum of HPAM are shown in Figures 5(a) and 5(b), respectively.In Figure 5(a), the chemical shift value at 2.12 ppm is assigned to the -CH-CH 2 -protons.The characteristic peak of the -CH-CH 2 -protons appears at 1.55 ppm.In Figure 5(b), the chemical shift value at 183.12 ppm belongs to C 4 .The chemical shift value at 179.58 ppm is due to C 3 .The characteristic peak at 41.95 ppm belongs to C 2 .The chemical shift value at 34.99 ppm is assigned to C 1 .

Elemental Analysis of the Hyperbranched Polymer and HPAM.
The elemental analysis of the hyperbranched polymer and HPAM was carried out by Vario EL III elemental analyzer.The content of different elements can be calculated by detecting the gases, which are the decomposition products of these copolymers at high temperature.Theoretical values of the hyperbranched polymer are 50.49%(C%), 6.53% (H%), 30.12% (O%), and 12.86% (N%); found values of the hyperbranched polymer are 45.46% (C%), 6.12% (H%), 26.73% (O%), and 11.36% (N%).Theoretical values of HPAM are 50.50%(C%), 6.60% (H%), 29.03% (O%), and 13.87% (N%); found values of HPAM are 45.91% (C%), 6.04% (H%), 26.19% (O%), and 12.03% (N%).).As shown in Figure 6, it could be obviously observed that there were space net structures in the images of the hyperbranched polymer solutions.Moreover, it could be found that the microscopic reticular structures of the hyperbranched polymer solutions were much more compact than those of HPAM solutions in the same scan size.The much denser networks of the hyperbranched polymer solutions may help to reduce the effect of shear on the hyperbranched polymer molecular chain and improve the viscosity retention rate of the hyperbranched polymer at high shear rate.

3.5.
Weight-Average Molecular Weight. 2 mg/L copolymer solution was prepared using distilled water and filtered by a 0.5 m Millipore Millex-LCR filter.As shown in Figure 7,    reveal that the hyperbranched polymer can withstand higher salt concentration than HPAM.This characteristic may be well explained by the special network structure which can enhance the interaction between the hyperbranched polymer chains, and crimping degree of the polymeric chains will be smaller than HPAM at the same salt concentration.Thus the hyperbranched polymer exhibits higher apparent viscosity and retention rates.
3.8.Shear-Resistance.Shear-resistance of the polymer solutions was conducted on HAAKE RS 6000 rotational rheometer at 80 ∘ C by changing the shear rate from 170 s −1 to 500 s −1 and from 500 s −1 to 170 s −1 around.As shown in Figure 10, the viscosity retention rate of the HPAM and the hyperbranched polymer was 61.95% and 91.64%, respectively, when one cycle was completed.The phenomena may support the microscopic reticular structures of the hyperbranched polymer which can reduce the effect of shear on the hyperbranched polymer molecular chain during shear process and restore the structures of the hyperbranched polymer after being sheared.compared with HPAM, the hyperbranched polymer revealed stronger ability of establishing flow resistance and reducing water cut in polymer flooding.This phenomenon may support the sweep efficiency which is obviously improved by the hyperbranched polymer due to its excellent temperatureresistance, salt-resistance, and shear-resistance.

Conclusions
A novel hyperbranched polymer possessing microscopic reticular structure was successfully synthesized using AM, AA, NVP, and dendrite functional monomer as raw materials under mild conditions.Compared with HPAM, the hyperbranched polymer exhibits obvious advantages in temperature-resistance, salt-resistance, and shear-resistance due to the introduction of pyrrole ring which can reduce the influence of high temperature on the polymer molecular chain and the introduction of the reticular structures which can be favorable to decrease the crimping degree of polymeric chains under high shear rate and high salinity.Thus, the EOR capability of the hyperbranched polymer is improved remarkably even in a harsh condition.

3. 2 . 1 H
NMR and13 C NMR Analyses.The 1 H NMR spectrum and13 C NMR spectrum of the dendrite functional monomer are shown in Figures3(a) and 3(b), respectively.In Figure 3(a), the chemical shift value at 2.44 ppm is assigned to the -CH 2 -CH 2 -C(O)-NH-protons.The chemical shift value at 2.60 ppm is due to the -CH 2 -N(CH 2 -CH 2 -C(O)-) 2 protons.The -CH 2 -CH 2 -C(O)-NH-protons appear at 2.79 ppm.The chemical shift value at 3.04 ppm is assigned to the -NH-CH 2 -CH 2 -protons.The -NH-CH 2 -CH 2protons appear at 3.42 ppm.The chemical shift value at 5.95 ppm is due to the -C(O)-CH=CH-C(O)-protons.In Figure 3(b), the chemical shift value at 171.11 ppm is due to C 6 .The chemical shift value at 164.96 ppm belongs to C 5 .The chemical shift value at 134.49 ppm is due to C 4 .
) and 4(b), respectively.In Figure4(a), the chemical shift value at 3.23 ppm is due to the -NH-CH 2 -CH 2 -protons and the -C(O)-CH 2 -CH 2 -CH 2 -protons.The chemical shift value at 2.65 ppm is assigned to the -CH 2 -N(CH 2 -CH 2 -C(O)-) 2 protons.The -CH 2 -CH 2 -C(O)-NH-protons and the -CH (C(O)-)-CH(C(O)-)-protons appear at 2.49 ppm.The chemical shift value at 2.16 ppm is assigned to the -C(O)-CH 2 -CH 2 -CH 2 -protons, the -CH 2 -CH 2 -C(O)-NH-protons, and the -CH 2 -protons which are obtained from the carbon double-bonds of AM, AA, and NVP.The -CH-protons, which are free radical polymerization products of the carbon double-bonds of AM, AA, and NVP, appear at 1.54 ppm.In Figure 4(b), the chemical shift value at 182.96 ppm belongs to C 8 .The chemical shift value at 179.79 ppm is due to C 7 .The chemical shift value at 178.63 ppm is assigned to C 6 .The chemical shift value at 44.80 ppm is due to C 5 .The characteristic peak at 42.30 ppm belongs to C 4 .The characteristic peak from 35.11 to 36.95 ppm is due to C 3 .The chemical shift value at 31.80 ppm is assigned to C 2 .And the characteristic peak of C 1 is observed at 17.62 ppm. 1 H NMR spectrum and 13 C NMR spectrum indicated that the hyperbranched polymer was successfully synthesized.

Figure 3 :
Figure 3: (a) 1 H NMR spectrum of the dendrite functional monomer in D 2 O; (b) 13 C NMR spectrum of the dendrite functional monomer in D 2 O.

Figure 4 :
Figure 4: (a) 1 H NMR spectrum of the hyperbranched polymer in D 2 O; (b) 13 C NMR spectrum of the hyperbranched polymer in D 2 O.

Figure 5 :
Figure 5: (a) 1 H NMR spectrum of HPAM in D 2 O; (b) 13 C NMR spectrum of HPAM in D 2 O.

Table 1 :
TDS and chemical composition of the brine.

Table 2 :
The parameters of cores and the results of core flooding tests.Figure 7: / VV () versus  2 for the hyperbranched polymer and HPAM.

Table 2
, the EOR of the hyperbranched polymer solutions and HPAM solutions was 23.51%, and 16.67%, respectively.This phenomenon may support higher viscosity retention rate of the hyperbranched polymer contributes to expand injection water sweeping volume and enhance oil recovery.As shown in Figure11,