Ultrastable Hydrogel for Enhanced Oil Recovery Based on Double

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An ultrastable hydrogel for enhanced oil recovery based on double-groups crosslinking Lifeng Chen, Guicai Zhang, Jijiang Ge, Ping Jiang, Xiaoming Zhu, Yunling Ran, and Shengxia Han Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02124 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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An ultrastable hydrogel for enhanced oil recovery based on double-groups crosslinking Lifeng Chen, Guicai Zhang,* Jijiang Ge, Ping Jiang, Xiaoming Zhu, Yunling Ran, Shengxia Han College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China ABSTRACT: Acrylamide copolymer hydrogels used in profile modification applications in oilfield share a common problem, termed syneresis, which decreases the efficiency of profile modification. In this paper, sodium tripolyphosphate (STPP) was found to be an effective syneresis inhibitor for the hydrogel formulated with acrylamide copolymer of acryloyloxyethyl trimethyl ammonium chloride (AM/DAC), and an ultrastable hydrogel for enhanced oil recovery in high-temperature and high-salinity petroleum reservoirs was obtained based on double-groups crosslinking. Experimental investigations, including DLS, FTIR, NMR, SEM and core flood test, have been conducted to elucidate the mechanism of STPP inhibiting the hydrogel syneresis in the aspect of the reaction between STPP and AM/DAC. The result showed that the AM/DAC crosslinks with STPP based on the hydrolysis reaction of the ester group and STPP, whereby the new bond of C-O-P is formed. For this reason, the viscosity and hydrophilicity of AM/DAC was significantly increased by STPP, and the generated double-groups crosslinking (AM/DAC crosslinked with phenol-formaldehyde, AM/DAC crosslinked with STPP) made more AM/DAC molecule chains crosslinked together, thereby the stronger grid structure was formed.

1

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Therefore, the increase of the hydrophilicity and the improved stability of grid structure enhanced the water-holding capacity of the hydrogel, leading to the decrease of the hydrogel syneresis and the increase of the water-shutoff efficiency. 1. INTRODUCTION Profile modification is a means of enhancing oil recovery by diverting flood water into previously unswept zones. The hydrogel is emplaced into the highly-permeable, flooded-out layers of the formation proximate to the wellbore, and reduces the permeability of the target zone to the flooding fluid, thereby modifying the flow profile and diverting injected fluids into zones of greater residual oil content. Since the profile modification is characterized by quick response time, low treatment costs, low risk, and frequently favorable payback, it has been widely used to improve the oil recovery.1-3 Hydrogels formulated with the partly hydrolyzed polyacrylamide (HPAM) and metal ions (Cr3+, Al3+) have been used extensively as water-shutoff agents in the wild reservoir condition.4-6 However, these hydrogels usually share a common phenomenon, termed syneresis, in which the solvent phase separates from the hydrogel phase. HPAM is susceptible to be degraded due to the effect of temperature and salt,7 and the degradation of molecular chains leads to the destruction of the grid structure of the hydrogel, which makes the solvent phase separates from the hydrogel phase.8 Increasing the dosage of HPAM can decrease the hydrogel syneresis, but high concentration of HPAM leads to high injection pressure, which brings adverse effect to the site construction in oilfield.9,10 Therefore, many new polymers have been 2


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developed to prepare the hydrogels with high stability. Although a fairly wide number of new polymers have been described in the literatures,11-15 only a relatively small number

of

them,

including

acrylamide

copolymers

of

2-acrylamido-2-methyl-propanesulfonate (AMPS), N-vinyl pyrrolidone (N-VP), and N-vinyl acetamide (N-VA), have been commercialized. These novel polymers were generally found to possess excellent thermal stability, but the syneresis problem of the hydrogels formulated with these polymers is still inevitable, since their dosage in the hydrogels is limited by their unreasonable cost. Besides, more and more high-temperature (≥120℃) and high-salinity (≥50000mg/L) petroleum reservoirs need the profile modification application, 16,17 but there is nearly no polymer which can be applied successfully in these harsh conditions. Hence, obtaining an effective syneresis inhibitor for the hydrogel formulated with acrylamide copolymers is important for the profile modification application to enhance oil recovery. AM/DAC, which is synthesized through the copolymerization of acrylamide and acryloyloxyethyl trimethyl ammonium chloride (DAC), is originally used as a flocculant for oilfield sewage. Compared with the common polyacrylamide, the stability is improved since the DAC group with large steric hindrance is introduced into the polyacrylamide chain. Additional, the reasonable price is more attractive to reduce the production cost of crude oil. As a result, AM/DAC has been widely used as the clay stabilizing agent, thickener and filtrate reducer for drilling fluid at difficult reservoir conditions.18 Therefore, in this paper, AM/DAC is employed to prepare a new hydrogel to be applied in profile modification at high-temperature and 3


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high-salinity petroleum reservoirs. However, we found that AM/DAC hydrogel also suffered syneresis problem with the temperature of 130℃ and the salinity of 58500mg/L NaCl. Many additives were investigated to be a syneresis inhibitor, but the result showed only STPP could effectively inhibit the syneresis of hydrogels formulated with AM/DAC. Previous researches have showed that STPP is a green and mild crosslinking agent, which can react with the chitosan, and the reaction product has been used to make nanoparticles applied as the drug carrier recently.19-22 The mechanism of the above crosslinking is the electrostatic attraction between protonated amine moiety NH3+ of chitosan and P–O− moiety of phosphate group. In this paper, the ultrastable hydrogel at the temperature of 130℃ and the salinity of 58500mg/L NaCl is firstly obtained by the STPP-AM/DAC system, and the syneresis-inhibiting mechanism of STPP on the AM/DAC hydrogel is investigated. This investigation provides a clear understanding of the effect of STPP on AM/DAC, and the findings can be utilized to improve the stability of the AM/DAC hydrogels applied in the profile modification and other stimulation treatments. 2. EXPERIMENTAL SECTION 2.1. Materials. STPP (Figure 1), phenol, thiourea and formaldehyde used in this paper are all analytically pure, and purchased from Sinopharm. AM/DAC (viscosity-average molecular weight: 8*106, DAC composition: 15%, Figure 2) was obtained from Beijing Hengju Chemical Group Corporation. It should be noted that the material concentration in the paper is on a weight basis. 4


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O +

Na O

-

O

P O O-Na

+

P O

O P

CH2 CH -

+

O Na

O- Na+ O Na+

Figure 1. Chemical structure of STPP

m

CH2 CH

n

CH3

C O

+ C O CH2 CH2 N CH3Cl

NH2

O

CH3

Figure 2. Chemical structure of AM/DAC

2.2. Measurement of syneresis rate. First, the polymer stock solution was prepared by dissolving solid polymer in water. A container with a known amount of water was vigorously stirred to create a deep vortex. Polymer was slowly added to the shoulder of the vortex to effectively wet the polymer beads. The container was sealed to minimize evaporation and was stirred continuously for 24 h to ensure complete dissolution of polymer. The crosslinker, whose amount was carefully tuned, was dissolved in water to prepare a crosslinker solution. Finally, the gelling solution was prepared by mixing the polymer stock solution and crosslinker solution. (The constituent of different gelling solutions will be respectively showed in the “Results and discussion” part) After the gelling solution was prepared, it was sealed in a bottle and put into an oven. Syneresis rate is defined as the decrease in the hydrogel weight at a given time relative to the initial hydrogel weight. 2.3. Measurements of viscosity. The 0.5% AM/DAC solutions with 0, 0.1%, 0.5% and 1% STPP were prepared respectively, and then they were sealed in a bottle and put into an oven for different times. The viscosity of these solutions was measured at a shear rate of 1.22s-1 by the Brookfield viscometer (DV-II+Pro) at 25℃. 2.4. Measurements of DLS. For the dynamic light scattering (DLS) measurements, the polymer solutions were carried out in the light scattering vials. All glassware was 5


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kept dustfree by rinsing in hot acetone prior using. The solutions were filtered through membrane filters (pore size=0.8 µm) directly into the vials, and this process was carried out in a dustfree glovebox. The determination concentration of AM/DAC solution is 5mg/L, which is obtained by diluting the aged AM/DAC solution with or without STPP. DLS measurements were carried out at 25℃ using a commercial multi-angle light scattering BI-200SM (Brookhaven Instruments Corporation) equipped with a digital correlator BI-9000AT. The scattering angle and wavelength were fixed to be 90° and 532nm respectively. The result of hydrodynamic radius (Rh) was directly obtained by Dynamic Light Scattering Software Ver. 5.74. 2.5. Measurements of FTIR. FTIR spectra were obtained on a Nicolet 6700 FTIR Spectrometer. The 0.5% STPP solution and 0.5% AM/DAC solutions with 0.5% STPP in sealed bottles were all firstly put at 60℃ for 120h, and then they (including the AM/DAC particles without heat treatment) were dried respectively by the lyophilization method (vacuum freeze drying plant: Cammal-16LSC). The FTIR specimen was prepared by mixing the above dry sample (20mg) with KBr (2000mg), and spectral analysis was performed over the range 4000-400 cm-1. 2.6. Measurements of NMR. The STPP solution (w=0.5%) and the composite solution composed of STPP (w=0.5%) and AM/DAC (w=0.5%) in sealed bottles were all firstly put at 60℃ for 120h, and then they were dried by the lyophilization method. 31

P solid state NMR spectra of the dry samples were measured with a Bruker

AVANCE III 400M instrument using a cross polarization pulse sequence, and operating at 161 MHz, with a 4 mm, 3-channel, magic angle spinning (MAS) probe. 6


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The contact time of the cross polarization/ magic angle spinning experiment was set to be 100µs. The acquisition time was 50ms, the recycle delay was 5s, and all NMR experiments were performed at room temperature. The spectra were processed and analyzed using Bruker Topspin V3.0. 2.7. Measurements of SEM. After the AM/DAC and AM/DAC-STPP solutions in sealed bottles were put at 60℃ for 72h, a drop of solution was directly placed in a SEM sample cup, and then the micro-morphology was directly investigated by the desktop SEM. The micro-morphology of hydrogel was investigated by FESEM with the refrigeration system of Emitech K1250X. The hydrogels were prepared as the formula of 0.5%AM/DAC+0.1%phenol+0.1%formaldehyde+0.3%thiourea+x%STPP (x=0, 0.5). The FESEM samples were prepared by a cryogenic preparation method and gold/palladium was used for sputter coating. 23 Determinations were conducted at accelerating voltage of 15 kV and working distance from 5 mm to 10 mm. 2.8. Sandpack flow experiment. The plugging ability of gels was evaluated by the sandpack flow experiment. The sandpack experimental procedure was showed in the following steps: (1) Clean quartz sand with different mesh size was used to fill the sandpack, and then the sandpack was saturated with the brine (58500mg/L NaCl). (2) The brine was injected into the sandpack at the flow rate of 1cm3·min-1 to measure the permeability to the brine (kw0). (3) After that, the gelling solution of 0.5 PV (pore volume) was injected into sandpack with the same flow rate, and then the sandpack was kept at 130℃ for 30 days. (4) When the heat-treatment of the gel is completed, the sandpack was flooded with the brine again to measure the permeability kw1. (5) 7


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Finally, the effectiveness of the gel in decreasing permeability was evaluated by calculating

the

plugging

efficiency

(PE)

with

the

following

equation:

PE=(kw0-kw1)/kw0×100%.

3. RESULTS AND DISCUSSION 3.1. Effect of STPP on the hydrogel syneresis. AM/DAC was used to prepare a new hydrogel for profile modification application. However, this hydrogel suffers serious syneresis problem on high-temperature and high-salinity condition, and the syneresis rate on the 30th day reaches up to 55.5% (Figure 3). In the previous report, 24 the alkaline salt was used as the delayed crosslinker to inhibit the hydrogel syneresis. Therefore, some alkaline salts, including sodium lactate, sodium salicylate, sodium oxalate, sodium acetate, sodium citrate, sodium erythorbate and STPP were employed in the AM/DAC hydrogel to improve the hydrogel stability, and the results (Figure 3) show that only STPP can inhibit the syneresis effectively. In order to further investigate the effect of STPP on the syneresis of AM/DAC hydrogel, more different concentrations of STPP were selected to add in the hydrogel respectively, and the result is showed in Figure 4. It can be seen that the syneresis decreases with the increase of the STPP in the hydrogel. When the concentration of STPP reaches to 0.5%, the syneresis rate on the 30th day is only 1.1%. For the hydrogel with 1% STPP, there is no syneresis phenomenon on the 30th day. Therefore, STPP is an effective syneresis inhibitor for AM/DAC hydrogel, and increasing the dosage can enhance the effect of inhibiting syneresis. 8


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60

Syneresis rate (%)

50 40 30 lactate salicylate oxalate acetate citrate erythorbate STPP

20 10 0

0.0

0.1

0.2

0.3

0.4

0.5

Concentration (%)

Figure 3. Effect of the concentration of alkaline salts on the AM/DAC hydrogel syneresis on the 30th day (Composition: 0.5% AM/DAC + 0.1% phenol + 0.1% formaldehyde + 0.3% thiourea + x% alkaline salt; x=0-0.5; temperature:130℃; salinity: 58500mg/L NaCl)

60 0 0.03 0.05 0.1 0.2 0.3 0.5 1

50

Syneresis rate (%)

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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40 30 20 10 0 -5

0

5

10

15

20

25

30

Time (day)

Figure 4. Effect of the concentration of STPP on the AM/DAC hydrogel syneresis (Composition: 0.5% AM/DAC + 0.1% phenol+ 0.1% formaldehyde+ 0.3% thiourea + x% STPP, x=0-1; 9


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temperature:130℃; salinity: 58500mg/L NaCl)

3.2. Reaction mechanism of STPP and AM/DAC. Generally, the stability of hydrogel is mainly determined by the property of polymer in hydrogel. Hence, the influence of STPP on the property of AM/DAC was investigated, whereby the effect of STPP on the hydrogel syneresis may be clarified. 3.2.1. Effect of STPP on the viscosity of AM/DAC. As showed in Figure 5, the viscosity of AM/DAC without STPP decreases with the increase of the heating time, which results from the thermal degradation of AM/DAC. However, the viscosity of AM/DAC with STPP has an increase stage firstly, and then decreases slowly. The viscosity of AM/DAC with 0.1% STPP rises slowly, but it increases by about four times on the 10th day. When the concentration of STPP is 0.5% and 1%, the tackifying effect is more significant, and the ultimate viscosity can reach up to a higher value. As it is generally accepted, the addition of common salts (NaCl, CaCl2, etc.) reduces the viscosity of the polymer solution, since the salt can compress the electronic double-layer and decrease the hydrophilicity of polymer, leading to the polymer molecule get coiled. 25 However, STPP can increase the viscosity of AM/DAC. Hence, STPP is an uncommon salt for the AM/DAC solution, and it must have some especial effect on AM/DAC.

10


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250

Viscosity retention (%)

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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200 150 0% 0.1% 0.5% 1%

100 50 0 0

5

10

15

20

25

30

Time (day)

Figure 5. Effect of the concentration of STPP on the viscosity of AM/DAC

As showed in Figure 6, the influence of temperature on the tackifying effect of STPP (0.5%) on the AM/DAC solution was investigated. The viscosity-increasing rate at 40℃ is low, and the largest viscosity is obtained on the 10th day. It indicates that the interaction between STPP and AM/DAC is a chemical reaction rather than a physical reaction, since it generally needs only little time to finish a physical reaction. Along with the increasing temperature, the viscosity-increasing rate rises, which shows the chemical reaction between STPP and AM/DAC is an endothermic reaction. The highest viscosity retention at 80℃ and 95℃ is attained on the 5th day and 1st day respectively, whereafter, the viscosity at 80℃ and 95℃ suffers a obvious decrease, which may result from the thermal degradation of AM/DAC. However, the viscosity at 40℃ and 60℃ decreases slowly after it reaches to the highest value, which indicates STPP is a long-acting tackifier for AM/DAC below 60℃. 11


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300

40℃ 60℃ 80℃ 95℃

250


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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200

150

100

50 0

5

10

15

20

25

30

Time (day)

Figure 6. Effect of temperature on the viscosity of AM/DAC with 0.5% STPP

Additional, the influence of NaCl on the tackifying effect of STPP (0.5%) on the AM/DAC solution was also researched at 60℃, and the result was showed in Figure 7. In the presence of NaCl, STPP still has the tackifying effect on AM/DAC, and the tackifying effect even is enhanced with the increase of NaCl. When the concentration of NaCl is 58500mg/L, the highest viscosity retention can reaches up to 477%, whereas 252.7% for the AM/DAC solution without NaCl. Besides, the highest viscosity retention of AM/DAC solution with 0 and 5850mg/L NaCl is obtained on the 5th day, while the one of AM/DAC solution with 29250mg/L and 58500mg/L NaCl is attained on the 1st day. This indicates NaCl is benefit to the reaction between STPP and AM/DAC. In the presence of NaCl, the electronic double-layer of AM/DAC molecule is compressed and hydration layer thickness is decreased. As a result, the active group in AM/DAC molecule may become easy to react with STPP, 12


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whereby the highest viscosity retention and the reaction rate between STPP and AM/DAC rise along with the increase of NaCl in AM/DAC solution.

500


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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0mg/L 5850mg/L 29250mg/L 58500mg/L

400

300

200

100 0

5

10

15

20

25

30

Time (day)

Figure 7. Effect of NaCl on the viscosity of AM/DAC with 0.5% STPP

3.2.2. Effect of STPP on the hydrophilicity of AM/DAC. In general, the increase of the polymer viscosity is caused by two factors:

26,27

(1) improving the

hydrophilicity of the polymer molecule by introducing the hydrophilic group to the polymer molecule, and increasing the polymer molecular weight can both results in a higher viscosity; (2) the formation of the three dimensional network structure among polymer molecules, resulted from the increase of the polymer concentration or the effect of crosslinkers, can increase the movement resistance of the whole polymer solution system, whereby the viscosity can be improved. In the above research, the concentration and molecular weight of AM/DAC is invariable and there is no addition of crosslinkers into the AM/DAC solution, so the effect of STPP on the hydrophilicity 13


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of AM/DAC was investigated to get a better understanding on the tackifying effect of STPP. The 0.5% AM/DAC solution with 0%, 0.1%, 0.5% and 1% STPP sealed in a bottle was respectively heat-treated at 60 ℃ for different days, and then the hydrodynamic radius (Rh) of AM/DAC was measured. As showed in Figure 8, the average hydrodynamic radius (aRh) of AM/DAC without STPP decreases in the heat-treated process, and it reduces from 131.3nm at the initial time to 45.4nm on the 30th day. But for the AM/DAC with 0.1% STPP, it is interested that the aRh increases with time, reaching up to 186.2nm from 134.6nm on the 10th day, and it can even increase to 222.9nm on the 5th day and 235.1nm on the 1st day when the concentration of STPP is 0.5% and 1% respectively. This indicates that more STPP results in higher Rh and increases the reaction rate between STPP and AM/DAC. Hence, the phenomenon of STPP inhibiting the syneresis can be partly clarified: STPP can increase the Rh of AM/DAC, which generally means that the binding force of the AM/DAC to the water molecule is enhanced, as a result, it is difficult for the water to escape from the grid structure of hydrogel.

14


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240 200

aRh (nm)

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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160 0 0.1% 0.5% 1%

120 80 40 0

5

10

15

20

25

30

Time (day)

Figure.8 Effect of the concentration of STPP on the aRh of AM/DAC

Generally, when the polymer solution is mixed with common salts, such as NaCl, CaCl2 and MgCl2, the aRh of polymer is decreased to be lower than that of single polymer. It is because that the common salts reduce the electrostatic repulsion between polymer molecules, and then the molecules get coiled.28 Besides, the salt itself has the hydration, and it competes with the polymer to adsorb the water, which will lead to the decrease of the aRh of polymer. Obviously, the effect of STPP on the aRh of AM/DAC can’t be explained by the above “Salt Effect”, and some other mechanisms must be existed in the STPP-AM/DAC system. To our best knowledge, it can be inferred that when STPP is exposed to the AM/DAC molecule, it is “adsorbed” on the AM/DAC molecule chain by the chemical reaction. Since the STPP is highly hydrophilic, the hydration layer of STPP undoubtedly contributes to increase the hydration layer thickness of AM/DAC. Besides, different AM/DAC molecule may 15


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link together via the bridge connection of STPP. As a result, the aRh of AM/DAC with STPP increases and becomes higher than that of AM/DAC without STPP. The effect of temperature on the aRh of AM/DAC with 0.5% STPP is showed in Figure 9. When the temperature is 40℃, the growth rate of the aRh is low, and the highest aRh (185.8nm) is obtained on the 10th day. With the rising temperature, the growth rate of the aRh increases, and the peak value gets higher. This indicates that increasing the temperature is benefit to the reaction between AM/DAC and STPP. However, due to the instability of AM/DAC itself (as showed in Figure 5 and 8), the aRh decreases after the peak value at high temperature condition (such as 80℃, 95℃). When the temperature is low (40℃ and 60℃), the aRh decreases slowly, which indicates the chemical bond formed between AM/DAC and STPP is steady.

40℃ 60℃ 80℃ 95℃

240

aRh (nm)

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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200

160

120 0

5

10

15

20

25

30

Time (day)

Figure 9. Effect of temperature on the aRh of AM/DAC with 0.5% STPP

3.2.3. FTIR research on AM/DAC-STPP system. To clarify the reaction between 16


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AM/DAC and STPP, FTIR was conducted, and the result was showed in Figure 10. For the spectrum of the initial AM/DAC in Figure 10a, the characteristic adsorption peak at 1730 cm−1 assigns to the C=O of the ester group.29 The characteristic adsorption peak at 1730 cm−1 disappears in Figure 10b, which indicates the ester group of AM/DAC is hydrolyzed. Besides, due to the reaction between AM/DAC and STPP, two new absorption peaks appear at 1560 cm−1 and 1060 cm−1, which can be assigned to the C-O bond in carboxyl and the C-O-P bond.30 Thus, the AM/DAC crosslinks with STPP basing on the hydrolysis reaction of the ester group, whereby the new bond of C-O-P is formed. Due to the effect of the bond energy and molecular conformation, the P-O bond in the STPP chain (P-O-P-O-P) is easier to be destroyed than the P=O bond, that is, STPP is apt to be hydrolyzed, and then the resulting bond tends to react with the highly reactive groups.31,32 As a result, it is inferred that the reaction mechanism between AM/DAC and STPP is showed in Figure 11. When only one P-O bond in P-O-P-O-P is broken (as shown in Reaction “a”), the residual “bi-phosphate group” crosslinks with the reaction product resulting from the hydrolysis of the ester group in AM/DAC. As the crosslinked “bi-phosphate group” in the AM/DAC chain further hydrolyzes, the resulting “mono-phosphate group” reacts with the AM/DAC once again (as shown in Reaction “c”), resulting in the intramolecular or intermolecular crosslinking of AM/DAC molecules. Additional, if two P-O bonds in P-O-P-O-P are broken (as shown in Reaction “b”), the resulting “mono-phosphate group” directly leads to the intramolecular or intermolecular crosslinking of AM/DAC molecules. 17


Energy & Fuels

Therefore, a large number of AM/DAC molecules intertwine together due to the crosslinking of STPP.

STTP

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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a 1730

b 1060

1560

4000

3000 2000 Wavenumbers (cm-1)

1000

Figure 10. FTIR spectra of STPP, a) AM/DAC, and b) AM/DAC+STPP

O-

O

O

P O

P O

O-

O

O P

O-

O-

-

a

CH2 CH

CH2 CH

m

C

C O CH2 CH

m

CH2 CH

C O

C

NH2

O

CH3

n

O CH2

CH3

-

O

O-

P O

-

c

O-

P

O

b

(AMDAC)

O-

O P O

O

NH2

+ CH2 N CH3Cl-

O

O n

CH2 CH

O

O

O

P O

P O

P

O-

O-

O-

C O-

m

O

NH2

CH2 CH

O n

CH2 CH

m

CH2 CH

n

C

O P O

C

C O

O

O-

O

NH2

Figure 11. Reaction mechanism between AM/DAC and STPP

3.2.4. NMR research on AM/DAC-STPP system. To further verify the reaction between AM/DAC and STPP, 31P solid state NMR spectrum was measured, and the 18


O-

P

O

AMDAC

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result was showed in Figure 12. As showed in the NMR curve of the STPP, the chemical shift at 3.00ppm and -6.85ppm assign to the P-O bond and the P=O bond in STPP respectively.33,34 Due to the reaction between STPP and AM/DAC, these two absorption peaks merge into a wide and high one at -1.73ppm, and a new characteristic peak at -15ppm appears, which indicates the phosphorus in STPP molecule has reacted with AM/DAC and produced a novel chemical bond. According to the previous reports,35,36 the characteristic peak at about -15ppm can be assigned to the C-O-P bond. It is therefore concluded that the crosslinking reaction occurs between STPP and AMDAC and produces the C-O-P bond.

3.00

STPP STPP+AM/DAC

-1.73 -6.85

-15.16

30

20

10

0

-10

-20

-30 PPM

Figure 12. 31P NMR spectrum of STPP and AM/DAC-STPP

3.2.4.

Micro-morphology

research

on

AM/DAC

and

its

hydrogel.

Micro-morphology of AM/DAC and AM/DAC-STPP solutions was showed in Figure 13. From Figure 13a, it can be seen that the AM/DAC appears in a typical dendritic 19


Energy & Fuels

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shape and the branched chain diffuses in an arbitrary direction. Additional, the branched chain is tenuous. When the AM/DAC was aged at 60℃ for 72h, it appeared as cotton fiber (Figure 13b), which resulted from the break of molecular chains and the hydrolysis of ester group. As for the AM/DAC-STPP system, its initial micro-morphology (Figure 13c) is similar to the one of the single AM/DAC. However, due to the reaction between AM/DAC and STPP, the branched chain of the AM/DAC-STPP system grows sturdily (Figure 13d), and the maximum diameter can reach up to 5µm whereas 1µm for the single AM/DAC system. This indicates that the entanglement among AM/DAC molecules is enhanced due to the crosslinking effect of STPP. However, the homogeneous grid structure isn’t generated in the AM/DAC-STPP system, so the STPP is a mild crosslinking agent.

a

b

10µm

10µm

20


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Energy & Fuels

c

d

10µm

10µm

Figure 13. Micro-morphology of AM/DAC and AM/DAC-STPP solutions (a. AM/DAC, 0h; b. AM/DAC, 72h;

c. AM/DAC-STPP, 0h; d. AM/DAC-STPP, 72h.)

a

b

c

d

Figure 14. Micro-morphology of the hydrogels with 0 and 0.5% STPP (a, b: 0% STPP; c, d: 0.5% STPP)

Micro-morphology of the hydrogels with 0 and 0.5% STPP was showed in Figure 21


Energy & Fuels

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14. From Figure 14a and 14b, micro-morphology of the hydrogel without STPP is the grid structure, but the grid structure seems to be fragile. However, it can be seen in Figure 14c and 14d that the micro-morphology of the hydrogel with STPP is the uniform grid structure, and it appears in pentagon or hexagon, whose chemical energy is the lowest,37 so the grid structure of hydrogel formed by AM/DAC-STPP system is steady. It is also observed that the side of the grid structure formed by AM/DAC-STPP system is obviously stronger than that of the grid structure without STPP, and the side thickness of the former can be up to about 1µm whereas 0.3µm for the one of the latter. In this case the grid structure of the hydrogel with STPP is difficult to be destroyed, and the water existed in the grid structure can’t escape easily. Therefore, the syneresis of hydrogel with STPP is inhibited.

22


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ClN+

O

O

NH2 O

O

2HN

N+

O

Cl-

OH HP: CH2OH

CH2OH CH2OH

2HN

O

N+

O

NH2 O Cl-

Cl-

O

O N+

2HN

O N+

O

NH2 O

O

: crosslink bond O

Cl

O

-

O N+

Cl-

OH

O

O

O

NH2 O

O

Cl-

O

N+

2HN

O

Cl-

O O O CH2OH -O-P-O-P-O-P-OO- O- OCH2OH N+ N+

-

Cl

NH2 O

O

O

CH2OH

O

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

O

Page 23 of 31

2HN

O

N+

Cl-

Figure 15. Schematic illustration of the hydrogel in the absence and presence of STPP

Hence, the mechanism of STPP inhibiting the hydrogel syneresis is clarified: (1) Due to the strong hydration capability of STPP,38 the Rh of AM/DAC molecule is significantly improved after the crosslinking reaction between STPP and AM/DAC, that is, the hydrophilicity of AM/DAC is enhanced, so the water-holding capacity of hydrogel is increased. (2) As showed in Figure 15, the amido of AM/DAC crosslinks with the hydroxymethyl phenol (HP) obtained from the reaction between phenol and 23


Energy & Fuels

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formaldehyde, and forms the network of the hydrogel. In the presence of STPP, the crosslinking reaction between STPP and AM/DAC changes the gel from the above single-group crosslinking to double-groups crosslinking, leading to the increase of interaction force among the AM/DAC molecules, and so the water in hydrogel is firmly fixed in the grid. (3) Long-chain aliphatic phosphates, especially the phosphate polymer containing amino group, have excellent thermal stability, and they are typically used as the fire-retardant material.39 The crosslinking reaction between STPP and AM/DAC generates the phosphate polymer containing amino group, whereby the thermal stability of the crosslinked AM/DAC increases and the macromolecular chain is not easy to be destroyed, therefore the gel stability at high temperature is improved.

3.3. Core Flood Test. Sandpack flow experiments were conducted on sandpacks with diameter of 2.5cm and length of 20cm, and the effect of STPP on the plugging ability was evaluated. As showed in Table 1, the PE of Gel 1# without STPP is only 38.35% whereas 95.06% for the one of Gel 4# with 1% STPP, and the PE rises with the decrease of the syneresis, resulted from the increase of STPP. It shows that the gel stability and plugging ability was positive correlation, that is, lower syneresis rate resulted in higher plugging ability. Besides, the syneresis rate of Gel 3# and Gel 4# is nearly the same, but the PE of Gel 4# is much higher than that of Gel 3#. This indicates that the crosslinking reaction between STPP and AM/DAC can not only inhibit the gel syneresis, but also enhance the gel viscoelasticity which is benefit to improve the water-shutoff efficiency. 24


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Energy & Fuels

Table 1 PE of gels in singular sandpack on Day30 Variable

Gel 1#

Gel 2#

Gel 3#

Gel 4#

STPP concentration, %

0

0.1

0.5

1

Syneresis rate, %

55.55

36.55

1.1

0

Permeability, kw0/µm2

4.85

5.14

4.72

5.07

Permeability, kw1/µm2

2.99

1.97

0.69

0.25

PE, %

38.35

61.67

85.38

95.06

4. CONCLUSIONS STPP is a mild crosslinking agent to inhibit the syneresis of AM/DAC hydrogel. The mechanism of STPP inhibiting the syneresis of AM/DAC hydrogel is the following: the AM/DAC crosslinks with STPP basing on the hydrolysis reaction of the ester group and STPP, whereby the new bond of C-O-P is formed. For this reason, the polymer viscosity is increased, and the hydrophilicity of polymer is enhanced due to the strong hydrophilicity of STPP, improving the water-holding capacity of the hydrogel. The crosslinking reaction between STPP and AM/DAC changes the gel from the single-group crosslinking (AM/DAC crosslinked with phenol-formaldehyde) to double-groups crosslinking (AM/DAC crosslinked with phenol-formaldehyde, AM/DAC crosslinked with STPP), leading to the increase of interaction force among the AM/DAC molecules. As a result, more polymer chains twins together and forms stronger grid structure, and so the water in hydrogel is firmly fixed in the grid.

AUTHOR INFORMATION

25


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Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS Financial support by the Natural Science Foundation of China (NSFC) (Grant 51474234 and 51474235) and Shandong Provincial Natural Science Foundation, China (ZR2014EZ002) is gratefully acknowledged.

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