Terpolymer Gel System Formed by Resorcinol

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Terpolymer gel system formed by resorcinol-hexamethylenetetramine (HMTA) for water management in extremely high temperature reservoirs Daoyi Zhu, Jirui Hou, Qi Wei, Xuan Wu, and Baojun Bai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03188 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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Terpolymer gel system formed by resorcinol-hexamethylenetetramine (HMTA) for water management in extremely high temperature reservoirs Daoyi Zhu† ‡, Jirui Hou†, Qi Wei†, Xuan Wu†, Baojun Bai*‡

† Enhanced Oil Recovery Institute, China University of Petroleum, Beijing 102249, China

‡ Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, MO 65401, United States

*Tel: +1 (573) 341 4016. E-mail: [email protected].

ABSTRACT：：

An in-situ terpolymer gel system formed by resorcinol-hexamethylenetetramine (HMTA) was developed and systematically evaluated for water management in extremely high temperature reservoirs. Suitable gelation time and favorable gelation performance were obtained by adjusting terpolymer and/or cross-linker system concentrations. With the increase of terpolymer, the resorcinol-HMTA concentration increased both the gelation time and gelation performance. The gel system was prepared by deionized water and maintained good thermostability in a high-salinity environment. Very low concentrations of NaCl, KCl and CaCl2

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can delay gelation time. After the critical concentrations are reached, these inorganic ions will boost the cross-linking reaction; however, the presence of MgCl2 shortens gelation time. The gel system in ampoules was kept stable at 150 °C for five months, and DSC (differential scanning calorimetry) measurement indicated the gel system could be used for water management at temperatures up to 240 °C. Uniformly distributed 3D network microstructures and dendritic structures among the gel grid pores could further increase the network structure strength and firmly lock water within the gel even under extremely high temperatures. The use of the gelation mechanism of an in-situ terpolymer gel system formed by resorcinol-HMTA can help petroleum engineers control gelation time and performance.

1. INTRODUCTION Excessive water production caused by mobility-induced viscous fingering, high-permeability matrix-rock directional trends, and fracture channeling can negatively affect oil recovery rate in the process of any recovery methods, especially during water flooding and oil-recovery flooding operations1, 2. Therefore, water management has become an increasingly important objective in oilfields. There are three general ways to improve water shut-off performance: increasing the viscosity of the flooding fluid, reducing the permeability of high-permeability flow paths and increasing the permeability of low-permeability3-7. However, compared with other water management techniques, by increasing the viscosity of oil recovery drive fluid, water management techniques that involve reducing the permeability of high-permeability flow


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channels are normally using low or median volumes, which is amazing especially in this period of relatively low oil prices8.

Polymer gels are a fluid-based system in which the gelant solution can form a continuous 3D solid-like structure in the reservoir. As such, they have proven to be one of the most effective and popular permeability-reducing materials used to manage water problems. This is due to their ability to control gelation time, adjust strength, and offer long-time stability—all at a low cost 9-12

. However, as the resources and recoverable reserves of conventional reservoirs decrease,

more and more reservoirs with high temperature, high salinity or serious heterogeneity are being explored and developed. In addition, thermal recovery has been widely applied for the development of heavy oil reservoirs or oil sands. However, viscous fingering and steam gravity override caused by formation heterogeneity and mobility differences between formation fluids and steam can lead to early steam channeling. Moreover, lean zones (or thief zones) with high water saturation will cause great heat loss, etc13, 14. Therefore, it is of major importance to developing a gel system for extremely high temperature reservoirs (above 150 °C) and steam flooded reservoirs.

Generally, commonly used gel systems are mainly composed of polymer and inorganic or organic cross-linkers15. Polymers include acrylamide polymer and xanthan biopolymer, which have been widely used in many EOR projects requiring polymer methods16,

17

. However,

common polymers degrade or precipitate at high temperatures. Degradation is also a problem in


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high salinity reservoirs18-20. New polymers designed to obtain gel systems that can be used for extra high temperature reservoirs. Recent studies21, 22 have shown that gels formed by nonionic polyacrylamide and phenolic resin at 80 °C can be applied in reservoirs with a temperature below 143 °C. Gels formed by associating low-hydrolysis polymers, which can be applied in reservoirs with temperatures less than 95 °C. Ultra-low-hydrolysis polyacrylamides and low molecular weight polyacrylamides have been reported as capable of forming suitable gel systems that can be applied to high temperature reservoirs2.

Inorganic cross-linkers, including Cr3+, Al3+, Zr4+, can cross-link with polymers through an ionic bond between the carboxylate ions (− COO-) of the polymer and multivalent ion complexes23-25, while organic cross-linkers, including phenol, formaldehyde, etc., cross-link with polymers through dehydration condensation reactions between the amide groups (− CONH2) of the polymer and the hydroxyl groups (− CH2OH) of the cross-linker9, 26, 27. Thus, organically cross-linked gel systems usually have stronger thermal stability than inorganically cross-linked gel systems. However, if the reservoir temperature is above 60–70 °C, the inorganic cross-linker will quickly react with polymer, giving the gelant solution poor propagation ability and the gel system poor thermal stability28, 29. Organic cross-linking reactions are dehydration condensation reactions, which are made by forming covalent bonds between a polymer and a cross-linker system. Hence, in-situ organically cross-linking gel systems have been widely used for water management with reservoir temperatures above 60–70 °C. Polyethylene imine (PEI) is one of the


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most popular cross-linkers as it is environmentally friendly with low toxicity30, 31. However, it is very expensive and its temperature application limitation is below 130–140 °C. The phenol-formaldehyde cross-linker system is popular because it is cheap, but the phenol is toxic and carcinogenic; plus, the gelation time is too short at high temperatures32. To overcome this problem, scholars and engineers have been committed to finding new cross-linker systems with lower toxicity and controllable gelation time. For example, the cross-linker system composed of hydroquinone (HQ) and hexamethylenetetramine (HMTA) has demonstrated the HQ-HMTA gel system to have a longer gelation time, better thermal stability and more effective water management32-34. However, their application in extra high temperature reservoirs is limited because common polymers degrade heavily at temperatures above 90 °C.

The original research presented in this article began with systematic studies on an in-situ terpolymer gel system formed by resorcinol-HMTA. The first step was a screening of polymers composed of different monomer(s) and different degrees of hydrolysis. Then, research was carried out to determine the effects of gel systems with different terpolymer and cross-linker concentrations on gel performance (gelation time, viscosity of bulk gel, storage modulus G′ and loss modulus G″). The effects of different temperatures and salt concentrations were also tested. A long-term thermal stability test of bulk gel system in ampoules was also conducted and a differential scanning calorimetry (DSC) test was used to determine the thermal stability of bulk gel. An environmental scanning electron microscope (ESEM) was also used in this study for


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further observation of the microstructures in the gel system. The end result of all these studies culminated in a better understanding of the gel system’s gelation mechanism when the gels made up of terpolymer and resorcinol-HMTA. The final proposal uses this new information to recommend more effective water management in enhanced oil recovery (EOR).

2. EXPERIMENTAL SECTION 2.1 Materials Four anionic polymers (ZP-1, ZP-2, ZP-3 and ZP-4) from Beijing Yuangyang Huanyu Petroleum Technology Co., Ltd., China were used, and they were synthesized from different monomer(s) (acrylamide, AM, 2-acrylamido-2-methyl propane sulfonic acid, AMPS, N-Vinypyrrolidone, NVP). They were also subjected to different degrees of hydrolysis. Their average molecular weight (Mw), hydrolysis degree (HD), monomer(s) and molecular formula are shown in Table 1 and Figure 1. The resorcinol and hexamethylenetetramine (HMTA) is from Shanghai Meclin Biochemical Technology Co., Ltd., China. The antioxidant and heat stabilizer (thiourea and cobalt salt) is from the Beijing Yuangyang Huanyu Petroleum Technology Co., Ltd., China. Sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2) are all analytically pure (AP) and are from Sinopharm Co., Ltd., China. Deionized water was used in all experiments.


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Table 1. Properties and compositions of anionic polyacrylamide Mw (million Dalton)

HD (%)

Monomers

ZP-1

400-600

20-25

AM

ZP-2

500-700

20-25

AM

ZP-3

300-500

10-15

AM/AMPS

ZP-4

300-500

15-20

AM/AMPS/NVP

Polymer

Figure 1. Molecular formula of different anionic polymer. (a)ZP-1 or ZP-2, (b)ZP-3, (c)ZP-4.


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2.2 Gelant Solution Preparation Polymer gelant solutions (before being gelled) contained different concentrations of polymers and resorcinol-HMTA, 0.05 % antioxidant and 0.05% heat stabilizer. Deionized water was used as a solvent in all experiments. The gelant solutions were prepared as follows: 10 g polymer solid particles were added into 1,000 mL deionized water and stirred at the rate of 400±20 rpm for 4 h, and then aged for 24 h. The polymer solution (1.0 %) was then diluted to the required concentration at room temperature. Then, pre-weighted resorcinol, HMTA, antioxidant, and heat stabilizer were added to the diluted polymer solution and stirred for 20 min to produce a homogeneous gelant solution. Finally, about 30 mL gelant solution was slowly transferred into a thick-walled ampoule (Beijing Glass Group Co., China) through a plastic conduit. The ampoule neck was sealed by an alcohol blaster burner after the ampoule was vacuumed for 2-3 h. Then, a cross-linking reaction was initiated when the gelant solution was heated in the oven at the desired temperatures ranging from 80 to 160 °C.

2.3 Determination of Gelation Time A gel strength code method was adopted to determine gelation time, which is by visual inspection35-37. Gel strength of different flowing, suspension and tongue states were divided into nine categories, from A to I35. In this method, ampoules with polymer gel were inverted every 30 min until the gel strength did not change any more, and this time was marked as the gelation time.


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2.4 Viscosity and Rheological Properties of Bulk Gel To quantify the strength of the bulk polymer gels, the viscosity (η) and viscoelasticity (storage modulus G′ and loss modulus G″) were measured by the plate-plate sensing system (PP20) of the HAAKE RS600 rheometer (Thermo Electron Co., Germany) at a temperature of 60 °C. Because the system is not closed, and the moisture in the gel will be volatile in high temperature conditions, so the viscoelasticity cannot be accurately measured. 60 °C is chosen as the experiment temperature as the purpose of this study is to study the law of polymer concentration on gel strength. All rheological properties were obtained in a controlled rate mode (CR). The shear rate was 1 s-1, the frequency ranged from 0.01 Hz to 10 Hz, and shear stress was determined by stress scanning and frequency scanning.

2.5 Differential Scanning Calorimetry (DSC) Test A differential scanning calorimetry (SDT Q600, TA Co., USA) was used to study the thermal stability of the bulk polymer gel (pre-frozen dried sample), together with the syneresis of bulk polymer gel (aqueous condition). The temperature ranged from 30 °C to 400 °C and scanned at 10 °C /min. The atmosphere was Ar. Syneresis is defined by the following equation ᇱ

ܵ = ܹ ൗܹ × 100% ܹ ᇱ is the weight of aged gel without free water and ܹ is the weight of gelant solution.


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2.6 Microstructure of Bulk Gel The bulk gel microstructure was observed by an environmental scanning electron microscope (ESEM, Quanta 200F, FEI Co., USA). In the experiment, a small amount of pre-frozen dried gel sample was placed on a copper plate surface attached with conductive adhesive, and then the gel surface was sprayed with gold to make it more conductive.

3. RESULTS AND DISCUSSION 3.1 Polymer Screening In order to improve the temperature resistance of the polymer, anionic polymers with different side chain structures have been designed and studied in recent years

38-40

. In our study,

temperature and shear resistance groups, such as –SO3-, C-N, COO-, etc., were selected as the side chain structures. In other words, AM, AMPS, and/or NVP were selected as monomers and thus polymers ZP-1 and ZP-2, copolymer ZP-3 and terpolymer ZP-4 were chosen to prepare a high-quality polymer gel system (see Table 1).

Polymer screening experiments were initially carried out to study whether those four polymers can form bulk gel in ampoules at the temperature of 150 °C. During the experiments, 1.0 % aged polymer solution of different polymers were diluted into 0.6 % and 0.8 %, respectively. Then, 0.6 % resorcinol and HMTA were added with the same concentration, 0.05 % antioxidant and 0.05% heat stabilizer to prepare eight gelant solutions. After being vacuumed in


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ampoules for 2 h, they were heated at 150 °C, and checked every 30 min. As the temperature was extremely high, the viscosities of gelant solution prepared by polymer ZP-1 or ZP-2 and copolymer ZP-3 were slightly increased, but they could not form a bulk gel; hence, the polymer began to degrade sharply after 12 h. Only gelant solutions prepared with terpolymer ZP-4 could form solid-like bulk gel under this harsh environment.

As we can see from Table 1, polymer ZP-1 and ZP-2 were synthesized with AM, and their HD was almost the same; their Mw was 400–600 and 500–700 million Daltons, respectively, but neither of them could form the needed bulk gel, because at high temperatures (usually higher than 90 °C), the end of the polymer chains fall off causing the monomers to disconnect one by one according to the chain reaction mechanism41. Copolymer ZP-3 synthesized with AM and AMPS also could not form a bulk gel at 150 °C; however, terpolymer synthesized with AM, AMPS and NVP could. The relationship between the viscosity of 0.8 % polymer (ZP-3 and ZP-4) solutions and aging is shown in Figure 2. The viscosity of ZP-3 solution decreased sharply and it remained at 71% of its original viscosity after being heated for 4 h, while the viscosity of the ZP-4 solution did not change significantly. Viscosities of both polymer solutions dropped dramatically from 4 h to 8 h being heated at a temperature of 150 °C, causing them to remain at 6.24% and 26.97% of their original viscosities, separately. It’s amazing that after 8 hours of heating, the viscosity of ZP-4 solution was higher than that of ZP-3. This means the terpolymer synthesized with an additional NVP group and thus gained a better thermal stability; in turn, this


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might also explain why the gelant solution composed of ZP-4 terpolymer was able to form bulk gel at a high temperature. Monomer AMPS had a large pendant group and the monomer NVP had a rigid fine-membered ring structure, which could enhance the thermal stability of ZP-4 solution. In addition, the amide group in the monomers AMPS and NVP were secondary amide groups and tertiary amide groups (Figure 1), which are less susceptible to hydrolysis than primary amide groups in monomer AM42.

200

T =150 160

Viscosity (mPas)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120

80

40

ZP-3 solution(0.8 wt%) ZP-4 solution(0.8 wt%)

0 0

2

4

6

8

10

12

Time (h)

Figure 2. Viscosity of polymer solution vs. time curve.

3.2 Effect of Terpolymer Concentration From the above results, the terpolymer ZP-4 was selected as the most suitable polymer to form a bulk gel at a temperature of 150 °C in this study. Figure 3 is the contour map of the


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gelation time of the gel systems composed of different terpolymer concentrations (ranging from 0.4 % to 1.0 %) and the cross-linker system concentration (ranging from 0.5 % to 0.8 %), which was processed by using Surfer 8.0 (Golden Software Co.). The gelant solutions were heated in a 150 °C oven. 0.8

15

11

8

3

14

11

9

3

0.6 14

12

10

4

0.75 Cross-linker concentration(wt%)

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0.7

0.65

0.55

0.5 0.4

13

15

0.5

0.6

10

0.7

0.8

4

0.9

1

Polymer concentration(wt%)

Figure 3. Contour map of gelation time of the gel system composed of different concentrations of terpolymer and the cross-linker system. Solid squares represent specific experiments, numbers next to the square represent the gelation time (h). Experimental temperature was 150 °C.

During experiments to determine the effect of terpolymer concentration, the concentration of the cross-linker system (resorcinol-HMTA) was fixed at 0.6 %. The antioxidant and heat stabilizer were both 0.05 %. As we can see from Figure 3, if the terpolymer concentration was below 0.4 %, the gelation time would exceed 14 h. In this condition, the polymer chain was not


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dense enough to form a continuous three-dimensional (3D) network structure and/or the gel strength was too weak to keep the gel stable for more than 10 days. Moreover, it had a very high syneresis. The effect of terpolymer concentration on gelation performance (e.g., gelation time, viscosity of bulk gel, storage modulus G′ and loss modulus G″) is shown in Figure 4 and microstructures of bulk gel systems with different terpolymer concentrations at the gelation time are shown in Figure 5. As shown in Figure 4, when the terpolymer concentration was 0.4 %, the gelation time was 14 h, but its viscosity and G′ and G″ were too weak to form a stable bulk gel at 150 °C. Figure 5(a) shows the grid sizes of the network structure of this gel system to be nearly 20–30 µm, so the network structure of the bulk gel was sure to break during overly long thermal vibrations under high temperature; plus, water in the large pore structures could easily escape from the grids.

30 10000

16

25 14

10

1000

8

Gelation time Viscosity G' G''

6

15

10

5

4 0.3

0.4

0.5

0.6

0.7

0.8

0.9



100 1.0

0

G'/G'' (MPa)

20

12

Viscosity(mPa.s)

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 4. Effect of terpolymer concentration on gelation performance (gelation time, viscosity of bulk gel, storage modulus G′ and loss modulus G″). Concentration of cross-linker system was 0.6 %, and experimental temperature was 150 °C. (a)

(b)

(c)

(d)

Figure 5. Microstructures of gel systems composed of 0.6 % cross-linker system, 0.05% antioxidant, 0.05% heat stabilizer, and different terpolymer concentrations: a)0.4% tepolymer, b)0.6% terpolymer, c) 0.8% terpolymer and d)1.0% terpolymer. All structures are magnified up to 5,000 times.

If the terpolymer concentration was higher than 1.0 %, the gelation time was shorter than 4 h; hence, the gelation time was too short to make the gelant solution flow into the targeted formations. Because high temperature reservoirs are always located at deeper depths, the gelant solution had a relatively long time to flow into the target areas. Moreover, if the terpolymer


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concentration was 1.0 %, more polymer strands would stretch in the aqueous solution. They can quickly form gel as soon as they react with cross-linker systems under high temperature, so grid sizes of the bulk gel were not reduced, but increased instead, as shown in Figure 5(d), which in turn decreased the gel water holding capacity. Therefore, applicable terpolymer concentration ranged from 0.4 % to 0.9 %.

From Figures 3, 4 and 5, when the terpolymer concentration ranged from 0.4 % to 0.9 %, the gelation time varied from 7 h to 14 h, which is suitable for oilfield application. The higher the terpolymer concentration is, the shorter the gelation time, and the stronger the bulk gel. When the terpolymer concentration increases, more cross-linking sites are created in the polymer side chains, so that the cross-linker systems can react more with the amide groups (− CONH2) of terpolymer. Figures 5(b) and (c) show that the microstructures of the bulk gel samples were composed of 0.6 % and 0.8 %, respectively. With the increase of terpolymer concentration, more and more dendritic structures gradually took shape and were distributed between the pores of grids, further increasing the network structure strength of the polymer gel and firmly locking the water in the gel even under high temperatures.

3.3 Effect of Cross-linker Concentration Cross-linker system concentration can also affect the gelation performance of the bulk gel. Figure 6 shows the relationship between gel performance and cross-linker system concentrations. Experimental results showed that if the cross-linker system concentration was lower than 0.5 %,


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the gelation time would be more than 20 h, the amide groups (− CONH2) of the terpolymer could form a 3D network structure by cross-linking with the hydroxyl groups (− CH2OH) of the cross-linker. If the cross-linker system concentration was very low, the reaction rate of cross-linking will decreased correspondingly, because under the high temperatures, − CONH2 of the terpolymer gradually becomes – COOH and the long chain backbones of the polymer will begin to separate, hence, it is very difficult to form bulk gel. However, if the cross-linker system concentration is above 0.8 %, the gelation will be shorter than 3 h, which is not applicable for oilfield operation. As we can see from Figure 6, when the cross-linker system concentrations ranged from 0.5 % to 0.6 %, the gelation times were both 10 h, the viscosities of bulk gel changed from 4963 mPa·s to 6427 mPa·s, G′ varied from 18.31 MPa to 23.77 MPa and G″ varied from 1.55 Mpa to 2.46 MPa. The higher the cross-linker system concentration, the shorter the gelation time and the stronger the gel strength. In addition, Figure 6 also provides a reagent dosage guidance for petroleum engineers that allows them to adjust the cross-linker system concentration to meet different water management requirements.


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30

100 10000 90

25

60 50

1000


40 30 20

Viscosity (mPa.s)

70

20

15

10

G'/G'' (MPa)

80

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

10 0 0.2

0.3

0.4

0.5

0.6

0.7

100 0.8

0

Cross-linker concentration(wt%)

Figure 6. Effect of cross-linker system concentration on gelation performance (gelation time, viscosity of bulk gel, storage modulus G′ and loss modulus G″). Concentration of terpolymer in gelant concentration was 0.6 %, and experimental temperature was 150 °C.

3.4 Effect of Temperature Temperature was also one of the most important factors to determine the gelation process and performance of the polymer gel43. To investigate the effect of different temperatures on gelation performance, a gelant solution composed of 0.6 % terpolymer ZP-4 and 0.6 % resorcinol-HMTA were selected as the evaluation sample. Because this gel system is designed for high-temperature reservoirs, the experimental temperature ranged from 80 °C to 160 °C. As shown in Figure 7, with the increase of temperature, the opportunity for effective collision of terpolymer and resorcinol-HMTA molecules increased, so the cross-linking rate became faster and the gelation


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time became shorter.

40

Gelation time 30

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

10

0 80

100

120

140

160

Temperature ( )

Figure 7. Effect of temperature on gelation process. Gelant solution composed of 0.6 % ZP-4 and 0.6 % resorcinol-HMTA were selected as the representative samples.

An Arrhenius-type equation44 shows the relationship between gelation time and temperature: ‫ݐ‬୥ = ‫ ݁ܣ‬ா౗ /ோ் where ‫ݐ‬୥ is the gelation time, h; ‫ ܣ‬is the frequency factor of the effective collision between activated molecules, h; ‫ܧ‬ୟ is the apparent activation energy between activated molecules and reactant molecules; kJ/mol, ܴ is the universal gas content; kJ/(mol K), and ܶ are the temperatures, K. After transformation, ln൫‫ݐ‬୥ ൯ and 1ൗܶ are in a straight-line relationship.


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ln൫‫ݐ‬୥ ൯ = ‫ܧ‬ୟ /ܴܶ + ln‫ܣ‬ which is shown in Figure 8, with a slope of 3294.4. From the above equation, the slope is ‫ܧ‬ୟ /ܴ, so the apparent activation energy, ‫ܧ‬ୟ , is 27.39 kJ/mol.

From Figure8, we can also see that when the temperature is higher than 140 °C, the dispersion degree of data points is greater. This may be due to the complicated chemical reactions under extremely high temperatures.

4.0 y = a + b*x No Weighting

Equation Weight

3.5

Residual Sum of Squares 0.18404 0.96055 Pearson's r 0.90331 Adj. R-Square Value Intercept -5.68407 Slope 3294.41763

B

Standard Err 1.20723 476.93067

3.0

ln(tg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 44

2.5

ln(tg)=Ea/RT+lnA Ea=Rdln(tg)/d(1/T) 2.0

1.5 0.0023

Ea=27.39 kJ/mol

0.0024

0.0025

0.0026

0.0027

0.0028

0.0029

1/T

Figure 8. Arrhenius plot for the reaction under different temperatures.

3.5 Effect of Salt Concentration Inorganic salt concentration is another important factor that affects the gelation process and performance of polymer gels. If the salt concentration is not suitable, the gelation time may be


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Energy & Fuels

too fast. This means the gelant solution cannot form nor can the gel system keep stable for a long time. In this experiment, gelant solutions were prepared with 0.6 % ZP-4, 0.6 % resorcinol-HMTA, 0.05 % antioxidant, and 0.05 % heat stabilizer. Then four different salts (NaCl, KCl, CaCl2 and MgCl2) of different concentrations were added, which had been heated in a 150 °C oven. Figure 9 shows the effect of different salts and concentrations on gelation performance. The concentration of NaCl ranges from 0.00005 mol/L to 4 mol/L, and concentrations of other salts range from 0.00005 mol/L to 0.01 mol/L.

As we can see from Figure 9, salts, even with relatively low concentrations, will affect the gelation time and thermal stability of bulk gels. Based on the thermal stability experiments in ampoules, the gel systems still had a strong performance when the concentration of NaCl, KCl, CaCl2 and MgCl2 reached 0.5 mol/L, 0.1 mol/L, 0.001 mol/L and 0.0005 mol/L, respectively. The negative effect of these four salts on the gelation time and gel performance can be ranked as follows: MgCl2> KCl > NaCl > CaCl2 when the concentrations were less than 0.001 mol/L. When the salt concentrations were more than 0.001 mol/L, they were ranked as follows: CaCl2 ≈ MgCl2> NaCl > KCl. A specific analysis of various salts follows.

NaCl, is a very common salt used to investigate the influence of salinity, so its concentration was chosen from 0.00005 mol/L to 4 mol/L. In other words, salinity ranged from 2.9 mg/L to 234,000 mg/L. As can be seen from Figure 9, if the concentration was less than 0.005 mol/L (292.2 mg/L), the gelation time and gel performance would slightly increase. With the increase


Energy & Fuels

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of salt concentration, the hydrodynamic radius of the polymer decreases due to the salt sensitivity of polymer molecules. But if the concentration is more than 0.005 mol/L, the gelation time will decrease. With the increase of ion concentration, the ion compression of the electrical double layer of terpolymer becomes stronger; thus, repulsion forces between terpolymers will decrease; thus, when the salinity of the gelant solution increases, the cross-linking rate becomes more rapid. When the concentration of NaCl was 0.05 mol/L (2,922 mg/L), the gelation time was 6 h, and the gel had a very low syneresis after being heated in a 150 °C oven for five months. When the concentration of NaCl was 1 mol/L (58,440 mg/L) and 2 mol/L (116,880 mg/L), the gel began syneresis after 25 days’ heating and 17 days’ heating, respectively, and their syneresis reached a peak value after 30 d (S ≈ 5%) and 23 d (S ≈ 20%), respectively. However, when the concentration reached 4 mol/L (233,760 mg/L), the gelation time was 1 h, and the gel began syneresis after 6 days’ heating and syneresis reached a peak value after 30 d (S ≈ 60%). Hence, the gel system can keep thermal stability for a long time if the salt concentration is less than 2 mol/L, but salinity will shorten the gelation time. For this reason, it is better to keep the salinity of gelant solution lower than 100,000 mg/L when applying this gel system into high-temperature reservoirs. When the concentration is lower than 0.01 mol/L (584.4 mg/L), the effect of KCl is almost the same as NaCl; however, the degree of impact is smaller.

Ca2+ and Mg2+ are both divalent. With the increase of MgCl2 concentration, the gelation time of gelant solution decreased sharply. Because the charge/size ratio of a divalent cation is twice


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that of the monovalent ions, so a divalent cation can provide stronger forces to compress the electric double layers of terpolymer molecules, and make the reaction of cross-linking faster. However, compared with Mg2+, Ca2+ can ion-complex with – COOH of terpolymer, so when the concentration is relatively smaller (lower than 0.001 mol/L), Ca2+ can impede the reaction between − CONH2 of the terpolymer and − CH2OH of the cross-linker system, which make the gelation time longer. So, it is very important for EOR engineers to measure the salt concentration of injection water and formation water.

20

NaCl KCl CaCl2

15

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

MgCl2

No salts line

10

5

0 10-5

10-4

10-3

10-2

10-1

100

101

Concentration (mol/L)

Figure 9. Effect of different salts and concentrations on gelation performance. Horizontal dotted line represents the gelation time of gelant solution prepared by deionized water.

3.6 Thermal Stability of Bulk Gels From the results of long-term thermal stability in ampoules, we can also see that the gel


Energy & Fuels

system can keep stable at 150 °C for 5 months with the syneresis below 5%. To investigate the energy consumption property of the bulk gel, DSC (differential scanning calorimetry) measurements were carried out45. Figure 10 shows the DSC curve of the bulk gel composed of 0.6 % ZP-4 and 0.6 % resorcinol-HMTA. As we can see from Figure 10, the temperature turning point is about 240° C, which suggests that the gel system will gradually be destroyed before the temperature reaches 240 °C. Because molecular reactions accelerate when the temperature gradually increases, the chemical bonds in the gel system were gradually destroyed. When the temperature exceeds 240 °C, the gel system is unstable and is gradually destroyed, which indicates that the bulk gel can keep a stable structure at high temperature, even below the temperature of 240 °C.

-1.0 100 -1.5

-2.5 80 -3.0 70

Sample: 1.3200 mg Method: 30-400-Ar-10k/min Sample tray: Alumina 70ul

Step: -35.5343% -0.4691 mg

-3.5

60 50

100

150

200

250

Temperature (

300

Heat flow (w/g)

-2.0

90

w′′/w (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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350

-4.0 400

)

Figure 10. DSC curve of the gel system of 0.6 % ZP-4 and 0.6 % resorcinol-HMTA.


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Energy & Fuels

3.7 Gelation Mechanism Cross-linking reaction of the in-situ organic polymer gel system is related to the amide groups (-CONH2) of the polymer and the hydroxyl groups (− CH2OH) of the cross-linker system. Figure 11 uses the above analysis to explain the gelation mechanism of the in-situ terpolymer gel system formed by resorcinol-HMTA.

First, at a high temperature of more than 80 °C, HMTA gradually decomposes to formaldehyde and NH3 as shown in step 1 of Figure 11. It's worth noting that the heat stabilizer can react with NH3 to form an ion complex, which allows free NH3 gas to be absorbed.

Second, resorcinol reacts with the formaldehyde generated from the above step, forming 2,4-bis(hydroxymethyl)benzene-1,3-diol or 4,6-bis(hydroxymethyl)benzene-1,3-diol, which is shown in step 2 of Figure 11.

Third, resorcinol substituted by hydroxymethyl generated from the above step, is further carried out to the polycondensation reaction to form cross-linked clusters (many different structures), which are shown in step 3 of Figure 11.

Finally, the amide groups (− CONH2) of the polymer cross-link with the hydroxyl groups (− CH2OH) of the cross-linked clusters (only one of the structures is indicated), this cross-link reaction is a kind of dehydration condensation reaction, which forms the C-N between the polymer side chains and cross-linker system (step 4 of Figure 11) and, thus, forms a stable


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Page 26 of 44

network structure in high temperatures (Figure 5).

Step 1:

Step 2: OH

OH

OH CH2OH

+ H2C

or

O OH

OH

(resorcin)

HOH2C

CH2OH

Step 3:


OH

CH2OH

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Step 4:

Figure 11. Gelation mechanism of cross-linking reaction between terpolymer and resorcinol-HMTA.

CONCLUSIONS In this work, an in-situ terpolymer gel system formed of resorcinol-HMTA was systematically


Energy & Fuels

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studied. This system can be used for EOR water management in extremely high-temperature reservoirs (above 150 °C). The gelation time and gelation performance can be controlled by adjusting the terpolymer concentrations, ranging from 0.4 % to 1.0 % and the cross-linker concentration, ranging from 0.5 % to 0.8 %, respectively. The gelation time varies from 7 h to 14 h. With the increase of terpolymer and resorcinol-HMTA concentration and temperature, the gelation time and gelation performance will both increase. The gel system has a good gelation performance and long-term thermal stability with different NaCl concentrations (i.e., salinities), which shows the gel system can form in high salinity injection water and can keep stable in a high salinity environment. Very low concentrations of NaCl, KCl and CaCl2 can delay the gelation time. Above these critical concentrations, inorganic ion will boost the cross-linking reaction. Ampoules test results show that the gel system can keep stable at 150 °C with very low syneresis for five months, and DSC measurement suggests that the gel system can keep stable up to 240 °C, which shows the gel system can be applied in extremely high temperature reservoirs. Microstructures of the bulk gel shows that a 3D network structure can be formed by a terpolymer gel system formed of resorcinol-HMTA, which means many dendritic structures are distributed between the grid pores. This gel system gridwork can further increase the network structure strength of the polymer gel and lock the water within the gel firmly even under high temperatures. The gelation mechanism of an in-situ terpolymer gel system formed by resorcinol-HMTA is composed of four steps, designed to help petroleum engineers adjust gelant solution composition. They can also add some reaction inhibitors or accelerators to control any


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step and thus control the gelation performance.

AUTHOR INFORMATION Corresponding Author

*Tel: +1 (573) 341 4016. E-mail: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Science and Technology Major Project (2016ZX05009-004),

the

National

Science

and

Technology

Major

Project

(2016ZX05014-004-006HZ) and the China Scholarship Council (201606440051).

REFERENCES (1) Lake, L. W.; Johns, R.; Rossen, B.; Pope, G. Fundamentals of enhanced oil recovery. Society of Petroleum Engineers: 2014. (2) Sydansk, R. D. Reservoir conformance improvement. Society of Petroleum Engineers: 2011. (3) Sydansk, R. D.; Seright, R. S. When and where relative permeability modification water-shutoff treatments can be successfully applied. Spe. Prod. Oper. 2007, 22 (02), 236-247. (4) Bai B, Liu Y, Coste J P, et al. Preformed particle gel for conformance control: transport mechanism through porous media. Spe. Reserv. Eval. Eng. 2007, 10 (02): 176-184. (5) Elsharafi, M. O.; Bai, B. Effect of back pressure on the gel pack permeability in mature


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reservoir. Fuel, 2016, 183, 449-456. (6) Elsharafi, M. O.; Bai, B. Experimental work to determine the effect of load pressure on the gel pack permeability of strong and weak preformed particle gels. Fuel, 2017, 188, 332-342. (7) Sang, Q.; Li, Y.; Yu, L.; Li, Z.; Dong, M. Enhanced oil recovery by branched-preformed particle gel injection in parallel-sandpack models. Fuel, 2014, 136, 295-306. (8) Goudarzi, A.; Almohsin, A.; Varavei, A.; Delshad, M.; Bai, B.; Sepehrnoori, K. New experiments and models for conformance control microgels. SPE Improved Oil Recovery Symposium 2014; SPE Paper 169159. (9) Hunter, B. L.; Buell, R. S.; Abate, T. A. Application of a polymer gel system to control steam breakthrough and channeling. SPE Western Regional Meeting 1992; SPE Paper 24031. (10) Hubbard, S.; Roberts, L. J.; Sorbie, K. S. Experimental and theoretical investigation of time-setting polymer gels in porous media. Spe. Reser. Eng. 1988, 3 (04), 1257-67. (11) Sydansk, R. D. Acrylamide-polymer/chromium (III)-carboxylate gels for near wellbore matrix treatments. SPE Advanced Technology Series 1993, 1 (01), 146-152. (12) Liu, Y.; Dai, C.; Wang, K.; Zhao, M.; Gao, M.; Yang, Z.; Fang, J.; Wu, Y. Investigation on preparation and profile control mechanisms of the dispersed particle gels (DPG) formed from phenol-formaldehyde crosslinked polymer gel. Ind. Eng. Chem. Res. 2016, 55 (22), 6284-6292. (13) Wang, C.; Liu, H.; Wang, J.; Hong, C.; Dong, X.; Meng, Q.; Liu, Y. A novel high-temperature gel to control the steam channeling in heavy oil reservoir. SPE Heavy Oil Conference-Canada 2014; SPE Paper 170049. (14) Hunter, B. L.; Buell, R. S.; Abate, T. A. Application of a polymer gel system to control steam breakthrough and channeling. SPE Western Regional Meeting 1992; SPE Paper 24031. (15) Sandiford, B. B.; Dovan, H. T.; Hutchins, R. D. High temperature stable gels. U.S. Patents 5246073, 1993. (16) Wang, D.; Zhang, J.; Meng, F.; Liu, H.; Li, L.; Han, B.; Qi, L. Commercial test of polymer flooding in Daqing Oil Field Daqing Petroleum Administrative Bureau. International Meeting on Petroleum Engineering 1995; SPE Paper 29902. (17) Davison, P.; Mentzer, E. Polymer flooding in North Sea reservoirs. SPE J. 1982, 22 (03), 353-362. (18) Wellington, S. L. Biopolymer solution viscosity stabilization-polymer degradation and


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antioxidant use. SPE J. 1983, 23 (06), 901-912. (19) Samuelson, M. L.; Constien, V. G. Effects of high temperature on polymer degradation and cleanup. SPE Annual Technical Conference and Exhibition 1996; SPE Paper No. 36495.. (20) Caulfield, M. J.; Qiao, G. G.; Solomon, D. H. Some aspects of the properties and degradation of polyacrylamides. Chem. Rev. 2002, 102 (9), 3067-3084. (21) Zhao, G.; Dai, C.; Chen, A.; Yan, Z.; Zhao, M. Experimental study and application of gels formed by nonionic polyacrylamide and phenolic resin for in-depth profile control. J. Pet. Sci. Eng. 2015, 135, 552-560. (22) Liu, Y.; Dai, C.; Wang, K.; Zhao, M.; Zhao, G.; Yang, S.; Yan, Z.; You, Q. New insights into the hydroquinone (HQ)-hexamethylenetetramine (HMTA) gel system for water shut-off treatment in high temperature reservoirs. J. Ind. Eng. Chem. 2016, 35, 20-28. (23) Flynn Jr, C. M. Hydrolysis of inorganic iron (III) salts. Chem. Rev. 1984, 84 (1), 31-41. (24) Zhang, L.; Pu, C.; Sang, H.; Zhao, Q. Mechanism study of the cross-linking reaction of hydrolyzed polyacrylamide/Ac3Cr in formation water. Energy Fuels, 2015, 29, (8), 4701-4710. (25) Zhang, L.; Zheng, L.; Pu, J.; Pu, C.; Cui, S. Influence of HPAM molecular weight on the cross-linking reaction of HPAM/Cr3+ and transportation of HPAM/Cr3+ in micro-fractures. Energy Fuels, 2016, 30 (11), 9351-9361. (26) Bai, Y.; Xiong, C.; Wei, F.; Li, J.; Shu, Y.; Liu, D. Gelation study on a hydrophobically associating polymer/polyethylenimine gel system for water shut-off treatment. Energy Fuels 2015, 29 (2), 447-458. (27) Bai, Y.; Wei, F.; Xiong, C.; Li, J.; Jiang, R.; Xu, H.; Shu, Y. Effects of fracture and matrix on propagation behavior and water shut-off performance of a polymer gel. Energy Fuels 2015, 29, (9), 5534-5543. (28) Zolfaghari, R.; Katbab, A. A.; Nabavizadeh, J.; Tabasi, R. Y.; Nejad, M. H. Preparation and characterization of nanocomposite hydrogels based on polyacrylamide for enhanced oil recovery applications. J. Appl. Polym. Sci. 2006, 100 (3), 2096-2103. (29) Zhao, G.; Dai, C.; You, Q.; Zhao, M.; Zhao, J. Study on formation of gels formed by polymer and zirconium acetate. J. Sol-gel. Sci. Techn. 2013, 65 (3), 392-398. (30) Al-Muntasheri, G. A.; Nasr-El-Din, H. A.; Zitha, P. L. Gelation kinetics and performance evaluation of an organically crosslinked gel at high temperature and pressure. SPE J. 2008, 13 (03), 337-345.


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(31) Zhao, J.; Jia, H.; Pu, W.; Liao, R. Influences of fracture aperture on the water-shutoff performance of polyethyleneimine cross-linking partially hydrolyzed polyacrylamide gels in hydraulic fractured reservoirs. Energy Fuels 2011, 25, (6), 2616-2624. (32) Jia, H.; Pu, W.; Zhao, J.; Liao, R. Experimental investigation of the novel phenol− formaldehyde cross-linking HPAM gel system: based on the secondary cross-linking method of organic cross-linkers and its gelation performance study after flowing through porous media. Energy Fuels 2011, 25, (2), 727-736. (33) Dovan, H. T.; Hutchins, R. D.; Sandiford, B. B. Delaying gelation of aqueous polymers at elevated temperatures using novel organic crosslinkers. International Symposium on Oilfield Chemistry 1996; SPE Paper 37246. (34) Dovan, H. T.; Sandiford, B. B.; Hutchins, R. D. Method for modifying gelation time of organically crosslinked, aqueous gels. U.S. Patent 5617920, 1997. (35) Sydansk, R. D.; Argabright, P. A. Conformance improvement in a subterranean hydrocarbon-bearing formation using a polymer gel. U.S. Patent 4683949, 1987. (36) Hashmat, M. D.; Sultan, A. S.; Rahman, S.; Hussain, S. M. Crosslinked polymeric gels as loss circulation materials: an experimental study. SPE Kingdom of Saudi Arabia Annual Technical Symposium and Exhibition 2016; SPE Paper 182740. (37) You, Q.; Zhang, P.; Bai, S.; Huang, W.; Jia, Z.; Zhou, C.; Li, D. Supramolecular linear polymer formed by host-guest interactions of β-cyclodextrin dimers and polyacrylamide end-capped with adamantane. Colloids Surf. A 2015, 484, 130-135. (38) Beuermann, S.; Buback, M.; Hesse, P.; Lacík, I. Free-radical propagation rate coefficient of nonionized methacrylic acid in aqueous solution from low monomer concentrations to bulk polymerization. Macromolecules 2006, 39, (1), 184-193. (39) Liwei, W.; Wen, Z.; Bo, C.; Yongjun, L.; Xiaohui, Q. 220 °C Ultra-temperature fracturing fluid in high pressure and high temperature reservoirs. Offshore Technology Conference Asia 2016; SPE Paper 26364. (40) Funkhouser, G. P.; Norman, L. R. Synthetic polymer fracturing fluid for high-temperature applications. International Symposium on Oilfield Chemistry 2003; SPE Paper 80236. (41) Villetti, M. A.; Crespo, J. S.; Soldi, M. S.; Pires, A.; Borsali, R.; Soldi, V. Thermal degradation of natural polymers. J. Therm. Anal. Calorim. 2002, 67, (2), 295-303. (42) Mudiyanselage, T. K.; Neckers, D. C. Highly absorbing superabsorbent polymer. J. Polym.


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Sci. Pol. Chem. 2008, 46, (4), 1357-1364. (43) Sengupta, B.; Sharma, V. P.; Udayabhanu, G. Gelation studies of an organically cross-linked polyacrylamide water shut-off gel system at different temperatures and pH. J. Petrol. Sci. Eng. 2012, 81, 145-150. (44) Jordan, D. S.; Green, D. W.; Terry, R. E.; Willhite, G. P. The effect of temperature on gelation time for polyacrylamide/chromium (III) systems. SPE J. 1982, 22, (04), 463-471. (45) Fang, J.; Zhang, X.; He, L.; Zhao, G.; Dai, C. Experimental research of hydroquinone (HQ)/hexamethylene tetramine (HMTA) gel for water plugging treatments in high-temperature and high-salinity reservoirs. J. Appl. Polym. Sci. 2017, 134, (1).


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Figure 1. Molecular formula of different anionic polymer. (a)ZP-1 or ZP-2, (b)ZP-3, (c)ZP-4.


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200

T =150 ℃ 160

Viscosity (mPa·s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120

80

40

ZP-3 solution(0.8 wt%) ZP-4 solution(0.8 wt%)

0 0

2

4

6

8

10

12

Time (h)

Figure 2. Viscosity of polymer solution vs. time curve.


Energy & Fuels

0.8

15

11

8

3

14

11

9

3

14

12

10

4

0.75 Cross-linker concentration(wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.7

0.65

0.6

0.55

0.5 0.4

13

15

0.5

0.6

10

0.7

0.8

4

0.9

1


Figure 3. Contour map of gelation time of the gel system composed of different concentrations of terpolymer and the cross-linker system. Solid squares represent specific experiments, numbers next to the square represent the gelation time (h). Experimental temperature was 150 C.


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30 10000

16

25 14

10

1000

8


6

15

10

G'/G'' (MPa)

20

12

Viscosity(mPa.s)

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

4 0.3

0.4

0.5

0.6

0.7

0.8

0.9

100 1.0

0


Figure 4. Effect of terpolymer concentration on gelation performance (gelation time, viscosity of bulk gel, storage modulus G′ and loss modulus G″). Concentration of cross-linker system was 0.6 %, and experimental temperature was 150 C.


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(a)

(b)

(c)

(d)

Figure 5. Microstructures of gel systems composed of 0.6 % cross-linker system, 0.05% antioxidant, 0.05% heat stabilizer, and different terpolymer concentrations: a)0.4% tepolymer, b)0.6% terpolymer, c) 0.8% terpolymer and d)1.0% terpolymer. All structures are magnified up to 5,000 times.


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100

30 10000

90 25

60 50

1000


40 30 20

Viscosity (mPa.s)

70

20

15

10

G'/G'' (MPa)

80

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5

10 0 0.2

0.3

0.4

0.5

0.6

0.7

100 0.8

0

Cross-linker concentration(wt%)

Figure 6. Effect of cross-linker system concentration on gelation performance (gelation time, viscosity of bulk gel, storage modulus G′ and loss modulus G″). Concentration of terpolymer in gelant concentration was 0.6 %, and experimental temperature was 150 C.


Energy & Fuels

40

Gelation time 30

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

10

0 80

100

120

140

160

Temperature (℃)

Figure 7. Effect of temperature on gelation process. Gelant solution composed of 0.6 % ZP-4 and 0.6 % resorcinol-HMTA were selected as the representative samples.


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4.0 Equation Weight

3.5

y = a + b*x No Weighting

Residual Sum of Squares 0.18404 0.96055 Pearson's r 0.90331 Adj. R-Square Value B

Intercept -5.68407 Slope 3294.41763

Standard Err 1.20723 476.93067

3.0

ln(tg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.5

ln(tg)=Ea/RT+lnA Ea=Rdln(tg)/d(1/T) 2.0

1.5 0.0023

Ea=27.39 kJ/mol

0.0024

0.0025

0.0026

0.0027

0.0028

0.0029

1/T

Figure 8. Arrhenius plot for the reaction under different temperatures.


Energy & Fuels

20

NaCl KCl CaCl2

15

Gelation time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MgCl2

No salts line

10

5

0 10-5

10-4

10-3

10-2

10-1

100

101

Concentration (mol/L)

Figure 9. Effect of different salts and concentrations on gelation performance. Horizontal dotted line represents the gelation time of gelant solution prepared by deionized water.


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-1.0 100 -1.5

90

-2.0

-2.5 80 -3.0 70

Sample: 1.3200 mg Method: 30-400-Ar-10k/min Sample tray: Alumina 70ul

Step: -35.5343% -0.4691 mg

-3.5

60 50

100

150

200

250

300

Heat flow (w/g)

w/w (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

350

-4.0 400

Temperature (℃)

Figure 10. DSC curve of the gel system of 0.6 % ZP-4 and 0.6 % resorcinol-HMTA.


Energy & Fuels

Step 1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Step 2:

Step 3:

Step 4:

Paragon Plus Environment Figure 11. Gelation mechanismACS of cross-linking reaction between terpolymer and resorcinol-HMTA.

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Terpolymer Gel System Formed by Resorcinol

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