Synthesis and Performance of an Acrylamide Copolymer Containing Nano-SiO 2 as Enhanced Oil Recovery Chemical

A novel copolymer containing nano-SiO 2 was synthesized by free radical polymerization using acrylamide (AM), acrylic acid (AA), and nano-SiO 2 functional monomer (NSFM) as raw materials under mild conditions. The AM/AA/NSFM copolymer was characterized by infrared (IR) spectroscopy, HNMR spectroscopy, elemental analysis, and scanning electronmicroscope (SEM). It was found that theAM/AA/NSFMcopolymer exhibited higher viscosity than theAM/AAcopolymer at 500 s shear rate (18.6mPa⋅ s versus 8.7mPa⋅s). It was also found thatAM/AA/NSFMcould achieve up to 43.7% viscosity retention rate at 95C.Mobility control results indicated that AM/AA/NSFM could establish much higher resistance factor (RF) and residual resistance factor (RRF) than AM/AA under the same conditions (RF: 16.52 versus 12.17, RRF: 3.63 versus 2.59). At last, the enhanced oil recovery (EOR) of AM/AA/NSFM was up to 20.10% by core flooding experiments at 65C.

Recently, many studies have demonstrated that performance of composite material could be significantly improved by combination or copolymerization with a functional monomer containing nano-SiO 2 .The composite material containing Nano-SiO 2 , such as polyethylene terephthalate [9], styrene butadiene rubber [10], polyaniline [11], polyimide [12], and nylon 6 [13], may exhibit more satisfactory thermal stability, toughness, and strength owing to the effect of physical adsorption, hydrogen bond, Si-O bond, and C-Si bond [10,12,[14][15][16].However, there are no papers about the application of nano-SiO 2 in polymer for flooding to develop temperature tolerance, salt tolerance, and shear resistance of copolymer.

Preparation of Nano-SiO
2 Functional Monomer.Firstly, 83.6 mL ethanol, 1.5 g nano-SiO 2 , 13.6 mL distilled water, and 1.3 mL ammonia were added into a 250 mL round-bottom flask, and the mixture was dispersed with supersonic wave for 30 min.Then 2.0 mL VTES was added into the stirred solution in the round-bottom flask, and the reaction time was 18 h at 30 ∘ C.After reaction, the product was NSFM which was separated by centrifugation and washed with distilled water [21][22][23].
2.3.Synthesis of AM/AA/NSFM.0.05 g NSFM, 6.50 g AM, 3.45 g AA, and a certain amount of distilled water were added into a 100 mL three-necked flask, respectively, and the pH value of the mixture was regulated to 7.0 using sodium hydroxide solution; then the solution with 20% total monomer mass concentration was prepared.0.05 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 assembled with a nitrogen (N 2 ) inlet.Then the copolymerization was carried out at 45 ∘ C under N 2 atmosphere for 6 h.Finally, the AM/AA/NSFM copolymer was obtained after ethanol washing, drying, and pulverizing.The synthesis of AM/AA/NSFM is shown in Scheme 1 [20].The AM/AA copolymer was synthesized by using the same method.weight-average molecular weight (  ) of the copolymers was obtained by using a BI-200SM wide angle dynamic/static laser light scattering apparatus.

Intrinsic Viscosity. The intrinsic viscosity of copolymer
was measured with an Ubbelohde viscometer at 25 ∘ C. The test temperature was controlled using a constant temperature bath.The flux time was reproducible to 0.05 s using a stopwatch.The copolymer solutions, at five different concentrations (0.1000, 0.0667, 0.0500, 0.0333, and 0.0250 wt%), were prepared with distilled water.The specific viscosity is calculated via the following equation [6,24,25]: where  sp is the specific viscosity of copolymer;  0 is flux time of distilled water, s; and  is flux time of copolymer solution, s.
Then the intrinsic viscosity is calculated with the Huggins equation [6,26]: where [] is intrinsic viscosity, mL/g;  is concentration of copolymer solution, wt%; and   is the Huggins constant.

Rheological Property and Viscoelasticity.
Rheological property and viscoelasticity measurements of the copolymers were conducted on a HAAKE RS 600 Rotational Rheometer (Germany).The shear rate was from 0.007 s −1 to 500 s −1 , and the temperature was 65 ∘ C with a heating rate of 1.5 ∘ C/min, while the test system was binocular tube and the rotor was DG41Ti in rheological measurements.The scanning range of frequency () was 0.01-10 Hz, and the stress was 0.1 Pa by using the same test system and rotor in viscoelasticity measurements.

Mobility Control Ability.
The mobility control ability of the copolymer solutions is characterized by the resistance factor (RF) and the residual resistance factor (RRF) [27][28][29].
The RF is calculated with the following equation: where   is aqueous phase permeability, mD;   is polymer phase permeability, mD;   is the viscosity of aqueous phase, mPa⋅s; and   is the viscosity of polymer phase, mPa⋅s.
The RRF is calculated with the following equation: where  wb is aqueous phase permeability before polymer flooding, mD;  wa is aqueous phase permeability after polymer flooding, mD.2.8.Core Flooding Tests.Two Berea sandstone cores were used for core flooding experiments.The cores were dried at 65 ∘ C, and then their length, diameter, porosity, and gas permeability were measured by using a SCMS-B2 core multiparameter measurement system.A Hassler core holder was used with 3.5 MPa confining pressure and 1.5 MPa backpressure.The core, after being saturated with brine, was saturated with crude oil (52.5 mPa⋅s at 65 ∘ C) at 0.1-0.2mL/min until irreducible water saturation ( wi ) was obtained.After 72 h of aging, the core was flooded by the brine at 0.2 mL/min until water cut was up to 95%, and then the copolymer solution (0.2 wt%) was injected at 0.2 mL/min until water cut reached 95% once more [6,29].All the core flooding procedures were conducted at 65 ∘ C. Chemical composition and total dissolved solids (TDS) of the brine are listed in Table 1.The maximum work pressure of the ISCO 260D syringe pump is 50 MPa, and its minimum and maximum displacement velocity is 0.001 and 50.000 mL/min, respectively.The EOR is calculated with the following equation: where EOR is enhanced oil recovery, %;  is the oil recovery of the whole displacement process, %; and   is the oil recovery of water flooding, %.Flow chart of the core flooding experiments is shown in Figure 1.

Results and Discussion
3.1.IR Spectra Analysis.The structures of AM/AA and AM/ AA/NSFM were confirmed by IR spectra as illustrated in Figure 2. The AM/AA/NSFM which was prepared using acrylic acid, acrylamide, and NSFM by free radical polymerization was confirmed by strong absorptions at 3419 cm −1 (-NH stretching vibration and -OH stretching vibration), 2942 cm −1 (-CH 2 stretching vibration), 1675 cm −1 (C=O stretching vibration), 1402 cm −1 (C-N stretching vibration), 1100 cm −1 (Si-O-Si asymmetric stretching vibration), and 780 cm −1 (Si-O-Si symmetric stretching vibration) in the spectrum of AM/AA/NSFM [18,20].The peak at 3419 cm −1 was broad in the IR spectrum of AM/AA/NSFM partly due to the hydroxyl on nano-SiO 2 surface [20].As expected, the IR spectra confirmed the presence of different monomers in AM/AA/NSFM.(0.05 wt%) were prepared with distilled water and cooled with liquid nitrogen, and then these samples were evacuated in order to keep original appearance of the copolymers as far as possible.As shown in Figures 4 and 5, the molecular chains of copolymer were obviously changed when NSFM was introduced into the AM/AA copolymer.Compared with the images of AM/AA, the molecular coils of AM/AA/NSFM were composed of many micro-nano structure units, and the force between these units could be heightened due to Si-O and C-Si bonds.In addition, this structure may increase retention of AM/AA/NSFM on the rock face which is favorable to mobility control and EOR.

Weight-Average Molecular Weight.
Five different concentrations (0.001, 0.002, 0.004, 0.006, and 0.008 wt%) copolymer solutions were prepared with distilled water and filtered using a 0.5 m Millipore Millex-LCR filter before static laser light scattering (SLLS) experiments.The   of AM/AA and AM/AA/NSFM can be calculated with the following equation [30]: where  is a constant;  is the concentration of copolymer solution, g/mL;  vv () is the Rayleigh ratio; ⟨  ⟩ is the average radius of gyration, nm; and  = (4/  ) sin(/2) with ,   , and  being the scattering angle, the wavelength of light in vacuo, and the solvent refractive index, respectively.The   of AM/AA and AM/AA/NSFM is (1.33 ± 0.30) × 10 7 g/mol and (1.32 ± 0.45) × 10 7 g/mol, respectively (for details, see Supporting Material available online at http:// dx.doi.org/10.1155/2013/437309).extrapolated to zero concentration.According to the Huggins equation, the -intercept is the intrinsic viscosity of the copolymers.The results revealed that the intrinsic viscosity of AM/AA and AM/AA/NSFM was 733.9 and 789.5 mL/g, respectively.

Shear Resistance.
The viscosity versus shear rate curves of AM/AA and AM/AA/NSFM (0.2 wt%) are shown in Figure 7(a).It was clearly found that AM/AA and AM/AA/ NSFM revealed non-Newtonian shear-thinning behavior.Hence, with the increase of the shear rate (from 0.007 to 500 s −1 ), the viscosity of copolymer solutions dropped obviously.The results indicated that AM/AA/NSFM had better viscosifying property than AM/AA, and the viscosity of AM/AA/NSFM was higher than that of AM/AA at 500 s −1 shear rate (18.6 mPa⋅s versus 8.7mPa⋅s).Furthermore, AM/AA and AM/AA/NSFM were investigated by changing the shear rate from 124 s −1 to 500 s −1 and from 500 s −1 to 124 s −1 around (Figures 7(b) and 7(c)).Compared with AM/AA, AM/AA/NSFM had higher retention rate of viscosity (85% versus 68%) when one cycle was completed.This phenomenon may support the Si-O and C-Si bonds in AM/AA/ NSFM which can improve the shear tolerance of the copolymer.The structures of AM/AA/NSFM may be restored after being sheared.

Viscoelasticity Measurements.
The viscoelasticity curves of AM/AA and AM/AA/NSFM solutions (0.2 wt%) are shown in Figure 8.When the frequency was lower than 1 Hz, the viscous modulus (G  ) of AM/AA/NSFM was higher than the elastic modulus (G  ); when the frequency was higher than 1 Hz, the situation was just the opposite.However, the G  of AM/AA was higher than G  in the entire frequency scan range.Compared with AM/AA, AM/AA/NSFM exhibited higher G  and G  under the same conditions.This phenomenon may support the micro-nano structure units in AM/AA/NSFM can enhance the acting force of polymer molecular coils.pressure was collected by a pressure sensor with precision of 0.0001 MPa.The flow characteristic curves of AM/AA and AM/AA/NSFM in porous media are shown in Figure 11.
As shown in Figure 11, the AM/AA/NSFM solution could establish much higher RF and RRF than that of the AM/AA solution under the same conditions (RF: 16.52 versus 12.17, RRF: 3.63 versus 2.59).This is to say that the AM/AA/NSFM solution has stronger mobility control ability which is favorable to enhance oil recovery due to the higher viscosity retention rate and microstructure.In addition, it was found that AM/AA/NSFM revealed higher retention than AM/AA (83 mg versus 55 mg) by material balance calculations (for details, see Supporting Materials).This may support that the huge surface area of micro-nano structure units of AM/AA/NSFM can enhance the adsorption which may play an important role in improving mobility control.
3.12.Enhanced Oil Recovery.As shown in Table 2, the EOR of AM/AA/NSFM solution (0.2 wt%) was 20.10% compared with water flooding at 65 ∘ C.However, the EOR of AM/AA solution (0.2 wt%) was 14.22% under the same conditions.The EOR results showed that AM/AA/NSFM revealed more superior ability of oil displacement.As shown in Figure 12, compared with AM/AA, AM/AA/NSFM exhibited stronger ability of reducing water cut and establishing flow resistance in polymer flooding process.This phenomenon may support that the sweep efficiency is obviously improved  by AM/AA/NSFM due to the excellent mobility control capability in porous media.

Conclusions
A novel copolymer containing nano-SiO 2 was synthesized by free radical polymerization using AM, AA, and NSFM as raw materials.The AM/AA/NSFM copolymer was characterized by IR spectrum, 1 H NMR spectrum, elemental analysis, and scanning electron microscope.The solution properties, such as rheological property, viscoelasticity, temperature tolerance, salt tolerance, mobility control ability, and oil displacement efficiency of the copolymer, were investigated under different conditions.The results indicated that the copolymer containing nano-SiO 2 possessed moderate or good shear resistance, temperature tolerance, and mobility control ability as EOR chemical.

Figure 6 :
Figure 6: The  sp / versus  relationships of AM/AA and AM/AA/ NSFM.

Figure 7 :
Figure 7: (a) Effect of shear rate on viscosity; (b) shear resistance of AM/AA; (c) shear resistance of AM/AA/NSFM.The copolymers solutions (0.2 wt%) were prepared with distilled water.

Figure 11 :Figure 12 :
Figure 11: Flow characteristic curves of AM/AA and AM/AA/ NSFM solution (0.2 wt%).The length and internal diameter of the core barrel were 25.0 cm and 2.5 cm, respectively.

Table 1 :
Composition and TDS of brine.

Table 2 :
The relevant core properties and the results of core flooding experiments.
CopolymerCores Length (cm) Diameter (cm) Porosity (%) Permeability (mD)  wi (%)  (%)   (%) EOR (%) , with the increase of salt concentration (NaCl, CaCl 2 , and MgCl 2 ⋅6H 2 O), the viscosity of copolymers decreases rapidly and then kept at a low value.It was found that AM/AA and AM/AA/NSFM had less satisfactory salt tolerance to Na + or Ca 2+ than to Mg 2+ under the same conditions.Compared with AM/AA, AM/AA/NSFM exhibited no obvious advantage in salt tolerance due to the shrinking of copolymer chain with the increase of salt concentration.3.11.Mobility Control Ability.The core barrel was packed with quartz sand which was washed by hydrochloric acid and distilled water for several times.The injection rate of brine (sodium chloride concentration was 0.5 wt%) and polymer solution prepared with the brine was 2.0 mL/min with the ISCO 260D syringe pump.Experiments were carried out at 65 ∘ C in an incubator with precision of 0.1 ∘ C. The injection