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Investigation on Polymer Reutilization Mechanism of Salt-tolerant Modified Starch on Bohai Offshore Oilfield Caili Dai, Shuai Yang, Xuepeng Wu, Yifei Liu, Dongxu Peng, Kai Wang, and Yining Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00840 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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Energy & Fuels

Investigation on Polymer Reutilization Mechanism of Salt-tolerant Modified Starch on Offshore Oilfield Caili Dai1, Shuai Yang1*, Xuepeng Wu1, Yifei Liu1, Dongxu Peng1, Kai Wang2,3, Yining Wu1†

1. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, 266580, People’s Republic of China 2. China National Offshore Oil Corporation Research Institute, Beijing, 100028, People’s Republic of China 3. State Key Laboratory of Offshore Oil Exploitation, Beijing, 100028, People’s Republic of China

*

Shuai Yang

†

Yining Wu

Email: [email protected] Tel: +86-532-86981157 Fax: +86-532-86981157 Email: [email protected] Tel: +86-532-86981152 Fax: +86-532-86981152 1

ACS Paragon Plus Environment

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ABSTRACT Salt-tolerant modified starch (SMS) is a novel green chemical agent used to reutilize the residual partially hydrolysed polyacrylamide (HPAM) in offshore reservoirs for enhanced oil recovery. In this study, laboratory experiments are conducted to investigate the mechanism of flocculation and conglomeration between the synthetic SMS and HPAM at different concentrations. The experimental results show that SMS reacts with the residual HPAM to form a floc by charge neutralization and adsorption bridging mechanisms when the concentration of residual HPAM is low. With the increase of the residual HPAM concentration, SMS blends with the HPAM to form a gel-like conglomeration (GLC) with high viscosity and strength. SEM and ESEM images of the microstructures of the floc and GLC show irregular multilayer network structures. Enhanced oil recovery tests in parallel-core models prove that the injection of an SMS slug after HPAM flooding can effectively improve the sweep efficiency and residual oil in the low permeability zone. Laboratory core tests provide credible proofs for the large-scale application of this polymer reutilization system in offshore oilfields. Key words: modified starch, flocculation, conglomeration, parallel cores, enhanced oil recovery

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1. Introduction Polymer flooding, as a technically and commercially proven enhanced oil recovery (EOR) process, plays an important and effective role in the exploitation of onshore and offshore oilfields worldwide1-4. However, a large pore path appears in long-term polymer flooding, especially in high-heterogeneity reservoirs. This effect results in the waste and pollution of injected polymer and a high polymer concentration of output fluid5-6. Dominguez and Miao et al.7-8 once reported that because of the effect of retention and the existence of the reservoir heterogeneity about 55% of the polymer which did not play a role of oil displacement remains in the formation after polymer flooding, especially in a large pore path. The residual polymer is mainly distributed in the high permeability zone and a small amount existed in the relatively low permeability zone9. Moreover, oil displacement efficiency of polymer flooding would decrease gradually caused by the lower viscosity of polymer solution. There were two main reasons for the lower viscosity. One is the irreversible degradation by high shearing action encountered in pumps and near the wellbore area10; the other is the degradation and hydrolytic action as salinity and temperature increasing. In view of the above problems, a polymer reutilization technique11 has been proposed. This technique took advantage of the reaction between residual polymer (molecular weight and high degree of hydrolysis) and polymer reuse agent to profile the reservoir heterogeneity and simultaneously decrease the polymer concentration of the produced fluid. In the previous work12-14, due to the different concentrations and properties of the

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residual polymer, flocculating agents and crosslinking agents were investigated in the lab. Stabilized sodium clay as a flocculating agent and organic chromium as a fixed agent were applied to react with the residual polymer in the pores to generate floc and gel units that could plug the large pore path and control the conformance efficiency. Meanwhile, the polymer reutilization technique which is a useful EOR method of reusing the residual polymer by injecting the polymer reuse agent was successfully applied to oil development after polymer flooding in the Daqing oilfield of China. However, the common flocculating agent of stabilized sodium clay14 has a few disadvantages, such as its lack of stable state, limited migration distance, and difficulty in breaking down. Additionally, the strength of the dynamic gel in the pore generated by the crosslinking agent and residual polymer is too low and is insufficient to block the pore. To resolve these problems, a type of modified cationic starch (MCS) has been synthesized from N-(3-chloro-2-hydroxypropyl) trimethyl ammonium chloride in the researchers15-16. It could react with the partially hydrolysed polyacrylamide (HPAM) remaining in the reservoir formation to form gels in situ, leading to an additional enhancement in the oil recovery of approximately 10%17-18. Due to the poor performance of the common modified cationic starch (MCS) in offshore oilfields, a novel salt-tolerant modified starch (SMS) for polymer flooding with a high salt tolerance, stability and low damage has been developed in necessary. This type of modified starch has seldom been investigated and reported. Furthermore, previous research on the interaction mechanism of anionic and cationic polyelectrolytes (polymer or surfactant) indicated that they would cause different

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phase behaviours including flocculation and conglomeration19-22. At present, the mechanism about polymer reutilization has only been studied by atomic force microscopy on stabilized sodium clay and organic chromium12, but the study of the polymer reutilization mechanism is not comprehensive and deep. In addition, there have been few scientific studies and reports on the reaction and mechanism between salt-tolerant modified starch (SMS) and residual HPAM based on the condition of the oilfield exploration. In this paper, we comprehensively study the mechanism of flocculation and conglomeration between the synthetic SMS and residual polymer at different concentrations. The microstructure of the floc and gel-like conglomeration (GLC) unit were observed by scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM). A series of artificial parallel-core flooding tests were carried out for enhanced oil recovery. In addition, a residual polymer reutilization mechanism was proposed in detail. These tests provided scientific evidence and technical support for an effective polymer flooding and polymer reutilization technique in oil and gas fields. 2. EXPERIMENTAL 2.1 Materials The salt-tolerant modified starch (SMS) was obtained by the reaction of cationic modified starch with 1, 3-propane sultone. The details of the synthesis and the reaction conditions are as follows: 0.6 g propane sultone was dissolved in 100 mL isopropanol, and 10.0 g cationic modified starch was added to the solution in a 250

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mL three-necked, round-bottomed flask. Then, 10 mL 1 N sodium hydroxide was added to the flask, and the reaction was maintained at 80 ℃ for 20 hours. A white powder was produced by filtrating, washing and drying at 40 ℃ for 24 hours. The reaction proceeded as depicted in Scheme 1. It has been proved that sulfonic acid group could improve the ability of dissolution and temperature salt-tolerance23-24 by the hydrogen-bond interaction between the oxygen from sulfonic acid group and water molecules. Partially hydrolysed polyacrylamide (HPAM) with an average weight of 22,000,000 g/mol and a hydrolysis degree of 26.55% was supplied by SNF Floerger of France. The chemical structure of HPAM is shown in Figure 1.

Scheme 1. Schematic representation for synthesis of seawater base cationic modified starch

Figure 1. Chemical structure of HPAM.

The oil was collected from Bohai offshore oilfield of China. The viscosity of the oil used in the experiments was 17.0 mPa·s-1 at 57℃. The density of oil was 958 6


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kg/m3. The injected water in offshore was mixed by offshore salt water and fresh water and it was prepared with the inorganic salt of NaCl, KCl, CaCl2, NaHCO3, Na2SO4, and MgCl2·6H2O in laboratory. The ionic concentrations of the inorganic salt are listed in Table 1. The total salinity is the sum of the ionic concentrations. All cores were artificial sand cores. Table 1. Components of the Injected Water.

Ccation(mg/L)

Canion(mg/L)

Total salinity(mg/L)

19198.9

Na+

Ca2+

Mg2+

Cl-

SO42-

HCO3-

5964.0

531.8

589.6

11347.9

164.5

601.1

2.2 Methods 2.2.1 Preparation of residual polymer Residual polymer solution was prepared by a stock HPAM solution with shear and hydrolysis actions to simulate the properties of the polymer in formation. The stock solution (5000 mg/L) was first sheared with a Warning blender for 20 minutes at 13000 r/min, and the molecular weight decreased from 22,000,000 g/mol to 5,000,000 g/mol after the shear action. Then, sodium hydroxide (1.0 mN) was added to the sheared polymer to raise the degree of hydrolysis from 26.55% to 35.75%. The pH of the polymer solution was 7.83. Depending the concentrations of injected and produced (average) polymer were 1500 and 177 mg/L in the Bohai offshore oilfield (S1), the concentrations of residual polymer solutions range from 200 mg/L to 1500 mg/L to simulate the Bohai practical production condition. 2.2.2 Zeta potential measurements 7


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The zeta potential serves as an important piece of evidence in characterizing the electrostatic interaction and properties between the anionic/cationic polymer and solution mixture20, 21, 25. A residual polymer solution with a concentration of 800 mg/L and an SMS solution of 15000 mg/L were prepared, and then a specific volume of the SMS solution (typically 5.0 ~ 40.0 mL) was added to 20.0 mL residual polymer solution. The mixtures were shaken in incubator shakers (KS4000i, IKA group equipment Co. Ltd., Germany) at 200 r/min for 6 hours, and the Zeta potential of the supernatant was measured by using a Zetasizer instrument (Nano ZS90, Malvern Instruments Ltd., Worcestershire, UK) at 25 °C. 2.2.3 Particle size distribution measurements The particle size distribution experiments were performed using a laser particle size analyser (Bettersize2000, Dandong Baxter instrument Co., Ltd., China). The maximum size range of this instrument was specified to be from 0.02 µm to 2000 µm. The seawater-based cationic modified starch (SMS) and flocculant were put into a stainless steel cell with an automatic test mode at 25 °C. Each test was run five times for 120 s, and the average results were recorded. 2.2.4 Scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM) measurements Scanning electronic microscope (SEM, Hitachi S-4800, Hitachi company, Tokyo, Japan) was used to observe the microstructure of the flocculation in the solution and core. The steps of method26 were as followed: 0.5 PV of 800 mg/L residual polymer solution and 0.5PV of 15000 mg/L SMS solution were injected into an artificial core

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(Ø25×80 mm), then the core was placed at certain temperature for 12 hours. The core was quickly transferred to a vacuum cup of liquid nitrogen and frozen at -80 °C for 2 hours, sequentially, the core was put into lyophilizer for 24 hours and the vacuum pressure was 7~8 Pa. After freeze-drying, the core was crushed at 30~40 mm away from the inlet, and then the flocculation was investigated with the SEM with an accelerating voltage of 3.0 kV. The microstructure of GLC by mixture was observed by an environmental scanning electron microscopy (ESEM, Quanta 200 FEG, FEI Company, America). A bit of gel sample was placed onto a covered ESEM grid at 25 °C. The temperature was set 0 °C and the pressure changed from 313 Pa to 455 Pa. Determinations were conducted at a voltage of 15 kV, with a working distance range of 5 mm to 10 mm. 2.2.5 Oil displacement experiment Parallel-core oil displacement tests (Figure 2) were performed to simulate the processes of diverting the fluid flows and enhancing oil recovery by injecting SMS after the polymer flooding. The permeabilities of the cores (Ø25×200 mm) were 0.5 µm2 and 3.0 µm2, respectively. The displacement systems were HPAM flooding and HPAM+SMS flooding. The cores were first saturated with brine water, which was then displaced with oil at a flow rate of 0.2 mL/min at 57 ℃ until no more water was produced. Based on economic reasons in Bohai offshore oilfield, the flooding method shifts from water flooding to chemical flooding when water cut reaches around 70%27. Then, water flooding was performed until the effluent water was cut by approximately 70%, and different chemical slugs were injected into different parallel cores. The pore

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volume of polymer and SMS slug were 0.3PV and 0.2PV, respectively. Afterward, water flooding was continued until the water cut of the production fluid approached 98%. The injection rate of the water and chemical flooding was set as 1.0 mL/min. Each parallel-core oil displacement test was repeated at three times in order to reduce experimental error. The method to calculate the total values is as follows. Record the volume of oil and produced fluid in the parallel cores every 5 minutes at first. The volume of water is the difference value between the volume of oil and produced fluid. Then the volume of total oil and produced fluid was calculated by adding the volume of every 5 minutes together. The total recovery was the ratio of the volume of total produced oil and initial saturated oil in the cores.

Figure 2. The diagram of the parallel-core oil displacement process.

3. RESULTS AND DISCUSSION 3.1 Flocculation by polymer mixtures To investigate the mechanism of flocculation after mixing the SMS into the residual polymer, the zeta potential, particle size distribution and SEM of the floc unit were

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measured and observed. Figure 3 shows the variation of the zeta potential as a function of the volume of SMS solution. The zeta potential of the original residual polymer at 800 mg/L was -15.3 mV and that of the SMS at 15000 mg/L was 26.5 mV. When the volume was less than 15 mL, the zeta potential increased rapidly with the added SMS. Thus, as the volume exceeded 15 mL, the zeta potential first increased slightly and approached 12.3 mV at 40 mL, as the SMS was net positive. The charge neutralization occurred between the low-concentration residual polymer and the SMS when the zeta potential of the mixture was close to zero. A large number of floc units were generated from the mixture because of the destabilization effect. From the results of the zeta potential, the best mass ratio of cationic and anionic components was determined to be approximately. 15 Charge Neutralization

10 5

Zeta/mV

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0 -5 -10 -15 -20

0

5

10

15

20

25

30

35

40

45

Volume of SMS/mL Figure 3. Curve of zeta potentials varying with SMS volume.

The particle size distribution of the SMS solution itself and the polymer mixture above were determined using a laser particle size analyser. Figure 4 (a) ~ (c) shows the particle size distribution expressed as cumulative and volume fraction functions. 11


Energy & Fuels

The cumulative resulting plots appear as S-shaped curves with the polymer mixtures ranging from 1.5 to 500µm in size. The particle size at which the cumulative distribution is 50% is identified as the median diameter (D50). As shown in Figure 4, the D50 of the SMS is of 2.3µm which is relatively small compared with the D50 of polymer mixture (110µm). The nearly 50-fold increase in the median diameter demonstrates that the SMS and residual polymer could be flocculated to form floc units. The data concerning the particle size distribution between the three curves also supports this. The effects of the residual polymer concentration (400 and 800 mg/L) on the particle size distribution are shown in Figure 4 (b) and (c), respectively. When the concentration increased from 400 mg/L to 800 mg/L, the D50 increased from 110.5 to 142.7 µm, and the size range also increased. This phenomenon occurred because of the combined effect of charge neutralization and polymer bridging in the polymer mixture with the increasing residual polymer concentration. 100 a b c

80

Cumu/%

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60 D50

40 20 0 0.01

0.1

1

10

100

1000

Size/µm Figure 4. Particle size distributions of the SMS solution itself and the polymer mixtures: (a:15000 mg/L SMS solution; b:400 mg/L HPAM+15000 mg/L SMS solution; c:800 mg/L HPAM+15000 mg/L SMS solution) 12


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To obtain a better understanding of how the SMS reacts with the residual polymer at a low concentration in the pore, the microstructure of the floc units generated by 800 mg/L residual polymer and 15000 mg/L SMS in aqueous solution and porous media were investigated by SEM. From Figure 5 (a), it is clearly observed that the floc unit skeletons present an irregular multilayer network structure of strips and clumps. Figure 5 (b) is a 700× magnification of yellow region in Figure 5 (a). It shows that the floc unit has a certain thickness and some filaments attached to the floc unit skeletons. The floc unit skeletons have a size of approximately 10~1000 µm, which is significantly larger than the size of the residual polymer (0.02~0.08µm) 28 and SMS (0.5~10 µm) skeletons. The SEM results provide further evidence of the flocculation interaction between the SMS and residual polymer. As shown in Figure 5 (c), a large number of network floc units hang over the pore due to the residual polymer from dissolution, formed by charge neutralization and bridging effects. In addition, some floc units lie in the surface of the core particles as the residual polymer of retention forms through adsorption and charge neutralization effects. Figure 5 (d) is a 2000× magnification image of yellow region in Figure 5 (c). Figures shows that the micro structure of a small floc unit is compact and thick, which results in the plugging effect of the high permeability zone and the profiling of the reservoir heterogeneity.

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Figure 5. Microstructures of the floc unit formed from 800mg/L HPAM and 15000 mg/L SMS in aqueous solution (a) 30×, (b) 700×, and porous media: (c) 300×, (d) 2000×

3.2 Conglomeration by polymer mixtures With the increase of the polymer concentration, a novel phenomenon of conglomeration occurred, and a milky gel was formed by the polymer mixtures. Figure 6 (a) and (b) shows the states of the SMS solution formed with 15000 mg/L SMS and the gel-like conglomeration (GLC) formed by 1500 mg/L HPAM and 15000 mg/L SMS, respectively. The apparent viscosity of the GLC was observed to be much higher than that of the SMS solution, and the GLC could hang on a glass rod. The rheological viscosity and ESEM of the gel were measured to study the mechanism of blending. The rheological viscosity of the polymer and mixture were determined by a

14


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rheometer (Physica MCR301, Anton Paar Company, Graz, Styria, Austria) at different shearing rates.

Figure 6. Photographs of SMS aqueous solution (15000mg/L) and a GLC of 1500mg/LHPAM and 15000 mg/L SMS at room temperature.

Figure 7 shows the rheological viscosity curves of the 1500mg/L HPAM solution, 15000mg/L SMS solution and the formed GLC. The viscosities of the gel were always higher than those of HPAM and SMS at different shear rates ranging from 5 r-1 to 1000 r-1. This thickening effect is due to the presence of carboxyl groups (-COOH-) and ammonium (NH4+) groups in the mixture. The interaction of two groups through coulombic force could cause the hydrodynamic volume of the gel to become larger and the molecular chain conformation to become more stretched29. The hydration of the carboxyl groups (-COOH-) and alkaline HPAM solution with a pH of 7.8 increases the viscosity of the mixture30. With the increasing shearing rate, the rheological viscosity of the three systems first decreases quickly and then reaches a plateau. It can be assumed that polymer molecular chains are sheared off, which weakens the thickening effect, so the viscosity decreases. Nevertheless, the gel-like

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conglomeration’s retention value of viscosity is much higher than that of HPAM and SMS at the same shearing rate. 1000

Viscosity/(mPa·s ·s) )

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1500 mg/L HPAM 15000 mg/L SMS GLC

100

10

1

1

10

100 -1

Shear rate/(r )

1000

Figure 7. Rheological curves of HPAM solution, SMS solution and the GLC systems.

ESEM is another excellent method to accurately investigate the microstructures of the GLC, as no pretreatment process is needed, so the sample microstructures can be observed in their natural state. Figure 8 shows an ESEM micrograph of the GLC sample composed of 1500 mg/L HPAM and 15000 mg/L SMS. Compared to the microstructure of the salt-tolerant polymer31, a three-dimensional network structure of the GLC with tighter lattices and a higher strength was observed. Nevertheless, the appearance of the network structure was irregular with different pore sizes of tens of micrometers, unlike the common gel32. It is speculated that the distribution of surface charges in the SMS leads to different thickening effects, resulting in the irregular structures. The network structure is conducive to the water holding capacity, which contributes to the low water separating proportion and good stability of the gel.

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Figure 8. ESEM micrographs of GLC composed of 1500 mg/L HPAM and 15000 mg/L SMS

3.3 Enhanced oil recovery by SMS injection in parallel-core models To investigate the effect of injecting SMS for enhanced oil recovery by flocculation and conglomeration, two tertiary oil recovery tests were carried out using parallel-core models to investigate the effectiveness of the SMS injection after HPAM flooding for enhancing the oil recovery in heterogeneous reservoirs. In the tests, the HPAM concentration was 1500 mg/L, and the SMS concentration was 15000 mg/L. The oil saturation and oil recovery in different stages of the two tests were summarized in Table 2. The oil saturation of high and low permeability core were 79.5% and 75.1%, respectively. The permeability of high and low permeability core was approximately 3.0 µm2 and 0.5 µm2 (S2).

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100

Water Flooding

100

Subsequent Water Flooding

HPAM Flooding

80

Water-Cut/%

(a)

80 60

60 40

52.8%

20

Water Cut, % Oil Recovery, %

0 0.0

0.3

0.6

0.9

1.2

40 20

Oil Recovery/%

0

Pore Volume 100

Water Flooding

HPAM Flooding

100

SMS Subsequent Water Flooding Flooding

Water-Cut/%

80

(b)

60

80 60

64.0% 40

40 Water

20 0 0.0

Water Cut, % Oil Recovery, %

0.3

0.6

0.9

1.2

20

Oil Recovery/%

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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0 1.5

Pore Volume Figure 9. Cumulative oil recovery and water-cut of HPAM flooding (a) and the HPAM/SMS flooding (b) slugs with injected volume in pore volume.

The cumulative oil recoveries and water cut of the aforementioned two parallel-core displacement tests are plotted as a function of the pore volume of fluid injected in Figure 9. The cumulative oil recovery covers the stages of the initial water flooding, HPAM flooding, SMS flooding and subsequent water flooding. The results of the tests indicate that the water cut of core (a) rapidly increased in the water flooding stage after the HPAM flooding. The total recovery was 52.8% when the

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Energy & Fuels

water cut reached 98%. This phenomenon contributed to the heterogeneity of the cores, and the polymer could not sweep the pore of the low-permeability core. However, for core (b), the water cut was decreased by 10.1% in the subsequent stage after SMS flooding, and the degree of total recovery was 64.0% when the water cut reached 98%. This recovery is an increase of 11.2% over the oil recovery of the HPAM flooding. You12 et al. studied the stabilized sodium clay injection to reuse the residual polymer in the heterogeneous models. Compared with stabilized sodium clay flooding, the HPAM/SMS flooding could increase more oil production by high profile and enhanced oil ability. The purpose of SMS injection is to react with the residual polymer to plug the high-permeability zone to improve the sweep efficiency of the heterogeneous cores, that is, to improve the oil recovery of the low-permeability core. Because of the great heterogeneity of the cores, the injected HPAM fluid almost entirely entered the high-permeability core. A large amount of floc or gel-like units were generated in the high permeability core, and their effect on the sweep efficiency continues during the subsequent water flooding after the SMS injection. The results presented in Table 2 show that the incremental recovery of the low-permeability core from core (b) is much higher than that from core (a). The ultimate oil recoveries of the core (from high permeability to low permeability) are 84.1%/50.1% and 88.7%/74.3%. The results indicate that the injection of an SMS slug can improve the effective sweep efficiency and enhance the recovery of residual oil from the low-permeability zones.

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Table 2. Recovery in different stage of displacement oil（（average）） Recovery/% Subsequent Model layer

k/µm2

So/%

Water

HPAM

SMS water

flooding

flooding

flooding flooding

high permeability

3.1

79.3

42.3

57.8

0

84.1

low permeability

0.5

75.5

17.3

45.2

0

50.1

total

-

77.4

23.6

40.1

0

52.8

high permeability

3.0

80.2

42.5

60.5

70.0

88.7

low permeability

0.5

75.1

17.0

42.1

45.2

74.3

total

-

77.9

24.1

40.5

45.5

64.0

3.4 Mechanism of residual polymer reutilization for enhanced oil recovery (EOR) Based on the above discussions, a residual polymer reutilization mechanism for enhanced oil recovery system was proposed, as shown in Figure 10. After long-term polymer flooding, a large pore occurs in the high permeability zone, resulting in ineffective injection of water. In addition, there are amount of residual polymer in the large pore by the forms of adsorption and dissolution (Figure 10 (a)). To re-utilize the residual polymer and enhanced oil recovery, SMS are injected and the SMS flow preferentially into large pores and floc unit is formed, blocking the pore throats of the high permeability zone (Fig.10 (b)). The blocking feature of formed floc will increase the flow pressure in the formation and the further water is diverted into low permeability zone, which can increase the sweep efficiency significantly and promote

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the displacement of the remaining oil (Fig.10 (c)). The function of injecting the SMS to react residual polymer was the similar with the injection of self-thickening polymer31, elastic microsphere33 and branched-preformed particle gel34 (B-PPG) systems.

Figure 10. Schematic diagram of the mechanism of residual polymer reutilization for EOR. (a) water flowing into the high permeability zone and residual polymer in the large pore after polymer flooding; (b) blocking the high permeability zone by formed flocs; (c) fluid diverting and increasing sweep efficiency of low permeability zone; (d) formed floc by residual polymer and SMS solution in the large pore

Furthermore, a schematic diagram of the interaction between the residual polymer and flocculating agent (SMS) is shown in Figure 10 (d) based on the SEM and ESEM micrographs. When the concentration of residual polymer is low, the SMS reacts with the residual polymer by the charge neutralization mechanism. With the increase of the residual polymer concentration, more floc units were generated to form 21


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nets by the adsorption bridging mechanism. The nets could trap the free residual polymer molecules. When the residual polymer concentration further increased, there were no floc units but rather a three-dimensional network structure of the GLC with a tighter lattice and higher strength. As the volume increases, the reaction efficiency with the residual polymer increases. Meanwhile, the three-dimensional network structure could be converted into smaller floc units due to the dilution effect of the injected water in the subsequent water flooding. The floc units could achieve the aim of the deep profile control for oil displacement. Conclusions In this work, flocculation and conglomeration between the synthetic SMS and residual HPAM with different concentrations were systematically studied. The major conclusions are summarized as follows: (1) Mechanism of polymer reutilization between the synthetic SMS and HPAM with different concentrations was systematically studied by the forms of floc and gel-like conglomeration. (2) The result of Zeta potential showed that charge neutralization occurred between the residual HPAM and SMS. The median diameter, which was increased nearly 50-fold, demonstrated that the SMS and residual polymer could form floc units. The SEM micrograph of the floc indicated that the floc unit skeletons present an irregular multilayer network structure of strips and clumps. (3) The gel-like conglomeration (GLC) formed by SMS conglomerating with the high-concentration residual HPAM is due to the presence of carboxyl (-COOH-) and 22


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ammonium (NH4+) groups through coulombic forces. The ESEM micrograph of the GLC shows its three-dimensional network structure with tighter lattices and higher strength. (4) An increase of 11.2% in the HPAM/SMS flooding compared with the oil recovery of the HPAM flooding indicates that the injection of an SMS slug could improve the effective sweep efficiency and enhance the residual oil recovery in low-permeability zones. The function of injecting the SMS to react residual polymer was the similar with the injection of self-thickening polymer, elastic microsphereand branched-preformed particle gel (B-PPG) systems. (5) As the concentration of residual HPAM was low, the SMS reacted with the HPAM by the charge neutralization. With the increasing of the residual HPAM concentration, more floc units were generated to form nets by the adsorption bridging. When the concentration of residual HPAM further increased, there were no floc units; rather, a three-dimensional network structure gel formed with tighter lattices and a higher strength. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (No. 2-9-2014-007), and the National Science Fund for Distinguished Young Scholars (No. 51425406). The authors would like to thank Dr. Wenli Luo of State Key Laboratory of Enhanced Oil Recovery (Research Institute of Petroleum Exploration and Development) for his support of Environmental Scanning Electron Microscopy. The authors express their appreciation to technical reviewers for their 23


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For Table of Contents Use Only Investigation on Polymer Reutilization Mechanism of Salt-tolerant Modified Starch on Offshore Oilfield Caili Dai, Shuai Yang, Xuepeng Wu, Yifei Liu, Dongxu Peng, Kai Wang, Yining Wu Description: Schematic diagram of the mechanism of residual polymer reutilization for EOR. (a) water flowing into the high permeability zone and residual polymer in the large pore after polymer flooding; (b) blocking the high permeability zone by formed flocs; (c) fluid diverting and increasing sweep efficiency of low permeability zone; (d) formed floc by residual polymer and SMS solution in the large pore.


Investigation on Polymer Reutilization Mechanism ... - ACS Publications

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