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Effect of Different Phenolic Compounds on Performance of Organically Cross-Linked Terpolymer Gel Systems at Extremely High Temperatures Daoyi Zhu, Jirui Hou, Xianxing Meng, Zigang Zheng, Qi Wei, Yuguang Chen, and Baojun Bai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01386 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017
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Effect of Different Phenolic Compounds on Performance of Organically Cross-Linked Terpolymer Gel Systems at Extremely High Temperatures Daoyi Zhu*1,3, Jirui Hou*1, Xianxing Meng4, Zigang Zheng5, Qi Wei1, Yuguang Chen1, Baojun Bai*2,3
1. Enhanced Oil Recovery Research Institute, China University of Petroleum, Beijing 102249, China 2. Karamay Campus, China University of Petroleum-Beijing, Karamay, Xinjiang 834000, China 3. Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, MO 65401, United States 4. School of Chemistry and Chemical Engineering, Ankang University, Ankang, Shaanxi 725000, China 5. Research Institute of Exploration and Development, Changqing Oilfield Company, CNPC, Xi’an, Shaanxi 710018, China
*Email:
[email protected] (B. Bai);
[email protected] (J. Hou);
[email protected] (D. Zhu)
Abstract The effect of four different phenolic compounds (i.e., phenol, catechol, resorcinol, and hydroquinone) on the performance of organically cross-linked terpolymer gel systems at the temperature of 150 °C was investigated. The phenol-based gelant systems were not able to form visible bulk gels at this extremely high temperature because the cross-linked clusters between phenol and hexamethylenetetramine (HMTA) only contained a small number of hydroxyl groups for cross-linking reactions. The catechol- and hydroquinone- based gelants were able to form relatively strong bulk gels because the amount of the cross-linked clusters between these two phenolic compounds and HMTA increased significantly. This increment also contributed to the decrease of the grid sizes of the gel network structures and the emergence of dendritic structures on them, thereby significantly increasing the viscosity, storage modulus and thermal stability of the obtained gels. However, these two gel systems could not be maintained for long; syneresis began after only 3 to 12 days of the systems being held at 150 °C. When phenol was replaced by resorcinol, bulk gels with excellent strength and long-term thermal stability were able to form at 150 °C. The use of the
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gelation mechanism of the cross-linking reactions between the terpolymer and different cross-linker systems can help researchers and petroleum engineers better understand the differences between the different cross-linker systems and thus develop more suitable polymer gel systems for water management in extremely high-temperature reservoirs.
1. Introduction Polymer gel systems that involve the use of acrylamide polymers and cross-linker systems have become the most widely applied method to improve oil recovery (IOR) that addresses the excessive water production problems in oil and gas reservoirs1-4. They are fluid-based systems that can form a continuous, three-dimensional (3D), solid-like network structure in the reservoir. Polymer gels operate, for the most part, either by diverting fluid flow from high-permeability, low-oil saturation flow paths to low-permeability, high-oil saturation flow paths, thus promoting improved sweep efficiency and incremental oil production, or by reducing oil-production operating costs by controlling excessive production of water or gas
5-9
. However, as the resources and recoverable
reserves of conventional reservoirs decrease, more and more hydrocarbon resources are gradually produced from high-temperature and high-salinity reservoirs10-12. Moreover, most heavy oil reservoirs worldwide use thermal methods to improve oil recovery; however, viscous fingering and steam gravity override caused by permeability anisotropy and mobility differences between displaced and displacing fluids can lead to early steam channeling and decreased oil production13, 14. Therefore, it is of great importance to investigate the performance and mechanisms of polymer gel systems for extremely high-temperature reservoirs and steam-flooded reservoirs. Polymer gel systems are composed mainly of acrylamide (AM)-based polymers and inorganic or organic cross-linker systems1, 15, 16. At elevated temperatures, partially hydrolyzed polyacrylamide (HPAM) degrades more quickly because the molecular chain of the polymer will be broken through redox and hydrolysis reactions under the conditions of heat, oxygen and other residual impurities17. In order to develop suitable polymer gel systems and enhance their thermal stability at high temperatures, many AM-based copolymers and terpolymers have been developed in recent years18, 19. For gel applications with inorganic cross-linkers, commercial copolymers synthesized by AM and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) were used to crosslink with chromium malonate20. The experimental gelation time was reported to be 0.25 to 42 days at 120 °C; however, no long-term stability data were provided. Albonico et al.21 also reported a gel system composed of a
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copolymer (AM/AMPS) with a molecular weight of 2 to 3 million Daltons and either chromium malonate or glycolate, the gelation times of which were 0.25 to 42 d and 0.01 to 16 d, respectively, at 120 °C. In addition, the gel systems were able to be kept thermally stable for aging for 2 to 7 months and 1.5 to 3 months, respectively. However, gel systems generated by inorganic cross-linkers typically have lower thermal stability because inorganic ions cross-link with the – COO- group of HPAM through ionic bonds. The bond is unstable at high temperatures. Moreover, a large number of uncross-linked amide groups also accelerate the degradation reactions of polymer molecules at high temperatures. Organic cross-linking involves the formation of covalent bonds between functional groups of the polymer, connecting two or more polymer chains12, 15, 22. Therefore, organically cross-linked gel systems have been widely used for water management at high-temperature reservoirs, even those cross-linked with common HPAM. Falk23 reported an organic cross-linking system that used phenol and formaldehyde as the cross-linker; the gelant started to form solid gels at only 50°C. However, this gel system has not been widely used because of the toxicity of phenol, and even more so, the carcinogenic character of formaldehyde. Numerous studies have been undertaken to replace the phenol and/or formaldehyde with lower-toxicity chemicals that can generate phenol and formaldehyde in situ, thereby forming stable gel systems at high temperatures. Moradi-Araghi24 found that HMTA can be used in place of formaldehyde to produce stable gels. HMTA can thermally break down to ammonia and formaldehyde, thus elongating the gelation time, as shown in Fig. 1. Other studies have reported that phenol can be replaced by catechol, resorcinol and hydroquinone, etc.25-30, as shown in Fig. 2. Hutchins et al.26 investigated an organic cross-linking polymer system suitable for high-temperature gel treatments. This system was formulated by PAM, hydroquinone, HMTA and 2% NaHCO3. The gels were able to be kept stable for 12 months at 149°C and 5 months at 176.7°C. This gel system was applied in many wells, but one major limitation was that the gelation time was only one to several hours at 176.7°C. Sengupta et al.31 studied the gelation time of another gel system at different temperatures ranging from 85 to 120°C. The gelant was formulated with 1% PAM (5 million Daltons), 0.4% hydroquinone and 0.5% HMTA. The increase in gelation temperature resulted in the decrease in gelation time, from 72-80 h to only 5 h, due to the increasing cross-linking rate at elevated temperatures. Chen at al.30 developed a thermal-resistant, salt-tolerant gel composed of copolymer (AM/AMPS), phenol, HMTA and resorcinol. The gel syneresis was only 12.5% after aging for 100 days under difficult environmental conditions (temperature=130 °C,
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salinity=223,000 mg/L). Core-flooding results have illustrated that the plugging efficiency of the gel, which has been heat-treated for 100 days under the previously noted harsh conditions, is more than 90%. Liu et al.32 reported a gel system composed of PAM and hydroquinone/HMTA. The gelation time could be controlled within the range of 6.3 to 15.5 h, and the gel strength could be adjusted from 0.033 to 0.071 MPa at 110 °C. Although different phenols have been studied for the development of high-temperature polymer gel systems, the performances (e.g., gel strength) of the gel systems composed by them are not exactly the same. As far as we know, the differences between different phenolic cross-linking agents has not been systematically studied yet, especially in extremely high temperatures.
Fig. 1 Thermal break-down process of HMTA to form formaldehyde.
Fig. 2 Four types of phenolic compounds used as cross-linkers. The original research presented in this paper began with systematic studies on high-temperature terpolymer gel systems formed by HMTA and four different kinds of phenolic compounds. This terpolymer was systematically evaluated in our previous study33. The effects of different phenolic compounds (phenol, catechol, resorcinol and hydroquinone) on the gelation time, gel strength and long-term thermal stability of the organically cross-linked terpolymer gel systems was studied in ampules at 150 °C. Then, the viscosity (η) and viscoelasticity (i.e., storage modulus G′ and loss modulus G″) of different gel systems were measured and then compared with each other. This allows further quantitative analysis on the basis of the qualitative results obtained in the previous
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ampule experiments. Differential scanning calorimetry (DSC) tests were carried out to compare the thermal stability of different bulk gels. Environmental scanning electron microscopy (ESEM) was also used in this study for further observation of the microscopic morphological differences between different gel systems. Finally, the gelation mechanisms of the polymer gel systems formed by the different phenolic compounds were compared; the previous performance differences can be explained by these differences in these mechanisms. These studies culminated in a better understanding of the development of the gel systems used for water management in extremely high-temperature reservoirs.
2. Experimental Section 2.1 Materials The terpolymer (ZP-4) used as the primary component of the gel systems was from the Beijing Yuanyang Huanyu Petroleum Technology Co., Ltd., China. It was synthesized from three monomers, AM, AMPS, and N-vinylpyrrolidone (NVP), as shown in Fig. 3. Its average molecular weight was 3 to 5 million Daltons, and its degree of hydrolysis was 15 to 20%. The HMTA and phenolic compounds (i.e., phenol, catechol, resorcinol and hydroquinone) from the Shanghai Macklin Biochemical Technology Co., Ltd., China were used as the organic cross-linker system. The antioxidant and heat stabilizer33 were from the Beijing Yuanyang Huanyu Petroleum Technology Co., Ltd., China. Deionized water was used in all experiments.
Fig. 3 Molecular structure of the terpolymer.
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2.2 Experimental method The research roadmap of the experiments conducted in this paper is depicted in Fig. 4. In our study, gelant composed of different concentrations of polymer and/or different types of cross-linkers were aged at 150 °C to form bulk gels and the gelation time was determined by the Sydansk Gel Code method. Then, the viscosity, rheological properties, microstructures and thermal stability of these bulk gels formed in ampules at 150 °C were tested and compared to study their different gelation mechanisms. The detailed experimental procedures are described as follows.
Fig. 4 Roadmap of the experimental study. 2.2.1 Gel Preparation and Determination of the Gelation Time Firstly, 2% terpolymer stock solutions were prepared in deionized water. Gelant was prepared by mixing 0.4 to 1.0% terpolymer, 0.3% HMTA, 0.3% phenolic compound, 0.05% antioxidant and 0.05% heat stabilizer in sequence while stirring vigorously with a magnetic stirrer. Then, approximately 30 mL of gelant solutions were slowly transferred to thick-walled ampules through a plastic conduit. The neck of the ampule was sealed after the ampule was vacuumed for 2 to 3 h. Finally, gelation reactions were initiated when the gelant solutions were aging in an oven at the desired temperature of 150 °C. The Sydansk Gel Code method34 was employed to investigate the gelation time, which was
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determined by visual inspections. By this method, the qualitative gel strength of different flowing and suspension states were assigned a letter code of A to J. The gel strength codes range from highly flowing gels (Code B) to rigid rubbery gels (Code J). During bottle testing, gelant were simply formulated and placed in ampules at the desired temperature. The ampules containing the gel systems were inverted every 30 min until the gel strength no longer changed, and this moment was marked as the gelation time33. The long-term thermal stability of the gel system could also be roughly observed. The syneresis (S) of the bulk gel can be defined by the following equation: ሺܹ − ܹ ᇱ ሻൗ ܵ= ܹ × 100% where ܹ ᇱ is the weight of the aged gel, and ܹ is the weight of the gelant solution. 2.2.2 Viscosity and Rheological Properties of Bulk Gel A rheometer (HAAKE RS600, Thermo Electron Co., Germany) was applied to quantify the strength of the bulk gel systems. The viscosity (η) and viscoelasticity (storage modulus G′ and loss modulus G″) were measured at the temperature of 60 °C33. The temperature of 60 °C was chosen as the test temperature because the gel system was not closed and the moisture in the gel would be volatile in high-temperature conditions, the viscosity and rheological properties cannot be accurately measured. All rheological properties were obtained in a controlled rate mode (CR). The shear rate was 7.1 s-1, the frequency was 0.1 Hz, and the shear stress was 1 Pa. 2.2.3 Microstructure of Bulk Gel An environmental scanning electron microscope (ESEM, Quanta 200F, FEI Co., USA) was used to observe the microstructures of the bulk gel systems composed of different phenolic compounds. In the sample preparation process, a small sample of pre-frozen, dried gel was placed on a copper plate surface and then sprayed with gold to increase conductivity. 2.2.4 DSC Test A differential scanning calorimeter (DSC, SDT Q600, TA Co., USA) was used to study the thermal stability of the bulk polymer gel (pre-frozen, dried sample). The temperature ranged from 30 °C to 400 °C and scanned at 10 °C /min.
3. Results and Discussion 3.1 Effect of Phenol Type on Gelation Time and Thermal Stability of Bulk Gel This research was based on our previous study33, which used HMTA and resorcinol as the
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organic cross-linker system, with cross-linker concentration ranging from 0.2% to 0.8%. It was reported that the storage modulus (also called the elastic modulus) G′ reached its maximum when the total concentration of organic cross-linking system was 0.6%. Therefore, in this study, the concentrations of the HMTA and phenolic compounds were both set as 0.3%, and the experimental temperature was 150 °C. There were four kinds of phenolic compounds, specifically, phenol, catechol, resorcinol and hydroquinone, investigated separately, with HMTA as the organic cross-linker system. The gelation time and long-term thermal stability of the gel systems prepared with different terpolymer concentrations were studied, and the results are shown in Table 1. Table 1 Gelation time and thermal stability of gel systems composed by different cross-linkers. Cross-linker
0.3% phenol + 0.3%HMTA
0.3% catechol + 0.3%HMTA
0.3% resorcinol + 0.3%HMTA
0.3% hydroquinone + 0.3%HMTA a
Polymer (%) 0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0
Gelation time(h)
Gel stability
-a
S=100% (5d)
14 14 10 4 14 12 10 4 16 16 12 6
Syneresis after 5 days of aging, S≈40% (8d) Syneresis after 10 days of aging, S≈40% (15d) Syneresis after 10 days of aging, S≈40% (15d) Syneresis after 12 days of aging, S≈40% (15d) Syneresis after 10 days of aging, S≈15% (30d) S
