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IR spectra of (a) PAM particles, (b) starch-g-PAM particles and (c) SN-PPG particles. 209x172mm .... remediation for large fractures or Super-K channe...
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Investigation and Characterization of Robust Nanocomposite Preformed Particle Gel for Enhanced Oil Recovery Yifu Long, Ze Wang, Haifeng Ding, Jiaming Geng, and Baojun Bai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00778 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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Figure 1. Polymerization of AM-co-AA-Na with the presence of MBA 242x73mm (300 x 300 DPI)

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Figure 2. Experimental design and flowchart of gel optimization and evaluation 165x176mm (144 x 144 DPI)

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Figure 3. Dynamic Rheological Measurement (a) HAAKE Rheoscope1 rheometer, (b) gel disc for rheology testing and (c) PP 35 Ti plate-plate geometry. 618x467mm (96 x 96 DPI)

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Figure 4. Experimental setting-up of core flooding test. 135x70mm (220 x 220 DPI)

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Figure 5. The effect of crosslinker concentrations on swelling ratio and modulus. 261x172mm (300 x 300 DPI)

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Figure 6. The effects of initiator concentrations on swelling ratio and modulus. 263x171mm (300 x 300 DPI)

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Figure 7. The effects of starch concentrations on swelling ratio and modulus. 265x166mm (300 x 300 DPI)

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Figure 8. Starch grafting polymerization (a) schematic of PAM and SN-PPG configurations, (b) amylose configuration, (c) amylopectin configuration. 224x117mm (96 x 96 DPI)

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Figure 9. The effects of NaOH concentrations on swelling ratio and modulus. 266x185mm (300 x 300 DPI)

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Figure 10. The effects of Na-MMT concentrations on swelling ratio and modulus. 267x169mm (300 x 300 DPI)

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Figure 11. Mechanism of Na-MMT nanocomposite including exfoliation and intercalation. 141x95mm (220 x 220 DPI)

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Figure 12. The effects of Na2SO3 concentrations on swelling ratio and modulus. 257x170mm (300 x 300 DPI)

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Figure 13(a). DSC thermograms of 40k 238x194mm (300 x 300 DPI)

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Figure 13(b). DSC thermograms of SN-PPG 240x200mm (300 x 300 DPI)

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Figure 14. Sectional and longitudinal view of fractured core before (a, b) and after (c, d) gel treatment 108x103mm (220 x 220 DPI)

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Figure 15. Schematic of SN-PPG plugging fracture aperture (a) gel-pack formation in fracture aperture, (b) hydrogen bonding enforced particle retention, (c) chasing fluid diverted by gel pack 118x90mm (220 x 220 DPI)

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Figure 19. Relationship of core permeabilities, ∆E and Frr 238x177mm (300 x 300 DPI)

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Figure 20. SEM images of pristine PAM gel (a, b) and images of SN-PPG gel (c, d) 164x119mm (220 x 220 DPI)

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Figure 21. IR spectra of (a) PAM particles, (b) starch-g-PAM particles and (c) SN-PPG particles 209x172mm (300 x 300 DPI)

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Figure 16. Oil recovery, pressure gradient and water cut of core#1 368x159mm (300 x 300 DPI)

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Figure 17. Oil recovery, pressure gradient and water cut of core#2 389x166mm (300 x 300 DPI)

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Figure 18. Oil recovery, pressure gradient and water cut of core#3. 375x160mm (300 x 300 DPI)

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1 2

Investigation and Characterization of Robust Nanocomposite Preformed Particle Gel for Enhanced Oil Recovery

3

Yifu Long, Ze Wang, Haifeng Ding, Jiaming Geng, and Baojun Bai*

4 5 6 7

Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA *To whom correspondence should be addressed. E-mail: [email protected] 1. Abstract

8

A robust starch grafting nanocomposite-preformed particle gel (SN-PPG) with superior

9

viscoelasticity and thermal stability is developed to improve the oil recovery. The effects of

10

crosslinker, initiator, starch, sodium montmorillonite (Na-MMT) and thereof additives were

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systematically studied. Through introducing starch and Na-MMT, the storage modulus of swollen

12

gel was ameliorated from 570 to 1420 Pa. At 80

13

aging 90 d, meanwhile showed an inflection temperature of 187.3

14

Calorimetry (DSC) measurements, which indicated an excellent thermostability. Moreover, the

15

SN-PPG displayed a reinforced pH and salinity tolerance compared with the conventional PPG. In

16

core-flooding tests, the SN-PPG successfully blocked the fracture aperture and improved the oil

17

recovery by 29.86-38.59 %. Furthermore, the characterizations involving Fourier Transform-Infrared

18

Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) confirmed the decoration of

19

starch grafting and Na-MMT nanocompositing. The results turned out the robust SN-PPG could

20

be a candidate to remedy the conformance problem and improve the oil recovery for high-

21

temperature and high-salinity reservoir.

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, the SN-PPG exhibited thermally stable after in the Differential Scanning

2. Introduction

23

Excessive water production is one of the most severe problems during the development of mature oilfields.

24

Among various remediation aiming at water-production control, gel treatment has been proven one of the

25

most cost-effective approaches. In a gel treatment, a well-designed amount of liquid-based or particles-

26

based polymeric agent will be pumped to seal or block the thief-zone which was comprised by fractures or

27

high-permeability channels. Subsequently, the injected water will be diverted towards the unswept zone

28

where is rich of remaining oil rather than being produced from the production well [1]. Since the first

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successful operation in 1985 [2], technologies regarding gel treatments have boosted and thereby were

30

categorized via their treating materials involving in-situ gels, preformed particle gel (PPG), and

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microgels[3]. Nevertheless, the in-situ gel treatment has been widely implemented; this technology

2

possessed its inherent defects. For instance, the gelation time and gel quality are interfered by dilution,

3

shear thinning and chromatography intensively [4], as the gelant is injected as a liquid form and the gelation

4

occurs in subterranean. Meanwhile, the applications of microgel or nanoparticles have been impeded as the

5

high cost of fabrication and the consistency of product quality. In contrast, PPG treatment, the technology

6

that overcame those drawbacks, is increasingly attractive for both researchers and engineers.

7

In decade usage of PPGs, poly(acrylamide) (PAM) or poly(acrylamide-co-acrylate) formulated the major

8

component. Despite the successful utilization, the pristine PAM gels were found prone to ‘extrusion’ due

9

to its poor rheological property [5, 6]. In actual gel treatment, extrusion is a negative phenomenon denoting

10

the plugging agents are too weak to withstand high pressure difference, which always results in ineffective

11

remediation for large fractures or Super-K channels [7].

12

It would be advantage of introducing starch to improve the viscoelasticity of hydrogel. Starch is an

13

abundant biopolymer which could be found from all complex plants in varying degrees [8]. It is hardy and

14

resistant to chemicals and relatively stable at moderate temperature [9]. Before the application for enhanced

15

oil recovery (EOR), starch as oilfield polymeric additive was initially deployed for fluid-loss control and

16

shale stabilization [10, 11]. Its unique reactivity and property opened up the possibility as polysaccharide

17

plugging agent. In 1998, Maria et al. reported modified starch crosslinked by organic and inorganic

18

crosslinkers which could function as water shutoff agent [12]. Hou and his group members did extensive

19

work with starch-based in-situ gel in which the gelant was comprising of modified starch, monomer,

20

crosslinker and initiator [13, 14, 15]. Ru et al. studied the gelation induced by electrostatic attraction [16,

21

17] between cationic starch and negatively charged, hydrolyzed poly(acrylamide) (HPAM). Albeit starch

22

and starch derivatives have been extensively studied, as far as we know, this versatile material has not been

23

introduced to particulate gel.

24

A successful gel treatment required hydrogel to be durable and long-term thermal stable. In subterranean

25

formation, the amide group in conventional HPAM gel underwent severe hydrolysis when subjected to high

26

temperature. Consequently, the formation of a more hydrophilic pendant, carboxylate group, was

27

accelerated, thus not only rendered PPG an excessive swelling but also resulted in a deterioration of

28

viscoelasticity. Given this, it is of critical essentiality to highlight the gel thermal-stability for the potential

29

industrial application. A viable approach to promote the thermal-stability was surface nonorganic-

30

modification via nano-composite technology [18, 19]. In our work, a species of nano-sized, layered silicate,

31

sodium montmorillonite (Na-MMT), was exploited by in-situ polymerization for the decoration of pristine

32

PAM gel. One step further, the addition of nanoclay can ameliorate the mechanical integrity of hydrogel

33

since clay nanoparticle may function as ‘physical crosslinker’ [20, 21].

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In this work, starch grafting polymerization and in-situ polymerization were integrated for the synthesis

2

of starch grafting nanocomposite-preformed particle gel (SN-PPG). The effects of different constituent

3

were symmetrically investigated. A comparison study of pH effect, salinity effect, and thermal stability

4

were conducted between SN-PPG and a pristine superabsorbent polymer that is under the trademark of

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LiquidBlock 40k. Core flooding experiments were carried out to examine the plugging performance, which

6

was followed by a comprehensive characterization.

7 8

3. Experimental

9 10

Materials and Preparation.

11 12

Acrylamide (AM, 98%), was purchased from Alfa Aesar. N,N’-methylene bisacrylamide (MBA, 99%)

13

and soluble starch (ACS Certified) were obtained from Sigma-Aldrich. Ammonium persulfate (APS, ACS

14

Certified), sodium sulfite (Na2SO3, ACS Certified), sodium hydroxide (NaOH, ACS Certified), and sodium

15

chloride (NaCl, ACS Certified) were supplied by Fisher Chemical. The nanoclay, a type of sodium-

16

montmorillonite (Na-MMT) in nano-size, was provided by Wyoming BentoniteSM with a trade name of

17

Hydrogel®. All abovementioned materials were used as received without further purification. The

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potassium salt of crosslinked polyacrylic acid/polyacrylamide copolymer, 40k, was supplied by Emerging

19

Technologies Inc. (Greensboro, NC, USA).

20

The preparation of SN-PPG (PPG23) started with dissolving of 20.94 g monomer, AM, 0.0564 g

21

crosslinker, MBA, and 0.19 g Na2SO3 in 30 ml water. 4.00 g Na-MMT was slowly added with vigorous

22

stirring. Stirred for 4-6 h, the nanoclay suspension was subsequently allowed for an ultrasonic dispersing

23

for 30 mins. In the meantime, 9.85 g starch and 15 ml water were added into a 100 mL flask followed by a

24

dropwise addition of NaOH solution (15 wt%). Due to the existence of strong intramolecular and

25

intermolecular hydrogen bonding, starch granules are resistant to water penetration at low temperature, 25

26

likewise. Heating in aqueous condition to weaken the strong bonding is a typical approach to obtain

27

starch solution, namely starch gelatinization. Herein, the gelatinization of starch was carried out with

28

mechanical stirring and 80

heating for 2 h.

29

The flask was then moved to ambient for cooling of 2 h. The mixing of starch solution and clay suspension

30

took place using high-speed blending, followed a dropwise addition of APS solution (14 wt%).

31

Furthermore, the container was kept in 40

32

screened in ambient, thereby gel particles with certain size were obtained and well prepared for the further

33

utilization.

heating for 6-8 h. The formed bulk gel was cut, dried and

34

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n

O

O O

O CH2

H2N

-O

O CH2

Na+

O N H

N H

APS

O

NH2

NH

O-Na+

NH2

NH

O-

O

O

40 C O

Na+

m

1 2

AM

MBA

AA-Na

Crosslinked poly(AM-co-AA-Na)

Figure 1. Polymerization of AM-co-AA-Na with the presence of MBA

3 4

Optimization and Evaluation of SN-PPG

5 6 7

The experimental investigation started from the optimization followed by the evaluation of SN-PPG and finalized with a series of characterization as showed in the flowchart Figure 2.

8

The optimum formulation was obtained through altering the concentration of each component and

9

investigating the effects of variables in which the swelling capacity and rheology properties were

10

predominated parameters. Finally, the specimen with tunable swelling ratio and viscoelasticity was

11

considered as optimum SN-PPG and furtherly performed with comprehensive evaluations.

12

13 14

Figure 2. Experimental design and flowchart of gel optimization and evaluation

15

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Table 1. Formulations of materials used in specimen preparation.

Material concentration (wt%) Specimen

Na-

AM

MBA

APS

NaOH

Starch

PPG 1

20.94

0.0188

0.0498

N/A

N/A

N/A

N/A

PPG 2

20.94

0.0376

0.0498

N/A

N/A

N/A

N/A

PPG 3

20.94

0.0564

0.0498

N/A

N/A

N/A

N/A

PPG 4

20.94

0.0752

0.0498

N/A

N/A

N/A

N/A

PPG 5

20.94

0.094

0.0498

N/A

N/A

N/A

N/A

PPG 6

20.94

0.0564

0.0166

N/A

N/A

N/A

N/A

PPG 7

20.94

0.0564

0.0332

N/A

N/A

N/A

N/A

PPG 8

20.94

0.0564

0.0664

N/A

N/A

N/A

N/A

PPG 9

20.94

0.0564

0.083

N/A

N/A

N/A

N/A

PPG 10

20.94

0.0564

0.0498

N/A

3.28

N/A

N/A

PPG 11

20.94

0.0564

0.0498

N/A

6.57

N/A

N/A

PPG 12

20.94

0.0564

0.0498

N/A

9.85

N/A

N/A

PPG 13

20.94

0.0564

0.0498

N/A

13.12

N/A

N/A

PPG 14

20.94

0.0564

0.0498

0.147

9.85

N/A

N/A

PPG15

20.94

0.0564

0.0498

0.293

9.85

N/A

N/A

PPG 16

20.94

0.0564

0.0498

0.44

9.85

N/A

N/A

PPG 17

20.94

0.0564

0.0498

0.585

9.85

N/A

N/A

PPG 18

20.94

0.0564

0.0498

0.44

9.85

2

N/A

PPG 19

20.94

0.0564

0.0498

0.44

9.85

4

N/A

PPG 20

20.94

0.0564

0.0498

0.44

9.85

6

N/A

PPG 21

20.94

0.0564

0.0498

0.44

9.85

8

N/A

PPG 22

20.94

0.0564

0.0498

0.44

9.85

4

0.095

PPG 23

20.94

0.0564

0.0498

0.44

9.85

4

0.19

PPG 24

20.94

0.0564

0.0498

0.44

9.85

4

0.285

PPG 25

20.94

0.0564

0.0498

0.44

9.85

4

0.38

3

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MMT

Na2SO3

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Swelling test

2 3

Dry SN-PPG were first blending cut to smaller particulates sized in 20-30 mesh (0.60-0.85 mm). The

4

particles in designed volume, Vb, were hydrated in 1 wt.% NaCl aq while measured for swollen volume,

5

Va, and thereby the equilibrium swelling ratio, SR, was achieved via Eq (1).

6

To assess the salinity effect on swelling ratio, NaCl brine of different concentrations, 0 wt.% (DI water),

7

0.25 wt.%, 0.5 wt.%, 1 wt.% and 2 wt.% were used. In like fashion, pH effect on swelling capacity was

8

investigated with “buffer fluid” which was adjusted by HCl and NaOH for acidic and basic circumstances.

9 10

SR 

V V

a

(1)

b

11

12 13 14 15 16

Figure 3. Dynamic Rheological Measurement (a) HAAKE Rheoscope1 rheometer, (b) gel disc for rheology testing and (c) PP 35 Ti plate-plate geometry. Rheology property

17 18

The rheology properties of SN-PPG were investigated using HAAKE Rheoscope1 rheometer (Germany)

19

with plate-plate geometry (PP35 Ti). All sets of rheological measurements were carried out in ambient with

20

the solvent trap to prevent vaporization. The testing specimen were prepared by crafting the swollen bulk

21

gel into a cubic gel disc of 12 mm* 12 mm*3 mm in volume.

22

Dynamic Rheological Measurement (DRM) [21] involved stress sweep and frequency sweep tests was

23

exploited in this study. In term of DRM, stress and frequency dependent, oscillatory shearing was imposed

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on gel specimen through which the viscoelasticity of specimen was quantified. The dynamic stress sweep

2

was carried out with stress varied from 0.1 pa to 10000 Pa, and a fixed frequency of 6.28 rad-1 (1 Hz).

3

Afterward, frequency sweep was conducted with an oscillatory frequency varied from 0.01 to 100 rad-1

4

(0.00159-15.9Hz) and constant stress determined by stress sweep. Finally, the storage modulus (G’) and

5

loss modulus (G’’) of each specimen were determined as the average value of plateau within Linear

6

Viscoelastic Region (LVR) [23].

7 8

Thermal stability test

9 10

The thermal stability of particles was evaluated in both aqueous condition [24] and water-free condition

11

[25]. In aqueous tests, SN-PPG and PAM particles were hydrated in 1 wt% NaCl aq, and measured for

12

equilibrium SR at elevated temperatures, namely 40, 60, and 80

13

aged in heating ovens. After 90 d aging, SR and viscoelasticity of particles were re-measured and furtherly

14

compared with virgin particles.

. The specimens were then sealed and

15

Moreover, gel thermal stability was investigated via Differential Scanning Calorimetry (DSC). DSC

16

experiments were conducted on Q-2000 Differential Scanning Calorimeter (TA Instruments) at a heating

17

rate of 5

18

mL/min was purged through the DSC cell. All experiments were performed in sealed aluminum pans.

19

Sample weights varied between 5 and 10 mg. The thermograms were recorded and analyzed via Universal

20

Analysis 2000 (TA Instruments).

/min. The heating temperature was elevated from 40

to 240

. Nitrogen at a flow rate of 50

21 22

Core flooding test

23 24

Table 2. Parameters of sandstone cores. Matrix permeability (mD)

Core code

Length (mm)

Diameter (mm)

Porosity (%)

#1

124.9

50.32

18.3

16.89

41.9

6.1

#2

125.0

50.50

149.6

16.78

42.0

6.1

#3

124.8

50.34

361.0

17.25

42.8

6.1

25

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Pore volume (cm3)

Fracture volume (cm3)

Energy & Fuels 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 2

Figure 4. Experimental setting-up of core flooding test.

3 4

To investigate the plugging performance of SN-PPG in fracture aperture, three sets of core flooding tests

5

were carried out using Berea sandstone cores with different permeabilities. The fractured core was prepared

6

and treated as follows:

7

(1) The sandstone core was oven dried at 120

for 24 h to remove the residual water, followed by a

8

measurement of fundamental parameters including length (L), diameter (D), and dry weight (M) in ambient.

9

(2) The core was vacuumed for 4 h and then saturated with 1 wt% NaCl brine for 6 h. The saturated core

10

was weighted to obtain pore volume (PV) and porosity (φ).

11

(3) Mounted into the core holder and confined by constant pressure of 500 psi, the core was then flooded

12

by 1 wt% NaCl brine of four different flow rates (2.0 mL/min, 3.0 mL/min, 4.0 mL/min, 5.0 mL/min),

13

while the real-time injection pressure was recorded. Until the steady-state flow was established, the matrix

14

permeability (K) could be calculated via Darcy’s law.

15

(4) The core was subsequently saturated by oil that possessed a viscosity of 14.3 cp in ambient. The

16

injection of oil was maintained at 0.8 mL/min until no water produced, while the original oil saturation

17

(OOIP) and irreducible water saturation (Swi) were achieved.

18

(5) The oil-saturated core was taken out from the core holder. A longitudinal fracture was cut along the

19

core carefully and smoothly using a band saw, whereupon, two pieces of incompressible steel (125 mm*4.8

20

mm*1.2 mm) were attached on segment surface to prop the fracture. Once assembled, the fractured core

21

was wrapped using Teflon® to ensure its firmness. The fracture volume (FV) was obtained.

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Page 31 of 51 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

Energy & Fuels

1

(6) The fractured core was re-mounted, confined and brine flooded via a flow rate of 2 mL/min. Brine

2

was injected until the water cut reached 98%, in the meantime, oil recovery was quantified according to the

3

oil production from the outlet. This process was the first water flooding.

4

(7) The gel treatment was carried out after the first water flooding. Without carrier fluid, swollen SN-PPG

5

particles (0.48-0.8 mm) was placed in the accumulator and then injected into fracture aperture at a flow rate

6

of 1 mL/min. SN-PPG of 12.3 FV in volume had been injected for the fracture remediation

7

(8) Second water flooding took place subsequently, which was similar to the first water flooding. Brine

8

was injected at a flow rate of 2 mL/min until water cut raised up to 98% so that the residual resistance factor

9

(Frr) and cumulative oil recovery can be calculated based on injection pressure and oil production

10 11 12

respectively. Residual resistance factor was calculated with the following equation in which Pa represented pressure difference after gel treatment, and Pb stood for pressure difference before gel treatment.

13 14 15

F

rr



P P

a

(2)

b

The incremental oil recovery was quantified via the following equation where ES, EF represented cumulative oil recovery of second water flooding and first water flooding respectively.

E  ES  EF

16

(3)

17 18

Characterization

19 20

Fourier transform-infrared spectroscopy (FT-IR)

21 22

FT-IR spectra were characterized using Nexus 470 FT-IR (Thermo Electron Corp.). To prepare testing

23

pellets, PAM (PPG3), starch-g-PAM (PPG16), and SN-PPG (PPG23) were first oven dried, and then mixed

24

with KBr at a ratio of 1:100 (wt/wt). The characteristic peaks were collected with a setting of 16 signal-

25

averaged scans at resolution of 2 cm-1 in mid-IR region (4000–400 cm−1).

26 27

Scanning electron microscopy (SEM) study

28 29

The microstructure was characterized with Hitachi S-4700 Field Emission Scanning Electron Microscope.

30

Swelled gel particles (PPG3 and PPG23) were frozen with liquid nitrogen prior to a 12 h freezing-dry

31

process. The dried polymeric networks were carefully placed on conductive tape that was attached on the

32

stainless-steel stub. The specimen was allowed for a spray of Au/Pd nano-particles for 3 min. Finally, SEM

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Energy & Fuels

1

imaging was performed using a field emitter source via a mixed detector at 15 kV accelerating voltage, and

2

9.5 µA current.

3 4

4. Results and discussions

5 6

Effect of Crosslinker Concentration

7 8

The monomer and initiator concentrations were initially controlled at 20.94 wt% and 0.0498 wt% (PPG1-

9

5). Through altering crosslinker concentration, optimum crosslinker concentration was achieved upon

10

swelling ratio and storage modulus.

11

As displayed in Figure 5, G’ was enhanced by the increase of MBA; meanwhile, the SR was diminished.

12

According to Flory–Rehner theory, both gel strength and swelling ratio were related to the crosslinking

13

density of gel network. In PPG1 and PPG2, MBA concentration was insufficient to buildup crosslinking

14

points and establish rigid 3-D network. The absence of rigid network would not interfere water diffusion.

15

Instead the larger “pore structure” in network facilitated water retention. Despite that PPG4 and PPG5

16

characterized superior G’, these specimens were excessively crosslinked and revealed unfavorably brittle.

17

In general, the moderate concentration, 0.0564 wt%, formulated in PPG3 was considered appropriated and

18

optimum. Thus crosslinker concentration was maintained at 0.0564 wt% in the following studies. 15

1200

SR

G'

G''

100 90

800 9

80 70 60

600 6

50 40

400 3

200

Modulus (Pa)

1000

12

Swelling ratio

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 32 of 51

30 20 10

0 0.00

19 20 21 22

0.02

0.04

0.06

0.08

0.10

0 0.12

0

Crosslinker concentration (wt%)

Figure 5. The effect of crosslinker concentrations on swelling ratio and modulus. Effect of Initiator Concentration

23

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Page 33 of 51

1

Figure 6 showed the effect of initiator concentration on swelling ratio, storage, and loss modulus. It was

2

found that the initiator concentration of 0.0459 wt% was an evolution point for all curves. Lower than this

3

concentration, gel strength increased but SR diminished, however, the tendency was vas versus above this

4

concentration. This phenomenon was induced by the configuration variation during polymerization.

5

Initially, the insufficient initiator resulted in a low concentration of radicals and therefore a low degree of

6

polymerization. Few macromolecules rendered the chain entanglement not strong and the network not well-

7

established. For a macroscopic view, the G’ and G’’ revealed small values. Moreover, the stability of gels

8

may not be favorable at this point. The initiator concentration of 0.0459 wt% led to an elastic gel network

9

and the maximum gel strength of 570 Pa. When the initiator concentration was beyond 0.0459 wt%, the

10

concentration of the radicals was so high that chain termination and transfer occurred easily before adequate

11

chain propagation. Producing macromolecules with lower chain length, the chain entanglement and

12

network elasticity were alleviated. The initiator concentration of 0.0459 wt% was optimum concentration

13

and formulated in the following investigation.

14 18

SR

G'

G''

1000 100 90 800

80 70

12 600 9

60 50

400 6

40

Modulus (Pa)

15

Swelling ratio

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

Energy & Fuels

30 200

3

20 10

0 0.00

15 16

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 0.10

0

Initiator concentration (wt%)

Figure 6. The effects of initiator concentrations on swelling ratio and modulus.

17 18

Effect of Starch Concentration.

19 20

Starch is one type of carbohydrate, obtained by the photosynthesis of plants. It contains two different

21

components, amylose and amylopectin, which have been discussed and reviewed by many authors [10-16].

22

Amylose (Figure 8, b) is a relatively long, linear α-glucan containing around 99% (1-4)-α-linkages and

23

around 1% (1-6)-α-linkages, while amylopectin (Figure 8, c) has a highly-branched structure containing

24

about 95% (1-4)-α-linkages and about 5% (1-6)-αlinkages [26].

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1 2

The starch of various concentrations was introduced to the pristine PAM gel (PPG3) via grafting reaction shown in Figure 8, a.

3 2800 200

15

SR

G'

G''

2400 12

160 2000 140

9

1600 120 100 1200

6

800 3 400 0

0 0

4 5

180

2

4

6

8

10

12

14

80

Modulus (Pa)

Swelling ratio

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 34 of 51

60 40 20 0

16

Starch concentration (wt%)

Figure 7. The effects of starch concentrations on swelling ratio and modulus.

6 7 8

As shown in Figure 7, G’ was subjected to a rapid increase when starch concentrations were 3.28 wt% or 6.57 wt%, then slightly diminished once starch concentration exceeded 6.75 wt%.

9

Through grafting reaction, an “encapsulation” configuration formed as the cartoon descript in Figure 8,

10

a. This configuration was subsequently confirmed by SEM characterization. Taking advantage of this

11

special configuration, the entanglement within polymer chains was enhanced. It was noted that this

12

enhanced entanglement has contributed to a strengthened viscoelasticity, namely a superior gel strength. In

13

contrast to G’ and G’’, SR diminished due to the intermolecular hydrogen bonding formed by -OH group

14

along starch molecules.

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Page 35 of 51

1 2 3

Figure 8. Starch grafting polymerization (a) schematic of PAM and SN-PPG configurations, (b) amylose configuration, (c) amylopectin configuration.

4 5

Furthermore, an excessive addition (e.g., 13.12 wt%) could drive the starch as a chain transfer agent

6

interfering propagation and termination, thus impaired on molecular weight of PAM. The starch

7

concentration therefore was finalized at 9.85 wt% was as the optimum concentration.

8 Effect of NaOH Concentration. 2800 200

12

SR

G'

G'' 2400

10

160 2000

8 1600

120

6 1200

80

Modulus (Pa)

9

Swelling ratio

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

Energy & Fuels

4 800 40

2

400 0

0 0.0

10 11

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

NaOH concentration (wt%)

Figure 9. The effects of NaOH concentrations on swelling ratio and modulus.

12

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1

NaOH of various concentrations was added to facilitate starch gelatinizing. The experimental results

2

(Figure 9) indicated that the introducing of 0.147-0.44 wt% NaOH resulted in an improvement of gel

3

strength. In case more NaOH was added, the G’ was subjected to a rapid reduction.

4

At low concentrations, the hydroxyl ions created basic circumstance to facilitate the hydrolysis reaction

5

of starch. This hydrolysis reaction accounted for a favorable disassociation of intramolecular hydrogen

6

bonding, rendering starch more prone to participating in the grafting reaction. The viscoelasticity was

7

improved since starch made-up the PAM skeleton and reinforced chain entanglement.

8

Excessive NaOH (e.g. 0.585 wt%) adjusted the solution to highly basic in which AM underwent sever

9

hydrolysis, leading the system more hydrophilic but less elastic. Under the stimulus of OH-, α-1,4 glycosidic

10

bond in the starch molecule, tended to break and thereby generate the smaller molecule, glucose. What was

11

unfavorable for gel strength was that the stiffness and tangle of glucose were inferior to starch.

12

Eventually, NaOH concentration was controlled at 0.44 wt% and deployed in following investigations.

13 14

Effect of Na-MMT Concentration

15 16

On the benchmark of PAM-grafting-starch gel (PPG16), the nano-sized sodium montmorillonite was

17

introduced and formulated with various concentrations. As Figure 10 exhibited, 4.0 wt% was demonstrated

18

an evolution point of G’ curve, in the meantime, SR and G’’ were rarely impaired by Na-MMT loading.

19

To interpret this phenomenon, it was of necessity to understand the decoration mechanism of nanoclay.

20

Two derived theories dominated in the decoration mechanism of nanoclay; one is exfoliation, and the other

21

is intercalation [27], which have been descript in Figure 11.

22 9.0

2800

SR

G'

G''

8.5

2400

180

140

1600

120

7.0

100 1200

6.5 800 400

5.5

80

Modulus (Pa)

2000

7.5

6.0

60 40 20

0

5.0

23 24

200

160

8.0

Swelling ratio

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 36 of 51

0

2

4

6

Na- MMT concentration (wt%)

8

0

Figure 10. The effects of Na-MMT concentrations on swelling ratio and modulus.

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1 2

Based on our previous work [18], exfoliation theory was considered predominating and thus highlighted

3

in this work. The exfoliated Na-MMT nanoparticles functioned as the physical crosslinker where the

4

nanoclay associated with the negatively charged PAM via electrostatic attraction. Owing to this association,

5

the entanglement between polymer chains was reinforced and thus the elasticity was improved.

6

In case excessive Na-MMT was added, such as PPG20 and PPG21, Na-MMT performed not only a

7

physical crosslinker but also a chain transfer agent that interfered the propagation of polymerization. An

8

increasing order of chain transfer reactions resulted in the deterioration of molecular weight and chain

9

length, thereby lead to a reduction of entanglement and a less elastic polymeric network. Taking account

10

of both SR and gel strength, an appropriate Na-MMT concentration was determined as 4 wt%.

11 12

Figure 11. Mechanism of Na-MMT nanocomposite including exfoliation and intercalation.

13 14

Effect of Na2SO3 Concentration

15 16

In this work, Na2SO3 was introduced as oxygen scavenger and biocide agent [28]. According to the result

17

shown in Figure 12, both G’ and SR were impaired by the Na2SO3 concentration. This phenomenon was

18

induced

19

(Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2ꞏnH2O, as a type of layered silicate, contained various cations including

20

Ca2+, Mg2+, Al3+ in its lattice. Once sulfite group and nanoclay encountered, the sulfite group and were

21

prone to complexing with cations, thereby prevented the attraction between PAM and Na-MMT. Hence,

22

G’ exhibited a reduction in tendency. Albeit the introduction of Na2SO3 brought impacts on the SR and gel

by

the

sensitivity

of

Na-MMT

to

sulfite

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group

(SO32-).

Na-MMT,

Energy & Fuels

1

elasticity, it functioned essentially in avoiding oxygen inhibition and bacterial decomposition. The

2

concentration of Na2SO3 was finalized at 0.19 wt%, and the recipe of PPG23 was considered the optimum

3

formulation of starch-grafting-polyacrylamide nanocomposite gel.

4 12

2800

SR

G'

G''

200

2400

10

160 2000

8

1600

120

6 1200

80

Modulus (Pa)

Swelling ratio

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 38 of 51

4 800 40

2

400

0

0 0.0

5 6

0.1

0.2

0.3

0.4

0

Na2SO3 concentration (wt%)

Figure 12. The effects of Na2SO3 concentrations on swelling ratio and modulus.

7 8

Thermal stability

9 10

In aqueous testing (Table 3), small increments of the SR were found for 40k particles aged at 40 and 60

,

11

while a significant increase was observed at 80

12

In contrast, such an excessive swelling was not observed for SN-PPG. At 80

13

underwent an increment from 14.5 to 16. The increments were even smaller at lower temperatures.

14

Moreover, residual modulus factor, R, defining as the ratio of complex modulus after aging (G*a) to

15

complex modulus before aging (G*b) was introduced to describe the strength variation. The complex

16

modulus (G*) was obtained via Eq. (5) that was adapted according to Song’s work[22].

. After 90 days aging, the SR increased from 55 to 83.5. , the SR of SN-PPG just

*

17

20

a

G

b

(4)

*

G* 

18 19

RG

The R value of 40k particles aged at 80

G   G  ' 2

'' 2

(5)

was as small as 44.67%, indicating gel strength deteriorated

seriously in virtue of particle excessive swelling. At 80

, the amide groups (-CONH2) in polymer chain

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1

were subjected to severe hydrolysis generating substantial carboxylate groups (-COO-) which were

2

relatively hydrophilic. The increase of hydrophilicity rendered the network absorb more water, which broke

3

the equilibrium condition and weakened the gel strength. In fact, this result was consistent with the

4

statement of Moradi-Araghi, the polyacrylamides can be used for treatment of the reservoirs with

5

temperatures below 75

[29].

6

Alternatively, the SN-PPG maintained the modulus effectively in which the R values of all cases were in

7

the scope of 90.14%~100%. The introduction of starch may account for the temperature tolerance of SN-

8

PPG as the grafting configuration impeded the mobility of polymer chain, and thereby alleviated the

9

excessive swelling of gel. On the other aspect, the temperature growth induced a re-distribution of electrical

10

charges on polymer chains and clay platelets, leading a compression of polymer chains or formation of a

11

card house structure by clay plates [30]. Hence, the interaction between Na-MMT plates and polymers also

12

contributed to thermal stability [31].

13 14

Table 3. Properties of 40k and SN-PPG before and after aging 40k

Temperature

SN-PPG

SR before

SR after

R value

SR before

SR after

R value

aging

aging

(%)

aging

aging

(%)

23

40

40

100

10

10

100

40

43

48.5

83.47

11

11

98.67

60

47.5

59.5

69.33

12.5

13

95.38

80

55

83.5

44.67

14.5

16

90.14

( )

15 16 17

The DSC curves of 40k and SN-PPG were illustrated in Figure 13. It was observed that a heat absorption

18

took place and increased with temperature growth before inflection temperature. This was induced by the

19

evaporation of water in network [25]. When the heating temperature exceeded inflection temperature, the

20

heat flow diminished dramatically. This rapid reduction implied a network damage or even destruction.

21

Herein, the inflection temperature was highlighted for gel thermal stability. It was pronounced that the

22

inflection temperature of 40k particles was 174.8

23

temperature of 187.3

24

improved due to the introduction of starch and Na-MMT.

. Meanwhile, SN-PPG characterized a higher inflection

. This result indicated the thermal stability of gel particles was significantly

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Energy & Fuels

16

16 14

174.8 C

(b)12 10

12

187.3 C

11

Heat flow (W/g)

20

18

Heat flow (W/g)

(a) 20

8

10 9 8 7

4 0

12

170

172

174 176 178 Temperature (C)

10 8

180

Heat flow (W/g)

Heat flow (W/g)

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 40 of 51

6 4

8

6

184

185

186 187 188 Temperature (C)

189

6

4

2

2 0

0

40

80

120

200

240

280

40

80

120

Temperature (C)

1 2 3

160

160

200

240

280

Temperature (C)

Figure 13. DSC thermograms of 40k(a) and SN-PPG(b). pH effect

4 5

The SR of SN-PPG and 40k varied in dependence to the pH value of external solution. As Table 3

6

showing, the 40k particles were subjected to a notable reduction in SR when pH value was lower than 7. In

7

an acidic condition such as pH-3, the SR of 40k decreased by 43%. At this point, the repulsion between

8

negatively charged carboxylate groups (-COO-) was reduced by neutralization effect. As the repulsion

9

weakened, the originally expanded chain spacing got collapsed, and thus the SR decreased.

10

Alternatively, the SR of SN-PPG was hardly impaired by acidic condition in which the SR only decreased

11

by 11.1% from pH-6 to pH-3. Due to the incorporation of starch, the polymer network of SN-PPG was

12

enriched of no-ionic hydroxyl groups (-OH). These hydroxyl groups had different response to H+ when

13

comparing with carboxylate groups (-COO-). Hence, SN-PPG was not that sensitive to acidity.

14 15

Table 4. pH effect on the swelling ratios of SN-PPG and 40k particles pH value Specimen 3

4

5

6

7

8

SN-PPG

20

21.5

22.5

22.5

21.5

21

40k

158

220

242.5

248.5

278

274.5

16 17

Salinity effect

18

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By increasing NaCl concentration, substantial Na+ ions were introduced and screened the fixed charges

2

among HPAM polymeric chain. This ion-shielding effect reduced the electrostatic repulsion tremendously,

3

collapsed chain spacing and thus resulted in a deterioration of SR.

4

The effects of NaCl concentration on the SR were exhibited in Table 4. It was noted that both SN-PPG

5

and 40k particles were affected by salinity, whereas SN-PPG were not as sensitive as 40k particles. When

6

the salinity was increased to 5 wt%, the SR of 40k particles deteriorated to 9.5% of its original SR. On the

7

contrary, SN-PPG held 44.2% of its original SR. The superior salt tolerance of SN-PPG derived from the

8

introduction of starch, especially the hydroxyl group (-OH) in its molecule. Due to the nature of hydroxyl

9

group (-OH), it associated water via hydration while marginally disturbed by ions in external solution, and

10

therefore the salt sensitivity of SN-PPG was weakened.

11 12

Table 5. Salinity effect on the swelling ratios of SN-PPG and 40k Salinity (wt%) Specimen 0

0.25

0.5

1

5

SAC

21.5

12.5

10.8

10

9.5

40k

278

69.5

52

40

26.5

13 14

Core flooding test

15

The images of models before and after gel treatment were displayed Figure 14, and furthermore, a

16

schematic image, Figure 15, was presented to elucidate the performance of SN-PPG. The water cut,

17

cumulative oil recovery, and pressure gradient were illustrated in Figure 16-18, while more detailed results

18

were summarized in Table 6. Herein, we first analyzed the mutual phenomenon of these cores, and then

19

discussed the underlying distinction.

20

It was found the injection brine was preferential to flood along fracture in first water flooding, owing to

21

the severe heterogeneity induced by fracture. The pressure gradient hence featured a low magnitude, and

22

furthermore, the deficiency of the brine slurry led a large portion of oil left in unswept matrix. Only 3.691-

23

13.031% OOIP was recovered, which was likely to happen in the flooding process of a heterogeneous

24

reservoir [32] especially where features or conduits distributed.

25

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1 2

Figure 14. Sectional and longitudinal view of fractured core before (a, b) and after (c, d) gel treatment.

3 4

During gel treatment, the pressure gradient revealed a gradual increase, implying a process of particle

5

accumulating and packing [19]. It was noted that a pronounced increment of cumulative oil recovery took

6

place, which can be attributed to a leak-off from SN-PPG dispersion and water imbibition by core segments.

7

Possibly yielded by particle dehydration and surface free-water, the leak-off fluid expanded water swept

8

volume and recovered substantial remaining oil, namely 29.86-38.59 % of OOIP.

9

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1 2 3

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Figure 15. Schematic of SN-PPG plugging fracture aperture (a) gel-pack formation in fracture aperture, (b) hydrogen bonding enforced particle retention, (c) chasing fluid diverted by gel pack.

4 5

In second water flooding, the pressure gradient increased rapidly at the beginning. This reflected an

6

effective fracture plugging as the gel pack resisted the flow of chasing brine intensively. A sudden drop

7

followed which indicated the imposed pressure exceeded the breakthrough pressure that SN-PPG could

8

withstand [33]. After this leap, the partial recovery of pressure gradient denoted the formation of wormholes

9

or new flow paths inwards the gel pack as shown in Figure 14 (d). By that time, the pressure gradient

10

tended to stabilize, and a steady flow has been established. Eventually, 14.94-21.64 % remaining oil was

11

produced due to the fluid diversion from aperture to matrix by SN-PPG.

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1st water flooding

Gel treatment

2nd water flooding 1500

80 1200

Water cut Cumulative oil recovery Pressure gradient

60

900

40

600

20

Pressure gradient (psi/ft)

Cumulative oil recovery and water cut (%)

100

300

0 9.0

0 0.0

1.5

3.0

4.5

6.0

7.5

Pore volumes (PV)

1 2

Figure 16. Oil recovery, pressure gradient and water cut of core#1.

3 1st water flooding

Gel treatment

2nd water flooding

100 1500 80 1200

Water cut Cumulative oil recovery Pressure gradient

60

900

40

600

20

300

0 0

4 5

2

4

6

8

10

12

Pore volumes (PV)

Figure 17. Oil recovery, pressure gradient and water cut of core#2.

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0 14

Pressure gradient (psi/ft)

Cumulative oil recovery and water cut (%)

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

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1st water flooding

2nd water flooding

Gel treatment

100 1500 80 1200

Water cut Cumulative oil recovery Pressure gradient

60

900

40

600

20

300

0 0.0

1

Pressure gradient (psi/ft)

Cumulative oil recovery and water cut (%)

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

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1.5

3.0

4.5

6.0

7.5

0 9.0

Pore volumes (PV)

2

Figure 18. Oil recovery, pressure gradient and water cut of core#3.

3 4

In accordance with the previous work [34], the Frr of 40k could only reach a magnitude of 193.5.

5

However, the SN-PPG were demonstrated a large magnitude of Frr, 623.12-1107.33, indicating a robust

6

plugging performance of SN-PPG. On one aspect, the superior elasticity rendered SN-PPG resist extruding

7

from the aperture and withstand significant differential pressure [35]. On the other aspect, the strong

8

interaction between sandstone surface and SN-PPG contributed to its advancement. As Figure 15 shows,

9

strong hydrogen bonding formed due to the existence of hydroxyl group (-OH) in starch molecule and the

10

silanol group (-SiO) on rock surface. The hydrogen bonding reinforced particle retention, and thereby

11

facilitated particles against extrusion.

12 13

Table 6. Results of core flooding tests. Oil recovery (%)

Core code

1st water flooding

Gel treatment

Cumulative 2nd water flooding

oil recovery

Breakthrough pressure

(%)

(psi/ft)

Frr

#1

3.69

26.17

21.64

51.51

881.66

1107.33

#2

12.19

24.69

16.15

53.04

937.80

630.00

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#3

13.03

25.56

14.94

52.97

656.96

623.12

1 2

To be noted, higher Frr and E were obtained in less permeable cores as Figure 19 shown. In accordance

3

to Eq. (2), the differential pressure played a determinant role for Frr. Since the gel injection volume was

4

controlled, the permeabilities of gel packs might have similar values. Meanwhile, the permeability of core#3

5

matrix was relatively large compared with other cores, thus the mixed permeability of treated model like

6

core#3 kept above the other cores [36]. Moreover, the relative permeability of brine in core#3 matrix was

7

supposed to be lower than that in core#1 and 2 as its Sor in higher magnitude. Therefore, different flow

8

hindrance and relative permeability lead to a difference in ∆p among core#1, 2 and 3.

9

Provided the number of EF, more remaining oil was left in the adjacent aperture area such as core #1,

10

namely richer oil could be displaced in potential. In addition, the chased fluid in core #1 or #2 was prone

11

to redirecting towards remaining oil zone due to the smaller permeability contrast between the gel pack and

12

matrix [36]. Hence, larger E was attained in cores of low K.

13 1400 1200 45

Allometric1 Model y = a*x^b Equation 1.02675 Reduced Chi-Sqr 0.94577 Adj. R-Square

1000 800

Value Standard Error 57.5107 2.78961 -0.06445 0.0106

a b

B

40 600

Allometric1 Model y = a*x^b Equation Reduced Chi-Sq 6725.0313 0.91273 Adj. R-Square

400

a b

C1

Value Standard Erro 2070.9875 396.56901 -0.21897 0.04689

35

Incremental oil recovery (%)

50 Residual resistance factor Incremental oil recovery

Residual resistance factor

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 46 of 51

200 0 0

14 15

50

100

150

200

250

300

350

30 400

Matrix permeability (mD)

Figure 19. Relationship of core permeabilities, ∆E and Frr.

16 17

Characterization Results

18 19

The microstructures of both PAM (PPG3) and SN-PPG (PPG23) were illustrated in Figure 20 which

20

supported the existence of three-dimensional networks in hydrogels. Comparing SEM images of PAM and

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1

SN-PPG, the distinction was notable that the microscale pore structure of PAM was thin and rigid. In

2

contrast, the microstructure of SN-PPG appeared to be strengthened and velvety. As abovementioned, the

3

special “encapsulation” configuration of SN-PPG may account for this phenomenon. Grafting to the matrix

4

of PAM, starch molecule attached, twined and encapsulated the polymeric chain of PAM so that made up

5

the pristine network of PAM.

6

7 8

Figure 20. SEM images of pristine PAM gel (a, b) and images of SN-PPG (c, d).

9 10

Infrared spectroscopy was deployed to characterize the function groups in PAM (Figure 21, a), starch-g-

11

PAM (Figure 21, b), and SN-PPG (Figure 21, c). The spectra of PAM particles showed two broad bands

12

at 3420 and 2914 cm-1 which were in virtue of N-H and C-H vibration stretching. The stretching of C=O in

13

amide group resulted in a band at 1668 cm-1 while CH2 scissoring induced a bend at 1459 cm-1. For starch-

14

g-PAM particles, the characteristic absorption peak at 1072 cm-1 derived from C–O stretching in the

15

anhydrous glucose ring [37]. This indicated the presence of starch in the particles. Beyond the function

16

groups mentioned above, SN-PPG revealed distinctions. The characteristic band at 906 cm-1 was attributed

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1

to the bending vibration of Al-Al-OH [38]. Furthermore, the band at 526 cm-1 indicated the bending

2

vibration of Si-O-Si. The presence of Na-MMT accounted for these absorbance bands.

3 35

Transmittance (%)

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

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30

a

25

b

20

c

15 10 5 0 4000

4 5

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Figure 21. IR spectra of (a) PAM particles, (b) starch-g-PAM particles and (c) SN-PPG.

6 7

5. Conclusions

8

In this paper, the robust preformed particle gel, SN-PPG, was successfully prepared. The effects

9

of crosslinker, initiator, starch, Na-MMT and thereof additives were systematically studied. The

10

thermal stability, pH and salinity effects were investigated and compared with the pristine PPG.

11

The core-flooding tests were carried out using fractured-core model. The characterization was

12

performed with SEM and FT-IR. Herein, the conclusions have been summarized as follows: (1)

13

Taking advantage of starch grafting polymerization and nanocompositing, the SN-PPG obtained

14

the excellent viscoelasticity in large magnitude. (2) In the thermal stability tests, the SN-PPG

15

effectively maintained its viscoelasticity rather than degradation while the DSC results indicated

16

the inflection temperature of particles was improved from 174.8

17

the investigations of pH and salinity effects, the SN-PPG presented superior tolerances to the acidic

18

and high-salinity conditions. (4) The SN-PPG particles were turned out a robust plugging

19

performance in which the SN-PPG not only plugged the fracture aperture but also improved the oil

20

recovery by 29.86-38.59 % of OOIP. (5) The surface modification induced by starch and Na-MMT

21

was observed through SEM meanwhile the IR spectra supported the incorporation of starch and

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to 187.3 . (3) According to

Page 49 of 51 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

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1

clay. In general, SN-PPG particles were demonstrated a potential plugging agent for conformance

2

control.

3 4

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