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Low Frequency Vibration Assisted Catalytic Aquathermolysis of Heavy Crude Oil Hongxing Xu, Chunsheng Pu, and Feipeng Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef301057t • Publication Date (Web): 29 Aug 2012 Downloaded from http://pubs.acs.org on September 8, 2012
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Low Frequency Vibration Assisted Catalytic Aquathermolysis of Heavy Crude Oil Hongxing Xu*, Chunsheng Pu, Feipeng Wu School of Petroleum Engineering, China University of Petroleum, Qingdao 266555, People’s Republic of China
ABSTRACT: Low frequency vibration was applied to assist the catalytic aquathermolysis reaction of heavy oil for the first time. The optimum vibration parameters were firstly optimized by orthogonal experiments: vibration acceleration is 3m.s-2, vibration time is 90min, and vibration frequency is 20Hz, and the efficient consequences of the parameters are as follows: vibration acceleration > vibration time > vibration frequency. Under the optimum vibration parameters, heavy oil viscosity could be reduced by 88.2% after reaction, and the viscosity bounce rate of treated oil is 4.9%. To evaluate the vibration’s performance, the structure and group compositions of the oil before and after reaction were characterized by modern chemical analysis techniques, such as column chromatography, elemental analysis, gas chromatography and Fourier transform infrared spectrometer. It is found that vibration cannot initiate new reactions in the process of catalytic aquathermolysis, but it can promote the original reactions and deepen the reaction degree such as dealcoholization reaction, hydrogenation reaction, ring opening reaction and alkyl side chain removal reaction et al. Compared to catalytic aquathermolysis reaction, vibration assisted catalytic aquathermolysis can further decrease the average molecular weight of heavy oil, increase the saturate and aromatic contents, decrease the resin and asphaltene contents, improve the ratio of NH/NC and decrease the heteroatoms content of heavy oil. Vibration plays more important role in in-situ catalytic aquathermolysis reactions due to the fact that vibration could aid to ACS Paragon Plus Environment
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reduce the adsorption of catalyst and help the catalysts contact with heavy oil sufficiently in the porous media. The preliminary results proved that vibration assisted in-situ catalytic aquathermolysis technique is feasible and it has some practical value.
1. INTRODUCTION With the shortage of conventional hydrocarbon resources and the increase of world energy demands, heavy oil, a kind of unconventional hydrocarbon resource which accounts for a large proportion of world proved oil reserves, has attracted worldwide attention1. Notable features of heavy oil are its high viscosity and low quality, due to which heavy oil exploitation is usually difficult and expensive2. Thus, numerous techniques have been developed and adopted to enhance heavy oil recovery, such as thermal recovery, chemical flooding, microbial recovery, etc., in which steam stimulation is the most popular and effective one in use3−6. In the processes of steam injection to recover heavy oils, it is generally believed that the main function of the injection steam is to reduce the viscosity by changing the viscosity-temperature characteristics of the heavy oils.7 However, many research results show that the injection steam does not only reduce the viscosity of heavy oil, but also reacts with some components of the heavy oil and of the reservoir minerals, thereby leading to changes in the heavy oils’ properties and compositions.
8–10
Hyne et al.
firstly studied the details of the chemical reactions between steam, heavy oil and minerals, and described all of these reactions as “aquathermolysis”.11 They found that, after aquathermolysis, the amounts of saturates and aromatics increased, while the amounts of resin and asphaltene decreased. The results favor in situ upgrading of heavy oils. Nevertheless, further research results show that these changes can regress rapidly. This is because the heteroatoms S, N, and O in heavy oil molecules can interact with other groups by hydrogen bonding or van der Waals forces, and polymerize to form larger molecules with strong intermolecular forces, thus leading to the rapid viscosity regression12. The termination of this regression is a key factor in reducing
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the viscosity of heavy oil. These results also show that aquathermolysis has little effect on extra-heavy oil only in the presence of the injection steam7. Hence, a number of scientists including Hyne conducted a lot of researches in various catalysts that should be added to catalyze the aquathermolysis13-23. Upon catalytic aquathermolysis, more heavy components of the heavy oil pyrolyzed and the structure changed to a larger degree, especially some heteroatom containing structures, and these changes could hardly regress and eventually lead to the reduction of the viscosity of heavy oil, thereby benefiting the oil exploitation, transportation and upgrading12. All the reactions between heavy oil, steam and catalysts were described as “catalytic aquathermolysis”. Catalytic aquathermolysis, which provided a new thought for exploitation of heavy oil, did reveal noteworthy effect in laboratory static experiments, the viscosity reduction rate of heavy oil could reach to 80-90%, or even higher7,23, however, this technique still has insufficiencies and is far from satisfactory in field tests. The major difficulties encountered in operating in situ catalytic aquathermolysis processes are as following: First, the reservoir temperature after steam injection gradually lowers with the increasing depth of the oil floor, thus, leading to insufficient energy supplied for catalysis. Second, the “processing radius” of catalytic aquathermolysis is limited due to the adsorption and retention of catalysts in the near wellbore formation. Third, the catalysts can not contact with heavy oil molecules sufficiently in the reservoir, which decreases their catalytic efficiency24. The first insufficiency can be remedied by increasing the downhole steam dryness fraction or synthesizing a catalyst which can maximize its catalysis at lower temperature, but these details are beyond the scope of this paper. To overcome the other insufficiencies, low frequency vibration is applied to assist catalytic aquathermolysis of heavy oil. Low frequency vibration stimulation, which is one of the most important oil production technologies with physical method, goes back to the 1950s. Since then, numerous literatures have reported successful vibration stimulation both in laboratory investigations and field tests25-29. As demonstrated by previous researches, the vibration force introduced in the reservoir is thought to effect the fluid flow characteristics in one or more ways: by diminishing ACS Paragon Plus Environment
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capillary forces; reducing adhesion between the rock and fluids; or causing oil droplets to cluster into “streams” that flow with the waterflood; altering rock relative permeability; reducing oil viscosity, etc.. The combination of low frequency vibration treatment and in situ catalytic aquathermolysis is a new attempt to enhance heavy oil recovery, and it has never been reported in literatures both at home and abroad. With the assist of vibration stimulation, catalytic aquathermolysis may be facilitated by taking advantage of vibrational energy for the chemical reactions and reducing the near wellbore consumption of catalyst due to the mechanisms mentioned above, and thus increasing the “processing radius” of in situ catalytic aquathermolysis. In addition, different from ultrasonic stimulation and acoustic stimulation, low frequency vibration can propagate thousands of meters, thus it makes possible to magnify the processing scope of this composite technology to several well groups, not single or several wells. The most important is that vibration may help the catalysts contact with heavy oil sufficiently in the porous media, which will improve the utilization rate of catalysts and increase the catalytic efficiency. Composite technology is a potential research area for enhanced heavy oil recovery. In our previous publication30, ultrasonic was used to assist catalytic aquathermolysis of heavy oil, and the experimental results were very encouraging. Since the experimental device, mode and mechanism of action of low frequency vibration are entirely different with that of ultrasonic, in the present paper, experimental investigations are presented on low frequency vibration assisted catalytic aquathermolysis of heavy oil whose viscosity is about 1390Pa·s. Firstly, static experiments are carried out to evaluate the effects of vibration parameters on the viscosity reduction rate of heavy oil, and the optimum vibration parameters were optimized. Then, the changes in the contents, structure and group compositions of the heavy oil before and after reaction are analyzed. In addition, dynamic tests of vibration assisted in-situ catalytic aquathermolysis are conducted to investigate the feasibility and effect of this composite technique. 2. EXPERIMENTAL SECTION 2.1. Material. Heavy oil and water used in the experiments are collected from the Shengli oilfield (Oil properties and water analysis results are shown in Table S1 and Table S2 in Supporting Information, respectively). Analytical reagents, such as heptane, petroleum ether, toluene, ethanol and xylene are 4 ACS Paragon Plus Environment
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purchased from Qingdao Century Star Chemical Reagent Co., Ltd. Silica gel and alumina used in chromatographic separation are obtained from Shanghai 54th Chemical Reagent Co., Ltd.. Artificial cuboid cores with size of 45mm×45mm×300mm and gas permeability of 1500×10-3µm2 from Northeast Petroleum University are used in flooding experiments. Catalyst used in the experiments was prepared as follows: 55g of dodecyl benzene sulfonic acid was put into an agitated reactor and stirred for 10 min, and then raised the temperature to 120℃ for 20min. Meanwhile, 6.6g of copper (II) chloride dihydrate was dissolved by 80g of water in a 250mL threenecked flask, after that 9.5g of sodium hydroxide solution with sodium hydroxide concentration of 30% was added into the flask. After they interacted with each other under stirring for 30min at room temperature, the mixture separated into two layers after standing for 30min and the upper solution layer was removed. Next, the bottom solution layer was added into the agitated reactor and stirred for 10 min, and then the temperature was raised to 120℃ for 1h. After the reaction, the products were cooled to room temperature and intermediate products were obtained in the form of liquid. 2.2. Apparatus. Schematic of the static experimental setup for vibration assisted catalytic aquathermolysis of heavy oil is shown in Figure 1 (Picture is shown in Figure S1 in Supporting Information). The reactor with a heating jacket is fixed on the vibrostand by screws, thus it will be able to vibrate with the vibrostand. Vibration parameters such as vibration acceleration, frequency and vibration time are set by the computer controlling system. In this setup, computer, vibration signal controller, vibration power amplifier, vibrostand and vibration acceleration transducer compose a cycle control system to make sure the vibrostand vibrates as designed.
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Figure 1. Schematic of the static experimental setup for vibration assisted catalytic aquathermolysis of heavy oil
Figure 2. Schematic of the dynamic experimental setup for vibration assisted in situ catalytic aquathermolysis of heavy oil Schematic of the dynamic experimental setup for vibration assisted in situ catalytic aquathermolysis of heavy oil is shown in Figure 2. The dynamic flow apparatus includes a Teledyne ISCO pump (Teledyne ISCO 100DX, USA), core holder, confining pressure pump, steam generator, high pressure vessels and oven etc. The ISCO pump is used to inject the fluid across the core. The core holder is specially designed, which can hold core with the size of 45mm×45mm×300mm. The steam generator is used to create steam by applying heat energy to fluid in the pipeline. The catalyst solution is injected into ACS Paragon Plus Environment
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steamline by a pump through a non-return valve. The core holder and high pressure vessels are placed in the oven to maintain a particular temperature when necessary. To analyze the changes in contents, SARA and structures of the oil before and after reaction, DV-III Ultra-Brookfield rheometer (Brookfield, USA), Elementar Vario EL III elemental analyzer (Elementar, Germany), Knauer K-7000 Vapor permeability tester (Knauer, Germany), Agilent 7890 gas chromatograph (Agilent, USA) and EQUINOX 55 Fourier transform infrared spectrometer (BRUKER OPTICS, USA) are used in the experiments. 3. EXPERIMENTAL PROCEDURE 3.1. Optimization of Vibration Parameters. Based on the catalytic aquathermolysis conditions optimized in our previous works, orthogonal experiments were designed to optimize the vibration parameters that having the best effect of reducing heavy oil viscosity. Factors and levels of orthogonal experiments are shown in Table 1, and the experimental program is shown in Table 2. For every experiment, 100g of heavy oil sample, 43g of water (oil/water=7:3) and 0.3g of catalyst were added into the high pressure reactor. The original pressure was kept at 3MPa by aerating with N2. The reaction went on for 24h at the temperature of 220℃. Meanwhile, the vibrostand was started and the vibration parameters were set as designed. After reaction, heavy oil was taken out and put aside for 24h to separate the reaction layer of the oil phase and water phase completely. Then, repeat the above steps after having changed the vibration parameters. Finally, the optimum vibration parameters were optimized by comparing the viscosity reduction rate of heavy oil after reaction. For comparative analysis, contrast experiment was carried out using similar procedures, except that the vibrostand was no longer needed, to compare with the experiment conducted under the optimum vibration parameters. In addition, viscosity bounce rate of heavy oil after catalytic aquathermolysis reaction with and without low frequency vibration was studied by measuring the viscosity of treated oil with time. The viscosity bounce rate ∆ηt% was calculated as follows: ∆ηt%=((ηt-η)/η)×100, where η
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(Pa·s) is the viscosity of oil immediately after reaction, and ηt (Pa·s) is the viscosity of treated oil with time. Table 1. Factors and Levels of Orthogonal Experiments A item
B
C
vibration frequency(Hz) vibration acceleration(m.s-2) vibration time(min)
1
10
1
30
2
20
2
60
3
30
3
90
Table 2. Orthogonal Experiments Program and Results item
A
B
C
viscosity reduction(%)
1
10
1
30
83.5
2
10
2
60
84.9
3
10
3
90
86.4
4
20
1
60
85.3
5
20
2
90
87.2
6
20
3
30
86.7
7
30
1
90
85.9
8
30
2
30
84.8
9
30
3
60
87.5
Viscosity accuracy of DV-III Ultra-Brookfield rheometer is ±1.0% for a specific spindle running at a specific speed. Through calculation, experimental error of viscosity reduction rate is less than 0.2%. 3.2. Analysis of Heavy Oil Properties before and after Reaction. Heavy oil samples before and after reaction under optimum vibration parameters are analyzed as follows: The viscosity of heavy oil was recorded by DV-III Ultra-Brookfield rheometers at 50◦C according to Industrial Specification of China Petroleum SY/T 6316. The viscosity reduction rate ∆η% was calculated as follows: ∆η%=((η0η)/η0)×100, where η0 (Pa·s) is the viscosity of the oil before reaction, and η (Pa·s) is the viscosity of the oil after reaction. Average relative molecular weight of heavy oil is measured by VPO method using Knauer K-700 vapor permeability tester. Four group compositions were separated from the heavy oil
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samples by column chromatography following Industrial Specification of China Petroleum SY/T 5119. Elemental analysis of the heavy oil was performed on Elementar Vario EL III elemental analyzer, in which the oxygen content was calculated by difference method. The GC analyses of heavy oil and saturated hydrocarbon fraction were carried out on an Agilent 7890 gas chromatograph. Changes in the structures of heavy oil, resins and asphaltenes were analyzed by FT-IR on an EQUINOX 55 Fourier transform infrared spectrometer. 3.3. Dynamic Test of Vibration Assisted In situ Catalytic Aquathermolysis of Heavy Oil. A cubic core sample was firstly evacuated and saturated with brine. Then, the core was placed inside the core holder at a confining pressure of 1000psi, and the initial liquid permeability was determined by flowing brine across the core at a constant flow rate of 1mL/min. Next, heavy oil was injected into the core to establish irreducible water saturation at 120℃, and the initial oil saturation was calculated. During the in situ catalytic aquathermolysis test, displacing fluid was translated into steam by the steam generator and injected into the core, whose outlet valve was in closed status, to simulate the process of steam huffpuff. Meanwhile, 0.5PV of catalyst solution was injected into the steamline through a non-return valve by a micro-pump. Then, shut down the import valve of the core and let the catalyst react with heavy oil in the core for 24 hours at 220℃. After that, open the import valve and record the oil production during the process, and the viscosity of the produced oil was measured at 50℃. In the test of vibration assisted in situ catalytic aquathermolysis, the core with a heating jacket was taken out of the oven and fixed on the vibrostand after having injected steam and catalyst. Then, it was treated by vibration under the optimum vibration parameters optimized by static experiments. After the treatment, the core was replaced into the oven. The remaining steps were identical to that used in the in situ catalytic aquathermolysis experiment. 4. RESULTS AND DISCUSSION 4.1. Optimization of vibration parameters. Results of the orthogonal experiments are shown in Table 2. From the experimental results, we have found that the efficient consequences of the factors are ACS Paragon Plus Environment
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as follows: vibration acceleration > vibration time > vibration frequency. The optimum vibration parameters are as follows: the vibration acceleration is 3m.s-2, the vibration time is 90min, and the vibration frequency is 20Hz. Experimental results of contrast experiments conducted without and with vibration are listed in Table 3. As can be seen from Table 3, the viscosity of heavy oil could be reduced by 88.2% under the optimum vibration parameters, while it is 81.6% without vibration, indicating the existence of synergistic effect between vibration and catalyst. The composite technology may help to pyrolyze the heavy content such as resin and asphaltene into light content such as saturates and aromatic, thus reducing heavy oil viscosity and improving heavy oil quality. In addition, average relative molecular weights of heavy oil after reactions without and with vibration were measured, respectively. The results show that molecular weight of heavy oil has decreased from 548 to 406, which have further confirmed the conversion of heavy content into light content. Table 3. Results of Contrast Experiments Conducted without and with Vibration
heavy oil samples
viscosity before viscosity after reaction viscosity reduction M reaction (mPa·s) (mPa·s) ratio (%)
after catalytic aquathermolysis
255760
after vibration assisted catalytic 1390000 aquathermolysis
164020
81.6
548
88.2
406
Viscosity bounce rates of heavy oil after reaction are illustrated in Figure 3. The figure demonstrates that viscosity bounce rates increase with time slightly, indicating the increase in viscosities of treated oil. Viscosity bounce rates with and without vibration tend to be stable 15 days later, and reach to 4.9% and 5.2%, respectively, proving the viscosity bounce rates are small. The results reveal that catalytic aquathermolysis with and without vibration all can reduce the viscosity of heavy oil irreversibly and the effect of reaction with vibration is slightly better than that without vibration.
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5 Viscosity bounce rate(%)
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3
2
viscosity bounce rate after vibration assisted catalytic aquathermolysis"
1
viscosity bounce rate after catalytic aquathermolysis
0 0
1
2
3
4
5
6
7 8 Time(d)
9
10 11
12 13
14 15
Figure 3. Viscosity bounce rates of heavy oil after catalytic aquathermolysis reaction with and without vibration 4.2. Group Compositions of Heavy Oil before and after Reaction. Table 4 lists the group compositions of heavy oil before and after reaction, which were determined by column chromatography. Table 4. Group Compositions of Heavy Oil before and after Reaction content (%) heavy oil samples saturates
aromatic
resin
asphaltene
15.2
26.6
28.4
29.8
24.9
34.3
21.3
19.5
after vibration assisted catalytic aquathermolysis reaction 25.7
36.7
19.7
17.9
before reaction after catalytic reaction
aquathermolysis
Relative standard deviation of column chromatography is less than 0.5%. It is observed from Table 4 that the content of saturates and aromatic increased by 9.7% and 7.7%, while the content of resin and asphaltene decreased by 7.1% and 10.3% after catalytic aquathermolysis reaction, respectively; In contrast, the content of saturates and aromatic increased by 10.5% and 10.1%, while the content of resin and asphaltene decreased by 8.7% and 11.9% after vibration assisted catalytic aquathermolysis, respectively. The experimental results indicate that vibration could promote the ACS Paragon Plus Environment
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catalytic aquathermolysis reaction of heavy oil and catalyze the heavy content (asphaltene and resin) to pyrolyze to the light content (saturate and aromatic). 4.3. Elements Analysis of Heavy Oil before and after Reaction. The elements analysis results of heavy oil before and after reaction are listed in Table 5. It can be seen that the content of O, N and S has decreased from 2.76, 1.96 and 3.28 to 2.06, 1.75 and 2.31, while the content of C and H has increased from 80.23 and 11.77 to 81.24 and 12.64 after catalytic aquathermolysis reaction, respectively. By comparison, the content of O, N and S has decreased to 1.89, 1.71 and 1.93, while the content of C and H has increased to 81.58 and 12.89 after vibration assisted catalytic aquathermolysis reaction. After calculation, the ratio of NH/NC of heavy oil has increased from 1.76 to 1.87 and 1.90 after catalytic aquathermolysis reaction without and with vibration, respectively. These changes demonstrate that vibration can promote the catalytic aquathermolysis reaction and enhance the reaction degree. Vibration is likely to assist the catalyst to act on the groups that contained O, N and S, and cleavage these heteroatoms from the oil molecules. It is not difficult to find out that content change of S is more evident than content changes of O and N after reaction. It can be explained that the bond energy of C-S is relatively low11. In contrast, the content change of N is much smaller due to relative stability of the C−N bond to thermolysis. Table 5. Elemental Analysis of Heavy Oil before and after Reaction content (%) NH/NC
heavy oil samples C
H
O
N
S
before reaction
80.23
11.77
2.76
1.96
3.28
1.76
after catalytic aquathermolysis reaction
81.24
12.64
2.16
1.75
2.21
1.87
after vibration assisted aquathermolysis reaction
81.58
12.89
1.89
1.71
1.93
1.90
catalytic
Standard error of element content is less than 0.1% according to the specifications of Elementar Vario EL III elemental analyzer. 4.4. GC Analysis of Heavy Oil and Saturated Hydrocarbon before and after Reaction. Figure 4 shows the GC spectrums of heavy oil before and after reaction. It is easy to see that, after catalytic ACS Paragon Plus Environment
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aquathermolysis reaction both with and without vibration, the GC spectrums reveal the appearance of new strong peaks due to the light hydrocarbons of the heavy oil. Compared with the spectrums of single catalytic aquathermolysis reaction, it is found that additional peaks appeared and the intensity of some original peaks obviously increased after catalytic aquathermolysis with vibration. By means of carbon number distribution analysis, the amount of light content with the carbon number less than C10 increased from 0.43% to 34.74%, while the hydrocarbon compounds over C25 decreased from 27.13% to 9.21% after catalytic aquathermolysis reaction. In contrast, the compounds under C10 increased to 37.5% and the compounds over C25 decreased to 7.59% after the reaction with vibration.
Figure 4. GC spectrum of heavy oil (a) before reaction, (b) after catalytic aquathermolysis reaction, and (c) after vibration assisted catalytic aquathermolysis reaction For a clearer illustration of GC analysis results, the GC spectrums of saturated hydrocarbon before and after reaction are obtained and shown in Figure 5. From this figure, it can be seen that additional peaks of C13-C18 and C21-C25 appeared and the peak intensity in the C13-C25 spectral region obviously increased after catalytic aquathermolysis reaction. In addition, the amount and intensity of the peaks less than C25 further increased after vibration assisted catalytic aquathermolysis. Experimental results indicated that vibration could assist to pyrolyze the large resin and asphaltene molecules into smaller ones such as saturates and aromatic hydrocarbons. Meanwhile, some parts of long-chain saturated hydrocarbons were also broken into short-chain ones with the assist of vibration. ACS Paragon Plus Environment
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These saturated hydrocarbons could act as some kind of solvent for the heavy oil. The above changes reduced the viscosity and thus improved the quality of the heavy oil.
Figure 5. GC spectrum of saturated hydrocarbon (a) before reaction, (b) after catalytic aquathermolysis reaction, and (c) after vibration assisted catalytic aquathermolysis reaction 4.5. FT-IR Spectra of Heavy Oil before and after Reaction. Figure 6 exhibits the FT-IR spectra of heavy oil before and after reaction. As seen from the figure, after catalytic aquathermolysis reaction, the absorption peak at 3380cm-1 weakened, which could be attributed to the occurrence of a series of alcohols reactions; The absorption peak at 1735cm-1 became weaker, indicating the breakage of C=O bonds and the occurrence of hydrogenation reaction; The bands at 1512cm-1 weakened, indicating a part of dealkylation existed; The absorption peak at 1024 cm-1 weakened and the absorption peaks at 1161cm-1 and 1265 cm-1 nearly disappeared, indicating the breakage of C–N, C–O and C–S bonds. The absorption peaks at 868 cm−1 became weaker, indicating the existence of ring opening reactions. The absorption peak at 796cm-1 weakened, indicating the occurrence of alkyl side chain removal reactions. The absorption peak at 1372 cm−1 enhanced, indicating the aggregation of same condensed aromatic rings. In addition to the changes of the absorption peaks that occurred in catalytic aquathermolysis reaction, same new changes were found after aquathermolysis reaction with vibration. The absorption peaks at ACS Paragon Plus Environment
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3380cm-1, 1735cm-1, 1372 cm−1, 868 cm−1 and 796cm-1 ulteriorly weakened, indicating that vibration probably deepened the reaction degree and promoted a series of reactions, such as dealcoholization reaction, hydrogenation reaction, ring opening reaction and alkyl side chain removal reaction. Particularly, the diminution of the absorption peak at 1372 cm−1 verified that vibration could probably inhibit the polymerization reaction.
Figure 6. FT-IR spectra of heavy oil (a) before reaction, (b) after catalytic aquathermolysis reaction, and (c) after vibration assisted catalytic aquathermolysis reaction In order to investigate the structures of heavy component in heavy oil before and after reaction, the FT-IR spectra of resin and asphaltene were studied. Figure 7 shows the FT-IR spectra of resin before and after reaction. Just as the structure changes in heavy oil, after catalytic aquathermolysis reaction, the bands at 3380, 1721, 1519, 1356, 1021, 878 and 815 cm-1 became weaker, and those bands further debilitated after vibration assisted catalytic aquathermolysis reaction. Figure 8 exhibits the FT-IR spectra of asphaltene before and after reaction. As shown in the figure, the structure changes of asphaltene are similar to that of resin, and vibration plays a role in promoting the catalytic aquathermolysis reaction, but the structure changes of resin are unapparent comparing with the asphaltene’s.
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Figure 7. FT-IR spectra of resin (a) before reaction, (b) after catalytic aquathermolysis reaction, and (c) after vibration assisted catalytic aquathermolysis reaction
Figure 8. FT-IR spectra of asphaltene (a) before reaction, (b) after catalytic aquathermolysis reaction, and (c) after vibration assisted catalytic aquathermolysis reaction It can be concluded that resin and asphaltene consist primarily of condensed aromatic rings, alkyl side chains, and heteroatoms, etc. All these changes indicated the probable pyrolysis of C-O, C-N, and C-S. A combination of this result with the result of the element analysis suggests that the pyrolysis of C-S and C-O were predominant during the reaction. ACS Paragon Plus Environment
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4.6. Dynamic Test of Vibration Assisted In situ Catalytic Aquathermolysis. In order to investigate the feasibility and effect of vibration assisted in situ catalytic aquathermolysis, dynamic tests both with and without vibration were carried out using self-developed apparatus shown in Fig.2, and the experimental results are listed in Table 6. As is shown in Table 6, after vibration assisted in-situ catalytic aquathermolysis, heavy oil recovery and viscosity reduction rate reached to 55.48% and 83.6%, which are 6.7% and 12.3% higher than that of catalytic aquathermolysis experiment, respectively, indicating not only can vibration promote catalytic aquathermolysis reaction in static experiments, but it can facilitate the reaction under dynamic conditions. The results preliminarily proved that vibration assisted in-situ catalytic aquathermolysis technique is feasible and it has some practical value. Table 6. Heavy Oil Recovery and Viscosity Reduction of Dynamic Tests
core number
item
heavy gas permeability porosity recovery (10-3µm2) (%) (%)
oil
1
catalytic aquathermolysis
1542
33.5
48.78
71.3
2
vibration assisted catalytic 1525 aquathermolysis
32.9
55.48
83.6
viscosity reduction ratio (%)
However, compared to the static experimental results listed in Table 2, heavy oil viscosity reduction rate decreased by 10.3% and 4.6% in dynamic experiments without and with vibration, respectively. This can be probably explained that the reaction conditions of dynamic tests were more complicated, and part of the catalyst will be unavoidably adsorbed by the rock and consumed near the entrance of the core, and most important, the catalyst cannot contact with the heavy oil adequately in the porous media. But obviously, with the assisted of vibration, the decrease of viscosity reduction ratio in dynamic experiment is 5.7% smaller than that without vibration, indicating that vibration plays more important role in in-situ catalytic aquathermolysis reaction. This might be due to the fact that vibration could aid to reduce the adsorption of catalyst in the porous media by decreasing adhesion between the rock and fluids and improving the liquid flow. On the other hand, vibration may help the catalysts contact with 17 ACS Paragon Plus Environment
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heavy oil sufficiently in the porous media, which will improve the utilization rate of catalysts and increase the catalytic efficiency. As illustrated by the static experimental results, vibration cannot initiate new reactions in the process of catalytic aquathermolysis, but it can promote the original reactions and deepen the reaction degree such as dealcoholization reaction, hydrogenation reaction, ring opening reaction and alkyl side chain removal reaction et al. This may be attributed to synergistic effect of catalyst and vibration. Vibration can provide vibrational energy to assist the chemical reactions. During the treatment of vibration, the whole reaction system is in an acceleration changing state, and such a frequent change of acceleration may lead to weak the intermolecular forces, and finally help to free the small molecules enwrapped in the heavy content of heavy oil. Moreover, under the action of catalyst, some changes in the electron density of molecular covalent bond will occur in heavy oil, which can lead to weak the van der Waals force and the hydrogen bonds between molecules, and thus it becomes easier to break the low energy bonds with the assistance of vibration. In brief, the synergistic effect of catalyst and vibration can weaken the stability of macromolecular organization, undermines the stability of certain C-C bond, as well as the C-R (R stands for the heteroatoms such as O, N, S in heavy oil) bonds to possibly produce small moleculars (such as saturated and aromatic hydrocarbon, furan, thiophene, pyridine et al.) and gas (such as sulferetedhydrogens, alkenes, hydroxybenzenes, alcohols, ethers, alkyls et al.). The total actions of above enventally reduce the viscosity of the heavy oil. Study results indicated that low frequency vibration assisted catalytic aquathermolysis of heavy oil is technical feasible and it worth trying to enhance heavy oil recovery. In field tests, this technology is lessintensive to reservoir temperature and pressure, however, it needs the downhole dryness fraction of steam greater than 20% to ensure the reservoir temperature is high enough for the reaction, and it requires the reservoir pressure is not too high too obstruct the injection of steam. 5. CONCLUSIONS Vibration was applied to assist the catalytic aquathermolysis reaction of heavy oil. The optimum vibration parameters were selected: vibration acceleration is 3m.s-2, vibration time is 90min, and 18 ACS Paragon Plus Environment
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vibration frequency is 20Hz, and the efficient consequences of the parameters are as follows: vibration acceleration > vibration time > vibration frequency. Experimental results show that heavy oil viscosity could be reduced by 88.2% after reaction under the optimum vibration parameters, and the viscosity bounce rate of treated oil is 4.9%. Vibration cannot initiate new reactions in the process of catalytic aquathermolysis as illustrated by FT-IR spectra, but it could promote the original reactions and deepen the reaction degree such as dealcoholization reaction, hydrogenation reaction, ring opening reaction and alkyl side chain removal reaction et al. Compared to catalytic aquathermolysis reaction, vibration assisted catalytic aquathermolysis can further decrease the average molecular weight of heavy oil, increase the saturate and aromatic contents, decrease the resin and asphaltene contents, improve the ratio of NH/NC and decrease the heteroatoms content of heavy oil. Vibration plays more important role in insitu catalytic aquathermolysis reactions due to the fact that vibration could aid to reduce the adsorption of catalyst and help the catalysts contact with heavy oil sufficiently in the porous media. The results preliminarily proved that vibration assisted in-situ catalytic aquathermolysis technique is feasible and it has some practical value. AUTHOR INFORMATION Corresponding Author *Telephone: (86) 532-86981736. Fax: (86)532-86981936. E-mail:
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors express their appreciation to the National Science and Technology Major Project (No.2009ZX05009) and the project supported by Shandong Provincial Natural Science Foundation of China (No.ZR2010EM014) for the financial support of this work. ASSOCIATED CONTENT ACS Paragon Plus Environment
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Supporting Information Table S1, Table S2 and Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org/. REFERENCES (1) Shah, A.; Fishwick, R.; Wood, J.; Leeke, G.; Rigby, S.; Greaves, M. A review of novel techniques for heavy oil and bitumen extraction and upgrading. Energy Environ. Sci. 2010, 3, 700−714. (2) Chao, K.; Chen, Y. L.; Liu, H. C.; Zhang, X. M.; Li, J. Laboratory experiments and field test of a difunctional catalyst for catalytic aquathermolysis of heavy oil. Energy Fuels 2012, 26, 1152−1159. (3) Weissman, J. G. Review of processes for downhole catalytic upgrading of heavy crude oil. Fuel Process. Technol. 1997, 50, 199−213. (4) Bagci, S.; Kok, M. V. In−situ combustion laboratory studies of Turkish heavy oil reservoirs. Fuel Process. Technol. 2001, 74, 65−79. (5) Xia, T. X.; Greaves, M. Upgrading Athabasca tar sand using toe−to−heel air injection. J. Can. Pet. Technol. 2002, 41, 51−57. (6) Pineda−Perez, L. A.; Carbognani, L.; Spencer, R. J.; Maini, B.; Pereira−Almao, P. Hydrocarbon depletion of athabasca core at Near Steam−Assisted Gravity Drainage (SAGD) Conditions. Energy Fuels 2010, 24, 5947−5954. (7) Liu, Y. J.; Fan, H. F. The effect of hydrogen donor additive on the viscosity of heavy oil during steam stimulation. Energy Fuels 2002, 16, 842-846. (8)Clark P.D.; Hyne J.B. Steam–oil chemical reactions: mechanisms for the aquathermolysis of heavy oil. AOSTRA J. Res. 1984, 1, 15–20. (9) Hyne, J. B. Aquathermolysis: A synopsis of work on the chemical reactions between water and heavy oil Sands during stimulated steam stimulation, AOSTRA Synopsis Report No. 50; 1986. (10) Clark P.D.; Hyne J.B. Studies on the chemical reactions of heavy oils under steam stimulation condition. AOSTRA J. Res. 1990, 29, 29–39. ACS Paragon Plus Environment
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