Heat–Generating Expandable Foamed–Gel Used for Water Plugging

gel system that is capable of spontaneous in–situ heat generation was developed. ... such circumstances, the gelation time is 40 hours, the gel stre...
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Heat-Generating Expandable Foamed Gel Used for Water Plugging in Low-Temperature Oil Reservoirs Ning Qi,*,† Boyang Li,† Guobin Chen,† Chong Liang,‡ Xinghua Ren,† and Mengfei Gao† †

School of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China Langfang Branch of Research Institute of Petroleum Exploration and Development, Langfang, Hebei 065007, People’s Republic of China

‡

ABSTRACT: Polymers are often used for chemical water plugging. When the reservoir temperature is lower than 50 °C, the reaction between polymers and cross-linking agents is very slow, which extensively prolongs the gelation time and even leads to unsuccessful gelation. To overcome such problems, a foamed-gel system that is capable of spontaneous in situ heat generation was developed. The optimal system was identified through the orthogonal test using the gel strength, gelation time, and gel volume as indexes. The test shows that, when the ambient temperature is fixed at 30 °C and the pH value is 6.8, the system performs well. Under such circumstances, the gelation time is 40 h, the gel strength reaches the G grade, and the volumetric expansion ratio at 10 MPa exceeds 130%. Nuclear-magnetic-resonance-based T2 spectra indicate that the foamed gel injected into the rock can effectively plug large pores and, therefore, offset the heterogeneity. It is also found that the foamed gel has great capacity for volumetric expansion-based water plugging. The synchronization between gelation and gas generation is the key to the heat-generating foamed gel. Experiments suggest the properties of the developed heat-generating expandable foamed gel can be manipulated by adjusting pH values to satisfy varied requirements for placement in different reservoirs. medium−low-temperature reservoirs was developed.7 Cao et al. used the modified starch−acrylamide graft copolymerization system as a new type of polymer gel plugging agent.8 We tested the second method from 2011 to 2013. With corn starch and acrylamide used as raw materials and ceric ammonium nitrate as the initiator, starch-grafted PAM presenting excellent gelation performance under low temperatures was formed. However, the generated agent is limited by its complicated production technology and onerous requirements for commercial production, and hence, the wide application of such agents to the oilfield operation is hard to achieve. The third method is to add heat-generating agents in conventional water-shutoff agents. As the heat-generating agent is injected into the reservoir, an exothermic reaction occurs. The in situ heat generation then allows for gelation of conventional plugging agents under desirable gelation pressures. It requires only conventional plugging agents and makes it feasible to plug water in low-temperature oil reservoirs. In the paper, this method is the focus. Thus far, the frequently used heat-generating system mainly includes four heat-generating systems, namely, nitrite and ammonium, hydrogen peroxide, polyhydroxy aldehyde oxidation, and metal oxidation. Among them, the reaction of nitrite and ammonium can produce the most heat. Ashton et al. and McSpadden et al. applied the in situ heat- and N2-generating processes in paraffin removal of oil wells, respectively.9,10 Apart from paraffin removal, the reaction of nitrite and ammonium is frequently used in thermal flooding, heavy oil viscosity reduction, energized fracturing, etc. in oilfields.11−14 In terms

1. INTRODUCTION In the late development stage, the heterogeneity of lowtemperature reservoirs tends to intensify. This will lead to rapid increases of water production and, hence, ineffective circulation of massive injected water. In such cases, chemical treatments are often used for formation blockage, and polymers account for the majority of the adopted chemical agents.1−5 These plugging agents are more suitable for high-temperature reservoirs, while the effect of plugging is not obvious in lowtemperature reservoirs. As a result of the low formation temperature, the reaction of polymers, e.g., polyacrylamide (PAM), with cross-linking agents suffers from a slow reaction rate. It tremendously prolongs the gelation process, reduces the gel strength, and even causes failure of gelation and, consequently, unsuccessful water plugging. Three methods are often used to tackle the difficulty of polymer gelation in low-temperature oil reservoirs. First, preheat the target reservoir, so that the reservoir temperature can rise above the minimum gelation temperature, with the help of hot water injection or short-term steam injection. However, most of the oilfields lack equipment for thermal injection, e.g., no available steam boilers for steam injection. In addition, heating the reservoir via hot water injection consumes a huge amount of water, which results in a long period of time for water flowback during production. In the meantime, an ineffective circulation of massive water is formed, and subsequent water treatment costs a lot. The second method is graft modification of polymers. For instance, starch-grafted PAM improves gelation performance under low temperatures. Zhang et al. prepared a hydrolyzed polyacrylamide (HPAM)/ polyatomic phenol/aldehyde gelling system combining the oilfield status.6 Li et al. applying advantageous property of modified starch, and the strong gel water plugging agent for © XXXX American Chemical Society

Received: September 11, 2017 Revised: December 21, 2017 Published: December 22, 2017 A

DOI: 10.1021/acs.energyfuels.7b02705 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels of profile control and water plugging, Wang et al. used the heatgenerating agents to produce N2 and obtained a foam system suitable for the oil reservoirs at 60 °C.15,16 However, 60 °C seems to be too high for some low-temperature oil reservoirs. Besides, when the reservoir temperature is lower than 50 °C, the reaction between polymers and cross-linking agents is overwhelmingly slow, which extensively prolongs the gelation time and even leads to unsuccessful gelation. Therefore, the onsite heat-generation-based water plugging technology for lowtemperature oil reservoirs is urgent to be developed. To obtain the foamed-gel system suitable for a lower temperature, the temperature of oil reservoirs is set to 30 °C in this study. The pressure of oil reservoirs is assumed to be 10 MPa, so that the volumetric expansion ratio can be calculated. The heat-generating expandable foamed-gel system for profile control and thief zone plugging developed in this paper17−21 uses PAM as the main agent and contains the in situ heating system of nitrite and ammonium. The proposed system has advantages of underground heat generation, high volumetric expansion, good plugging capacity, selective water shutoff, and easy preparation. It is capable of handling edge and bottom water invasion, plugging thief zones, and sealing flow channels between strata in low-temperature reservoirs.

Table 1. Evaluation Standards of Gel Strength Grades

0.1

V1

=

0.01(V2 − V1) + V1 V1

A B

1 2

C

3

D

4

E

5

F

6

G

7

H

8

description

classification

ungelatinized high-mobility gel, slightly sticking to the bottle wall flowable gel, with intensified wall sticking medium-mobility gel, adhesive, with intensified wall sticking low-mobility gel, with apparent wall sticking and intensified adhesion high-deformation immovable gel, much more adhesive, with a long tongue structure slowly deforming immovable gel, with good adhesion and a short tongue structure rigid gel, with maximum adhesion and no tongue structure

weak gel

medium gel

strong gel

The strength level noted as 1−8 in this paper is used for convenience in cases of plotting. 2.3. Workflow. (1) Prepare 50 mL of PAM solutions with a prescribed concentration. (2) Successively add cross-linking reagents and heat-generating reagents into PAM solutions, according to specific ratios, and then stir thoroughly for complete dissolution. (3) Place the prepared solution into the 100 mL cuvette, which is then water-bathed at 30 °C and atmospheric pressure. (4) Record the gelation time, gel strength, and gel volume of the system, (5) Identify the optimal system on the basis of orthogonal array tests, conduct NMR-based core plugging tests, observe changes of pore structures of core samples via the T2 spectrum, and analyze mechanisms of water plugging using heat-generating expandable foamed gel.

2.1. Reagents and Instruments. 2.1.1. Reagents. PAM dry powder (with the relative molecular weight of 18 × 106), ammonium chloride [analytical reagent (AR), manufactured by Sinopharm Chemical Reagent Co., Ltd., China (SCRC)], sodium nitrite (AR, SCRC), anhydrous sodium sulfite (AR, SCRC), sodium dichromate (AR, Tianjin Guangcheng Chemical Reagent Co., Ltd., China), hydrogen peroxide (AR, SCRC), manganese dioxide (AR, SCRC), glucose (AR, SCRC), acetic acid (AR, Xilong Scientific Co., Ltd., China), hydrochloric acid (AR, SCRC), etc. were provided. 2.1.2. Instruments. Nuclear magnetic resonance (NMR, manufactured by Suzhou Niumag Analytical Instrument Corporation, China), REX pH meter (Shanghai INESA Scientific Instrument Co., Ltd., China), BS electronic balance (Sartorius, Germany), digital display thermostatic water bath (Jintan Medical Instrument Manufacturer, China), electronic stirrer (Jintan Medical Instrument Manufacturer, China), 100 mL cuvette, beaker, etc. were provided. 2.2. Evaluation Index. 2.2.1. Gel Strength. The gel strength was qualitatively evaluated using the visual observation-based strength code. Gel was put into a bottle, which was then placed upside down, and the gel strength was assessed according to the variation of the length of the tongue-shaped structure formed by the system.22,23 The strength grade and corresponding code are shown in Table 1. 2.2.2. Gelation Time. The gelation time refers to the time required by the foamed-gel system to reach the F-grade strength. 2.2.3. Gel Volume. One of the technical advantages of the heatgenerating expandable foamed-gel system for water plugging is that the system allows for spontaneous expansion.24 The gas that stimulates the volumetric expansion is a byproduct (N2) of the reaction of the nitrite−ammonium heat-generating system. The gel volume is defined as the volume of the foamed gel after gelation. This paper used the volumetric expansion ratio based on 50 mL of the prepared base solution to evaluate the gel volume. For the sake of simplicity, the volume of the foamed gel at 10 MPa was calculated using the ideal gas equation of state

Vgas 10 + V1

strength levela

a

2. EXPERIMENTAL SECTION

k=

strength grade

3. RESULTS AND DISCUSSION 3.1. Optimization of Heat-Generating Systems. 3.1.1. Nitrite−Ammonium Heat-Generating Systems. A total of 50 mL of NaNO2 and NH4Cl solutions with identical concentrations of 0.5, 1.0, 1.5, and 2.0 mol/L were prepared. Two solutions with identical concentrations were added to the reaction flask, and 2.0 mL of diluted hydrochloric acid was dropped into the mixture as the catalyst. The flask was then sealed with the rubber stopper equipped with a thermometer, after rapid thorough stirring. The peak temperature and required time were recorded. Results are shown in Table 2. Table 2. Heat Generation Capacity of Nitrite−Ammonium Heat-Generating Systems reactant concentration (mol/L)

peak temperature (°C)

time required for heating (min)

0.5 1.0 1.5 2.0

35.4 60.6 83.2 90.8

42 35 17 13

Table 2 shows that, with the increasing concentration of the heat-generating reagent, the peak temperature increases, while the time required for heating reduces. 3.1.2. Hydrogen Peroxide Heat-Generating Systems. A total of 10 mL of hydrogen peroxide solutions with varied concentrations (0.5, 0.75, 1.0, and 1.5 mol/L) were prepared and poured into reaction flasks. A total of 1.0 g of manganese dioxide was added to the flask, which was then sealed with the rubber stopper equipped with a thermometer after rapid,

(1)

where k stands for the volumetric expansion ratio at 10 MPa, Vgas stands for the volume of gas generation at 0.1 MPa (mL), V1 stands for the volume of the base solution (mL), and V2 stands for the volume of the foamed gel after gelation (mL). B

DOI: 10.1021/acs.energyfuels.7b02705 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels thorough stirring. The peak temperature and required time were recorded. Results are shown in Table 3.

Table 5. Scheme of the Orthogonal Test value

Table 3. Heat Generation Capacity of Hydrogen Peroxide Heat-Generating Systems reactant concentration (mol/L)

peak temperature (°C)

time required for heating (min)

0.5 0.75 1.0 1.25 1.5

36.3 42.8 52.4 60.4 69.0

4 3 2 1 1

factor

1

2

3

HPAM concentration (%) cross-linking agent concentration (%) heat-generating agent concentration (mol/L) pH

0.40 0.15 1.00 5

0.50 0.23 1.50 6

0.60 0.30 2.00 7

30 °C, to observe the variation of the gelation performance of the system. 3.2.1. Effects of HPAM Concentrations. The effect of the polymer concentration on the system property was evaluated in a single-factor approach. The cross-linking agents were 0.4% Na2Cr2O7 + 0.6% Na2SO3, and the heat-generating agent concentration was 1 mol/L. The tests were repeated with polymer concentrations of 0.3, 0.4, 0.5, and 0.6%. The strength level noted as 1−8 in Figure 1 equals the strength grade noted as A−H, respectively. Results are shown in Figures 1 and 2.

It is shown in Table 3 that the heat generation of the hydrogen peroxide heat-generating system is fast. However, the peak temperature that the system can reach is far below that of the nitrite−ammonium system with the same concentration. Moreover, the hydrogen peroxide system is overwhelmingly reactive, which leads to potential safety risks. 3.1.3. Polyhydroxy Aldehyde Oxidation Heat-Generating Systems. A total of 10 mL of glucose solutions with different concentrations (1, 2, and 3 mol/L) were mixed with 10 mL of chromium trioxide in reaction flasks. A total of 1 mL of acetic acid was added to the flask, which was then sealed with the rubber stopper equipped with a thermometer after rapid, thorough stirring. The peak temperature and required time were recorded. Results are shown in Table 4. Table 4. Heat Generation Capacity of Polyhydroxy Aldehyde Oxidation Heat-Generating Systems reactant concentration (mol/L)

peak temperature (°C)

time required for heating (min)

1 2 3

29.4 42.8 61.5

42 27 20

Figure 1. Gel strength versus gelation time at different HPAM concentrations.

Table 4 demonstrates that, with the identical concentration, the peak temperature of the exothermal reaction of the polyhydroxy aldehyde system is far less that of the nitrite− ammonium system. Therefore, the nitrite−ammonium heatgenerating system is identified as the optimal heat-generating agent for the proposed water plugging system. It should be noted that all experiment data were measured under atmospheric pressure and unsealed conditions. In such cases, some heat loss occurs, and hence, the real peak temperature of each heat-generating system is higher than the measured peak temperature. 3.2. Evaluation of Properties of Heat-Generating Expandable Foamed Gel. In this paper, PAM dry powder was used as the polymer component, Na2Cr2O7 and Na2SO3 were used as cross-linking agents, and NH4Cl and NaNO2 were used as heat-generating agents. The gel strength and gelation time were defined as the evaluation indicators, and the orthogonal test with four factors and three values for each factor was designed and implemented to evaluate the effect of each factor on the property of the heat-generating expandable foamed-gel system. The scheme of the orthogonal test is shown in Table 5. To compare and analyze the sensitivity of the property of the heat-generating expandable foamed-gel system to each influential factor, all prepared gel fluids were water-bathed at

Figure 2. Gel volume versus gelation time at different HPAM concentrations.

It can be seen from Figures 1 and 2 that the gel strength of the system increases with the rising polymer concentration, while the gelation time reduces and the volumetric expansion slows. The minimum gelation time corresponds to the polymer concentration of 0.6%, yet the volumetric expansion is the slowest in such cases. This is mainly because the cross-linking reaction is accelerated as a result of the increase in the polymer concentration. The gelation time reduces, and the gel strength grows, which constrains the migration and dispersion of the generated gas in the gel and, consequently, leads to the loss of the volumetric expansion capacity of the foamed gel. 3.2.2. Effects of Cross-Linking Agent Concentrations. The effect of the cross-linking agent concentration on the system C

DOI: 10.1021/acs.energyfuels.7b02705 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels property was also evaluated in a single-factor approach. The polymer concentration was 0.6%, and the heat-generating agent concentration was 1 mol/L. Cross-linking agent concentrations were set at four levels: 0.15, 0.3, 0.6, and 0.9%. Test results are shown in Figures 3 and 4.

Figure 5. Gel strength versus gelation time at different heat-generating agent concentrations.

Figure 3. Gel strength versus gelation time at different cross-linking agent concentrations.

Figure 6. Gel volume versus gelation time at different heat-generating agent concentrations. Figure 4. Gel volume versus gelation time at different cross-linking agent concentrations.

Figures 5 and 6 suggest that the gel volume and volumetric expansion velocity grow as the heat-generating agent concentration increases. It should be noted that the gelation time declines with the growing heat-generating agent concentration, yet the stability of the foamed gel is compromised by excessive heat-generating agent concentrations. The thermal energy provided by the heat generation reactions can accelerate the synthesis of the multinuclear olation complex ion and promote the cross-linking reaction of the system. Besides, with the increase of heat-generating agents, the gas generation rate is also accelerated. Once the gas generation reaches a specific critical level (with a heatgenerating agent concentration of 1 mol/L), the massive generated gas will damage the spatial network structure of the gel and reduce the strength of the system. Hence, this study advises that the heat-generating agent concentration should not exceed 1 mol/L. 3.2.4. Effects of pH Values. The key to the heat-generating foamed gel is the synchronization between gelation and gas generation, both of which are very sensitive to pH values. Via a single-factor approach, the effect of the pH on the system property has been studied. The HPAM concentration was 0.6%; 0.4% Na2Cr2O7 + 0.6% Na2SO3 was adopted as the crosslinking agent; and the heat-generating agent had a concentration of 1 mol/L. Because the cross-linking and heatgenerating components in the system both require a nonalkaline chemical environment for desirable reactions, the pH value was adjusted to 5.5, 6, 6.5, 6.8, and 7.2 (with no hydrochloric acid). Test results are shown in Figures 7 and 8. Acid can be used as catalyst for the heat generation reaction. From Figures 7 and 8, it is shown that the heat generation reaction accelerates as the pH value decreases. In a short time, the amount of heat produced is much higher than that of heat

It is demonstrated in Figures 3 and 4 that the gelation and volumetric expansion are fast with low cross-linking agent concentrations. It is also observed that the foamed-gel system presents poor stability, although the strength of the system manages to reach the H grade. With a cross-linking agent concentration of 0.6%, stable foamed gel can be formed. However, with a higher cross-linking agent concentration (0.9%), the volumetric expansion rate of the foamed gel declines, which dampens the volumetric expansion capacity of the system. This can be explained by the fact that, with lower cross-linking agent concentrations, the cross linking of polymers is loose and, hence, the gel strength is insufficient to confine the gas produced by heat-generating agents; under such circumstances, massive gas escapes and instability of the foamed-gel system grows, although the volumetric expansion accelerates. Moreover, with higher cross-linking agent concentrations, the cross-linking of polymers speeds up and reaches higher levels. The resulting high gel strength imposes restrictions on the heat generation reactions and gas production, which reduces the volumetric expansion capacity of the foamed gel. Given the aforementioned, the cross-linking agent concentration of 0.6% is recommended for practice. 3.2.3. Effects of Heat-Generating Agent Concentrations. The single-factor method was again used to analyze the effect of the heat-generating agent concentration on the system property. The polymer concentration was 0.40%, and the cross-linking agent was 0.4% Na2Cr2O7 + 0.6% Na2SO3. The heat-generating concentrations in the evaluation were 0.5, 1, 1.5, and 2 mol/L. Test results are shown in Figures 5 and 6. D

DOI: 10.1021/acs.energyfuels.7b02705 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 7. Gel strength versus gelation time at different pH.

Figure 9. Photo of heat-generating expandable foamed gel (after gelation).

Table 7. Effects of Temperatures on the Gelation Time

Figure 8. Gel volume versus gelation time at different pH.

Table 6. Properties of Optimal Heat-Generating Expandable Foamed-Gel Systems gelation time (h)

volumetric expansion ratio (%)

strength grade

30

40

130

G

gelation time (h)

20 30 40 50 60

44 40 34 30 25

generating foamed gel spontaneously swells in the rock. By measurement of the relaxation characteristic of hydrogencontaining fluids inside the rock, the change of the rock pore structure can be observed and used to reflect the volumetric expansion capacity and process foamed gel, which provides evidence for analyzing the mechanism of volumetric expansionassisted water plugging. According to the above-mentioned procedure, the heatgenerating expandable foamed-gel solution was prepared. The cores prepared were 5.6 cm long and 2.54 cm in diameter. The average porosity of the cores was about 0.30. The solution, whose volume was equal to 1/3 of the core pore volume, was injected into the cores. Then, the cores were placed at different temperatures. As the gelation time of the heat-generating foamed gel greatly declines with the growing temperature, the core testing was set at 40 °C to accelerate the testing. According to Table 7, in general, the strength of the system can reach the F grade at 40 °C after 30 h. The variation of pore structures during the gelation process was observed through the measurement of the T2 spectrum. After gelation, the core sample was placed at 60 °C for 8 h before another T2 spectrum measurement, to determine whether the system still possesses certain volumetric expansion capacity after gelation. The pre- and post-foamed-gel injection NMR-based T2 spectra of the core sample are shown in Figure 10, which indicates that large pores are the majority and small pores are the minority in the original core sample. After injection of foamed-gel plugging agents and placement at 40 °C for 20 h, the first NMR testing after water plugging suggests that large pores (seen at x = 1000 in Figure 10) decrease and small pores (seen at x = 10−100) grow. This means that the injected foamed gel effectively plugs large pores and part of large pores shrink into small pores. The NMR testing was repeated after an extra 30 h placement at 40 °C. It is found that large pores continue to decline, which shows that the foamed gel injected into pores of rocks can achieve volumetric expansion-based

dissipation and the temperature will increase. Consequently, the reaction of gelation and gas generation speeds up, and the volume of the foamed-gel system increases rapidly. The higher value of pH seems to be beneficial to the accretion of the volume. However, at a higher temperature, the rate of gas generation is much larger than that of gelation. The gas is difficult to be bound in the foamed-gel firmly. After a period of time, the gas in the foamed gel will gradually escape and the volume will shrink, which were observed in the experiments. Therefore, keeping pH at a higher value, less than 7, is conducive to the formation of firmer foamed gel. Considering the volumetric expansion, gelation time, and effective utilization of generated heat, the pH value of 6.8 is recommended for the optimal system property. 3.2.5. Properties of the Optimal Heat-Generating Expandable Foamed-Gel System. The optimal composition of the heat-generating expandable foamed-gel system based on the orthogonal test is 0.6% HPAM + 0.6% cross-linking agent (Na2Cr2O7 + Na2SO3) + 1.0 mol/L heat-generating agent (NH4Cl + NaNO2). The property of this system is shown in Table 6, and the morphology of the gel is seen in Figure 9.

temperature (°C)

temperature (°C)

It can be seen from Table 7 that lower temperatures prolong the gelation time, but the system is still of effective gelation under such conditions. 3.3. Mechanism Analysis of Volumetric ExpansionAssisted Water Plugging. The NMR testing can reveal the change in the pore structure characteristic of rocks. The heatE

DOI: 10.1021/acs.energyfuels.7b02705 Energy Fuels XXXX, XXX, XXX−XXX

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(6) Zhang, J. L.; Tang, F. M.; Wang, S. J. Oilfield Chem. 2004, 21, 138−141. (7) Li, F. L.; Hou, J. R.; Liu, Y. H. Pet. Geol. Oilfield Dev. Daqing 2007, 26, 80−82. (8) Cao, G. Z.; Hou, G. R.; Yue, X. A. Pet. Geol. Recovery Effic. 2008, 15, 72−74. (9) Ashton, J. P.; Kirspel, L. J.; Nguyen, H. T.; Credeur, D. J. SPE Prod. Eng. 1989, 4, 157−160. (10) McSpadden, H. W.; Tyler, M. L.; Velasco, T. T. Proceedings of the SPE California Regional Meeting; Oakland, CA, April 2−4, 1986; DOI: 10.2118/15098-MS. (11) Khalil, C. N.; Franco, Z. A. Proceedings of the SPE Latin America Petroleum Engineering Conference; Rio de Janeiro, Brazil, Oct 14−19, 1990; DOI: 10.2118/21113-MS. (12) Yang, H.; Duan, Y. J. Pet. Sci. Technol. 2014, 122, 304−317. (13) Wu, J.; Zhang, N.; Wu, X.; Liu, X. Chin. J. Geochem. 2006, 25, 162−166. (14) Liu, S. Z.; Sun, A. S.; Liu, F. J. Oil Drill. Prod. Technol. 2003, 25, 83−106. (15) Wang, F.; Li, Z. M.; Li, S. Y. J. China Univ. Pet., Ed. Nat. Sci. 2017, 41, 116−123. (16) Wang, F.; Li, Z. M.; Li, S. Y. J. China Univ. Pet., Ed. Nat. Sci. 2016, 40, 130−135. (17) Schramm, L. L.; Isaacs, E. E. Foams in Enhancing Petroleum Recovery. In Foam Engineering: Fundamentals and Applications; Stevenson, P., Ed.; John Wiley & Sons, Ltd.: Chichester, U.K., 2012; Chapter 13, pp 283−305, DOI: 10.1002/9781119954620.ch13. (18) Simjoo, M.; Nguyen, Q. P.; Zitha, P. L. J. Ind. Eng. Chem. Res. 2012, 51, 10225−10231. (19) Puertas, F.; Palacios, M.; Manzano, H.; Dolado, J. S.; Rico, A.; Rodríguez, J. J. Eur. Ceram. Soc. 2011, 31, 2043−2056. (20) Verdolotti, L.; Lavorgna, M.; Lamanna, R.; Di Maio, E.; Iannace, S. Polymer 2015, 56, 20−28. (21) Deleurence, R.; Saison, T.; Lequeux, F.; Monteux, C. Soft Matter 2015, 11, 7032−7037. (22) Li, S.; Li, Z.; Li, B. J. Porous Media 2015, 18, 519−536. (23) Romero-Zeron, L.; Kantzas, A. J. Can. Pet. Technol. 2015, 44, 44−50. (24) Qi, N.; Wang, Y.; Liu, S. Chinese Patent ZL 2014106289763, 2016.

Figure 10. T2 spectra of rock during the expansion process of the heatgenerating expandable foamed gel.

water plugging. A third NMR testing was conducted after putting the core sample at 60 °C for 8 h, and similar phenomena were observed.

4. CONCLUSION The optimal formula of the heat-generating expandable foamed-gel system is 0.6% HPAM + 0.6% cross-linking agent (Na2Cr2O7 + Na2SO3) + 1.0 mol/L heat-generating agent (NH4Cl + NaNO2). The system best works with the pH value of 6.8. In such cases, the gelation time is 40 h, the strength grade is G, and the volumetric expansion ratio at 10 MPa can surpass 130%. The key to the heat-generating foamed gel is the synchronized gelation and gas generation. The property of the heat-generating expandable foamed gel can be regulated by adjusting pH values. On the basis of NMR-based T2 spectra of core samples, the injected foamed gel effectively plugs large pores and, hence, reduces the rock heterogeneity. It is also indicated that the foamed gel possesses excellent capacity of volumetric expansion-assisted water plugging.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: 86-532-86981303. E-mail: [email protected]. cn. ORCID

Ning Qi: 0000-0002-7602-8179 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Technologies Research and Development Program of China during the 13th Five-Year Plan Period (Project 2017ZX005030005) and the Shandong Provincial Natural Science Foundation, China (Project ZR201702180073). Their sponsorship is gratefully acknowledged.

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REFERENCES

(1) Li, J. F.; Qi, N.; Zhang, Q. Oil Drill. Prod. Technol. 2007, 29, 76− 78. (2) Dai, C.; Zhao, G.; You, Q.; Zhao, M. J. Appl. Polym. Sci. 2014, 131, 1001−1007. (3) Haugen, A.; Ferno, M. A.; Graue, A.; Bertin, H. J. Proceedings of the SPE Improved Oil Recovery Symposium; Tulsa, OK, April 24−28, 2010; DOI: 10.2118/129763-MS. (4) Wang, B.; Lin, M.; Guo, J.; Wang, D.; Xu, F.; Li, M. J. Appl. Polym. Sci. 2016, 133, 43666. (5) Gao, C. H. Proceedings of the SPE EOR Conference at Oil and Gas West Asia; Muscat, Oman, March 31−April 2, 2014; DOI: 10.2118/ 169652-MS. F

DOI: 10.1021/acs.energyfuels.7b02705 Energy Fuels XXXX, XXX, XXX−XXX