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Enhance oil recovery for steam flooding: Low temperature oxidative decomposition of heavy oil with air injection Changjiu WANG, Huiqing Liu, Zhanxi Pang, Jing Wang, Changyong Chen, Chunlei Wang, and Zhengbin Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01330 • Publication Date (Web): 02 Sep 2015 Downloaded from http://pubs.acs.org on September 5, 2015
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Enhance oil recovery for steam flooding: Low temperature oxidative decomposition of heavy oil with air injection WANG Changjiu*, LIU Huiqing, PANG Zhanxi, WANG Jing, CHEN Changyong, WANG Chunlei, Wu Zhengbin (Key Laboratory for Petroleum Engineering of the Ministry of Education, China University of Petroleum)
Abstract: With the increasing demand of energy, technical research of how to enhance oil recovery of steam flooding for heavy oil reservoirs has attracted widespread attention at present. Air-injection is an effective technology which has been used to improve the development effect of steam flooding in heavy oil reservoirs. Low temperature oxidation (LTO) reaction and high temperature combustion reaction are the main mechanisms of air-injection technology. High temperature combustion reaction can decompose the heavy component in heavy oil, but it requires a higher temperature condition which the steam flooding process cannot offer. And LTO reaction between air and heavy oil consumes the O2, so the safety risk of explosion caused by the mixture of O2 and hydrocarbon gas can be eliminated. Nevertheless, oil viscosity will increase. During the steam flooding process, aquathermolysis reaction occurs between heavy oil and high-temperature water, which decreases the content of heavy component in heavy oil. Besides, catalyst MnO2 promotes the reaction by decreasing the activation energy of the reaction. In this paper, several static oxidative decomposition experiments are conducted to study the change characteristics of pressure, gas composition, oil composition and oil viscosity after the reactions with different temperatures, pressures and water saturations. In addition, four dynamic displacement experiments are conducted to compare the displacement effect of different displacement methods, including N2-injection displacement, air-injection displacement, steam flooding, and air-injection assisted steam flooding. Experimental results show that air-injection can effectively improve the development effect of steam flooding in heavy oil reservoirs. Upgrading and viscosity reduction for heavy oil by the combination of LTO reaction and aquathermolysis reaction can slow down steam channeling and increase production rate, and then enhance the ultimate recovery of steam flooding. Keyword: steam flooding; LTO; aquathermolysis; heavy oil reservoirs
1. Introduction Steam flooding is one of the major thermal recovery methods for heavy oil reservoirs, and it has good development effect. But due to steam channeling in the development process, the issues
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such as thermal efficiency decline limit the improvement of steam flooding recovery. The increasing energy demand makes it more necessary to further enhance steam flooding recovery. Additive -assisted steam flooding is a major method for improving the development effect of steam flooding. The additive mainly includes non-condensable gas1-2, solvent3-5, surfactant6-7, and so on. Air becomes more and more popular as a kind of non-condensable gas to assist steam in the development of heavy oil reservoirs due to its abundant sources and low engineering cost8-10. It is generally believed that there are three major temperature ranges where different reactions occur during the process of air injection in heavy oil reservoirs. LTO reaction generally occurs at the temperature below about 350℃, and pyrolysis reaction is mostly found in the temperature range from about 350℃ to 450℃. The high temperature oxidation reaction needs higher temperature to occur11. LTO reaction not only consumes O2 in the air, which eliminates the safety risk of explosion caused by mixture of O2 and hydrocarbon gas, but also creates gases such as CO2 to enlarge formation energy by dissolving in the heavy oil. However, the liquid products of LTO reactions have viscosities greater than that of the original oil, which may block formation pore and reduce the flowing ability of heavy oil. This can potentially harm oil recovery, especially for heavy oil containing large amounts of asphaltene12-13. The pyrolysis reactions of resins and asphaltenes in heavy oil occur under high-temperature condition. In the reactions, saturates and aromatics are generated by the broken of molecular chains, which is beneficial for the upgrading for heavy oil, as well as oil recovery. But the pyrolysis reactions need higher temperature14-15. Conventional steam stimulation and steam flooding process are often operated under 300℃, and the pyrolysis level is low. Canadian academics Hyne and Viloria first propose the aquathermolysis reaction of heavy oil, and suggest the pyrolysis reaction is weaker in the temperature range between 200℃ and 300℃, probably because the water inhibits the reaction16. But a series of chemical reactions occur between the heavy oil and steam, including acid polymerization reaction, hydrodesulfurization reaction, water gas shift reaction, and so on17. During the process of heavy oil aquathermolysis reaction, the content of heavy component resins and asphaltene decreases, and the content of light component saturates and aromatics increases. This plays a significant role in reducing the heavy oil viscosity. Besides, CO2 produced by decarboxylation reaction and water gas shift reaction in the aquathermolysis reaction is also positive for reducing the heavy oil viscosity18-19. Other studies suggest metal salts or metal ions such as nickel, copper, manganese and iron have catalysis for the aquathermolysis reaction of heavy oil. Catalyst can decrease the activation energy of heavy oil pyrolysis reactions, and then promote the pyrolysis reaction of heavy component in heavy oil at low temperature20-22. The cracking reaction and polymerization reaction compete with each other in the aquathermolysis reaction. Catalyst can promote the cracking
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reaction and inhibit the polymerization reaction so that the aquathermolysis reaction occurs, which is beneficial for the upgrading for heavy oil23. Besides, solid catalysts with extraordinary performance have been employed to modulate the oxidation behavior of heavy oil, which can promote the cracking of heavy compounds at low temperature and enhance the heat production of the residue at high temperature24-28. Especially, the metal oxide catalyst MnO2 has excellent catalytic oxidation performance at low temperature by increasing the total weight loss, promoting the low temperature cracking, increasing the transfer rate of oxygen in oil phase, and decreasing the activation energies29-33. To study the development effect of air-injection assisted steam flooding in heavy oil reservoirs, static oxidation decomposition experiments and dynamic displacement experiments are conducted in this paper. Under the high-temperature steam condition, the O2 in the air is consumed by the LTO reaction with heavy oil. Meanwhile, the reaction produces CO2, CO and heavy component. But the aquqthermolysis reaction between high-temperature water and heavy oil can break down the heavy component in heavy oil, and produce more light component. The experiments results show that under the temperature condition of high-temperature steam and catalysis of MnO2, LTO reaction and aquqthermolysis reaction synergistically reduce the heavy oil viscosity and enhance the steam flooding recovery, and then improve the development effect of steam flooding in heavy oil reservoirs. 2. Experiment 2.1 Materials The heavy oil used in the experiments is obtained from Biqian 10 districts in Henan Oilfield, which is the class II of ordinary heavy oil. The physical parameters of crude oil are shown in Table 1. The high-pressure air is specially purified, and only has two compositions of 79.1% N2 and 21.9% O2. The MnO2 catalyst used in experiments is analytically reagent. Before the experiments, MnO2 catalyst is introduced into the heavy oil to give an amount of 0.1wt%. Then the mixture of heavy oil and MnO2 is obtained after vigorously mechanical stirring. Table 1. Physical parameters of heavy oil used in experiments Reservoir
Density
Viscosity
temperature
(25℃ ℃)
(50℃ ℃)
3
, ℃
, g/cm
, mPa· ·s
35
0.9554
865.7
SARA fractions, % Saturates
Aromatics
Resins
Asphaltenes
47.52
27.07
19.34
5.74
2.2 Apparatus and procedure Experiment 1: Static oxidative decomposition experiments in the oxidation-tube. To study the characteristics of oxidative decomposition reaction between air and heavy oil
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with steam injection, several static oxidative decomposition experiments under different reaction conditions are conducted in Experiment 1. The experiment apparatuses include four parts: injection system, high-temperature and high-pressure reaction vessel, data acquisition system, gas and liquid collection system. The reaction vessel includes two parts: temperature-control and heating system and high-temperature and high-pressure oxidation-tube. The latter has a volume of 100mL and can resist the high temperature of 350℃ and the high pressure of 50MPa. Besides, the iBrid MX6 multi-gas monitor is used in the experiments to analyze gas composition after the reaction. Dehydrate the oil samples before and after the reaction, and then conduct simulated distillation and the measurement of SARA fraction by column chromatography. The HAAKE RS6000 rheometer is used to measure the viscosity of heavy oil before and after the reaction. In Experiment 1, 10 static oxidative decomposition experiments under different temperatures (No. 1, No. 2, No. 3 and No. 4), initial system pressures (No. 5, No. 3, No. 6 and No. 7) and water saturations of oil sample (No. 8, No. 3, No. 9 and No. 10) are conducted. In addition, LTO reaction (No. 11) and aquathermolysis reaction (No. 12) are conducted in the experiments. The parameters of static oxidative decomposition experiments are shown in Table 2. Table 2. Parameters of static oxidative decomposition experiments No.
Oil, mL
Air, mL
Water saturation, %
Temperature, ℃
Initial pressure, MPa
1
40
60
25
150
2.5
2
40
60
25
200
2.5
3
40
60
25
250
2.5
4
40
60
25
300
2.5
5
40
60
25
250
1.5
6
40
60
25
250
3.5
7
40
60
25
250
4.5
8
40
60
0
250
2.5
9
40
60
50
250
2.5
10
40
60
75
250
2.5
11
40
60
0
150
2.5
12
40
60(only N2)
25
250
2.5
Experiment procedures are as follows: (1) Connect the experiment apparatus according to the flow chart as shown in Figure 1, and detect the gas tightness of system. (2) Inject heavy oil and water into the oxidation-tube according to the experiment scheme, and inject air to increase system pressure to the experiment pressure. Set the vessel at the experimental temperature. (3) Stop the experiment when it lasts for 120h after the system achieving the experimental
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temperature, and cool it to room temperature. Collect the gas and liquid products after the reaction. Data acquisition system monitors the change of temperature and pressure in the oxidation-tube during the experiment. (4) Measure the composition of the collected gas and oil, as well as oil viscosity. Clean and dry the oxidation-tube, and prepare for the next experiment.
Figure 1. Flow chart of static oxidative decomposition experiments. Experiment 2: Dynamic displacement experiments in the slim-tube. To validate the effect of the oxidative decomposition reaction between air and heavy oil to enhance the oil recovery of steam flooding in heavy oil reservoirs, four kinds of dynamic displacement experiments are conducted in Experiment 2, including N2-injection displacement, air-injection displacement, steam flooding, and air-injection assisted steam flooding. The displacement effects of four experiments are contrastively analyzed. Considering the short length of conventional sand pack, a long slim-tube which is 15m in length and 6mm in internal diameter is used in Experiment 2. The slim-tube is filled with 100µm quartz sand beforehand. Besides, the gas mass flowmeter is used to control the gas injection speed. Experiment procedures are as follows: (1) Connect the experiment apparatus according to the flow chart as shown in Figure 2, and detect the gas tightness of system. (2) Set the temperature of thermostat to 35℃ and the pressure of back-pressure valve to 3.5MPa. After the system temperature is steady, inject the heavy oil into the slim-tube at the rate of 1mL/min until full saturation. (3) Inject the 250℃ steam at 2mL/min (nitrogen or air at 12mL/min) into the slim-tube to conduct the displacement. Stop the experiment after steam (or gas) breakthrough occurs. Record the oil and water production during the experiment, and collect the produced gas when steam (or gas) breakthrough occurs. Data acquisition system monitors the change of pressure in the system during the experiment. (4) Measure the composition of the collected gas. Clean and dry the slim-tube, and prepare for the next experiment.
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Figure 2. Flow chart of dynamic displacement experiments. 3. Results and discussion 3.1. Pressure-drop In the experiment, the O2 in the air reacts with heavy oil under the condition of high-temperature steam, which consumes a lot of O2 and produces gases such as CO2, making the system pressure decline gradually28. Figure 3 shows the pressure-drop of oxidative decomposition experiments under different temperatures. It can be seen that system pressure drops quickly during the initial stage of reaction. But at the later stage system pressure drops slower as O2 concentration declines. Besides, the experimental temperature is higher, and the system pressure is higher when the system temperature is steady. It is the result of heating expansion of air and oil in the oxidation-tube. The rising temperature promotes the formation of radicals, and at the same time activates some groups that difficult to activate, which promotes the oxidative decomposition and consumes more O2. Therefore, the final pressure-drop is greater. 16
1.6
150℃ ℃
200℃ ℃
250 ℃
300℃ ℃
14
1.4
12
1.2
Pressure-drop, Mpa
System pressure, MPa
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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10 8 6
1 0.8 0.6
4
0.4
2
0.2 0
0 0
20
40
60 Time, hour
80
100
120
150
200
250
300
Temperature, ℃
Figure 3. Pressure-drop of oxidative decomposition experiments under different temperatures. Figure 4 shows the pressure-drop of oxidative decomposition experiments under different
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pressures. Similar to the influence law of experimental temperature on oxidative decomposition reaction, the experiment pressure is higher when the system temperature is steady, and the final pressure-drop is greater. It shows that rising oxygen partial pressure shortens the intermolecular distance, increases the concentration of reactants and activated molecules, increases the collision rate of oxygen molecule with the active groups in heavy oil, and ultimately results in more O2 consumption and greater pressure-drop. 18 1.5MPa
2.5MPa
3.5MPa
2.1
4.5MPa
16 1.8
Pressure-drop, Mpa
System pressure, Mpa
14 12 10 8 6
1.5
1.2
0.9
0.6
4 0.3
2 0 1.5
0 0
20
40
60 Time, hour
80
100
120
2.5
3.5
4.5
Initial pressure, MPa
Figure 4. Pressure-drop of oxidative decomposition experiments under different pressures. Figure 5 shows the pressure-drop of oxidative decomposition experiments under different water saturations. It can be seen that pressure-drop in the experiment with 25% water saturation is greater than that without water, which shows that water can promote the reaction and consume more O2. As the water saturation increases, oil content in oil sample declines. The decreasing of O2 consumption results in a lower pressure-drop. 12.6 0%
25%
50%
1.2
75%
12.2
Pressure-drop, Mpa
1
System pressure, Mpa
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11.8
11.4
0.8
0.6
0.4
11
0.2
10.6
0
10.2
0%
0
20
40
60 Time, hour
80
100
120
25%
50%
75%
Water saturation, %
Figure 5. Pressure-drop of oxidative decomposition experiments under different water saturations. 3.2. Gas composition After the static oxidative decomposition reaction, gases in the oxidation-tube are mainly N2,
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O2, CO2, CO and CH4. N2 is a kind of inert gas. Several conclusions can be obtained by analyzing the measurement results of gas compositions (as shown in Table 3) after oxidative decomposition reaction. Firstly, the content of O2 decreases with the production of CO2, CO and CH4 after the reaction. The consumptions of O2 in the reaction varies with different reaction conditions. Secondly, the results of reaction under different temperatures show that reaction becomes more powerful as the temperature rises, resulting in an increase of O2 consumption, as well as CO2 and CH4 production. But the CO production increases firstly and then decreases, which might be the result of the oxidization of CO into CO2 at higher temperature. Thirdly, the results of reaction under different pressures show that reaction becomes more powerful as the pressure rises, resulting in an increase of O2 consumption, as well as CO2, CO and CH4 production. Fourthly, the results of reaction under different water saturations show that water promotes the reaction, and there is more O2 consumption, as well as CO2, CO and CH4 production in the reaction with water saturation is 25%. But the O2 consumption and gas production decrease when water saturation is too large. Finally, the LTO reaction occurs under 150℃ in No. 11, which has higher reaction degree and more O2 consumption than that under 250℃. The result of No. 12 shows that degree of aquathermolysis reaction of oil sample under 250℃ is higher, and the reaction generates more CO2, CO and CH4. Table 3. Measurement results of gas compositions after the reaction Gas compositions No.
Oxygen
O2, %
CO2, %
CO, 10-6
CH4, %
consumption, %
1
16.4
1.97
401
2
22.01
2
14.9
2.66
623
3
29.19
3
9.7
3.92
604
5
54.07
4
4.8
5.29
557
7
77.51
5
15.2
1.91
527
3
27.75
6
8.1
4.02
766
6
61.72
7
6.0
4.83
838
7
71.77
8
14.4
3.42
771
2
31.58
9
12.9
2.52
563
4
38.76
10
14.8
2.21
477
4
29.67
11
13.2
3.55
702
2
37.32
12
0
3.53
256
3
--
3.3. Oil composition Measurement of SARA fractions and simulated distillation of heavy oil before and after oxidative decomposition reaction are conducted to obtain the change characteristics of oil
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composition under different reaction conditions. Table 4 shows the measurement results of SARA fractions of oil samples. The results illustrate some points. Firstly, the results of oxidative decomposition reaction under different temperatures show that the content of saturates increases after the reaction, while that of aromatics decreases. But the content of heavy component (resins and asphaltenes) increases firstly and then decreases as the temperature rises. This is because carbon bonds of aromatics and resins break to produce longer saturates at temperature below 200℃. Meanwhile, condensation reaction produces more macromoleculer asphaltenes to increase the content of asphaltenes. When the temperature exceeds 250℃, pyrolysis reaction of heavy component occurs. As a result, the content of saturates and aromatics increases. Secondly，the results of oxidative decomposition reaction under different pressures show that heavy component decreases after the reaction. The pressure is higher, the content of light component (saturates and aromatics) is more. This is because pyrolysis reaction activity of heavy component increases as pressure increases. Finally, the results of oxidative decomposition reaction under different water saturations show that the content of heavy component increases after the reaction with water saturation 0, which is because LTO reaction of aromatics and resins plays a main role, and produces more asphaltenes. The result of No. 11 also proves this point. But the content of heavy component in heavy oil sample with water decreases after the reaction due to the active property of water at the high temperature. It forms ions to break the long chains hydrocarbon, and promotes the pyrolysis reaction of heavy component29-30. The result of No. 12 shows that aquathermolysis reaction decreases the content of heavy component significantly. Aquathermolysis reaction and LTO reaciton further decrease the content of heavy component synergestically when heavy component mixed with O2. Table 4. Measurement results of the SARA fractions of oil samples SARA fractions No. Saturates, %
Aromatics, %
Resins, %
Asphaltenes, %
1
52.39
21.76
19.27
6.27
2
53.52
20.15
19.09
6.95
3
55.25
21.44
19.15
3.94
4
56.91
21.63
18.75
2.34
5
54.77
20.31
19.25
5.35
6
55.76
21.69
19.44
2.87
7
55.93
21.92
19.77
2.11
8
53.36
20.89
19.17
6.17
9
54.52
21.16
18.76
5.42
10
54.41
20.57
18.38
6.22
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11
53.13
21.01
19.31
6.31
12
49.34
26.35
19.21
4.72
To further analyze the change characteristic of heavy oil fraction before and after the reaction, simulated distillation is conducted on the original oil sample and oil samples after No. 11, No. 12 and No. 3. The yield-temperature curves are shown in Figure 6. Without water, only LTO reaction between heavy oil and O2 occurs in No. 11. It can be found that both of the yields of both low boiling point fraction and high boiling point fraction increase after the reaction. The change of oil composition after the reaction has a polarization trend, creating more light component and heavy component. This is because aromatics and resins mainly react with O2 to create more saturates and asphaltenes. Without O2, only aquathermolysis reaction between heavy oil and 250℃ water occurs in No. 12. It can be found that after the reaction, yield of low boiling point fraction changes a little, but that of high boiling point fraction decreases significantly. This is because during the aquathermolysis reaction, heavy component in heavy oil translates into light component due to broken of carbon bonds and hydrogen ion joining31-32. Oxidative decomposition reaction occurs with the existence of water and O2 in No. 3. LTO reaction and aquathermolysis reaction decompose the heavy component and produce light component synergistically. The yield of high boiling point fraction decreases significantly after the reaction. 100 Original sample
90
No.11 80
No.12
70 Yield, %(m/m)
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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No.3
60 50 40 30 20 10 0 150
200
250
300
350
400 450 Temperature, ℃
500
550
600
650
Figure 6. Results of simulated distillation on oil samples before and after the reaction. 3.4. Oil viscosity Reducing oil viscosity is one of the main mechanisms of enhancing heavy oil recovery, so it is important to study viscosity change law of heavy oil after the oxidative decomposition reaction. Heavy oil viscosity is closely related to its chemical composition, affected by the proportion of light and heavy component. As mentioned above, change of SARA fractions of heavy oil after the oxidative decomposition reactions leads to the change of oil viscosity directly. Compared with the
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viscosity of original oil sample, oil viscosities after the reactions under different reaction conditions show several features, which are coincident with the measurement results of SARA fractions. Firstly, as shown in Figure 7, when the reaction temperature is below 200℃, oil viscosity after the reaction is higher than that of original oil sample, and increases more as the temperature rises. Because LTO reaction plays a dominant role and produces more heavy component at this time. But when the temperature is over 250℃, oil viscosity after the reaction decreases significantly. At this point, aquathermolysis reaction of heavy component plays a leading role. Secondly, as shown in Figure 8, when the reaction temperature is 250℃, oxidative decomposition reaction becomes more violent to decrease the content of heavy component as pressure increases. As a result, oil viscosity decreases more. Finally, as shown in Figure 9, oil viscosity of heavy oil sample with water saturation 0 increases significantly after the reaction. When the water saturation is 25%, violent aquathermolysis reaction decreases oil viscosity significantly. The decrease range of oil viscosity becomes small as the water saturation of heavy oil further increases. 21000
Original sample 150℃ ℃
18000
200℃ ℃ 250℃ ℃
15000
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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300℃ ℃
12000
9000
6000
3000
0 15
25
35
45
55
65
75
85
95
Temperature, ℃
Figure 7. Oil viscosity before and after the oxidative decomposition reactions under different temperatures.
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16000 Original sample
14000
1.5MPa 2.5MPa
12000
Viscosity, mPa·s
3.5MPa 4.5MPa
10000
8000
6000
4000
2000
0 15
25
35
45
55
65
75
85
95
Temperature, ℃
Figure 8. Oil viscosity before and after the oxidative decomposition reactions under different pressures. 18000 Original sample
16000
0% 25%
14000
50% 75%
12000 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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10000 8000 6000 4000 2000 0 15
25
35
45
55
65
75
85
95
Temperature, ℃
Figure 9. Oil viscosity before and after the oxidative decomposition reactions under different water saturations. 3.5. Enhance oil recovery Figure 10 shows the oil recovery and water cut curves of slim-tube experiments with different displacement methods. It can be seen that during the N2-injection displacement experiment, gas channeling occurs so early at approximately 82min because of large N2-heavy oil mobility ratio and miscible phase difficulty, and the ultimate recovery is low. During the air-injection displacement experiment, air-heavy oil mobility ratio is large, too. But the injected 250℃ air can promote LTO reaction of heavy oil, consuming O2 in the air. Meanwhile, miscible phase occurs between the produced CO2 and N2 in the air, which slows down gas channeling.
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Finally, gas channeling occurs at 94min, and its oil recovery is higher than that of N2-injection displacement. Detect the gas composition from the outlet when gas channeling occurs, and the results is that the content of O2, CO2, CO, CH4 is 8.5%, 3.03%, 0.0542%, and 2% respectively. O2 in the front of air-injection displacement is consumed through contacting with heavy oil. During the steam flooding experiment, oil viscosity decreases significantly due to the heat transfer of high-temperature steam. Aquathermolysis reaction between heavy oil and high-temperature water further decreases oil viscosity. Steam channeling occurs at approximately 171min, later than the gas channeling time in air-injection displacement experiment, and the effect of steam flooding is better than that of air-injection displacement. During the air-injection assisted steam flooding experiment, under the condition of high-temperature steam, oxidative decomposition reaction between air and heavy oil decreases oil viscosity significantly synergistically with the heating effect of steam, and slows down the steam channeling. Ultimate recovery of air-injection assisted steam flooding is 27.94%, 24.81% and 8.13% higher than N2-injection displacement, air-injection displacement and steam flooding respectively. Volume effect of the air increases the production rate of steam flooding effectively. Besides, when steam channeling occurs, detection result of gas composition from the outlet shows that the content of O2 is 0, which results from the flue gas as the leading edge reaches the outlet. It shows that a violent oxidative decomposition reaction occurs between injected air and heavy oil under the temperature and pressure condition in the slim-tube, consuming O2 in the air. Gas product increases energy in the slim-tube and decreases
100
100
80
80
60
60
40
40
Water Cut, %
oil viscosity by dissolving in the heavy oil, which enhances the oil recovery.
Oil Recovery, %
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N₂-OR Air-OR Steam-OR 20
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Figure 10. Oil recovery and water cut curves of slim-tube experiments with different displacement methods. During the process of steam flooding in heavy oil reservoirs, air-injection not only has the effect of usual gas injection, but also improves the effect of steam flooding through oxidative
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decomposition reaction with heavy oil. LTO reaction can consume O2 in the air, which eliminates the safety risk of explosion caused by the mixture of O2 and hydrocarbon gas at the production well. Meanwhile, CO2 produced by the reaction has the displacement effect of flue gas when mixed with N239. Besides, the heat generated by LTO reaction can directly heat oil layer, and the oxygenated compounds generated by LTO can effectively decrease oil-water interfacial tension, which enhances oil displacement efficiency of steam. However, resin translates into asphaltene during the reaction, which potentially harms the flowing ability of heavy oil. With the high-temperature steam injection, aquathermolysis reaction occurs between heavy oil and steam, which decreases the heavy component in heavy oil and oil viscosity significantly, and then decreases the mobility ratio of heavy oil and steam. Meanwhile, volume effect of gas also enlarges the swept volume of steam, which enhances sweep efficiency of steam. Technology of air-injection assisted steam flooding in heavy oil reservoirs composites the reaction mechanism of LTO and aquathermolysis, and improves the development effect of steam flooding significantly. 4. Conclusions In this paper, upgrading and viscosity reduction effect of low-temperature oxidative decomposition reaction for heavy oil is studied through static oxidative decomposition experiments in the oxidation-tube. Besides, enhancing oil recovery effect of air-injection assisted steam flooding for heavy oil is studied through dynamic displacement experiments in the slim-tube. The main conclusions from this study are as follows: (1) When the air is injected into the heavy oil reservoirs, O2 in the air can be consumed by LTO reaction. But much heavy component is produced in the reaction, which potentially harms oil recovery, especially for heavy oil containing large amounts of asphaltenes. (2) Aquathermolysis reaction occurs between heavy component in heavy oil and high-temperature steam. The added catalyst further decreases activation energy of the reaction, which has an effect of upgrading and reducing viscosity of heavy oil. (3) As the temperature and pressure increase, oxidative decomposition reaction becomes more violent, which consumes more O2 and produces more gas product. LTO plays a dominant role in the reaction of heavy oil with water saturation 0. High-temperature water promotes the pyrolysis reaction of heavy component in heavy oil significantly. (4) Air-injection improves the oil recovery of steam flooding in heavy oil reservoirs through oxidative decomposition reaction. It increases the production rate of steam flooding and slows down the steam channeling. Author information Corresponding Author
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*E-mail:
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