Article pubs.acs.org/IECR
Investigating the Effect of Several Parameters on the Gelation Behavior of Partially Hydrolyzed Polyacrylamide−Hexamine− Hydroquinone Gels Upendra Singh Yadav and Vikas Mahto* Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826004, Jharkhand, India ABSTRACT: The polymer gel systems are mostly used for the control of excessive water production during enhanced oil recovery operations in mature oil fields. However, these polymer gels are not stable in the high-temperature and high-salinity reservoir. Keeping these in mind, an attempt was made to investigate the effect of several parameters on the gelation time of polymer gels for its suitable application in the oil fields. In this paper, a polymer gel comprised of partially hydrolyzed polyacrylamide as a water-soluble polymer and hydroquinone and hexamine as organic cross-linking agents of low toxicity was prepared to control excessive water production. The effect of various parameters such as polymer and cross-linker concentrations, temperature, pH, and salinity on the gelation time and gel strength was evaluated using the bottle testing method, and variations in the performance of the polymer system were analyzed under different gel compositions and environmental conditions.
1. INTRODUCTION During water flooding, water is injected into the injector well to sweep the reservoir fluid toward the production well, and in this way, fluid from the petroleum formation is recovered at the surface. However, the injected fluid (water) is bypassed into the producer well in the presence of high permeability, fractures, and a fracture network, and the same injected water is produced at the surface from the production well. The other sources of excess water production are casing leaks, the channel behind the casing, completion into or near the water zone, water coning, and wettability problems.1−5 Excessive water production in association with crude oil is one of the major production difficulties for the oil industries worldwide. The costs of lifting, handling, separation, and disposal of large amounts of produced water increase the operating cost of the well. Moreover, problems such as scale, corrosion, emulsion, bacteria, and sand production can arise as a result of excessive water production.6 Normally, control of excessive water production is carried out by adopting methods like cement/sand plugs, mechanical packers, sodium silicate gels, resins, polymer gels, etc. These methods are basically being used to stop or reduce water cut, thus improving the sweep efficiency and controlling conformance, which will improve/increase the economic life of the reservoir.7,8 Mechanical plugging and cementing are the only options for high-salinity and high-temperature reservoirs because their properties are not affected by the presence of multivalent ions in water as well as the temperature of the reservoir.9 It is estimated that, for each barrel of oil produced worldwide, an average of three barrels of water are produced.10 Disposal of the produced water increases the operating cost of crude oil production and decreases the economic life of a well. Therefore, there is a need to reduce excessive water production.11,12 The polymer gel treatment is one of the most useful chemical methods to reduce water production.13 Polymer gels are typically composed of a water-soluble polymer and cross© 2013 American Chemical Society
linking agents that are dissolved in water. After sufficient time, the gelant solution sets into a semisolid mass and behaves as flow-diverting or -blocking agent. The selection of a polymer gel system for a given well treatment strongly depends on the reservoir conditions such as the temperature, salinity, hardness, and pH of the water used for the preparation of the gelant.14 Other parameters to be considered for the proper selection of a given polymer gel system include the salinity of the formation water, permeability of the target zone, and lithology of the formation.15 The different polymers used for the development of polymer gel in the oil fields are polyacrylamide with different degrees of hydrolysis (partially hydrolyzed polyacrylamide, PHPA) and polysaccharide such as xanthan biopolymer.16,17 These polymers can be cross-linked with metallic/inorganic and organic cross-linkers to produce a three-dimensional polymer structure of the gel.18 The inorganic cross-linkers include chromium(III), aluminum(III), and zirconium(IV) and have been mostly utilized to cross-link PHPA. Inorganically crosslinked gels result from ionic bonding between the negatively charged carboxylate groups and multivalent cation. The organic cross-linkers were introduced to obtain gels that are stable over a wider temperature range.19 This is possible because, in this case, the cross-linking is done by a covalent bond, which is much more stable than ionic bonds. The covalent bonds often involve the amide groups on the polymer backbone. The organically cross-linked gels developed using phenol and formaldehyde cross-linkers are thermally stable under harsh environmental conditions. However, these cross-linkers are not environmentally friendly.20 To overcome the toxicity issues associated with formaldehyde and phenol, several less toxic substitutes of these cross-linkers are being used by the oil Received: Revised: Accepted: Published: 9532
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3.1. Effect of the Temperature on the Gelation Time. Different gelling solutions were prepared with various concentrations of polymer and cross-linking agent and kept for gelation at temperature ranges from 80 to 120 °C. Table 1
industries worldwide. The substitutes for formaldehyde are hexamine (HMTA), glyoxal, paraformaldehyde, acetaldehyde, trioxane, polyoxymethylene, etc., and the substitutes for phenol include hydroquinone (HQ), resorcinol, catechol, pyrogallol, phenyl acetate, etc.21−23 Other systems are based on a polyethyleneimine (PEI) cross-linker and a copolymer of acrylamide and tert-butylacrylate (PA-t-BA). PA-t-BA is a relatively low-molecular-weight polymer and is expected to provide rigid ringing gels. PEI cross-linking with PA-t-BA as water shutoff gels has been widely used in recent years.24 The present work involves bulk gelation studies with the help of the bottle testing method of the polymer gelant prepared from PHPA, HMTA, and HQ. The gelation time determined from this study may be useful in finding the depth up to which the gel can be placed in the formation. The most important variable was the temperature, but variations in gelation as a function of the pH, salinity, and the concentrations of the polymer and cross-linking agent(s) were also studied and reported in this paper.
Table 1. Effect of the Gelation Temperature on the Gelation Time at pH 8.0 polymer concn (wt %)
2. EXPERIMENTAL WORK 2.1. Material Used. The materials used for this work are partially hydrolyzed polyacrylamide (PHPA), hexamine (HMTA), hydroquinone (HQ), sodium chloride, hydrochloric acid, and sodium hydroxide. PHPA was procured from Oil and Natural Gas Corp. Ltd., Mumbai, India. HMTA was purchased from Otto Kemi Mumbai, India, and HQ was procured from Ranbaxy Fine Chemicals Ltd., New Delhi, India. Hydrochloric acid was purchased from Central Drug house (P) Ltd., New Delhi, India. Sodium chloride was purchased from Nice Chemical Pvt. Ltd., Cochin, India, and sodium hydroxide was purchased from S. D. Fine Chem Ltd., Mumbai, India. 2.2. Methodology and Experimental Procedure. Initially, a stock solution of the polymer was prepared in brine and constantly stirred on a magnetic stirrer until a uniform viscous solution was obtained. The polymer solution aged at normal temperature for almost 48 h to ensure proper dissolution of the polymer in brine. The solutions of crosslinkers were then prepared by adding preweighed samples of the chemicals in brine. The gelant was then prepared by mixing the HMTA and HQ cross-linkers into the polymer solution at a specified ratio. The solution was homogenized by constant stirring using a magnetic stirrer. The pH of the gelant was adjusted by using a 1 N NaOH and 1 N HCl solution. Further, the bottle testing method was used for the measurement of the gelation rate and gel strength because it is the faster and least expensive method to study gelation kinetics.25−27 In this method, the glass tubes containing gelants were kept in the hot air oven and were inspected visually by inverting the tubes at frequent time intervals to observe gel formation. The time for formation of a stiff/rigid gel was considered as the gelation time.28 The same experiment was repeated three to four times, and the constant value of the gelation time was taken in our study.
cross-linker concn (wt %)
no.
PHPA
HMTA
HQ
temperature (°C)
gelation time (h)
1 2 3 4 5
1.1 1.1 1.1 1.1 1.1
0.5 0.5 0.5 0.5 0.5
0.4 0.4 0.4 0.4 0.4
80 90 100 110 120
93 21 14 8.5 5.5
Figure 1. Effect of the gelation temperature on the gelation time at 1.0 wt % PHPA, 0.4 wt % HMTA, 0.3 wt % HQ, and pH 8.0.
and Figure 1 shows the effect of the temperature on the gelation behavior of this polymer gel solution. With an increase in the temperature, the gelation time of the developed gel system decreases. A possible explanation for this is rapid crosslinking between the amide group of PHPA and the methylol group of cross-linkers, which is due to either enhancement of the molecular mobility or formation of new cross-linking sites during chemical reactions at higher temperature conditions.29 The probabilities of polymer molecules and cross-linkers colliding with each other and the contact between different chains within a molecule also increase. It is a known fact that a high temperature increases hydrolysis of the available amide groups, giving rise to more cross-linking sites, which is found to increase the reaction rate and decrease the gelation time.30 Therefore, the gel strength becomes stronger at high temperature.16 The gelation time was correlated to the temperature according to Arrhenius’ equation:
3. RESULTS AND DISCUSSION Qualitative analysis of the gelation time was carried out, and it was assumed that the same structure would be possible when the physical behavior of the polymer gel is the same. The physical behavior of the polymer gel will be the same at the time of stiff gel formation.
GT = M exp Ea / RT 9533
(1)
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where GT is the gelation time in hours, M is the frequency factor (h), Ea is the activation energy (kJ/mol/K), R is the universal gas constant, and T is the gelation temperature (K). The laboratory practice for determination of the activation energy for any chemical products is determined from the slope of the plot of the reaction rate versus temperature. In our cases, the slope was determined by several gelation rate−temperature data points. Here, the same experiment for determination of the gelation time using tube inversion (the bottle testing method) three to four times reported the constant value of the gelation time for accuracy or error minimization, as stated in the experimental procedure. According to eq 1, a plot of the natural logarithm for t versus 1/T gives a straight line with a slope of Ea/R and an intercept of ln M. Figure 2 shows an Arrhenius
Figure 3. Effect of the polymer concentration on the gelation time in 0.4 wt % HMTA and 0.4 wt % HQ at pH 8.0 and a temperature of 100 °C.
Table 2. Effect of the Polymer Concentration on the Gelation Time at 120 °C and pH 8.0 polymer concn (wt %)
cross-linker concn (wt %)
no.
PHPA
HMTA
HQ
gelation time (h)
1 2 3 4
1.1 1.0 0.9 0.8
0.5 0.5 0.5 0.5
0.3 0.3 0.3 0.3
6 8 14 34
3.3. Effect of the Cross-Linker Concentration on the Gelation Time. HMTA and HQ cross-linkers are a multifunctional group that can build a complex network with the amide groups of PHPA and form a three-dimensional gel network structure. The cross-linker concentration has a significant effect on the gel strength. Cross-linker concentration increases versus gelation time decreases are shown in Figure 4
Figure 2. Arrhenius plot of the gelation time and gelation temperature in 1.1 wt % PHPA, 0.3 wt % HMTA, and 0.3 wt % HQ at pH 8.0.
plot for gels prepared in a 1.1 wt % polymeric solution, 0.3 wt % HMTA, 0.3 and wt % HQ cross-linkers at pH 8.0 and a brine concentration of 1.0 wt %. The gelation time can be obtained from eq 1. Figure 2 shows the Arrhenius relationship between gelation times at the above-mentioned temperatures. The slope of the graph depicting the value of Ea/R shows an upward trend, which reflects the endothermic nature of the gelation reaction. 3.2. Effect of the Polymer Concentration on the Gelation Time. The bottle testing results show the effect of the polymer concentration on the gelation time and also its significant effect on the physical properties of gel. Polymer concentration increases versus gelation time decreases are shown in Figure 3. The polymer concentration ranges from 0.8 to 1.1 wt %, and constant concentrations of the cross-linker (0.4−0.2 wt % HMTA and HQ) are depicted in Table 2. As the polymer concentration increases, it means that more crosslinking sites are available for the fast cross-linking reaction and, hence, the gel formation reaction increases, which leads to decreases of the gelation time. This trend is expected to be the same at all gelation temperatures under study. Thus, the required time to obtain a nonflowing polymer gel with a tolerable strength decreased when the polymer concentration was increased.31
Figure 4. Effect of the cross-linker concentration on the gelation time in 0.9 wt % PHPA at pH 8.0 and a temperature of 100 °C. 9534
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showed that the gelation time increases with an increase in the concentration of brine. The increase in the brine concentration screens the cross-linking sites; thus, the induction period increases, due to which gelation takes longer time; moreover, the gels formed are less elastic in nature. This is due to sodium cations forming a shield on the amide groups, which results in shrinkage of the polymer chains or masking of the cross-linking sites. Consequently, the number of active cross-linking sites decreased and thus the intensity of the cross-links was lowered. The gel formation takes much longer. Further, the gels prepared in saline water were visually weaker than those prepared in distilled water.22 However, sodium ions can delay gelation more than potassium ions, which is mainly because of the higher charge density of sodium ions compared to that of potassium ions.32 The salinity has a positive effect on the gelation time delay, which can be used as a retardation agent of this gel system in high-temperature reservoirs. For polymer gel treatments in high-temperature reservoirs, understanding the gelation time delay mechanism is helpful for developing various retardation agents for the cross-linking reaction. Most studies provide some ambiguous and vague explanations for the effect of the salinity on the gelation time. They ascribe the root cause for a superficial explanation to the contraction of charged PHPA- or acrylamide-based copolymer coils by the charge-screening effect. The produced carboxylate groups carry negative charges under high-pH conditions and therefore stretch the PHPA network, increasing the hydrodynamic volume of the polymer. The reason for the gelation time delay is due to the chargescreening effect for polymer coil contraction. Furthermore, the contractions of the carboxyl groups caused by a chargescreening effect are often rather concentrated and can provide higher cross-linking sites, which results in an increase of the gelation rate.17 3.5. Effect of the pH. Different polymer gel systems have different ranges of pH over which they can maintain their stability. The pH range over which the experiments were carried out was from 8.0 to 9.5. Table 5 shows that pH ranges
and Table 3. Several samples were prepared to investigate the effect of the cross-linker concentration on the network strength. Table 3. Effect of the Cross-Linker Concentration on the Gelation Time at 110 °C and pH 8.0 polymer concn (wt %)
cross-linker concn (wt %)
no.
PHPA
HMTA
HQ
gelation time (h)
1 2 3 4 5
1.0 1.0 1.0 1.0 1.0
0.4 0.4 0.3 0.3 0.2
0.4 0.3 0.3 0.2 0.2
18 20.3 24 41 49
The bottle testing results indicate that when the concentrations of both cross-linkers are decreased, the gelation rate and gel quality are also decreased. In other words, when the crosslinking agent concentration was increased, the stage of the polymer gel changed from a state of flowing gel to one of deformable nonflowing gel because cross-linking sites increase for the formation of gel in lesser time intervals.12 3.4. Effect of the Salinity. The gelation reaction between PHPA and the HMTA−HQ gel system strongly depends on the solution salinity. The effect of the salinity of the polymer gelant on the gelation rate at different gelation temperatures is given in Table 4 and Figure 5. Gelation occurs rapidly in the Table 4. Effect of the Salinity on the Gelation Time in 0.9 wt % PHPA, 0.5 wt % HMTA, and 0.3 wt % HQ at pH 8.0 gelation time (h) no.
salinity (wt %)
80 °C
90 °C
100 °C
110 °C
120 °C
1 2 3 4
1.0 2.0 3.0 4.0
231 235 241 248
93 98 106 112
55 61 69 75
26 32 40 47
15 18.5 22 25
Table 5. Change in the Gelation Time with the pH in 0.8 wt % PHPA, 0.3 wt % HMTA, and 0.3 wt % HQ at pH 8.0 gelation time (h) no.
pH
80 °C
90 °C
100 °C
110 °C
120 °C
1 2 3 4
8.0 8.5 9.0 9.5
450 448.5 451 452.5
309 308 310 312
169 168 170.5 172.5
85 83.5 86.5 88
43.5 42.5 44 45.5
between 8.0 and 9.5 have no significant effect on the gelation time. From the experiments carried out in the laboratory, it was found that HMTA/HQ cross-linked PHPA is stable up to pH 9.5. Above pH 9.5, proper gelation did not take place and syneresis also occurred because of excess cross-linking. The results indicate that a suitable solution pH range for PHPA, HMTA, and HQ cross-linking is between 8.0 and 9.5.
Figure 5. Effect of the salinity of brine on the gelation time in 0.8 wt % PHPA, 0.5 wt % HMTA, and 0.3 wt % HQ at pH 8.0.
4. CONCLUSION In this work, the gelation performance of the PHPA−HMTA− HQ gel system was comprehensively and deeply investigated. The main new understandings are as follows:
presence of salt compared to distilled water. The polymer gel samples prepared with constant polymer and cross-linker concentrations and varying brine salinity from 1.0 to 4.0 wt % 9535
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1. The gelation time can be controlled from a few hours to several days depending upon the gel composition and environmental conditions. 2. As the concentration of the polymer in the gelant solution increased, the gelation time decreased. 3. The increase in the cross-linker concentration caused a decrease in the gelation time, which can be used as a retardation agent. 4. Decreased gelling times are observed for high gelation temperatures of up to 120 °C. 5. The gelation time increased with an increase in the salinity, but excessive increases in the high concentrations of the polymer and cross-linkers result in syneresis. 6. The pH range over which the gel system under study gives the best results is 8.0−9.5.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +91-326-5498. Fax: +91-326-2296563. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors gratefully acknowledge financial assistance provided by the Council of Scientific and Industrial Research (CSIR), New Delhi, India, to the Department of Petroleum Engineering, Indian School of Mines, Dhanbad, India. Thanks are also extended to all individuals associated with the project.
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