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The catalytic effects of minerals on aquathermolysis of heavy oils
Fuel 83 (2004) 2035–2039 www.fuelfirst.com
The catalytic effects of minerals on aquathermolysis of heavy oils Hongfu Fan*, Yi Zhang, Yujuan Lin Department of Petroleum Engineering, Daqing Petroleum Institute, Xuefu Street, Daqing 163318, China Received 13 September 2003; revised 28 April 2004; accepted 30 April 2004; available online 26 May 2004
Abstract The catalytic effects of minerals on aquathermolysis of heavy oil were studied. The change of viscosity and average molecular weight of heavy oil with temperature, water content and reaction time are discussed in the paper. The results show that heavy oil can undergo aquathermolysis in steam injection condition. The results also show that minerals can accelerate the aquathermolysis of heavy oil and lead the viscosity and average molecular weight to decrease further. The reservoir minerals have the catalytic effects on the aquathermolysis of the heavy oils. The results provide a basic theoretical basis for down-hole catalytic upgrading of heavy oil during steam stimulation for heavy oil recovery. q 2004 Elsevier Ltd. All rights reserved. Keywords: Heavy oil; Aquathermolysis; Viscosity; Average molecular weight; Minerals
1. Introduction Steam stimulation (huff and puff) is the most popular and effective technology to recover heavy oil in the world. In the steam injection process, the viscosity reduced according to its viscosity –temperature characteristic properties, reduce the flowing resistance through the pore media of reservoir, and increase the yield and production rate. In addition to the fact that steam can reduce the viscosity of heavy oil, there are chemical reactions between steam and heavy oil. Hyne et al. [1] used the term ‘aquathermolysis’ to describe the chemical interaction of high-temperature, high-pressure water with the reactive components of heavy oil and tar sands bitumen, to distinguish this process from the term ‘hydrothermolysis’ which has become associated with the interaction of hydrogen at elevated temperature and pressure. There are many reports that deal with the catalytic effects of reservoir minerals in the generation process of oil and gases. Tao [2] investigated the catalytic effects of pillared interlayered clay and pointed out that the catalytic effect is dependent on the acidity of clay, like the zeolite molecular sieve catalyst, the acidic centre of the interlayered clay is the catalytic centre. Zhang [3] conducted the thermal experiment by mixing kerogen with illite, montmorillonite * Corresponding author. Tel./fax: þ 86-4596-504246. E-mail address: [email protected] (H. Fan). 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.04.010
and kaolinite, the results show that the minerals have clear effect on the composition and yield of hydrocarbon from thermal cracking. Brook [4] was the first man to postulate the theory that oil and gas comes from hydrocarbon source, rock by catalyzed and degraded processes. Horsfield [5] also believed that the reservoir minerals have effect on the hydrocarbon formation. Li [6] investigated the catalytic degradation of asphaltene, and the results show that montmorillonite and potassium carbonate have catalytic effect on the process in which asphaltene is converted to lighter hydrocarbons, and montmorillonite can accelerate the heavier hydrocarbon formation. Hyne et al. [1] pointed that some of the mineral components can accelerate the breakdown of organosulfur components present in heavy oil, producing carbon monoxide and other gases. The results of aquathermolysis of heavy oils lead to the saturate and aromatic increase, resin and asphaltene decrease, lowered the average molecular weight and viscosity of heavy oils, and improved the properties of heavy oils. Belgrave et al. [7] have investigated the kinetic models for the aquathermolysis of heavy oil and pointed out that the mineralogy play important role in the generation of CO2 and H2S. Clark et al. [8 – 10] studied the steam – oil chemical reaction and pointed out that some of metal ions, minerals and reservoir sands can result in the composition change of heavy oils. Monin et al. [11] have studied the thermal cracking of heavy-oil/mineral
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matrix systems at temperature and pressures encountered during thermal recovery. They pointed out that chemical reactions involving oil, possibly water, and mineral matrix may lead to significant change in composition of the heavy oil. Their works focus on four crude oils with different geochemical compositions. They observed that there are a large number of light hydrocarbons, CO2, and H2S were produced if mineral existed in the reaction system. Hamid et al. [12] also studied the geochemical changes of heavy crude oils during thermal recovery. They observed that during steam injection, both the rock matrix and the oil undergo chemical alteration due to the high temperature and pressure. They noted that chemical reaction could play an important role in thermal processes. For example, the formation of gaseous components such as carbon dioxide and hydrogen sulfide occurred. They also found that the rock matrix could promote the reactivity of the thermal alteration of heavy oil under steam injection, and produce the light hydrocarbons by cracking resins and asphaltenes. The asphaltene fraction decreased from 10 to about 4% without minerals, but decreased to 2% in the presence of sand and clay minerals during the steam stimulation condition. Fan et al. [13 – 19] investigated the feasibility of aquathermolysis of heavy oil under steam stimulation condition, the results show that after aquathermolysis, the viscosity reduced and there are synergetic effects of minerals and steam on the compositions and viscosity changes. All of these studies have shown that the mineralogy plays a major role in the process of thermal recovery of heavy crude oil. In order to investigate the catalytic effects of reservoir minerals on the aquathermolysis, the static experiments were carried out under steam stimulation condition.
2. Experimental 2.1. Pretreatment of the oil samples The oil samples before and after reaction were treated using the following processes. Oil, water and minerals were recovered from autoclave by manual methods and the oil phase was obtained by extraction into dichloromethane. The bulk of water used in the experiments was separated at this stage. Last traces of oil were obtained by Soxhlet extraction of minerals. The combined dichloromethane extracts were dried (MgSO4) and were filtered through Celite. The filtered solution was evaporated (rotary evaporation at water pump pressure and 40 8C over 3 h) yielding an oil phase free of water and minerals. 2.2. Experimental process The heavy oil sample and added different amount of water and reservoir minerals were put into the autoclave (internal volume, 500 ml), and the system was heated from
160 to 280 8C for the desired time period. At the end of the heating period, the autoclave was cooled. Then the viscosity, average molecular weight and the compositions of the oil samples were analyzed according to the following processes. 2.3. Viscosity determinations Oil viscosities were determined using a rotary viscometer (Haake RV-20, made in Germany). For the measurement, a SV2 sensor system and about 2 g of oil were used. Approximately, 2 g of the oil was placed in the sample cup and allowed to equilibrate to 50 8C over 20 min. Besides bringing the sample to the measurement temperature, this procedure allows any trace of dichloromethane to escape from the sample. Then the measurement was made according to procedures specified by the manufacture. 2.4. Average molecular weight The average molecular weights of heavy oil samples before and after treatment were measured using benzene as solvent by VPO methods, adding of 0.50 g oil sample into 1000 ml benzene and the experiment was carried out at 30 8C. 2.5. Oil composition analysis The compositions of oil samples were determined by high performance liquid chromatographic (HPLC) analysis of the deasphaltened separated oil. The asphaltenes were removed by precipitation with addition of 40-volume excess of dry hexane to the solution of the oil in dichloromethane. The HPLC analysis was carried out on a semipreparative basis using a Whatman Magnum-9 10 m silica column and ultra violet (UV) and refractive index (RI) detectors in series. The silica column was activated previously by overnight flushing (2 ml/min) with dry hexane. The saturate and aromatic fractions were obtained by elution with hexane, and after collection of the aromatic fraction, the resins were obtained by back-flushing the column with tetrahydrofuran (distilled from LiAlH4). The fractions were quantified gravimetrically after removal of solvent. A mass balance in the range 99– 101% was considered acceptable.
3. Results and discussion 3.1. Effect of the amount of water on viscosity and average molecular weight The effects of the amount of water on viscosity and average molecular weight of heavy oils at 240 8C and 48 h are given in Table 1. It is very clear that when there is no water in the reaction system, viscosity and average molecular weight are not changed. This shows that there
H. Fan et al. / Fuel 83 (2004) 2035–2039 Table 1 The effect of the amount of water on viscosity and average molecular weight wH2 O (%)
Viscosity (Pa s)
Average molecular weight
Feedstock 0 10 20 30 40 50
88.5 88.5 76.4 69.6 65.7 64.2 63.8
587 587 587 538 506 502 496
Temperature, 240 8C; time, 48 h.
was no reaction, at this reaction condition. Added 10 wt% water to the reaction system, after reaction, the viscosity of the oil sample vary from its original 88.5 Pa s to 76.4 Pa s, decreased by 13.7%. This tendency is very clear with the water content increased. When the amount of water exceeds 30 wt%, this tendency is not clear, because water acts as reactant in the reaction. The average molecular weight of heavy oil has the same tendency after reaction. 3.2. The effect of react time on viscosity and average molecular weight Added 100 g heavy oil sample and 30 wt% water into autoclave, under 240 8C, the experiments are conducted at different times. After reaction, the viscosity and average molecular weight were determined, and the results are given in Table 2. The viscosity and average molecular weight of the oil sample changed with the reaction time. This tendency was very clear before 36 h, but after reaction 36 h, this tendency became weak. The result indicates that the aqauthermolysis reaction is over after reaction 36 h. 3.3. The effect of temperature on viscosity and average molecular weight In order to investigate the effect of temperature on viscosity and average molecular weight of heavy oil in the process of aquathermolysis, the experiments were carried out at the temperature between 160 and 280 8C. The results are given in Table 3. The viscosity and average molecular weight are decreased with temperature. Table 2 The effect of reaction time on viscosity and average molecular weight Reaction time (h)
Viscosity (Pa s)
Average molecular weight
0 12 18 24 36 48
88.5 80.4 73.6 67.4 65.9 65.7
587 552 541 526 514 503
Temperature, 240 8C; wH2 O ; 30%.
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Table 3 The effect of temperature on viscosity and average molecular weight Temperature (8C)
Viscosity (Pa s)
Average molecular weight
160 180 200 240 260 280
82.4 76.6 70.2 65.7 64.6 61.5
587 542 526 503 494 486
Reaction time, 48 h; wH2 O ; 30%.
For example, the viscosity reduced from 88.5 to 65.7 Pa s, decreased by 25.8%, the average molecular weight decreased from 587 to 503, decreased by 14.3%, when the reaction was conducted at 240 8C. 3.4. The effect of reservoir minerals The minerals have been selected to be representative of reservoir rocks. Their characteristics are given in Table 4. The effect of reservoir minerals on the viscosity and average molecular weight of heavy oils are given in Table 5. The results show that the minerals have clear effect on the viscosity and molecular weight. When added 10 wt% minerals to the reaction system, the viscosity of the sample decreased from 88.5 to 55.8 Pa s, decreased by 36.9%. At the same time, we investigate the effect of reservoir on the hydrocarbon composition of oil sample. According to the oil composition analysis procedure, we conducted the analyses of the saturate, aromatic, resin and asphaltene of the heavy oil samples before and after reaction. The results are given in Table 6. It was very clearly seen that the saturate and aromatic increased, resin and asphaltene decreased when the minerals was added into the reaction system. Its original SARA composition of feedstock is 22.2, 27.4, 43.6 and 6.8%. When treated only with steam, the saturate increased to 26.5%, aromatic increased to 29.3%; resin decreased to 37.7% and asphaltene decreased to 6.5%. When minerals and steam are in coexistence, the saturate increased to 27.8%, aromatic increased to 32.2%; resin decreased to 33.8% and asphaltene decreased to 6.2%. Table 4 The properties of mineral Type mineral
Mineral
Amount (wt%)
Rock mineral
Quartz Potassic feldspar Plagioclase Calcite Dolomite Rhodochrosite Total Montmorillonite Illite Kaolinite Chlorite
53.7 19.0 13.9 1.0 1.4 1.3 9.7 92.9 4.5 1.2 1.4
Clay mineral Percentage of clay minerals
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Table 5 The effect of the minerals on viscosity and average molecular weight Reaction system
Viscosity (Pa s)
Average molecular weight
Untreated heavy oils Without any additives With water With water and minerals
88.5 88.5 65.7 55.8
587 587 503 475
4. The analysis of the catalytic mechanism of reservoir minerals The reservoir minerals are composed of clay minerals and non-clay minerals. According to geochemistry theory [20], the clay minerals, such as kaolinites and montmorillonite, are the main catalysts in the process of hydrocarbon and oil formation form hydrocarbon source rock organic compounds. Similar to kaolinites and montmorillonite, chlorites are aluminosilicates with layered structure, and have many similar structure and properties. It has been proven that chlorites have some catalytic effect in the formation process of hydrocarbon and oil from hydrocarbon source rocks. At the same time, when steam is injected into oil reservoirs, the steam can react with most of the rock minerals and clay minerals. Clay minerals are silica– aluminate compounds, under the high temperature, they can react with steam. For example, the reactions of montmorillonite and feldspar are as following [21]: 6Ca0:167 Al2:23 Si3:67 O10 ðOHÞ2 þ 6H2 O þ 12OH2 ¼ Ca2þ þ 14AlðOHÞ2 4 þ 2H4 SiO4 þ
KAlSi3 O8 þ 8H2 O ¼ K þ
AlðOHÞ2 4
ð1Þ þ 3H4 SiO4
ð2Þ
When the H4SiO4 forms, the Al3þ can insert to the surface of the H4SiO4, and produce a surface hydroxyl group with strong acidity [22]. Proton acid was yielded by water dissociation and adsorption on the surface of Al3þ. Hþ from water combine oxygen bond connected with Si4þ and leads to formation of the hydroxyl group with acidity, the hydroxyl group releases Hþ easily and gives it the characteristics of a Bronsted acid. Being the electrophilic property of Al3þ, it can get rid of charge and formed a electronic field and produce hydroxyl group from water. The SiOOHAl group was polarized by the asymmetry of the environment, thus yielding strong Table 6 The analysis results of the hydrocarbon composition of oil samples Reaction system
Saturate (wt%)
Aromatic (wt%)
Resin (wt%)
Asphaltene (wt%)
Feedstock Without any additive With water With water and mineral
22.2 22.1 26.5 27.8
27.4 27.4 29.3 32.2
43.6 43.6 37.7 33.8
6.8 6.9 6.5 6.2
Note: reaction temperature, 240 8C; reaction time, 24 h.
acidity [23]. As well known that many acid catalyzed organic reaction can carry out on the surface of clay minerals. According to geochemistry and organic chemistry theory, at high temperature clay minerals act as strong acid and the catalytic mechanism of hydrocarbon formation from minerals matrix is carbocation mechanism, i.e. the acidic centre in the surface of mineral matrix can promote kerogen to form carbocation, and the catalytic effect were occurred by the decomposition and transmission of carbocation. The effects of organic compounds, by electronic inductive effect, on the surface charge distribution of the asphaltene molecules in heavy oils, thus accelerated the aquathermolysis of heavy oil, and reduced the viscosity and average molecular weight of heavy oil. At the same time, the surface of the reservoir minerals are negatively charged for the substituting effect of crystal lattice, thus they can adsorb the cations, and make the reservoir minerals have the effect of the normal catalyst and supporter. There are some transition metal spices such as Ni and V in heavy oils. The amount of Ni and V in Liaohe heavy oil is 122.5 and 3.1 mg/l, respectively. According to the organic chemistry theory [24], when an extraneous polar nuclei is close to the reaction molecules, it can change the normal state of electronic cloud in covalent molecules, this process is called ‘dynamic inductive effect’, well known that, inorganic compounds and the transition metals are always located in the surface and pore of the asphaltene, when there are cation in some structure unit of asphaltene, the dynamic polarization occurred in the polar bond in asphaltene, and transformed to dynamic inductive effect, due to electrostatic effect surrounding the cations and anions. The dynamic inductive effect can transmit to the adjacent carbon– carbon bond, and change the electronic cloud, strengthen the polarity of carbon – carbon bond, reduce the active energy, lead to breakdown the C – C bond more easily. At the same time, for their 3-d electronic shell was not filled, the transition metal can adsorb the organic compounds in heavy oils extensively and thus lead to the breakdown of C –C, C – O, C – S and C – N bonds. According to catalytic theory [25], the transition metals and their oxide and sulfide have the catalytic effects for organic compounds. The catalytic effects of transition metals are different for different forms. For example, the oxide and sulfide of some transition metals are semiconductors, they have acidity, alkalinity and oxidation – reduction, and they show catalytic effect under different acid– base conditions. There is sulfur in heavy oil, the amount of sulfur in Liaohe heavy oil is about 0.3 –0.5 wt%, usually located in the condensed nuclei aromatics of heavy oils, under steam stimulation, the desulfurization occurred and formed sulfide. This can also accelerate the aquathermolysis. At the same time, the desulfurization of heavy oil may also lead to breakdown the C – S bond and hence reduced the viscosity and lowered the molecular weight of the oil sample.
H. Fan et al. / Fuel 83 (2004) 2035–2039
5. Conclusion Based on the experimental data and the theoretical analysis, following conclusions were made. 1. Heavy oil from Liaohe oilfield can undergo aquathermolysis in the steam stimulation process, and thus reduced the viscosity and molecular weight of the heavy oils. 2. The reservoir minerals have catalytic effects on aquathermolysis, when added 10 wt% reservoir minerals into the reaction system, the viscosity of heavy oil reduce from 88.5 to 55.8 Pa s, reduced by 36.9%, and the average molecular reduced from 587 to 475, reduced by 19.1%. 3. The catalytic mechanism of reservoir minerals are discussed in detailed.
Acknowledgements This study was supported by National Natural Science Foundation of China (Project No. 50374019).
References [1] Hyne JB, Greidanus JW. Aquathermolysis of heavy oils. Second International Conference on Heavy Crude and Tar Sands, Caracas, Venezuela; 1982. p. 25–30. [2] Tao LX. Pillared interlayered clay and its application in catalytic reactions. Bull Mineral Petrol Geochem 2002;21(4):220– 4. [3] Zhang ZF. The effect of minerals on the pyrolysis of kerogens. Pet Explor Dev 1994;21(5):29– 37. [4] Brook BT. Evidence of catalytic action in petroleum. Ind Eng Chem 1952;44(11):2570–7. [5] Horfield B. The influence of minerals on pyrolysis of kerogens. Geochem Cosmochem Acta 1980;44(1):119– 31. [6] Li SY. Study of characteristics and kinetics of catalytic degradation of asphaltene. Acta Sedimentol Sin 2001;19(1):136 –40. [7] Clark PD, Hyne JB. Studies on the chemical reactions of heavy oils under steam stimulation condition. AOSTRA J Res 1990;6(1):29–39.
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[8] Clark PD, Hyne JB. Steam-oil chemical reactions: mechanism for the aquathermolysis of heavy oil. AOSTRA J Res 1984;1(1):15–20. [9] Clark PD, Hyne JB, Tyrer JD. Chemistry of organo sulfur compound type occurring in heavy oil sands. 1. High temperature hydrolysis and thermolysis of therahydrothiophene in relation to steam stimulation processes. Fuel 1983;62(4):959 –62. [10] Clark PD, Hyne JB, Tyrer JD. Chemistry of organo sulfur compound type occurring in heavy oil sands. 2. Influence of pH on the high temperature hydrolysis of tetraothiophene and thiophene. Fuel 1984; 63(1):125– 8. [11] Moin JC, Audlbert A. Thermal cracking of heavy oil/mineral matrix system. SPE Reservoir Eng 1988;1243–50. [12] Pahlavan H, Rafiqul I. Laboratory simulation of geochemical changes heavy oils during thermal recovery. Petrol Sci Eng 1995; 12:219–31. [13] Fan H, Liu Y, Zhao X. Study on composition changes of heavy oils under steam treatment. J Fuel Chem Technol 2001;29(3):269–72. [14] Fan H, Liu Y, Zhao X. A study on heavy oil recovery by in-situ catalytic aquathermal cracking. Oilfield Chem 2001;8(1):13– 16. [15] Fan H, Liu Y, Zhao X. First field experimental of recovery heavy oil using downhole catalytic method in China. Oil Drilling Prod Technol 2001;23(3):42–4. [16] Fan H, Liu Y, Zhao X. Studies on the effect of metal ions on aquathermolysis reaction of Liaohe heavy oils under steam treatment. J Fuel Chem Technol 2001;29(5):430–3. [17] Fan H, Liu Y, Zhong L. Fundamental research on aquathermolysis for heavy oils recovery technology. J Daqing Pet Inst 2001;25(3):56– 69. [18] Fan H, Liu Y, Zhong L. Studies on the synergetic effects of mineral and steam on the composition changes of heavy oils. Energy Fuels 2001;15(6):1475–9. [19] Fan H, Liu Y. Downhole catalyst upgrades heavy oil. Oil Gas J 2002; 100(11):60 –3. [20] Goldstein TP. Geocatalytic reaction in the formation and maturation of petroleum. Am Assoc Pet Geol Bull 1983;67(1):152–9. [21] Li Y, Qian H. Calculation of the amount of minerals species dissolved and precipitated in steam huff –puff well. Oilfield Chem 1997;14(2): 143–7. [22] Gao Z. Zeolite catalyst and separation. Beijing: Chinese Petrochemical Press; 1999. 44–62. [23] Van Olphen H. An introduction to clay colloid chemistry for clay technologists, geologists and soil scientists. New York: Wiley/ Interscience; 1977. p. 57–56. [24] Shi Q. Inorganic chemistry and chemical analysis. Beijing: Higher education Press; 1998. p. 356 –379. [25] Li Y. Catalysis principles of catalyst. Tianjin University Press; 1991. p. 64 –108.
The catalytic effects of minerals on aquathermolysis of heavy oils Hongfu Fan*, Yi Zhang, Yujuan Lin Department of Petroleum Engineering, Daqing Petroleum Institute, Xuefu Street, Daqing 163318, China Received 13 September 2003; revised 28 April 2004; accepted 30 April 2004; available online 26 May 2004
Abstract The catalytic effects of minerals on aquathermolysis of heavy oil were studied. The change of viscosity and average molecular weight of heavy oil with temperature, water content and reaction time are discussed in the paper. The results show that heavy oil can undergo aquathermolysis in steam injection condition. The results also show that minerals can accelerate the aquathermolysis of heavy oil and lead the viscosity and average molecular weight to decrease further. The reservoir minerals have the catalytic effects on the aquathermolysis of the heavy oils. The results provide a basic theoretical basis for down-hole catalytic upgrading of heavy oil during steam stimulation for heavy oil recovery. q 2004 Elsevier Ltd. All rights reserved. Keywords: Heavy oil; Aquathermolysis; Viscosity; Average molecular weight; Minerals
1. Introduction Steam stimulation (huff and puff) is the most popular and effective technology to recover heavy oil in the world. In the steam injection process, the viscosity reduced according to its viscosity –temperature characteristic properties, reduce the flowing resistance through the pore media of reservoir, and increase the yield and production rate. In addition to the fact that steam can reduce the viscosity of heavy oil, there are chemical reactions between steam and heavy oil. Hyne et al. [1] used the term ‘aquathermolysis’ to describe the chemical interaction of high-temperature, high-pressure water with the reactive components of heavy oil and tar sands bitumen, to distinguish this process from the term ‘hydrothermolysis’ which has become associated with the interaction of hydrogen at elevated temperature and pressure. There are many reports that deal with the catalytic effects of reservoir minerals in the generation process of oil and gases. Tao [2] investigated the catalytic effects of pillared interlayered clay and pointed out that the catalytic effect is dependent on the acidity of clay, like the zeolite molecular sieve catalyst, the acidic centre of the interlayered clay is the catalytic centre. Zhang [3] conducted the thermal experiment by mixing kerogen with illite, montmorillonite * Corresponding author. Tel./fax: þ 86-4596-504246. E-mail address: [email protected] (H. Fan). 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.04.010
and kaolinite, the results show that the minerals have clear effect on the composition and yield of hydrocarbon from thermal cracking. Brook [4] was the first man to postulate the theory that oil and gas comes from hydrocarbon source, rock by catalyzed and degraded processes. Horsfield [5] also believed that the reservoir minerals have effect on the hydrocarbon formation. Li [6] investigated the catalytic degradation of asphaltene, and the results show that montmorillonite and potassium carbonate have catalytic effect on the process in which asphaltene is converted to lighter hydrocarbons, and montmorillonite can accelerate the heavier hydrocarbon formation. Hyne et al. [1] pointed that some of the mineral components can accelerate the breakdown of organosulfur components present in heavy oil, producing carbon monoxide and other gases. The results of aquathermolysis of heavy oils lead to the saturate and aromatic increase, resin and asphaltene decrease, lowered the average molecular weight and viscosity of heavy oils, and improved the properties of heavy oils. Belgrave et al. [7] have investigated the kinetic models for the aquathermolysis of heavy oil and pointed out that the mineralogy play important role in the generation of CO2 and H2S. Clark et al. [8 – 10] studied the steam – oil chemical reaction and pointed out that some of metal ions, minerals and reservoir sands can result in the composition change of heavy oils. Monin et al. [11] have studied the thermal cracking of heavy-oil/mineral
2036
H. Fan et al. / Fuel 83 (2004) 2035–2039
matrix systems at temperature and pressures encountered during thermal recovery. They pointed out that chemical reactions involving oil, possibly water, and mineral matrix may lead to significant change in composition of the heavy oil. Their works focus on four crude oils with different geochemical compositions. They observed that there are a large number of light hydrocarbons, CO2, and H2S were produced if mineral existed in the reaction system. Hamid et al. [12] also studied the geochemical changes of heavy crude oils during thermal recovery. They observed that during steam injection, both the rock matrix and the oil undergo chemical alteration due to the high temperature and pressure. They noted that chemical reaction could play an important role in thermal processes. For example, the formation of gaseous components such as carbon dioxide and hydrogen sulfide occurred. They also found that the rock matrix could promote the reactivity of the thermal alteration of heavy oil under steam injection, and produce the light hydrocarbons by cracking resins and asphaltenes. The asphaltene fraction decreased from 10 to about 4% without minerals, but decreased to 2% in the presence of sand and clay minerals during the steam stimulation condition. Fan et al. [13 – 19] investigated the feasibility of aquathermolysis of heavy oil under steam stimulation condition, the results show that after aquathermolysis, the viscosity reduced and there are synergetic effects of minerals and steam on the compositions and viscosity changes. All of these studies have shown that the mineralogy plays a major role in the process of thermal recovery of heavy crude oil. In order to investigate the catalytic effects of reservoir minerals on the aquathermolysis, the static experiments were carried out under steam stimulation condition.
2. Experimental 2.1. Pretreatment of the oil samples The oil samples before and after reaction were treated using the following processes. Oil, water and minerals were recovered from autoclave by manual methods and the oil phase was obtained by extraction into dichloromethane. The bulk of water used in the experiments was separated at this stage. Last traces of oil were obtained by Soxhlet extraction of minerals. The combined dichloromethane extracts were dried (MgSO4) and were filtered through Celite. The filtered solution was evaporated (rotary evaporation at water pump pressure and 40 8C over 3 h) yielding an oil phase free of water and minerals. 2.2. Experimental process The heavy oil sample and added different amount of water and reservoir minerals were put into the autoclave (internal volume, 500 ml), and the system was heated from
160 to 280 8C for the desired time period. At the end of the heating period, the autoclave was cooled. Then the viscosity, average molecular weight and the compositions of the oil samples were analyzed according to the following processes. 2.3. Viscosity determinations Oil viscosities were determined using a rotary viscometer (Haake RV-20, made in Germany). For the measurement, a SV2 sensor system and about 2 g of oil were used. Approximately, 2 g of the oil was placed in the sample cup and allowed to equilibrate to 50 8C over 20 min. Besides bringing the sample to the measurement temperature, this procedure allows any trace of dichloromethane to escape from the sample. Then the measurement was made according to procedures specified by the manufacture. 2.4. Average molecular weight The average molecular weights of heavy oil samples before and after treatment were measured using benzene as solvent by VPO methods, adding of 0.50 g oil sample into 1000 ml benzene and the experiment was carried out at 30 8C. 2.5. Oil composition analysis The compositions of oil samples were determined by high performance liquid chromatographic (HPLC) analysis of the deasphaltened separated oil. The asphaltenes were removed by precipitation with addition of 40-volume excess of dry hexane to the solution of the oil in dichloromethane. The HPLC analysis was carried out on a semipreparative basis using a Whatman Magnum-9 10 m silica column and ultra violet (UV) and refractive index (RI) detectors in series. The silica column was activated previously by overnight flushing (2 ml/min) with dry hexane. The saturate and aromatic fractions were obtained by elution with hexane, and after collection of the aromatic fraction, the resins were obtained by back-flushing the column with tetrahydrofuran (distilled from LiAlH4). The fractions were quantified gravimetrically after removal of solvent. A mass balance in the range 99– 101% was considered acceptable.
3. Results and discussion 3.1. Effect of the amount of water on viscosity and average molecular weight The effects of the amount of water on viscosity and average molecular weight of heavy oils at 240 8C and 48 h are given in Table 1. It is very clear that when there is no water in the reaction system, viscosity and average molecular weight are not changed. This shows that there
H. Fan et al. / Fuel 83 (2004) 2035–2039 Table 1 The effect of the amount of water on viscosity and average molecular weight wH2 O (%)
Viscosity (Pa s)
Average molecular weight
Feedstock 0 10 20 30 40 50
88.5 88.5 76.4 69.6 65.7 64.2 63.8
587 587 587 538 506 502 496
Temperature, 240 8C; time, 48 h.
was no reaction, at this reaction condition. Added 10 wt% water to the reaction system, after reaction, the viscosity of the oil sample vary from its original 88.5 Pa s to 76.4 Pa s, decreased by 13.7%. This tendency is very clear with the water content increased. When the amount of water exceeds 30 wt%, this tendency is not clear, because water acts as reactant in the reaction. The average molecular weight of heavy oil has the same tendency after reaction. 3.2. The effect of react time on viscosity and average molecular weight Added 100 g heavy oil sample and 30 wt% water into autoclave, under 240 8C, the experiments are conducted at different times. After reaction, the viscosity and average molecular weight were determined, and the results are given in Table 2. The viscosity and average molecular weight of the oil sample changed with the reaction time. This tendency was very clear before 36 h, but after reaction 36 h, this tendency became weak. The result indicates that the aqauthermolysis reaction is over after reaction 36 h. 3.3. The effect of temperature on viscosity and average molecular weight In order to investigate the effect of temperature on viscosity and average molecular weight of heavy oil in the process of aquathermolysis, the experiments were carried out at the temperature between 160 and 280 8C. The results are given in Table 3. The viscosity and average molecular weight are decreased with temperature. Table 2 The effect of reaction time on viscosity and average molecular weight Reaction time (h)
Viscosity (Pa s)
Average molecular weight
0 12 18 24 36 48
88.5 80.4 73.6 67.4 65.9 65.7
587 552 541 526 514 503
Temperature, 240 8C; wH2 O ; 30%.
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Table 3 The effect of temperature on viscosity and average molecular weight Temperature (8C)
Viscosity (Pa s)
Average molecular weight
160 180 200 240 260 280
82.4 76.6 70.2 65.7 64.6 61.5
587 542 526 503 494 486
Reaction time, 48 h; wH2 O ; 30%.
For example, the viscosity reduced from 88.5 to 65.7 Pa s, decreased by 25.8%, the average molecular weight decreased from 587 to 503, decreased by 14.3%, when the reaction was conducted at 240 8C. 3.4. The effect of reservoir minerals The minerals have been selected to be representative of reservoir rocks. Their characteristics are given in Table 4. The effect of reservoir minerals on the viscosity and average molecular weight of heavy oils are given in Table 5. The results show that the minerals have clear effect on the viscosity and molecular weight. When added 10 wt% minerals to the reaction system, the viscosity of the sample decreased from 88.5 to 55.8 Pa s, decreased by 36.9%. At the same time, we investigate the effect of reservoir on the hydrocarbon composition of oil sample. According to the oil composition analysis procedure, we conducted the analyses of the saturate, aromatic, resin and asphaltene of the heavy oil samples before and after reaction. The results are given in Table 6. It was very clearly seen that the saturate and aromatic increased, resin and asphaltene decreased when the minerals was added into the reaction system. Its original SARA composition of feedstock is 22.2, 27.4, 43.6 and 6.8%. When treated only with steam, the saturate increased to 26.5%, aromatic increased to 29.3%; resin decreased to 37.7% and asphaltene decreased to 6.5%. When minerals and steam are in coexistence, the saturate increased to 27.8%, aromatic increased to 32.2%; resin decreased to 33.8% and asphaltene decreased to 6.2%. Table 4 The properties of mineral Type mineral
Mineral
Amount (wt%)
Rock mineral
Quartz Potassic feldspar Plagioclase Calcite Dolomite Rhodochrosite Total Montmorillonite Illite Kaolinite Chlorite
53.7 19.0 13.9 1.0 1.4 1.3 9.7 92.9 4.5 1.2 1.4
Clay mineral Percentage of clay minerals
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Table 5 The effect of the minerals on viscosity and average molecular weight Reaction system
Viscosity (Pa s)
Average molecular weight
Untreated heavy oils Without any additives With water With water and minerals
88.5 88.5 65.7 55.8
587 587 503 475
4. The analysis of the catalytic mechanism of reservoir minerals The reservoir minerals are composed of clay minerals and non-clay minerals. According to geochemistry theory [20], the clay minerals, such as kaolinites and montmorillonite, are the main catalysts in the process of hydrocarbon and oil formation form hydrocarbon source rock organic compounds. Similar to kaolinites and montmorillonite, chlorites are aluminosilicates with layered structure, and have many similar structure and properties. It has been proven that chlorites have some catalytic effect in the formation process of hydrocarbon and oil from hydrocarbon source rocks. At the same time, when steam is injected into oil reservoirs, the steam can react with most of the rock minerals and clay minerals. Clay minerals are silica– aluminate compounds, under the high temperature, they can react with steam. For example, the reactions of montmorillonite and feldspar are as following [21]: 6Ca0:167 Al2:23 Si3:67 O10 ðOHÞ2 þ 6H2 O þ 12OH2 ¼ Ca2þ þ 14AlðOHÞ2 4 þ 2H4 SiO4 þ
KAlSi3 O8 þ 8H2 O ¼ K þ
AlðOHÞ2 4
ð1Þ þ 3H4 SiO4
ð2Þ
When the H4SiO4 forms, the Al3þ can insert to the surface of the H4SiO4, and produce a surface hydroxyl group with strong acidity [22]. Proton acid was yielded by water dissociation and adsorption on the surface of Al3þ. Hþ from water combine oxygen bond connected with Si4þ and leads to formation of the hydroxyl group with acidity, the hydroxyl group releases Hþ easily and gives it the characteristics of a Bronsted acid. Being the electrophilic property of Al3þ, it can get rid of charge and formed a electronic field and produce hydroxyl group from water. The SiOOHAl group was polarized by the asymmetry of the environment, thus yielding strong Table 6 The analysis results of the hydrocarbon composition of oil samples Reaction system
Saturate (wt%)
Aromatic (wt%)
Resin (wt%)
Asphaltene (wt%)
Feedstock Without any additive With water With water and mineral
22.2 22.1 26.5 27.8
27.4 27.4 29.3 32.2
43.6 43.6 37.7 33.8
6.8 6.9 6.5 6.2
Note: reaction temperature, 240 8C; reaction time, 24 h.
acidity [23]. As well known that many acid catalyzed organic reaction can carry out on the surface of clay minerals. According to geochemistry and organic chemistry theory, at high temperature clay minerals act as strong acid and the catalytic mechanism of hydrocarbon formation from minerals matrix is carbocation mechanism, i.e. the acidic centre in the surface of mineral matrix can promote kerogen to form carbocation, and the catalytic effect were occurred by the decomposition and transmission of carbocation. The effects of organic compounds, by electronic inductive effect, on the surface charge distribution of the asphaltene molecules in heavy oils, thus accelerated the aquathermolysis of heavy oil, and reduced the viscosity and average molecular weight of heavy oil. At the same time, the surface of the reservoir minerals are negatively charged for the substituting effect of crystal lattice, thus they can adsorb the cations, and make the reservoir minerals have the effect of the normal catalyst and supporter. There are some transition metal spices such as Ni and V in heavy oils. The amount of Ni and V in Liaohe heavy oil is 122.5 and 3.1 mg/l, respectively. According to the organic chemistry theory [24], when an extraneous polar nuclei is close to the reaction molecules, it can change the normal state of electronic cloud in covalent molecules, this process is called ‘dynamic inductive effect’, well known that, inorganic compounds and the transition metals are always located in the surface and pore of the asphaltene, when there are cation in some structure unit of asphaltene, the dynamic polarization occurred in the polar bond in asphaltene, and transformed to dynamic inductive effect, due to electrostatic effect surrounding the cations and anions. The dynamic inductive effect can transmit to the adjacent carbon– carbon bond, and change the electronic cloud, strengthen the polarity of carbon – carbon bond, reduce the active energy, lead to breakdown the C – C bond more easily. At the same time, for their 3-d electronic shell was not filled, the transition metal can adsorb the organic compounds in heavy oils extensively and thus lead to the breakdown of C –C, C – O, C – S and C – N bonds. According to catalytic theory [25], the transition metals and their oxide and sulfide have the catalytic effects for organic compounds. The catalytic effects of transition metals are different for different forms. For example, the oxide and sulfide of some transition metals are semiconductors, they have acidity, alkalinity and oxidation – reduction, and they show catalytic effect under different acid– base conditions. There is sulfur in heavy oil, the amount of sulfur in Liaohe heavy oil is about 0.3 –0.5 wt%, usually located in the condensed nuclei aromatics of heavy oils, under steam stimulation, the desulfurization occurred and formed sulfide. This can also accelerate the aquathermolysis. At the same time, the desulfurization of heavy oil may also lead to breakdown the C – S bond and hence reduced the viscosity and lowered the molecular weight of the oil sample.
H. Fan et al. / Fuel 83 (2004) 2035–2039
5. Conclusion Based on the experimental data and the theoretical analysis, following conclusions were made. 1. Heavy oil from Liaohe oilfield can undergo aquathermolysis in the steam stimulation process, and thus reduced the viscosity and molecular weight of the heavy oils. 2. The reservoir minerals have catalytic effects on aquathermolysis, when added 10 wt% reservoir minerals into the reaction system, the viscosity of heavy oil reduce from 88.5 to 55.8 Pa s, reduced by 36.9%, and the average molecular reduced from 587 to 475, reduced by 19.1%. 3. The catalytic mechanism of reservoir minerals are discussed in detailed.
Acknowledgements This study was supported by National Natural Science Foundation of China (Project No. 50374019).
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