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Cr3+ gel
Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Experimental research of syneresis mechanism of HPAM/Cr3+ gel Guicai Zhang, Lifeng Chen ∗ , Jijiang Ge, Ping Jiang, Xiaoming Zhu College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The over-crosslinking is occurred
SEM images of different gels. (a) Gel with 0% syneresis; (b) gel immersed with CaCl2 solution.
between C O bond in COO− of HPAM and Cr3+ . • The Ca2+ reacts with the C O bond in COO− of HPAM. • The syneresis water originates from the water bounded to carboxylate group of HPAM. • Syneresis inhibitor restrains the generation of the Cr3+ polynuclear olation complex.
a r t i c l e
i n f o
Article history: Received 1 April 2015 Received in revised form 1 June 2015 Accepted 25 July 2015 Available online 29 July 2015 Keywords: Gel HPAM Syneresis Crosslinking
a b s t r a c t Experimental investigations have been conducted to elucidate the syneresis mechanism of HPAM (partly hydrolyzed polyacrylamide)/Cr3+ gel. The gel prepared with HPAM of high hydrolysis degree is apt to result in the syneresis, since HPAM with more carboxylate group is easier to be over-crosslinked. The over-crosslinking, which contributes a lot to the syneresis, occurs between C O bond in COO− of HPAM and Cr3+ . The decreasing hydrophilicity of HPAM molecule, the increase of crosslinking density, and the complexation of the carboxylate group with Ca2+ are the main syneresis mechanisms of the gel influenced by the inorganic salt. The Ca2+ reacts with the C O bond in COO− , and the reaction produces a tabular structure when the concentration of Ca2+ is high, whereby the water in the initial gel is extruded. The produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of HPAM, and this confirms that the decreasing hydrophilicity of the carboxylate group in HPAM molecule is one of the important reasons to the gel syneresis. Sodium d-isoascorbate inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+ , whereby the crosslinking speed is decreased and the syneresis is suppressed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Fax: +86 53286981178. E-mail addresses: [email protected], [email protected] (L. Chen). http://dx.doi.org/10.1016/j.colsurfa.2015.07.048 0927-7757/© 2015 Elsevier B.V. All rights reserved.
Gels formulated with partly hydrolyzed polyacrylamide (HPAM) and Cr3+ have been used extensively as water-shutoff agents to reduce water production [1–5]. HPAM/Cr3+ gel placed in situ in water dominated channels reduces water permeability and therefore water production. The gelation criteria, kinetics of gelation,
G. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
transport properties of the gel fluid and long-term stability of the gel in the formation are generally the major concerns in the design of such a profile modification treatment, and the last one is an important part since it is directly associated with the effect of the water-shutoff treatment. Gels typically used in profile modification applications share a common phenomenon, termed syneresis, in which the solvent phase separates from the gel phase as a result of gel shrinkage. The rate of this separation determines the lifetime of the gel under any given set of circumstances. For HPAM/Cr3+ gel in reservoir, this shrinkage in volume reduces the ability of the gel to reduce water flow [6]. As such, syneresis is the mechanism whereby the effectiveness of the gel decreases with time. Therefore, inhibiting the syneresis of HPAM/Cr3+ gel is a key to extend the duration of water-shutoff treatment. When HPAM/Cr3+ gel is in contact with excess brine or high temperature over a long period of time, the amide group in HPAM hydrolyzes to form carboxylate groups. The interaction of the carboxylate group with the divalent cations results in a sharp reduction in polymer solubility, which is an important reason of the gel syneresis [7,8]. In order to decrease the syneresis, some novel polymers with comonomers meeting the criteria of resistance to hydrolysis and inactivity toward divalent cations have been investigated and used in the gel. Although a fairly wide number of new acrylamide polymers have been described in the literature [9–15]. only a relatively small number of them, including acrylamide copolymers of N-vinyl pyrrolidone (N-VP), 2-acrylamido-2-methyl-propanesulfonate (AMPS), and Nvinyl acetamide (N-VA), have been commercialized. Although the novel polymers were generally found to possess excellent thermal stability and resistance to brine, their suitability for petroleum applications is limited by their unreasonable cost. Besides, it is worth emphasizing that a more serious problem for most of the new polymers used in the gel treatments is their relatively high adsorption on reservoir rock. Hence, finding the syneresis mechanism and applying the common polyacrylamide in the gel for water-shutoff is a better way to improve the effectiveness of the profile modification treatment. The syneresis mechanisms of the water-shutoff gel have been studied by some researchers, and the over-crosslinking mechanism is generally accepted as the primary reason of the gel shrinkage. Gales et al. [16] investigated the stability of xanthan gum/Cr3+ gel, and they showed that the increase of Cr3+ used in the gel led to the high degree of syneresis. They therefore concluded that reducing the crosslinking density is facilitated to decrease the syneresis, and pointed out the appropriate amount of crosslinking agent was the precondition to ensure the gel possessing the excellent stability. Eriksen et al. [17] confirmed Gales’s point of view. They studied the stability of the polyacrylamide-formaldehyde gel at 120 ◦ C, and they found that reducing the crosslinking ratio of formaldehyde and polyacrylamide increases the crosslinking time, which results in the low crosslinking density and the decreasing syneresis. Albonico and Lockhart [18] researched the syneresis of the HPAM/Cr3+ gel under the effect of the Ca2+ and Mg2+ . They stated that Ca2+ and Mg2+ may react with the carboxylate in HPAM, which increased the crosslinking density and decreased the water solubility of HPAM. As a result, the water seperated out from the gel phase whereby the gel stability reduced. In summary, over-crosslinking is considered to be the main reason for the gel syneresis in the previous reports. However, the above conclusion was obtained by the inference based on the experiment result, and it still needs intuitive and sufficient evidence to be proved. Although the syneresis mechanism of the gel influenced by the Ca2+ has been speculated in the previous literature [18], it is necessary to be clearly clarified. In addition, to our best knowledge, the effect of the hydrophilicity of HPAM on the gel syneresis has not been investigated, so the research on the hydrophilicity will
97
Table 1 Hydrolysis degree and molecular mass of HPAM. HPAM
Molecular mass, 106
Hydrolysis degree (%)
H1 H2 H3
12 12 12
20 30 39
contribute a lot to elucidate the principle of gel syneresis. Therefore, a series of experiments were conducted in this work to further study the syneresis mechanism. This investigation will provide an improved understanding of the gel syneresis, which is conducive to select the formula of water-shutoff gels with high stability. 2. Experimental 2.1. Materials Sodium bichromate (Na2 Cr2 O7 ), sodium sulfite (Na2 SO3 ), sodium chloride (NaCl), calcium chloride (CaCl2 ), magnesium chloride (MgCl2 ) and ethylene diamine tetraacetic acid (EDTA) are all analytically pure, and purchased from Sinopharm. Besides, the additives, sodium d-isoascorbate, sodium oxalate, sodium lactate and sodium salicylate, are also obtained from Sinopharm. HPAMs were purchased from Beijing Hengju Chemical Group Corporation, and their hydrolysis degree and molecular mass are listed in Table 1. All concentrations in the paper are on a weight basis. 2.2. Measurements of gelation time and syneresis rate First, a 0.4% HPAM stock solution was prepared by dissolving solid HPAM in fresh water. A container with a known amount of water was vigorously stirred to create a deep vortex. HPAM was slowly added to the shoulder of the vortex to effectively wet the HPAM beads. The container was sealed to minimize evaporation and was stirred continuously for 24 h to ensure complete dissolution of HPAM. The crosslinker, whose amount was carefully tuned, was dissolved in fresh water to prepare a crosslinker solution. Finally, the gelling solution was prepared by mixing the HPAM stock solution and crosslinker solution. Aqueous gellable compositions were obtained through many screening procedures, that is, 0.2% HPAM + 0.3% Na2 Cr2 O7 + 0.6% Na2 SO3 . After the gelling solution was prepared, it (20 g) was sealed in a bottle and put into an oven at 60 ◦ C, and then the gelation time and syneresis rate were measured. The gel strength code method [19], which is showed in Table 2, was used to determine the gelation time, and the gelation time is considered as the period of time when gelling solutions in code A state turn to code G in this paper. Syneresis rate is defined as the decrease in the gel weight at a given time relative to the initial gel weight, and the onset of the syneresis is referred to the time from which the gel actually formed, not from the moment that the solutions were placed in the oven. 2.3. Effect of inorganic salts on the syneresis When the gel was formed in the bottle with stopper, the inorganic salt solution (NaCl: 0.05–0.5 mol/L; CaCl2 and MgCl2 : 0.001–0.1 mol/L) of equal quality (20 g) was put in the bottle. As a result, the effect of inorganic salts on the syneresis can be investigated by measuring the weight of the salt solution at the given time. 2.4. Measurements of differential scanning calorimetry (DSC) Two gel samples were prepared as the method in Section 2.2. When the syneresis of them reached to 0% and 25%, respectively,
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Table 2 Gel strength code. Gel strength code
Gel description
A B C D E F G H I
No detectable gel formed: No detectable gel formed: The gel appears to have the same viscosity as the original polymer solution Highly flowing gel: The gel appears to be only slightly more viscous than the initial polymer solution Flowing gel: Most of the gel flows to the bottle cap by gravity upon inversion Moderately flowing gel: Only a small portion (5–10%) of the gel does not readily flow to the bottle cap by gravity upon inversion Barely flowing gel: The gel can barely flow to the bottle cap and/or a significant portion (>15%) of the gel does not flow by gravity upon inversion Highly deformable nonflowing gel: The gel does not flow to the bottle cap by gravity upon inversion Moderately deformable non flowing gel: The gel deforms about half way down the bottle by gravity upon inversion Slightly deformable nonflowing gel: only the gel surface slightly deforms by gravity upon inversion Rigid gel: There is no gel surface deformation by gravity upon inversion
2.5. Measurements of scanning electron microscope (SEM) When the gel was formed, a sample was immersed with CaCl2 solution (0.1 mol/L) for 12 h. The gel with different color on the interface of the gel and the CaCl2 solution and the one without the immersing treatment by CaCl2 solution were dried by freeze-dry technique. The resulting samples were sputter coated for 90 s with gold and observed using a SEM (Hitachi S-4800, Hitachi High-Technologies Corporation, Tokyo, Japan). Determinations were conducted at accelerating voltage of 25 kV and working distance from 5 mm to 10 mm.
50
H1 H2 H3
40
Syneresis rate (%)
a part of them were taken to be measured by DSC. A Pyris Diamond DSC (PerkinsElmer) equipped with a cooling device was used to measure the phase transition of water sorbed in the gel samples. DSC cures were obtained by fast sample cooling from 40 ◦ C to −40 ◦ C at the scanning rate of 10 ◦ C/min. The crystallization temperatures of water sorbed in the sample were determined from the temperature at the maximum point of the corresponding enthalpy peaks.
30
20
10
0
0
5
10
15
20
25
30
Time (day) Fig. 1. Effect of different HPAM on the syneresis rate of gel.
3. Results and discussion
syneresis rate (syneresis rate is defined as the decrease in the gel weight at a given time relative to the initial gel weight), three different HPAMs were employed to prepare a gelling solution. Through many lab screening experiments, the robust and persistent gels could not be formed when the HPAM with the hydrolysis degree lower than 15% was used. As a result, the HPAM with the hydrolysis degree in the range of 20–39% was employed in the research. It can be observed from Fig. 1 that the gel syneresis process has two main stages: the first stage of rapid syneresis and the second stage of slow syneresis. The rapid syneresis may be mainly due to the high temperature and hot oxygen effect on inorganic chromium gel, which is usually used in lower temperature (around 50 ◦ C). Besides, the syneresis of the gel prepared with various HPAMs is obviously different. As to H1–H3, the hydrolysis degree increases orderly while the molecular mass is identical (Table 1), and the syneresis rate of the corresponding gel also increases sequentially. DiGiacomo et al. also obtained the same conclusion by investigating the changes in hydrolysis of polyacrylamide polymers with C-13 NMR. They found that the increase in the degree of hydrolysis parallels the syneresis of the gels [20]. Since the HPAM gel is resulted from the crosslinking between the carboxylates and Cr3+ , and higher hydrolysis degree means that there are more carboxylates in a HPAM molecular, as a result, the crosslinking density of the HPAM gel is large when the hydrolysis degree is high, and the water in the grid structure is easy to be expelled due to the high density of grid structure. Therefore, the gel prepared with HPAM of high hydrolysis degree is apt to produce the syneresis.
3.1. Effect of hydrolysis degree on the syneresis
3.2. Effect of inorganic salts on the syneresis
Hydrolysis degree (the number ratio of carboxyl group to amide and carboxyl groups in one HPAM molecule.) is the basic item to be evaluated when HPAM is used as the main agent in water plugging gel. In order to understand the effect of hydrolysis degree on the
The gel at the real reservoir condition is inevitably exposed in the formation water with inorganic salts, so it is necessary to study the effect of inorganic salts on the gel stability. Figs. 2, 3 and 4 show the syneresis law of the initial gels immersed in the NaCl,
2.6. Measurements of Fourier transform infrared spectroscopy (FTIR) The HPAM beads and the samples which were used in Sections 2.4 and 2.5 were employed to be measured by FTIR spectra. The above samples were all dried by freeze-dry technique, and the FTIR specimen were prepared by mixing the sample and KBr with the weight ratio of 1:100. FTIR spectra were obtained on an Nicolet 6700 FTIR Spectrometer, and spectral analysis was performed over the range 4000–400 cm−1 . 2.7. Effect of additives on the syneresis A certain amount of additive was used in the gelling solution, and the syneresis, gelation time and gel strength were measured as the method in Section 2.2. The measurements of spectrophotometry were as the following: the crosslinker solutions (0.3% Na2 Cr2 O7 + 0.6% Na2 SO3 ) with no additive, 0.1% sodium d-isoascorbate and 0.1% sodium oxalate were prepared, respectively. 5 g crosslinker solution, 0.5 g 0.025 mol/L H2 SO4 and 2.5 g 0.05 mol/L EDTA were mixed and aged at 80 ◦ C for 10 min, and then the absorbance of the mixed solution was measured at the wavelength of 538 nm by the spectrophotometer.
G. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
100
80 70
80
50
Syneresis rate (%)
Syneresis rate (%)
60
40 30 20
0.05 0.1 0.2 0.3 0.5
10 0 -10 -20
0
5
10
15
Time (day)
20
25
30
80 60 40
0.0001 0.001 0.01 0.05 0.1
0 -20 -40 0
5
10
15
20
25
40 20
0.0001 0.001 0.01 0.05 0.1
-20
0
5
10
15
Time (day)
20
25
30
Fig. 4. Effect of MgCl2 (mol/L) on the syneresis rate of gel.
100
20
60
0
Fig. 2. Effect of NaCl (mol/L) on the syneresis rate of gel.
Syneresis rate (%)
99
30
Time (day) Fig. 3. Effect of CaCl2 (mol/L) on the syneresis rate of gel.
CaCl2 and MgCl2 solution respectively. With the increasing concentration of inorganic salt, the syneresis is exacerbated. The reasons may be the followings: for the gel immersed in the water with highconcentration salt (0.2–0.5 mol/L NaCl, 0.01–0.1 mol/L CaCl2 and MgCl2 ), the repulsion between carboxyl groups is reduced by an attraction between cations (Na+ , Ca2+ and Mg2+ ) and the negatively charged carboxyl group. This causes the molecule to be compressed from its enlarged state and results in a reduction of the hydrated radius [21]. In another word, the hydrophilicity of the carboxyl group decreases, whereby a part of bound water may be transformed into free water. Since there is no interaction force between the free water and the gel, the free water is easy to be seperated out from the gel. However, the syneresis law of the gel immersed in NaCl, CaCl2 and MgCl2 solution is different when the salt concentration is low (0.05–0.1 mol/L NaCl, 0.0001–0.001 mol/L CaCl2 and MgCl2 ). As to the gel exposed to the NaCl and MgCl2 solution of low concentration, it swells at first then produces the syneresis. This indicates that the effect of the inorganic cations on the carboxyl group is no longer the determining factor for the volume changes of the gel. According to polymer network theories [16], the swelling or the syneresis of a polymer network is governed by the mixing potential and the elastic potential. The mixing potential favors the dispersion of network chains into the solvent and hence swelling, and the elastic potential represents the elastic force imposed by the crosslinks which resists the change of the network chains from their unstrained state. When the brine concentration is low, the gel with a low crosslinking density has the relatively large mixing
potential, which will cause swelling. During the swelling process, the increasing deformation of the network leads to an increased elastic potential while the increased gel volume causes a decrease in the mixing potential. At a certain degree of swelling, the elastic potential is equal to the mixing potential and the swelling equilibrium is reached. As the gel further crosslinks, the elastic potential exceeds the mixing potential, and the syneresis appears. Hence, the change of the crosslinking density determines the variation of the gel volume as the gel is exposed to the NaCl and MgCl2 solution of low concentration. Nevertheless, when the gel is immersed in the CaCl2 solution of low concentration, it nearly swells continuously rather than swelling at first then producing the syneresis, as showed in Fig. 3. The interaction between polyelectrolytes and the Ca2+ has been widely investigated, and it has been proposed that the carboxylate group in polyelectrolytes can be crosslinked with Ca2+ to form a complex compound [22–25]. So it can be theorized that the reaction between the carboxylate group and Ca2+ is the key factor on the stability of the gel in the low-concentration CaCl2 solution. Therefore, the syneresis mechanism of the gel influenced by the salt is complicated, and the decreasing hydrophilicity of HPAM molecule, the increase of the crosslinking density, and the complexation of the carboxylate group with Ca2+ codetermine the gel syneresis. When the gel was immersed in the CaCl2 solution for 12 h, a new material with different colour was generated on the interface of the gel and the CaCl2 solution (0.05 and 0.1 mol/L), as showed
Fig. 5. Gel immersed with CaCl2 solution of different concentrations for 12 h (from left to right: 0, 0.01, 0.05, 0.1 mol/L; the bottle is inverted and the gel are all with 0% syneresis).
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Fig. 6. SEM images of different gels. (a) Gel with 0% syneresis; (b) gel immersed with CaCl2 solution (the new material on the solid–liquid interface).
in Fig. 5. The new material is rigid, and it cannot flow even when the bottle with the gel was inverted. To clarify the difference of the microstructure between the new material and the gel with 0% syneresis, the observation analysis by the SEM was conducted. As shown in Fig. 6, the microstructures of the two materials in the same amplification are obviously different. The microstructure of the gel with 0% syneresis is the network structure, and there is large amount of pore space in the network structure to hold the water in gels. However, the microstructures of the new material is the tabular structure with many small holes, and there is not enough space to hold the water which is forced to be separated from the gel phase. This is the reason why the gel exposed to the high-concentration CaCl2 solution synereses quickly. However, why did the gel in the CaCl2 solution get stiff to produce the tabular structure? As it is generally accepted, the Ca2+ may take part in the crosslinking reaction with HPAM in the gel, but it is not clear that which functional group the Ca2+ reacts with. To answer the above question, FTIR measurements were conducted. The analysis objects of the FTIR measurement are HPAM, gel with 0% syneresis, gel with 25% syneresis and gel immersed with CaCl2 solution (the above new material analyzed by SEM), and the result is showed in Fig. 7. In order to better compare the quantitative changes in the major functional groups resulted from the syneresis and Ca2+ , the FTIR curves were analyzed by fitting multi-peaks of Gaussian form, and the corresponding peak height of different functional groups were listed in Table 3. For the gel with 0% syneresis and the one immersed with CaCl2 solution, the
differences of the peak height at 1110 cm−1 and 1410 cm−1 are small. But the peak height of the gel immersed with CaCl2 solution at 1570 cm−1 increases sharply compared with that of the gel with 0% syneresis. As the shoulder at about 1110 cm−1 , 1410 cm−1 and 1570 cm−1 is respectively ascribed to the stretching vibration of the C O bond in COO− , the C N bond in CONH2 and the C O bond in COO− [25], it can be theorized that the reaction between the C O bond in COO− and the Ca2+ is the reason why the new material is produced. This result is in good agreement with the standpoint of Fantinel et al. [26], and they showed that the COO− in polyacrylates can react with Ca2+ to form the complex chelating bidentate. Besides, some other conclusions can be obtained by closer inspection of the data in Fig. 7 and Table 3. The peak height at 1410 cm−1 of the gel immersed with CaCl2 solution and that of the one with 25% syneresis is lower than that of the gel with 0% syneresis. This shows that the C N bond in CONH2 decreased which means HPAM hydrolyzes in the process of syneresis and immersing. It is noteworthy that the influence of the Ca2+ on the increase of the hydrolyzation degree is more significant. The C N bond in CONH2 is transformed to the C O bond due to the hydrolyzation, so the amout of the C O bond should increase. However, the C O bond of the gel immersed with CaCl2 solution and that of the one with 25% syneresis do not increase but decrease. This result can prove that the over-crosslinking, which is generally accepted as the gel syneresis mechanism, occurs between the C O bond in COO− and Cr3+ .
Fig. 7. FTIR curve of HPAM and different gels.
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Table 3 Peak height of functional groups in different gels. Wavenumber (cm−1 )
Group assignments
Gel with 0% syneresis
Gel immersed with CaCl2 solution
Gel with 25% syneresis
1110 1410 1570
C O in COO− C N in CONH2 C O in COO−
0.8059 0.0544 0.0727
0.8000 0.0457 0.2455
0.7509 0.0469 0.0939
Table 5 The mass fraction of the different states of water in gel.
Gel with 0% syneresis Gel with 25% syneresis
Free water content (%)
Freezable hydration water content (%)
Non-freezable hydration water content (%)
54.27 89.22
22.91 0
22.82 10.78
non-freezable hydration water decreases, leading to the increase of the amount of free water. The reduction of the freezable hydration water and non-freezable hydration one may be due to the following reasons: (i) the abscission of the carboxylate group caused by the ageing effect, (ii) the decrease of the hydrophilicity of the carboxylate group influenced by the inorganic salt ions (Na+ , Cr3+ ). This indicates the produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of the polymer. Hence, it is confirmed once again that the decrease of the hydrophilicity of HPAM is one of the important reasons to the gel syneresis.
Fig. 8. DSC curve of two different gels.
3.3. Effect of HPAM hydrophilicity on the syneresis 3.4. Effect of additives on the syneresis The state of water in the polymer solution has been investigated by some researchers, and most of them divide this water into three types [27–30]: (i) Free water — in a “free” associated state with phase transition parameters close to those of melting ice; (ii) Freezable hydration water — weakly bound to the polymer, but capable of freezing at quite reduced temperatures, and (iii) Non-freezable hydration water — strongly bound to the polar groups of the polymer, not freezing, and not registered calorimetrically. Since the gel is obtained by the crosslinking between large amount of polymer solution and small amount of crosslinking agent, it is inferred that the three types of water also existed in the gel. Through the DSC measurement, the above inference has been confirmed. As showed in Fig. 8, the gel with 0% syneresis has two exothermic peaks which appears at −19.89 ◦ C and −17.09 ◦ C respectively, whereas the one with 25% syneresis has only one exothermic peak which appears at −16.92 ◦ C. According to the theory of Nishioka et al. [31], for the gel with 0% syneresis, the exothermic peak at −19.89 ◦ C results from the heat release of the freezable hydration water, the exothermic peak at −17.09 ◦ C is due to the heat release of the free water, and the exothermic peak at −16.92 ◦ C is also due to the heat release of the free water for the gel with 25% syneresis. Compared with the polymer solution, the phase transition temperature of the water in the gel decreases obviously due to the crosslinking reaction. The melting enthalpy obtained in the DSC measurement is showed in Table 4, and the mass fraction of a type of water was obtained as: Wn = Hn /H0 × 100%, where Hn is the melting enthalpy of one type of water, and H0 is assumed to be the same as that of pure water (333.5 J/g) [32]. As shown in Table 5, due to the syneresis, the freezable hydration water in the gel disappears and the amount of Table 4 The melting enthalpy obtained in the DSC measurement.
Gel with 0% syneresis Gel with 25% syneresis
Melting enthalpy, H1 (J/g)
Melting enthalpy, H2 (J/g)
180.99 297.54
76.40 0
In order to decrease the syneresis, some additives were used in the gelling solution, and the effect of additives on syneresis is shown in Table 6. Sodium d-isoascorbate is a syneresis inhibitor to extend the stability of gels, whereas the sodium oxalate cannot be defined as an effective additive. To clarify the mechnism of the additives influencing the gel stability, the syneresis at Day 60 and the gelation time were studied at different concentrations of additives. As shown in Tables 7 and 8, the syneresis decreases and the gelation is delayed along with the increase of the syneresis inhibitors. According to the published literature [33], the above result is due to the reaction between the inhibitor and Cr3+ . Since Cr3+ in water can form the polynuclear olation complex by the three process: complexation, hydrolysis and olation [34], the inhibitor may react with the Cr3+ to produce the Cr-inhibitor complexes. In this condition, the forming speed of the polynuclear olation complex decreases resulted from the above competition effect. As a result, the gelation time is prolonged, and the crosslinking rate decreases. Hence, the crosslinking density increases slowly, which improves the stability of the gel. In the previous report, Albonico and Lockhart [35] investigated the effect of acetate, 2-hydroxy butyrate, serine, glycolate, et al. on the gel syneresis, they obtained the same conclusion that these additives could inhibit the syneresis by prolonging the gelation time. Besides, the gel strength was also studied, and the results showed that the gel strength was nearly not influenced by sodium d-isoascorbate and sodium oxalate, so the additive would not decrease the water-shutoff efficiency of the gel. In order to obtain an improved understanding of the Cr-inhibitor complex, the heat treatment of the crosslinker solutions (with the additives) was conducted at 60 ◦ C. The crosslinker solutions with sodium d-isoascorbate and sodium oxalate aged for 4 h are shown in Fig. 9a and b respectively. It can be observed that the green “fluffy” precipitate is produced in the crosslinker solutions with no additives, 0.05% and 0.1% sodium oxalate. On the contrary, the crosslinker solutions with 0.2% sodium oxalate, 0.05% and 0.1% sodium d-isoascorbate are all still green clear liquid. By observing Tables 7 and 8 again, the good correlation between the syneresis
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Table 6 Effect of additives on syneresis of gel. 0.05% additive
None
Sodium d-isoascorbate
Sodium lactate
Sodium salicylate
Sodium oxalate
Syneresis at day 60 (%)
45.11
12.14
36.43
17.55
43.41
Fig. 9. Effect of additives on the Na2 Cr2 O7 –Na2 SO3 solution (without HPAM). (a) sodium d-isoascorbate: 0, 0.05%, 0.1% (from left to right). (b) sodium oxalate: 0.05%, 0.1%, 0.2%; (c) 0.1% sodium d-isoascorbate, 0.2% sodium oxalate.
Table 7 Effect of sodium d-isoascorbate on the syneresis and gelation time. Sodium d-isoascorbate (%) Syneresis at day 60 (%) Gelation time (h) Gel strength
0 45.11 1.9 H
0.05 12.14 6.1 H
0.1 6.22 20.2 G
Table 9 Absorbance of Cr3+ in crosslinker solutions with various additives at different times. 0.2 – >1000 –
“–” means that the result can not be obtained.
Table 8 Effect of sodium oxalate on the syneresis and gelation time. Sodium oxalate (%) Syneresis at day 60 (%) Gelation time (h) Gel strength
0 45.11 1.9 H
0.05 43.41 2.8 H
0.1 35.33 3.4 H
0.2 20.72 4.8 G
and the stability of the crosslinker solution is found, that is, the more steady the crosslinker solution, the lower the syneresis rate. Since there is no other reaction leading to the precipitation in the solution, the green “fluffy” precipitate is considered as a variant polynuclear olation complex with the overlong molecular chain. The effective syneresis inhibitor can delay the generation of the Cr3+ polynuclear olation complex by restraining a process of the complexation, hydrolysis and olation polymerization, thereby the chain of the polynuclear olation complex grows slowly, which results in the increase of the crosslinker stability. It is also observed in Fig. 9c that the crosslinker solution with 0.1% sodium d-isoascorbate is still clear after being aged for 24 h, whereas the one with 0.2% sodium oxalate generates the green precipitation. This further shows that the inhibiting effect of sodium d-isoascorbate on the growth of the polynuclear olation complex is much stronger than that of sodium oxalate. To further explain the above result, the absorbance of Cr3+ in different crosslinker solutions were measured according to the method of Ohls et al. [36] Cr3+ can form violet complex with EDTA in the temperature range of 70–80 ◦ C, whereas Cr6+ cannot react with EDTA at the same condition. The maximum absorption peak of the violet complex is at 538 nm, and the absorbance measured by the spectrophotometry is positively correlated with the content of Cr3+ . As shown in Table 9, the absorbance decreases over time,
Time (min)
None
Sodium oxalate
Sodium d-isoascorbate
0 10 30 60
0.992 0.867 0.786 0.754
0.976 0.855 0.776 0.727
0.823 0.764 0.709 0.651
it indicates that the individual Cr3+ is transformed into the polynuclear olation complex continuously, thereby less violet complex is formed due to the decrease of Cr3+ . Besides, the content of Cr3+ in the crosslinker solution with sodium d-isoascorbate is lower than that in the other two solutions. This shows that sodium disoascorbate coordinates more Cr3+ to form a certain kind of Cr3+ complex whereby there is less Cr3+ to produce the violet complex with EDTA. Hence, this result confirms that sodium d-isoascorbate can decrease the crosslinking speed by reacting with the Cr3+ . In summary, the mechanism of an efficient inhibitor suppressing the gel syneresis is as the follows: an additive inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+ . Since the HPAM crosslinks with the polynuclear olation complex to form the gel, the crosslinking density increases slowly due to the effect of the inhibitor. Therefore, the water will not be squeezed out from the gel quickly on account of the overcrosslinking. 4. Conclusions A series of experimental investigations have been conducted to elucidate the syneresis mechanism of HPAM/Cr3+ gel. The results indicate that: (1) The gel prepared with HPAM of high hydrolysis degree is apt to result in the syneresis, since HPAM with more carboxylate group is easier to be over-crosslinked. The over-crosslinking, which contributes a lot to the gel syneresis, occurs between C O bond in COO− of HPAM and Cr3+ . (2) The decreasing hydrophilicity of HPAM molecule, the increase of the crosslinking density, and the complexation of the
G. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
carboxylate group with Ca2+ are the main syneresis mechanism of the gel influenced by the inorganic salts. The Ca2+ reacts with the C O bond in COO− , and the reaction produces a tabular structure when the concentration of Ca2+ is high, whereby the water in the initial gel is extruded. (3) The produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of HPAM, and this indicates that the decreasing hydrophilicity of the carboxylate group in HPAM molecule is one of the important reasons to the gel syneresis. (4) Sodium d-isoascorbate inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+ , whereby the crosslinking speed is decreased and the syneresis is suppressed.
[14]
[15]
[16] [17]
[18] [19]
Acknowledgments Financial support by the National Natural Science Foundation of China (NSFC) (Grant 51474234 and 51474235), China Postdoctoral Science Foundation funded project (2014M551988), the Special Funds for Shandong Province Postdoctoral Innovative Projects (201302016), Shandong Provincial Natural Science Foundation, China (ZR2012EEM007) and the Innovation Project Foundation for Graduate Student in China University of Petroleum (East China) (Grant CX2015012) is gratefully acknowledged.
[20]
[21]
[22]
[23]
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Experimental research of syneresis mechanism of HPAM/Cr3+ gel Guicai Zhang, Lifeng Chen ∗ , Jijiang Ge, Ping Jiang, Xiaoming Zhu College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The over-crosslinking is occurred
SEM images of different gels. (a) Gel with 0% syneresis; (b) gel immersed with CaCl2 solution.
between C O bond in COO− of HPAM and Cr3+ . • The Ca2+ reacts with the C O bond in COO− of HPAM. • The syneresis water originates from the water bounded to carboxylate group of HPAM. • Syneresis inhibitor restrains the generation of the Cr3+ polynuclear olation complex.
a r t i c l e
i n f o
Article history: Received 1 April 2015 Received in revised form 1 June 2015 Accepted 25 July 2015 Available online 29 July 2015 Keywords: Gel HPAM Syneresis Crosslinking
a b s t r a c t Experimental investigations have been conducted to elucidate the syneresis mechanism of HPAM (partly hydrolyzed polyacrylamide)/Cr3+ gel. The gel prepared with HPAM of high hydrolysis degree is apt to result in the syneresis, since HPAM with more carboxylate group is easier to be over-crosslinked. The over-crosslinking, which contributes a lot to the syneresis, occurs between C O bond in COO− of HPAM and Cr3+ . The decreasing hydrophilicity of HPAM molecule, the increase of crosslinking density, and the complexation of the carboxylate group with Ca2+ are the main syneresis mechanisms of the gel influenced by the inorganic salt. The Ca2+ reacts with the C O bond in COO− , and the reaction produces a tabular structure when the concentration of Ca2+ is high, whereby the water in the initial gel is extruded. The produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of HPAM, and this confirms that the decreasing hydrophilicity of the carboxylate group in HPAM molecule is one of the important reasons to the gel syneresis. Sodium d-isoascorbate inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+ , whereby the crosslinking speed is decreased and the syneresis is suppressed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Fax: +86 53286981178. E-mail addresses: [email protected], [email protected] (L. Chen). http://dx.doi.org/10.1016/j.colsurfa.2015.07.048 0927-7757/© 2015 Elsevier B.V. All rights reserved.
Gels formulated with partly hydrolyzed polyacrylamide (HPAM) and Cr3+ have been used extensively as water-shutoff agents to reduce water production [1–5]. HPAM/Cr3+ gel placed in situ in water dominated channels reduces water permeability and therefore water production. The gelation criteria, kinetics of gelation,
G. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
transport properties of the gel fluid and long-term stability of the gel in the formation are generally the major concerns in the design of such a profile modification treatment, and the last one is an important part since it is directly associated with the effect of the water-shutoff treatment. Gels typically used in profile modification applications share a common phenomenon, termed syneresis, in which the solvent phase separates from the gel phase as a result of gel shrinkage. The rate of this separation determines the lifetime of the gel under any given set of circumstances. For HPAM/Cr3+ gel in reservoir, this shrinkage in volume reduces the ability of the gel to reduce water flow [6]. As such, syneresis is the mechanism whereby the effectiveness of the gel decreases with time. Therefore, inhibiting the syneresis of HPAM/Cr3+ gel is a key to extend the duration of water-shutoff treatment. When HPAM/Cr3+ gel is in contact with excess brine or high temperature over a long period of time, the amide group in HPAM hydrolyzes to form carboxylate groups. The interaction of the carboxylate group with the divalent cations results in a sharp reduction in polymer solubility, which is an important reason of the gel syneresis [7,8]. In order to decrease the syneresis, some novel polymers with comonomers meeting the criteria of resistance to hydrolysis and inactivity toward divalent cations have been investigated and used in the gel. Although a fairly wide number of new acrylamide polymers have been described in the literature [9–15]. only a relatively small number of them, including acrylamide copolymers of N-vinyl pyrrolidone (N-VP), 2-acrylamido-2-methyl-propanesulfonate (AMPS), and Nvinyl acetamide (N-VA), have been commercialized. Although the novel polymers were generally found to possess excellent thermal stability and resistance to brine, their suitability for petroleum applications is limited by their unreasonable cost. Besides, it is worth emphasizing that a more serious problem for most of the new polymers used in the gel treatments is their relatively high adsorption on reservoir rock. Hence, finding the syneresis mechanism and applying the common polyacrylamide in the gel for water-shutoff is a better way to improve the effectiveness of the profile modification treatment. The syneresis mechanisms of the water-shutoff gel have been studied by some researchers, and the over-crosslinking mechanism is generally accepted as the primary reason of the gel shrinkage. Gales et al. [16] investigated the stability of xanthan gum/Cr3+ gel, and they showed that the increase of Cr3+ used in the gel led to the high degree of syneresis. They therefore concluded that reducing the crosslinking density is facilitated to decrease the syneresis, and pointed out the appropriate amount of crosslinking agent was the precondition to ensure the gel possessing the excellent stability. Eriksen et al. [17] confirmed Gales’s point of view. They studied the stability of the polyacrylamide-formaldehyde gel at 120 ◦ C, and they found that reducing the crosslinking ratio of formaldehyde and polyacrylamide increases the crosslinking time, which results in the low crosslinking density and the decreasing syneresis. Albonico and Lockhart [18] researched the syneresis of the HPAM/Cr3+ gel under the effect of the Ca2+ and Mg2+ . They stated that Ca2+ and Mg2+ may react with the carboxylate in HPAM, which increased the crosslinking density and decreased the water solubility of HPAM. As a result, the water seperated out from the gel phase whereby the gel stability reduced. In summary, over-crosslinking is considered to be the main reason for the gel syneresis in the previous reports. However, the above conclusion was obtained by the inference based on the experiment result, and it still needs intuitive and sufficient evidence to be proved. Although the syneresis mechanism of the gel influenced by the Ca2+ has been speculated in the previous literature [18], it is necessary to be clearly clarified. In addition, to our best knowledge, the effect of the hydrophilicity of HPAM on the gel syneresis has not been investigated, so the research on the hydrophilicity will
97
Table 1 Hydrolysis degree and molecular mass of HPAM. HPAM
Molecular mass, 106
Hydrolysis degree (%)
H1 H2 H3
12 12 12
20 30 39
contribute a lot to elucidate the principle of gel syneresis. Therefore, a series of experiments were conducted in this work to further study the syneresis mechanism. This investigation will provide an improved understanding of the gel syneresis, which is conducive to select the formula of water-shutoff gels with high stability. 2. Experimental 2.1. Materials Sodium bichromate (Na2 Cr2 O7 ), sodium sulfite (Na2 SO3 ), sodium chloride (NaCl), calcium chloride (CaCl2 ), magnesium chloride (MgCl2 ) and ethylene diamine tetraacetic acid (EDTA) are all analytically pure, and purchased from Sinopharm. Besides, the additives, sodium d-isoascorbate, sodium oxalate, sodium lactate and sodium salicylate, are also obtained from Sinopharm. HPAMs were purchased from Beijing Hengju Chemical Group Corporation, and their hydrolysis degree and molecular mass are listed in Table 1. All concentrations in the paper are on a weight basis. 2.2. Measurements of gelation time and syneresis rate First, a 0.4% HPAM stock solution was prepared by dissolving solid HPAM in fresh water. A container with a known amount of water was vigorously stirred to create a deep vortex. HPAM was slowly added to the shoulder of the vortex to effectively wet the HPAM beads. The container was sealed to minimize evaporation and was stirred continuously for 24 h to ensure complete dissolution of HPAM. The crosslinker, whose amount was carefully tuned, was dissolved in fresh water to prepare a crosslinker solution. Finally, the gelling solution was prepared by mixing the HPAM stock solution and crosslinker solution. Aqueous gellable compositions were obtained through many screening procedures, that is, 0.2% HPAM + 0.3% Na2 Cr2 O7 + 0.6% Na2 SO3 . After the gelling solution was prepared, it (20 g) was sealed in a bottle and put into an oven at 60 ◦ C, and then the gelation time and syneresis rate were measured. The gel strength code method [19], which is showed in Table 2, was used to determine the gelation time, and the gelation time is considered as the period of time when gelling solutions in code A state turn to code G in this paper. Syneresis rate is defined as the decrease in the gel weight at a given time relative to the initial gel weight, and the onset of the syneresis is referred to the time from which the gel actually formed, not from the moment that the solutions were placed in the oven. 2.3. Effect of inorganic salts on the syneresis When the gel was formed in the bottle with stopper, the inorganic salt solution (NaCl: 0.05–0.5 mol/L; CaCl2 and MgCl2 : 0.001–0.1 mol/L) of equal quality (20 g) was put in the bottle. As a result, the effect of inorganic salts on the syneresis can be investigated by measuring the weight of the salt solution at the given time. 2.4. Measurements of differential scanning calorimetry (DSC) Two gel samples were prepared as the method in Section 2.2. When the syneresis of them reached to 0% and 25%, respectively,
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Table 2 Gel strength code. Gel strength code
Gel description
A B C D E F G H I
No detectable gel formed: No detectable gel formed: The gel appears to have the same viscosity as the original polymer solution Highly flowing gel: The gel appears to be only slightly more viscous than the initial polymer solution Flowing gel: Most of the gel flows to the bottle cap by gravity upon inversion Moderately flowing gel: Only a small portion (5–10%) of the gel does not readily flow to the bottle cap by gravity upon inversion Barely flowing gel: The gel can barely flow to the bottle cap and/or a significant portion (>15%) of the gel does not flow by gravity upon inversion Highly deformable nonflowing gel: The gel does not flow to the bottle cap by gravity upon inversion Moderately deformable non flowing gel: The gel deforms about half way down the bottle by gravity upon inversion Slightly deformable nonflowing gel: only the gel surface slightly deforms by gravity upon inversion Rigid gel: There is no gel surface deformation by gravity upon inversion
2.5. Measurements of scanning electron microscope (SEM) When the gel was formed, a sample was immersed with CaCl2 solution (0.1 mol/L) for 12 h. The gel with different color on the interface of the gel and the CaCl2 solution and the one without the immersing treatment by CaCl2 solution were dried by freeze-dry technique. The resulting samples were sputter coated for 90 s with gold and observed using a SEM (Hitachi S-4800, Hitachi High-Technologies Corporation, Tokyo, Japan). Determinations were conducted at accelerating voltage of 25 kV and working distance from 5 mm to 10 mm.
50
H1 H2 H3
40
Syneresis rate (%)
a part of them were taken to be measured by DSC. A Pyris Diamond DSC (PerkinsElmer) equipped with a cooling device was used to measure the phase transition of water sorbed in the gel samples. DSC cures were obtained by fast sample cooling from 40 ◦ C to −40 ◦ C at the scanning rate of 10 ◦ C/min. The crystallization temperatures of water sorbed in the sample were determined from the temperature at the maximum point of the corresponding enthalpy peaks.
30
20
10
0
0
5
10
15
20
25
30
Time (day) Fig. 1. Effect of different HPAM on the syneresis rate of gel.
3. Results and discussion
syneresis rate (syneresis rate is defined as the decrease in the gel weight at a given time relative to the initial gel weight), three different HPAMs were employed to prepare a gelling solution. Through many lab screening experiments, the robust and persistent gels could not be formed when the HPAM with the hydrolysis degree lower than 15% was used. As a result, the HPAM with the hydrolysis degree in the range of 20–39% was employed in the research. It can be observed from Fig. 1 that the gel syneresis process has two main stages: the first stage of rapid syneresis and the second stage of slow syneresis. The rapid syneresis may be mainly due to the high temperature and hot oxygen effect on inorganic chromium gel, which is usually used in lower temperature (around 50 ◦ C). Besides, the syneresis of the gel prepared with various HPAMs is obviously different. As to H1–H3, the hydrolysis degree increases orderly while the molecular mass is identical (Table 1), and the syneresis rate of the corresponding gel also increases sequentially. DiGiacomo et al. also obtained the same conclusion by investigating the changes in hydrolysis of polyacrylamide polymers with C-13 NMR. They found that the increase in the degree of hydrolysis parallels the syneresis of the gels [20]. Since the HPAM gel is resulted from the crosslinking between the carboxylates and Cr3+ , and higher hydrolysis degree means that there are more carboxylates in a HPAM molecular, as a result, the crosslinking density of the HPAM gel is large when the hydrolysis degree is high, and the water in the grid structure is easy to be expelled due to the high density of grid structure. Therefore, the gel prepared with HPAM of high hydrolysis degree is apt to produce the syneresis.
3.1. Effect of hydrolysis degree on the syneresis
3.2. Effect of inorganic salts on the syneresis
Hydrolysis degree (the number ratio of carboxyl group to amide and carboxyl groups in one HPAM molecule.) is the basic item to be evaluated when HPAM is used as the main agent in water plugging gel. In order to understand the effect of hydrolysis degree on the
The gel at the real reservoir condition is inevitably exposed in the formation water with inorganic salts, so it is necessary to study the effect of inorganic salts on the gel stability. Figs. 2, 3 and 4 show the syneresis law of the initial gels immersed in the NaCl,
2.6. Measurements of Fourier transform infrared spectroscopy (FTIR) The HPAM beads and the samples which were used in Sections 2.4 and 2.5 were employed to be measured by FTIR spectra. The above samples were all dried by freeze-dry technique, and the FTIR specimen were prepared by mixing the sample and KBr with the weight ratio of 1:100. FTIR spectra were obtained on an Nicolet 6700 FTIR Spectrometer, and spectral analysis was performed over the range 4000–400 cm−1 . 2.7. Effect of additives on the syneresis A certain amount of additive was used in the gelling solution, and the syneresis, gelation time and gel strength were measured as the method in Section 2.2. The measurements of spectrophotometry were as the following: the crosslinker solutions (0.3% Na2 Cr2 O7 + 0.6% Na2 SO3 ) with no additive, 0.1% sodium d-isoascorbate and 0.1% sodium oxalate were prepared, respectively. 5 g crosslinker solution, 0.5 g 0.025 mol/L H2 SO4 and 2.5 g 0.05 mol/L EDTA were mixed and aged at 80 ◦ C for 10 min, and then the absorbance of the mixed solution was measured at the wavelength of 538 nm by the spectrophotometer.
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100
80 70
80
50
Syneresis rate (%)
Syneresis rate (%)
60
40 30 20
0.05 0.1 0.2 0.3 0.5
10 0 -10 -20
0
5
10
15
Time (day)
20
25
30
80 60 40
0.0001 0.001 0.01 0.05 0.1
0 -20 -40 0
5
10
15
20
25
40 20
0.0001 0.001 0.01 0.05 0.1
-20
0
5
10
15
Time (day)
20
25
30
Fig. 4. Effect of MgCl2 (mol/L) on the syneresis rate of gel.
100
20
60
0
Fig. 2. Effect of NaCl (mol/L) on the syneresis rate of gel.
Syneresis rate (%)
99
30
Time (day) Fig. 3. Effect of CaCl2 (mol/L) on the syneresis rate of gel.
CaCl2 and MgCl2 solution respectively. With the increasing concentration of inorganic salt, the syneresis is exacerbated. The reasons may be the followings: for the gel immersed in the water with highconcentration salt (0.2–0.5 mol/L NaCl, 0.01–0.1 mol/L CaCl2 and MgCl2 ), the repulsion between carboxyl groups is reduced by an attraction between cations (Na+ , Ca2+ and Mg2+ ) and the negatively charged carboxyl group. This causes the molecule to be compressed from its enlarged state and results in a reduction of the hydrated radius [21]. In another word, the hydrophilicity of the carboxyl group decreases, whereby a part of bound water may be transformed into free water. Since there is no interaction force between the free water and the gel, the free water is easy to be seperated out from the gel. However, the syneresis law of the gel immersed in NaCl, CaCl2 and MgCl2 solution is different when the salt concentration is low (0.05–0.1 mol/L NaCl, 0.0001–0.001 mol/L CaCl2 and MgCl2 ). As to the gel exposed to the NaCl and MgCl2 solution of low concentration, it swells at first then produces the syneresis. This indicates that the effect of the inorganic cations on the carboxyl group is no longer the determining factor for the volume changes of the gel. According to polymer network theories [16], the swelling or the syneresis of a polymer network is governed by the mixing potential and the elastic potential. The mixing potential favors the dispersion of network chains into the solvent and hence swelling, and the elastic potential represents the elastic force imposed by the crosslinks which resists the change of the network chains from their unstrained state. When the brine concentration is low, the gel with a low crosslinking density has the relatively large mixing
potential, which will cause swelling. During the swelling process, the increasing deformation of the network leads to an increased elastic potential while the increased gel volume causes a decrease in the mixing potential. At a certain degree of swelling, the elastic potential is equal to the mixing potential and the swelling equilibrium is reached. As the gel further crosslinks, the elastic potential exceeds the mixing potential, and the syneresis appears. Hence, the change of the crosslinking density determines the variation of the gel volume as the gel is exposed to the NaCl and MgCl2 solution of low concentration. Nevertheless, when the gel is immersed in the CaCl2 solution of low concentration, it nearly swells continuously rather than swelling at first then producing the syneresis, as showed in Fig. 3. The interaction between polyelectrolytes and the Ca2+ has been widely investigated, and it has been proposed that the carboxylate group in polyelectrolytes can be crosslinked with Ca2+ to form a complex compound [22–25]. So it can be theorized that the reaction between the carboxylate group and Ca2+ is the key factor on the stability of the gel in the low-concentration CaCl2 solution. Therefore, the syneresis mechanism of the gel influenced by the salt is complicated, and the decreasing hydrophilicity of HPAM molecule, the increase of the crosslinking density, and the complexation of the carboxylate group with Ca2+ codetermine the gel syneresis. When the gel was immersed in the CaCl2 solution for 12 h, a new material with different colour was generated on the interface of the gel and the CaCl2 solution (0.05 and 0.1 mol/L), as showed
Fig. 5. Gel immersed with CaCl2 solution of different concentrations for 12 h (from left to right: 0, 0.01, 0.05, 0.1 mol/L; the bottle is inverted and the gel are all with 0% syneresis).
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Fig. 6. SEM images of different gels. (a) Gel with 0% syneresis; (b) gel immersed with CaCl2 solution (the new material on the solid–liquid interface).
in Fig. 5. The new material is rigid, and it cannot flow even when the bottle with the gel was inverted. To clarify the difference of the microstructure between the new material and the gel with 0% syneresis, the observation analysis by the SEM was conducted. As shown in Fig. 6, the microstructures of the two materials in the same amplification are obviously different. The microstructure of the gel with 0% syneresis is the network structure, and there is large amount of pore space in the network structure to hold the water in gels. However, the microstructures of the new material is the tabular structure with many small holes, and there is not enough space to hold the water which is forced to be separated from the gel phase. This is the reason why the gel exposed to the high-concentration CaCl2 solution synereses quickly. However, why did the gel in the CaCl2 solution get stiff to produce the tabular structure? As it is generally accepted, the Ca2+ may take part in the crosslinking reaction with HPAM in the gel, but it is not clear that which functional group the Ca2+ reacts with. To answer the above question, FTIR measurements were conducted. The analysis objects of the FTIR measurement are HPAM, gel with 0% syneresis, gel with 25% syneresis and gel immersed with CaCl2 solution (the above new material analyzed by SEM), and the result is showed in Fig. 7. In order to better compare the quantitative changes in the major functional groups resulted from the syneresis and Ca2+ , the FTIR curves were analyzed by fitting multi-peaks of Gaussian form, and the corresponding peak height of different functional groups were listed in Table 3. For the gel with 0% syneresis and the one immersed with CaCl2 solution, the
differences of the peak height at 1110 cm−1 and 1410 cm−1 are small. But the peak height of the gel immersed with CaCl2 solution at 1570 cm−1 increases sharply compared with that of the gel with 0% syneresis. As the shoulder at about 1110 cm−1 , 1410 cm−1 and 1570 cm−1 is respectively ascribed to the stretching vibration of the C O bond in COO− , the C N bond in CONH2 and the C O bond in COO− [25], it can be theorized that the reaction between the C O bond in COO− and the Ca2+ is the reason why the new material is produced. This result is in good agreement with the standpoint of Fantinel et al. [26], and they showed that the COO− in polyacrylates can react with Ca2+ to form the complex chelating bidentate. Besides, some other conclusions can be obtained by closer inspection of the data in Fig. 7 and Table 3. The peak height at 1410 cm−1 of the gel immersed with CaCl2 solution and that of the one with 25% syneresis is lower than that of the gel with 0% syneresis. This shows that the C N bond in CONH2 decreased which means HPAM hydrolyzes in the process of syneresis and immersing. It is noteworthy that the influence of the Ca2+ on the increase of the hydrolyzation degree is more significant. The C N bond in CONH2 is transformed to the C O bond due to the hydrolyzation, so the amout of the C O bond should increase. However, the C O bond of the gel immersed with CaCl2 solution and that of the one with 25% syneresis do not increase but decrease. This result can prove that the over-crosslinking, which is generally accepted as the gel syneresis mechanism, occurs between the C O bond in COO− and Cr3+ .
Fig. 7. FTIR curve of HPAM and different gels.
G. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
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Table 3 Peak height of functional groups in different gels. Wavenumber (cm−1 )
Group assignments
Gel with 0% syneresis
Gel immersed with CaCl2 solution
Gel with 25% syneresis
1110 1410 1570
C O in COO− C N in CONH2 C O in COO−
0.8059 0.0544 0.0727
0.8000 0.0457 0.2455
0.7509 0.0469 0.0939
Table 5 The mass fraction of the different states of water in gel.
Gel with 0% syneresis Gel with 25% syneresis
Free water content (%)
Freezable hydration water content (%)
Non-freezable hydration water content (%)
54.27 89.22
22.91 0
22.82 10.78
non-freezable hydration water decreases, leading to the increase of the amount of free water. The reduction of the freezable hydration water and non-freezable hydration one may be due to the following reasons: (i) the abscission of the carboxylate group caused by the ageing effect, (ii) the decrease of the hydrophilicity of the carboxylate group influenced by the inorganic salt ions (Na+ , Cr3+ ). This indicates the produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of the polymer. Hence, it is confirmed once again that the decrease of the hydrophilicity of HPAM is one of the important reasons to the gel syneresis.
Fig. 8. DSC curve of two different gels.
3.3. Effect of HPAM hydrophilicity on the syneresis 3.4. Effect of additives on the syneresis The state of water in the polymer solution has been investigated by some researchers, and most of them divide this water into three types [27–30]: (i) Free water — in a “free” associated state with phase transition parameters close to those of melting ice; (ii) Freezable hydration water — weakly bound to the polymer, but capable of freezing at quite reduced temperatures, and (iii) Non-freezable hydration water — strongly bound to the polar groups of the polymer, not freezing, and not registered calorimetrically. Since the gel is obtained by the crosslinking between large amount of polymer solution and small amount of crosslinking agent, it is inferred that the three types of water also existed in the gel. Through the DSC measurement, the above inference has been confirmed. As showed in Fig. 8, the gel with 0% syneresis has two exothermic peaks which appears at −19.89 ◦ C and −17.09 ◦ C respectively, whereas the one with 25% syneresis has only one exothermic peak which appears at −16.92 ◦ C. According to the theory of Nishioka et al. [31], for the gel with 0% syneresis, the exothermic peak at −19.89 ◦ C results from the heat release of the freezable hydration water, the exothermic peak at −17.09 ◦ C is due to the heat release of the free water, and the exothermic peak at −16.92 ◦ C is also due to the heat release of the free water for the gel with 25% syneresis. Compared with the polymer solution, the phase transition temperature of the water in the gel decreases obviously due to the crosslinking reaction. The melting enthalpy obtained in the DSC measurement is showed in Table 4, and the mass fraction of a type of water was obtained as: Wn = Hn /H0 × 100%, where Hn is the melting enthalpy of one type of water, and H0 is assumed to be the same as that of pure water (333.5 J/g) [32]. As shown in Table 5, due to the syneresis, the freezable hydration water in the gel disappears and the amount of Table 4 The melting enthalpy obtained in the DSC measurement.
Gel with 0% syneresis Gel with 25% syneresis
Melting enthalpy, H1 (J/g)
Melting enthalpy, H2 (J/g)
180.99 297.54
76.40 0
In order to decrease the syneresis, some additives were used in the gelling solution, and the effect of additives on syneresis is shown in Table 6. Sodium d-isoascorbate is a syneresis inhibitor to extend the stability of gels, whereas the sodium oxalate cannot be defined as an effective additive. To clarify the mechnism of the additives influencing the gel stability, the syneresis at Day 60 and the gelation time were studied at different concentrations of additives. As shown in Tables 7 and 8, the syneresis decreases and the gelation is delayed along with the increase of the syneresis inhibitors. According to the published literature [33], the above result is due to the reaction between the inhibitor and Cr3+ . Since Cr3+ in water can form the polynuclear olation complex by the three process: complexation, hydrolysis and olation [34], the inhibitor may react with the Cr3+ to produce the Cr-inhibitor complexes. In this condition, the forming speed of the polynuclear olation complex decreases resulted from the above competition effect. As a result, the gelation time is prolonged, and the crosslinking rate decreases. Hence, the crosslinking density increases slowly, which improves the stability of the gel. In the previous report, Albonico and Lockhart [35] investigated the effect of acetate, 2-hydroxy butyrate, serine, glycolate, et al. on the gel syneresis, they obtained the same conclusion that these additives could inhibit the syneresis by prolonging the gelation time. Besides, the gel strength was also studied, and the results showed that the gel strength was nearly not influenced by sodium d-isoascorbate and sodium oxalate, so the additive would not decrease the water-shutoff efficiency of the gel. In order to obtain an improved understanding of the Cr-inhibitor complex, the heat treatment of the crosslinker solutions (with the additives) was conducted at 60 ◦ C. The crosslinker solutions with sodium d-isoascorbate and sodium oxalate aged for 4 h are shown in Fig. 9a and b respectively. It can be observed that the green “fluffy” precipitate is produced in the crosslinker solutions with no additives, 0.05% and 0.1% sodium oxalate. On the contrary, the crosslinker solutions with 0.2% sodium oxalate, 0.05% and 0.1% sodium d-isoascorbate are all still green clear liquid. By observing Tables 7 and 8 again, the good correlation between the syneresis
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Table 6 Effect of additives on syneresis of gel. 0.05% additive
None
Sodium d-isoascorbate
Sodium lactate
Sodium salicylate
Sodium oxalate
Syneresis at day 60 (%)
45.11
12.14
36.43
17.55
43.41
Fig. 9. Effect of additives on the Na2 Cr2 O7 –Na2 SO3 solution (without HPAM). (a) sodium d-isoascorbate: 0, 0.05%, 0.1% (from left to right). (b) sodium oxalate: 0.05%, 0.1%, 0.2%; (c) 0.1% sodium d-isoascorbate, 0.2% sodium oxalate.
Table 7 Effect of sodium d-isoascorbate on the syneresis and gelation time. Sodium d-isoascorbate (%) Syneresis at day 60 (%) Gelation time (h) Gel strength
0 45.11 1.9 H
0.05 12.14 6.1 H
0.1 6.22 20.2 G
Table 9 Absorbance of Cr3+ in crosslinker solutions with various additives at different times. 0.2 – >1000 –
“–” means that the result can not be obtained.
Table 8 Effect of sodium oxalate on the syneresis and gelation time. Sodium oxalate (%) Syneresis at day 60 (%) Gelation time (h) Gel strength
0 45.11 1.9 H
0.05 43.41 2.8 H
0.1 35.33 3.4 H
0.2 20.72 4.8 G
and the stability of the crosslinker solution is found, that is, the more steady the crosslinker solution, the lower the syneresis rate. Since there is no other reaction leading to the precipitation in the solution, the green “fluffy” precipitate is considered as a variant polynuclear olation complex with the overlong molecular chain. The effective syneresis inhibitor can delay the generation of the Cr3+ polynuclear olation complex by restraining a process of the complexation, hydrolysis and olation polymerization, thereby the chain of the polynuclear olation complex grows slowly, which results in the increase of the crosslinker stability. It is also observed in Fig. 9c that the crosslinker solution with 0.1% sodium d-isoascorbate is still clear after being aged for 24 h, whereas the one with 0.2% sodium oxalate generates the green precipitation. This further shows that the inhibiting effect of sodium d-isoascorbate on the growth of the polynuclear olation complex is much stronger than that of sodium oxalate. To further explain the above result, the absorbance of Cr3+ in different crosslinker solutions were measured according to the method of Ohls et al. [36] Cr3+ can form violet complex with EDTA in the temperature range of 70–80 ◦ C, whereas Cr6+ cannot react with EDTA at the same condition. The maximum absorption peak of the violet complex is at 538 nm, and the absorbance measured by the spectrophotometry is positively correlated with the content of Cr3+ . As shown in Table 9, the absorbance decreases over time,
Time (min)
None
Sodium oxalate
Sodium d-isoascorbate
0 10 30 60
0.992 0.867 0.786 0.754
0.976 0.855 0.776 0.727
0.823 0.764 0.709 0.651
it indicates that the individual Cr3+ is transformed into the polynuclear olation complex continuously, thereby less violet complex is formed due to the decrease of Cr3+ . Besides, the content of Cr3+ in the crosslinker solution with sodium d-isoascorbate is lower than that in the other two solutions. This shows that sodium disoascorbate coordinates more Cr3+ to form a certain kind of Cr3+ complex whereby there is less Cr3+ to produce the violet complex with EDTA. Hence, this result confirms that sodium d-isoascorbate can decrease the crosslinking speed by reacting with the Cr3+ . In summary, the mechanism of an efficient inhibitor suppressing the gel syneresis is as the follows: an additive inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+ . Since the HPAM crosslinks with the polynuclear olation complex to form the gel, the crosslinking density increases slowly due to the effect of the inhibitor. Therefore, the water will not be squeezed out from the gel quickly on account of the overcrosslinking. 4. Conclusions A series of experimental investigations have been conducted to elucidate the syneresis mechanism of HPAM/Cr3+ gel. The results indicate that: (1) The gel prepared with HPAM of high hydrolysis degree is apt to result in the syneresis, since HPAM with more carboxylate group is easier to be over-crosslinked. The over-crosslinking, which contributes a lot to the gel syneresis, occurs between C O bond in COO− of HPAM and Cr3+ . (2) The decreasing hydrophilicity of HPAM molecule, the increase of the crosslinking density, and the complexation of the
G. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 483 (2015) 96–103
carboxylate group with Ca2+ are the main syneresis mechanism of the gel influenced by the inorganic salts. The Ca2+ reacts with the C O bond in COO− , and the reaction produces a tabular structure when the concentration of Ca2+ is high, whereby the water in the initial gel is extruded. (3) The produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of HPAM, and this indicates that the decreasing hydrophilicity of the carboxylate group in HPAM molecule is one of the important reasons to the gel syneresis. (4) Sodium d-isoascorbate inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+ , whereby the crosslinking speed is decreased and the syneresis is suppressed.
[14]
[15]
[16] [17]
[18] [19]
Acknowledgments Financial support by the National Natural Science Foundation of China (NSFC) (Grant 51474234 and 51474235), China Postdoctoral Science Foundation funded project (2014M551988), the Special Funds for Shandong Province Postdoctoral Innovative Projects (201302016), Shandong Provincial Natural Science Foundation, China (ZR2012EEM007) and the Innovation Project Foundation for Graduate Student in China University of Petroleum (East China) (Grant CX2015012) is gratefully acknowledged.
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