Energy Fuels 2010, 24, 2809–2816 Published on Web 04/23/2010
: DOI:10.1021/ef100230k
Catalytic Aquathermolysis Used for Viscosity Reduction of Heavy Crude Oils: A Review S. K. Maity,* J. Ancheyta, and G. Marroquı´ n Instituto Mexicano del Petr oleo, Eje Central L azaro C ardenas Norte 152, Col. San Bartolo Atepehuacan, M exico D. F. 07730 Received March 1, 2010. Revised Manuscript Received April 8, 2010
The catalytic aquathermolysis becomes an important area for investigation to solve some of the problems during exploration of heavy crude oil. It has been reported in 1982 by Hyne et al. that metals can accelerate the aquathermolysis and thereafter the uses of several catalysts on this reaction have been studied. It is believed that superheated water passes heat to the hydrocarbon, and some asphaltene molecules are broken down by thermal heat to small molecules. Hence the viscosity as well as flow properties of heavy oil are improved. Moreover, the added heat provides driving force or pressure so that the viscous oils can flow easily and increases the oil production. When the catalyst is present on this reaction system, the viscosity is reduced very deeply. In general the catalysts employed for aquathermolysis are mineral, watersoluble, oil soluble, and dispersed catalyst. The viscosity reduction with these catalysts is in the order of mineral < water-soluble catalyst < oil-soluble catalyst < dispersed catalyst. It has also been found that during aquathermolysis, the saturates and aromatics increase while the amount of asphaltene and resin decreases. The use of different hydrogen donors on aquathermolysis also improves the quality of the heavy crude oil. The most commonly used hydrogen donor is tetralin. Moreover, when tetralin is used with a catalyst, the viscosity is also reduced more effectively. The use of catalysts in the real oil field indicates that the catalysts can substantially reduce viscosity and hence the catalytic aquathermolysis process can be used successfully for exploration of heavy crude oils. However, the oil soluble and dispersed catalysts are slightly more active than the water-soluble catalyst. The cost of the former two types of catalysts may be higher than the preparation cost of simple water-soluble catalysts. Therefore, more research is needed so that the catalysts can be used for this process more economically. Another problem is the efficiency of these catalysts in the oil field. The activity of the catalysts depends on the homogeneity of the temperature in the oil floor. When the superheated water is injected into the oil reservoir, the oil surface temperature is high. However, temperature is gradually lower on the depth of the oil floor, and hence the catalyst loses its activity. So, further investigation is also necessary to address this aspect.
fields are discussed latter. The thermal cracking in the presence of water was first named as aquathermolysis by Hyne et al.2 This process is commonly used for the production of heavy crude oil and its transportation. It is simply believed that steam carrying thermal energy passes heat to the hydrocarbons. This heat energy breaks large molecules into smaller ones and as a consequence the reduction of viscosity and improvement in flow properties of heavy oils are noticed. It is a well-known fact that hydrocarbons are broken down at temperatures above 300 °C whether water is present or not. Hyne et al.2 proposed the following chemical reaction for aquathermolysis:
1. Introduction The production of heavy oil is increasing in coming years due to short fall of conventional light crude. However, it is very difficult to handle heavy crude oils due to its high viscosity. Therefore, several researchers are investigating on how to reduce viscosity and transport heavy oils easily from one place to another. In this respect, the “Huff and Puff” process is mostly used. In this process, superheated water is injected into the reservoir well so that it reduces the viscosity of the crude oil. It is believed that there is some cracking of the asphaltene molecules present in the crude oil due to the injected hot water. This process is used commercially for recovery of heavy oil as early as 1960s and since then it has been applied for recovery of several heavy oils and tar sands like Athabasca sands, California tar sand, Orinoco heavy oil, Utah tar, etc.1 The simplified flow diagram of this process is presented in Figure 1. In general, this process includes three steps-injection of superheated steam into the well, soak period where the well is closed for several days, and finally production of oil. More details about the use of this process in real oil
RCH2 CH2 SCH3 þ 2H2 O ¼ RCH3 þ CO2 þ H2 þ H2 S þ CH4 The principal aspect of this chemical reaction is that the C-S bond is broken down and hence it reduces viscosity of the heavy crude oil. Even a small fraction of bond breakage can lead to huge improvement of flow properties of heavy crude oils. The organosulfur compounds in heavy oil do not rupture in a single way as depicted above but in a complex sequence of
*To whom correspondence should be addressed. Fax: þ52(55) 91758429. E-mail:
[email protected]. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 4th ed.; CRC Press/Taylor & Francis Group: Boca Raton, FL, 2007. r 2010 American Chemical Society
(2) Hyne, J. B.; Greidanus, J. W.; Tyrer, J. D.; Verona, D.; Rizek, C.; Clark, P. D.; Clarke, R. A.; Koo, J. The Second International Conference on Heavy Crude and Tar Sands, Caracas, Venezuela, 1982; p 25.
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: DOI:10.1021/ef100230k pounds present in the crude. It is also important to consider the reactivity of the present sulfur compounds in the heavy oil, not only the amount of sulfur compounds. Hyne et al.2 noticed that the production of H2S is the same for Canadian and Venezuelan heavy crude oils although the sulfur content of the former is 5.0 wt % and the latter is only 2.3 wt %. It indicates that the reactivity of sulfur compounds of Venezuelan crude is more than that of Canadian crude. The viscosity of Canadian heavy oil is initially increased during aquathermolysis at lower temperature (200 or 240 °C). It is proposed that though viscosity of the heavy oil increases, the average molecular weight is not changed, indicating that the increase of the viscosity is not a result of polymerization during the reaction. The increase of viscosity (instead of decrease) at the initial stage explains that, even at low temperature, there may be slight production of light hydrocarbons. The loss of these light hydrocarbons may increase the viscosity of the liquid. However, this slight loss may not have a great effect on the average molecular weight. If the aquathermolysis is carried out for long time, for example, 60-90 days even at the lower temperature 240 °C, the viscosity is reduced as it is expected. When the reaction is carried out at 300 °C, the viscosity is normally coming down with the duration of the reaction. The opposite tendency is, however, observed when aquathermolysis is tested on Venezuelan heavy oil. The viscosity of the final product is gradually reduced with time. On the contrary, Rivas et al.5 noticed the reverse tendency. The viscosity of Cerro Negro bitumen (Venezuelan) decreases after 3 days of reaction while it increases after 20 days of reaction. In this case, an aqueous solution of nickel sulfate was used as an additive for aquathermolysis. Here it is worth mentioning that the sulfur content of the heavy crude oil used by Hyne et al.2 is around 2.5-5 wt %. From their investigation it was found that the viscosity reduction is mainly by C-S bond breaking. Later, the same group has studied the aquathermolysis on several aromatic and aliphatic sulfur compounds. Also, it has been clear from their studies that viscosity reduction is mainly due to the bond cleavage of C-S.6-10 Moreover, the viscosity reduction also depends on the concentration of organosulfur compounds present in the heavy oil. Now the question is whether this same aquathermolysis could be applied to the heavy crude oils having a lower sulfur content. The northeast part of China has a vast heavy oil field named as Liaohe. The sulfur content of the oil of this field is below 0.5 wt %. Therefore, aquathermolysis becomes the primary interest of Chinese researchers to solve the recovery problem of heavy oils and its transportation from this oil field. Because of this, most of the research papers published on the use of catalytic aquathermolysis are from China. From the foregoing discussion it will be clear that aquathermolysis can also be used for viscosity reduction of the heavy crude oils having a low sulfur content. Other processes like heat treatment, dilution by light fractions, emulsion, etc. are also used for viscosity reduction and transportation.11 However, the detailed review about these
Figure 1. A simplified flow diagram of the Huff and Puff process used commercially for recovery of heavy crude oils. (Adapted with permission from ref 1 Copyright 2006 Taylor & Francis Group.)
steps. At the same time, hydrogen is also produced as a bonus. It is important to note that this hydrogen comes from water. The produced hydrogen can take part for upgrading of heavy oil and hence improves oil quality. Carbon dioxide is another gas produced during aquathermolysis. An oil reservoir consists of several metals carbonate. These carbonates, mainly siderite (FeCO3), may produce carbon dioxide at the steam injection temperature. The studies of model compounds show that carbon dioxide is also produced during aquathermolysis of thiophene and thiolane. Thus, it suggests that at steam injection conditions, the production of carbon dioxide may be from both the heavy oil and from the metals carbonates present in the reservoir mineral. The production of carbon dioxide, from whatever sources, is beneficial for viscosity reduction. The aquathermolysis has various important benefits like (a) reduction of viscosity and hence improvement of its flow properties, (b) desulfurization, and (c) hydrogenation and hence upgrading of heavy oils. In addition, in situ viscosity reduction with slight upgrading of heavy crude oil in oil fields has other advantages as well. It enhances oil production and transportation from the reservoir to the refinery. It is not necessary to put in a separate catalytic process which is very costly, since in situ upgrading occurs inside the reservoir, so there is no need of an extra place in the catalytic process. In general, a catalytic process is designed for a specific application for a particular feed. However, the in situ process can be used in oil fields with minor variation of catalyst and hydrogen donor depending on the properties of heavy crude and final requirements.3,4 To confirm the C-S bond breakage during aquathermolysis, the production of gases by heavy oil and thiolane or thiophene aquathermolysis has been investigated.2 The major components of gases (CH4, CO2, H2S, and H2) for both cases (heavy oil and model compounds) are the same and even the production of these gases with the duration of the reaction also shows similar tendency. Thus, it suggests that the organosulfur components of the heavy oil are reacting chemically with water in the same manner as the model compounds do. The production of gases from aquathermolysis of thiolane is, however, almost 10 times higher than that from thiophene. Therefore, the production of gases from heavy oil hydrocarbons depends on the amount of the different sulfur com-
(5) Rivas, O. R.; Campog, R. E.; Borgee, L. G. SPE Annual Technical Conference and Exhibition, Houston, Texas, October 2-5, 1988; Paper No. 18076-MS. (6) Clark, P. D.; Hyne, J. B.; Tyrer, J. D. Fuel 1983, 62, 959. (7) Clark, P. D.; Hyne, J. B.; Tyrer, J. D. Fuel 1984, 63, 125. (8) Clark, P. D.; Hyne, J. B. Fuel 1984, 63, 1649. (9) Clark, P. D.; Dowling, N. I.; Hyne, J. B.; Lesage, K. L. Fuel 1987, 66, 1353. (10) Clark, P. D.; Lesage, K. L.; Dowling, N. I.; Hyne, J. B. Fuel 1987, 66, 1699. (11) Saniere, A.; Henaut, I.; Argillier, J. F. Oil Gas Sci. Technol. 2004, 59 (5), 455.
(3) Mohammad, A. A.; Mamora, D. D. SPE/PS/CHOA, International Thermal Operations and Heavy Oil Symposium, Society of Petroleum Engineers, Calgary, Alberta, Canada, October 20-23, 2008; Paper No. 117604. (4) Song, G.; Zhou, T.; Cheng, L.; Wang, Y.; Tian, G.; Zhang, J.; Pi, Z. Pet. Sci. 2009, 6 (3), 289.
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processes is out of the scope in this article. It has been pointed out above that aquathermolysis is a thermal process. Free radicals may be generated during breaking of the large bonds. Sometimes, these free radicals participate in polymerization reactions and form bigger molecules. It leads to production of more viscous materials instead of reducing the viscosity. The use of different minerals and catalysts is proposed to inhibit this kind of free radical formation and enhances the reduction of viscosity. Apart from the catalyst, several hydrogen donors have also been used to slightly improve the heavy oils quality. The use of aquathermolysis for the production and transportation of heavy and extra heavy oils is becoming an important research area. There are number of scientific publications on the various aspects of this process. The present contribution represents the first attempt of a critical review on this topic.
a high percentage of rock (consists of quartz, feldspar etc.) and clay mineral (mostly montmorillonite). When high temperature steam is injected to a reservoir, the montmorillonite and feldspar take part in the following chemical reactions:12 6Ca0:167 Al2:23 Si3:67 O10 ðOHÞ2 þ 6H2 O þ 12OH ¼ Ca2þ þ 14AlðOHÞ4 - þ 2H4 SiO4 KAlSi3 O8 þ 8H2 O ¼ Kþ þ AlðOHÞ4 þ 3H4 SiO4 It is assumed that Al3þ interacts with the produced H4SiO4 and generates a surface hydroxyl group with strong acidity. The Bronsted acidity is also produced by water dissociation and adsorption on the surface of Al3þ. The SiOOHAl group is polarized by asymmetry of the environment and yields a strong acidity. Therefore, a mineral reacting with steam water can yield a similar acidic environment as amorphous silica-alumina does. The hydrocracking activity of amorphous silica-alumina is a well-known fact. So, in a similar manner, the mineral can accelerate aquathermolysis and hence reduces viscosity of the treated heavy oils. Moreover, when the mineral is used with the metals, this mineral acts as support for metals providing a more effective catalyst. Ovalles et al.14 conducted a systematic study of the use of mineral on aquathermolysis. The mineral consists of various metals including iron, silica, alumina, etc. The effect of these components on viscosity reduction is in the order of mineral < SiO2 < Fe2O3 < silica-alumina. It also supports that due to the acidity of silica-alumina, the viscosity is reduced more effectively. Fan et al. in their other work15 has investigated the use of mineral in detail. In this work, the heavy oil, water, and mineral were heated in an autoclave at 160-280 °C for a given time. The results show that when water is not used in the reaction system, the viscosity and average molecular weight are not changed. However, with the use of 10 wt % of water, the viscosity decreases about 14%. It is also reported that the viscosity gradually decreases with added water (up to 30 wt %), and after that the improvement of it is not clearly observed. The usefulness of hot water in aquathermolysis has also been supported by Ogbuneke.16 It was found that the hot water, used on aquathermolysis, not only helps to crack the large hydrocarbon to smaller ones but also it helps to reduce coke formation during thermal cracking. The introduction of a small amount of water can depress the coke formation, and with an increasing amount of it, coke formation is also decreasing and it leveled off after 10 wt % of water. Fan et al.15 also observe that with increasing reaction time, temperature, and the amount of added mineral, the viscosity and average molecular weight of the heavy oil decrease. The more prominent changes are observed up to 36 h of reaction time. It is found that at 240 °C reaction temperature, the viscosity and the average molecular weight are decreased by 26 and 14 wt %, respectively. The great improvement of the viscosity is observed when the mineral is added to the reaction mixture. With 10 wt % of mineral, the viscosity is improved by around 37%. The saturate, aromatic, resin, and asphaltene content of the feed are 22.2, 27.4, 43.6, and 6.8, respectively. After reaction in the
2. Use of Catalysts in Aquathermolysis The benefits for the use of catalysts in aquathermolysis were reported as early as 1982 by Hyne et al.2 For an aquathermolysis study, authors use two types of reactors: one was a quartz tube and another was a vessel made of a high percentage of nickel and cobalt. When the results were compared, it was found that the viscosity reduction was more on the vessel than on the quartz tube. It was then obvious that the metals (Ni or Co) influenced aquathermolysis. Thereafter, several researchers investigated the effect of catalysts on aquathermolysis. The catalysts used, for aquathermolysis, can be roughly divided into four categories: mineral, water-soluble catalysts, oil-soluble catalysts, and dispersed catalysts. Several solvents are often used as hydrogen donors together with catalysts. The use of different types of catalysts and hydrogen donors is discussed below. 2.1. Mineral. The idea behind the study of mineral for aquathermolysis is that an oil reservoir consists of sand and mineral. The mineral present naturally on the reservoir can take part in the reaction at the steam injection conditions. Most probably, Clark et al.5 first reported that metal, mineral sands might change the reaction equilibrium during aquathermolysis. The effect of it during aquathermolysis has been systematically studied by Fan et al.12 For a laboratory test, 10 wt % of mineral and 10 wt % of water are taken with heavy crude oil in an autoclave and heated at 240 °C. Three different types of heavy oils have been tested for viscosity reduction. One catalyst prepared in the laboratory containing VO2þ, Ni2þ, and Fe3þ (1:1:5) has also been tested. When the catalyst is used with the mineral, the reduction of viscosity of heavy oil is more effective and it is reduced up to 86.3%. The average molecular weight has also been reduced from 648 to 385. However, if the mineral or catalyst is used alone, the viscosity reduction is not so effective (25.6% for the mineral and 76.4% for the catalyst). The high activity of the metal catalyst with the mineral has been explained in the following way. The clay minerals are negatively charged, and when metal VO2þ or Ni2þ is added, this metal cation is adsorbed on the clay surface. Therefore, the clay mineral can act as a support for metals cations like highly active conventional catalysts. The change of the reaction equilibrium during aquathermolysis in the presence of clay was previously also reported by Siskin.13 An oil reservoir is a large pore medium consisting of sands, clay minerals, and nonclay minerals. A typical mineral contains
(14) Ovalles, C.; Vallejos, C.; Vasquez, T.; Rojas, I.; Ehrman, U.; Benitez, J. L.; Martinez, R. Pet. Sci. Technol. 2003, 21 (1-2), 255. (15) Fan, H.; Zhang, Y.; Lin, Y. Fuel 2009, 83, 2035. (16) Ogbuneke, K. U.; Snape, C. E.; Andresen, J. M.; Rusell, C.; Crozier, S.; Sharpe, R. Am. Chem. Soc., Div. Pet. Chem. 2007, 52 (2), 118.
(12) Fan, H.; Zhang, Y.; Zhong, L. G. Energy Fuels 2001, 15, 1475. (13) Siskin, M.; Brons, G.; Katritzky, A. R.; Balasubramanian, M. Energy Fuels 1990, 4, 475.
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presence of water and mineral, the saturate increased to 27.8%, the aromatic increased to 32.2%; the resin decreased to 33.8%; and the asphaltene decreased to 6.2%. The sulfur content of the effluents is also reduced. 2.2. Water-Soluble Catalyst. Hyne et al.2 first observed that the presence of nickel and cobalt influences the aquathermolysis, and later it was supported by his co-workers. Clark et al.8 used vanadyl and nickel salts in the aquathermolysis of thiophene and tetrahydrothiophene. These metals strongly interact with these sulfur model compounds during the hydrolysis reaction and effectively remove sulfur from these molecules. The use of different transition metal cations has also been investigated by the same author.9 It is found that Al3þ, Sc3þ, VO2þ, Cr3þ, Ni2þ, and Cu2þ ions are very reactive for thiophene while Al3þ, VO2þ, Cr3þ, and Cu2þ ions are very effective for tetrahydrothiophene. The effect of group VIIIB metals on desulfurization of thiophene and tetrahydrothiophene has also been reported.10 The sulfur compound, metal salt, and water were put in a 1:0.1:10 molar ratio in a sealed quartz tube reactor. The reaction mixture was heated at 200 and 240 °C for 7 days. The aqueous solutions of Fe(II), Ru(II), Os(III), Co(II), Rh(III), Ir(III), Ni(II), Pd(II), Pt(II), and Pt(IV) have been studied as the catalyst. It has been observed that Pt(IV) is the most reactive metal, and it reduces the sulfur compound around 40 and 56% for thiophene and tetrahydrothiophene, respectively. Thiophene desulfurization was studied at 200 °C, whereas tetrahydrothiophene was studied at 240 °C. It is also reported that Ru(III), Rh(III), and Ir(III) are also very effective for removal of sulfur from thiophene. The water-soluble Fe(II) and Ru(III) catalysts are used for upgrading of bitumen at high temperature 375-415 °C.17 In a typical experiment, 80 g of bitumen and 20 g of water are placed with 10 g of iron(II) sulfate or 1.5 g of ruthenium(III) chloride. It is noticed that water-soluble iron catalyst minimizes the formation of insolubles, and it is mainly due to the increase of asphaltene content in the liquid products from 14.6 to 19.7 wt %. Moreover, it is surprising to note that in the iron/water system, the viscosity of the liquid product is reduced to 520 mPa s from 2140 mPa s in spite of increased asphaltene content. It suggests that there may not be any direct linkage of asphaltene with viscosity. While in the ruthenium/water system, the reduction of asphaltene content in the liquid products has been observed and it leads to an increase of insolubles formation. However, if the ruthenium catalyst is used in the presence of hydrogen, it diminishes the formation of the insolubles. The sulfur removal for Ru/water, Fe/water, and only water systems has also been studied. The sulfur removal from the feed is higher in the Fe/water or Ru/water systems than that of only water. The highest reduction of sulfur has been found on the Ru catalyzed reaction. The percentages of sulfur reduction for Ru, Fe, and the only water systems are 21, 18, and 14.7 wt %, respectively. Thus, the results show that the ruthenium catalyst is more active than iron since the first one is used in lesser amounts in the reaction system. It reveals that the iron catalyst helps to transfer H from H-rich organic molecules to radical intermediates. On the other hand, ruthenium(III) has very good affinity to coordinate with organic sulfur bond formation to a stable complex and it leads to weakening of the C-S bond. During C-S bond cleavage, the reactive intermediates are formed which undergo coupling and polymerization reactions. As a
result, the formation of higher molecular weight molecules is observed unless there is adequate hydrogen to stabilize these intermediates. The improvement of viscosity and average molecular weight were investigated by Zhong in the presence of the catalyst and the hydrogen donor.18 Liaohe extra heavy oil was treated with water, catalysts, and tetralin at the following conditions: temperature of 160-280 °C, pressure of 10-25 MPa, 24-240 h of reaction time. The catalysts used in this investigation are Fe(II), Co(II), Mo(II), Ni(II), and Al(II). Tetralin is used as the hydrogen donor during reaction. The metals Fe, Co, and Mo give almost similar results with respect to viscosity reduction (60%). It is also observed that with increasing Fe concentration, the reduction of viscosity increases. At 240 °C and 72 h of reaction, the viscosity is reduced to 40 and 60% in the presence of only tetralin and Fe, respectively. However, it is reduced to 90% when both tetralin and Fe are present in the reaction system. The sulfur content is also decreased to 1/3 in the presence of Fe and 2/3 in presence of both Fe and tetralin. The temperature effect on the viscosity reduction has also been examined, and it was reported that 240 °C is the optimum temperature that could be used for aquathermolysis. Hyne et al.2 observed that the production of gases drastically increases with increasing reaction temperature of aquathermolysis, suggesting that at higher temperature, thermal cracking is more predominant than that of catalytic aquathermolysis. That is why it is proposed that the aquathermolysis mainly occurs at 240 °C or below. The catalysis by keggin heteropoly acid salt during the aquathermolysis of heavy oil was investigated.19 The heteropoly acid salt has the capability of acidity, redox, and pseudo liquid phase reaction. The activity of this catalyst was studied in an autoclave reactor by placing 100 g of heavy oil, 43 g of water, and 0.3 g of K3PMo12O40 catalyst. The reaction was carried out at 200-280 °C temperature, for 24 h, at 3 MPa pressure. The viscosity of the heavy oil is reduced by 92% at 280 °C of reaction temperature. The reduction of the viscosity depends on the feed properties. For slightly heavy crude, the reduction is around 90%. It is also reported that the resins and asphaltenes decrease by 23 and 2.6%, respectively, while the saturate and aromatic increase by 16.5 and 6.5%, respectively. It is stated that the nano keggin catalyst promotes the pyrolysis of asphaltene and resin to the lighter fractions like saturates and aromatics. This catalyst also has the ability to restructure the hard associating structure and changes oxygen containing groups in such a way that it leads to the viscosity reduction effectively. It is also postulated that the catalyst not only breaks the C-S bond but also the C-O bond and hence reduces the viscosity of the treated oil. It is found that during the reaction, the olefins are hydrogenated to the saturates. Hydrogen NMR studies show that the aromaticity of asphaltene increases while the aromaticity of resins decreases. It suggests that this catalyst has very little effect on rearrangement of the framework of the asphaltene structure, and rather it reconstructs the side chain associated molecules. However, the catalyst can react effectively with the loosely bonded aromatic molecules on the resins, resulting in a decrease of the aromaticity by ring-opening. Thus, it (18) Zhong, L. G.; Liu, Y. J.; Fan, H. F. SPE International Improved Oil Recovery Conference in Asia Pacific, Kuala Lumpur, Malaysia, October 20-21, 2003; Paper No. 84863-MS. (19) Chen, Y.; Wang, Y.; Lu, J.; Wu, C. Fuel 2009, 88, 1426.
(17) Clark, P. D.; Kirk, M. J. Energy Fuels 1994, 8, 380.
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: DOI:10.1021/ef100230k Table 1. Effect of Catalysts and Hydrogen Donor on Viscosity Reductiona viscosity
1 2 3 4 5 6
removal of metals (%)
experiment
API gravity
at 50 °C (cP)
reduction (%)
V
Ni
H/C ratio
untreated pure steam injection 5 wt % tetralin injection 5 wt % tetralin premixed 15 wt % tetralin injection 5 wt % tetralin þ catalyst injection 5 wt % tetralin þ catalyst premixed
12.4 17 14.9 14.8 14.1 15.1 15.48
1445 111 67 110 45 109 50
92.3 95.4 92.4 96.9 92.5 96.5
17 25.7 27.5 34 26 34.5
16.4 23.5 29 36 29.5 36.5
1.62 1.58 1.67 1.63 1.58 1.61 1.67
a
Adapted with permission from ref 3 Copyright 2008 Society of Petroleum Engineers.
the production of light hydrocarbons (C2-C7) is from the pyrolysis of the unstable alkyl side chain. The saturated and aromatic hydrocarbons are increased, while the resin and asphaltene are decreased. Aromatization in a part of the cyclic hydrocarbons occurs, and it contributes to an increase of the aromatics in the final products. Some normal and isoalkyl side chains attached with the edge of condensed aromatic cores in the resin and asphaltenes are broken and converted to alkyl hydrocarbons, and hence the amount of saturate hydrocarbons increases. The IR studies show that heteroatoms associated with the condensed aromatic structure of heavy oil are removed during aquathermolysis and thus reduce the total ring number. The organometallic iron catalyst Fe(CH3COCHCOCH3) has been used for the viscosity reduction.3 This catalyst is highly soluble in tetralin, and its concentration in the tetralin is 750 ppm. Tetralin is used in this case as a hydrogen donor. Several experiments have been carried out to compare the effect of tetralin, catalysts, and the catalyst with tetralin on the upgrading of Jobo (Venezuelan) heavy crude oil. In the experiment, a stainless steel cell is used where a mixture of sand, water, and oil (88.49%, 3.89%, and 7.62%, respectively) is heated at 273 °C. The heated steam is injected at a rate of 5.5 mL/min. The catalyst activity of six different runs is compared in Table 1. The tetralin and/or tetralin-catalyst is used either as premixed with sand, the water-oil mixture, or introduced with steam. The API gravity of the treated oil increases compared with the untreated oil. The highest increase of API gravity is observed when the heavy oil is only treated with pure steam (run 1) and the lowest with 15 wt % tetralin (run 4). However, when tetralin is used with the catalyst, (runs 5 or 6), the increase of the API gravity is prominent. The viscosities of all the runs are lower than that of the untreated oil. The viscosity reduction for run 4 treated with 15 wt % tetralin is the highest. The mixture of tetralin with the catalyst also shows a considerable reduction of the viscosity. Although there is not a regular tendency for the improvement of API gravity and the reduction of viscosity, the use of tetralin with the catalyst substantially improves these properties. Metal removal of the treated oil is also presented in same table. The removals of vanadium and nickel are the highest when the oil is treated with the solution of tetralin-catalyst. The best improvement of hydrogen-tocarbon ratio is also observed for this catalyst solution. It has also been reported that the oil production with addition of 15 wt % tetralin is more than oil production by any other run. The pure steam injection (run 1) has the least oil production. The activities of the viscosity reduction of water-soluble and oil-soluble catalysts have been compared by Yafengu et al.22
is concluded that nano keggin catalyst can pyrolyze the heavy hydrocarbon to the smaller fractions by breaking of the C-S, C-C, and C-O bonds, and hence the chemical properties of the treated oil are changed in favor of liquid flow properties. The several changes of the chemical properties of heavy oil have been proved by IR studies. The IR bands at 2924 and 2358.1 cm-1 are assigned to C-H stretching vibration. The gaining intensity of these bands after reaction indicates that the saturate content increases by hydrogenation of the unsaturated content and pyrolysis of the long alkanes chain. The IR bands of asphaltene before and after the reaction suggest that olefins decrease or disappear because of hydrogenation. 2.3. Oil-Soluble Catalyst. The above-discussed catalysts are water-soluble, and these catalysts have a limitation to mix with oil. When catalyst is not mixed with the oil, it means that this catalyst cannot be used so effectively. This idea leads to the development of the oil-soluble catalyst. The effectiveness of the oil-soluble catalyst for viscosity reduction is reported by Wen et al.20 In this study, the molybdenum oleate organic catalyst was prepared by reacting MoO3 with oileic acid. It is an oil-soluble catalyst with 24.9 wt % of Mo content in the organic phase. This catalyst was used for viscosity reduction of Liaohe heavy oils. The catalytic activity was studied in an autoclave reactor by taking 75 g of feed, 0.4 g of catalyst, and 25 g of water at 240 °C for 24 h. The results show that this oil-soluble catalyst can reduce viscosity very effectively up to 90%. During reaction at 240 °C, a large amount of gases like CO2, H2S, and light hydrocarbons, mainly C2-C7, are produced. It is believed that these light hydrocarbons act as solvent and hence the reduction of viscosity is observed. Even CO2 can also reduce viscosity and improve the flow of the heavy oil. The improvement of the hydrogen to carbon ratio has also been observed. The amount of saturates and aromatics increases whereas the content of asphaltene and resin decrease. This indicates that some cyclic hydrocarbons may be converted to aromatics. The production of light hydrocarbons was also reported earlier by Fan et al.21 In this study, the production of light hydrocarbons, hydrogen sulfide, and carbon dioxide has been observed. It is noted that methane and hydrogen are produced as noncondensed gases, and the amount of these gases decreases with the added water and reaction time. It is stated that methane might react with water and forms hydrogen, carbon monoxide, and carbon dioxide in the presence of the catalyst. The produced hydrogen gas takes part on hydrogenation and hydrodesulfurization of heavy oil and hence improves heavy oil properties. It is assumed that (20) Wen, S.; Zhao, Y.; Liu, Y.; Hu, S. International Symposium on Oilfield Chemistry, Houston, Texas, February 28-March 2, 2007; Paper No. 106180-MS. (21) Fan, H.; Liu, Y.; Zhang, L.; Zhao, X. Fuel 2002, 81, 1733.
(22) Yufeng, H.; Shuyuan, L.; Fuchen, D.; Hang, Y. Pet. Sci. 2009, 6, 194.
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Energy Fuels 2010, 24, 2809–2816
: DOI:10.1021/ef100230k
For this study, asphaltene and resin have been separated from Liaohe heavy oil. The aquathermolysis of water-soluble and oil-soluble catalysts has been tested on these asphaltene and resin. The conversion of the catalysts is found in the order of no catalyst < NiSO4 < FeSO4 < nickel napthenate < iron napthenate. The conversion of asphaltene is 3.8-14.9% while the resin conversion is somewhat higher (8.1-22.9%). The first two catalysts are water-soluble while the last two are oil-soluble catalysts. Thus, it is found that the oil-soluble catalyst is better than the water-soluble catalyst. The H/C ratio of asphaltene and resin decreases due to the reaction. It is observed that the molecular weight of asphaltene or resin is found to be higher after reaction. It suggests that asphaltene and resin are partly aggregated by aquathermolysis. In the presence of the catalyst, however, this aggregation is less than that without the catalyst. 2.4. Dispersed Catalyst. There are also other types of catalysts used for reduction of viscosity. These catalysts are neither dissolved in water nor in oil, and the nano-nickel catalyst is one of them. The nano-nickel catalyst was prepared by using methylcyclohexane, water surfactant AEO9, n-octanol, Ni(NO3)2, and LiBH4 by a microemulsion process.23 The use of this catalyst was investigated on a Liaohe extra heavy oil. The particle of the catalyst is spheroidal in form with a mean size of 6.3 nm. The idea for use of this kind of catalyst is that it neither dissolves in water nor in oil, but it can be highly dispersed in water as well as in oil. The effective contact of the catalyst with both water and oil is very high. For the reaction, 100 g of extra heavy oil, 10 mL of nano-nickel catalyst, and 50 g of water are heated at 280 °C for 24 h at 6.4 MPa pressure. Substantial reduction of viscosity and an increase of the H/C ratio of the product have been observed. The viscosity of the feed decreases from 139 800 to 2 400 u/mPa s. It is stated that the viscosity reduction depends on the synergic effect of dilution, emulsification, and the catalyst. In this case, the added methylcyclohexane acts as a hydrogen donor. This hydrogen donor enhances the viscosity reduction and well as reduction of the sulfur content in the treated oil. At the reaction conditions, methylcylcohexane is converted to toluene and hydrogen. This hydrogen improves the H/C ratio of the upgraded crude oil. The reductions of asphaltene, resin, and sulfur are 16, 15, and 49%, respectively. Different catalysts named as aromatic sulfonic iron, aromatic bis-Schiff base, keggin K3PMo12O40, and naphthenic acidic iron have been tested for aquathermolysis.24 It is found that the aromatic sulfonic iron catalyst reduces the viscosity of the heavy crude oils more effectively compared with the others. The maximum viscosity reduction is observed around 93%. The viscosity reduction of heavy oil on Keggin K3PMo12O40 catalyst is 79%. So it is noted that aromatic iron is more active than the Keggin catalyst. This aromatic iron catalyst has also reduced resin and asphaltene by 10.44 and 4.22%, while the saturate and aromatic increased by 8.43 and 6.23%, respectively. It is stated that in general the conventional catalyst like the transition metal salt can reduce viscosity 80-90% at 280 °C and around 75-85% at 240 °C. In their other work,25 the detail studies have been conducted to test the efficiency of the aromatic sulfonic iron and the aromatic sulfonic molybdenum. It has
been noticed that the reduction of viscosity by molybdenum (99.3%) is higher than the iron (95.6%) catalyst. It has been concluded that during catalytic aquathermolysis, several reactions, like pyrolysis, depolymerization, hydrogenation, isomerization, ring-opening, oxygenation, alcoholization, esterification, and reconstruction, took place. Though these catalysts are very effective in the laboratory, these are not so effective in a real oil field. The efficiency of these catalysts in an oil field depends on the homogeneity of the temperature in the oil floor. When the superheated water is injected into the oil reservoir, the oil surface temperature is high. However, temperature is gradually lower on the depth of the oil floor and hence the catalyst loses its activity. Also, the watersoluble catalysts do not have a high ability for contact with heavy oil and hence do not produce the expected results. These deficiencies could be avoided if the aromatic sulfonic iron type of catalyst is used. This catalyst has two types of compositions; one is a high active metal cation and the other one is an amphilic anion which is aromatic sulfur in this case. The metallic ions are dispersed effectively into the oil-water while the amphiphilic anion penetrates into the large molecules of the resin and asphaltenes. As a result, it can damage the firm associating structure, breaking the close packed aggregation and reforming a loose structure. In this way, this catalyst can mix up very easily with the oil and water system and reduces the viscosity more effectively. It is also observed that this kind of amphiphilic anion is active even at low temperature. Therefore, it can be used successfully in the oil field. The activity result of this catalyst in the oil field will be discussed later. The use of an amine chelating complex for the viscosity reduction of Venezuelan heavy crude is also reported.26 The various kinds of amines and chelating agents were used in this invention. The viscosity is reported to be reduced by a maximum of 63%. Another type of catalyst used for viscosity reduction is a solid super acid. The use of two solid super acids SO42-/ZrO2 doped with Ni2þ or Sn2þ is reported by Ping et al.27 The reaction was carried out in an autoclave by using Shengli heavy oil with the catalyst (100:0.05 mass ratio) at 240 °C, 3-4 MPa for 24 h. The viscosity is reduced by 57.7 and 48.9% for the Ni2þ/SZ and Sn2þ/SZ catalysts, respectively. The above said reactions are performed without water. The effect of water content on the viscosity reduction was also tested, and it was found that these solid super acids did not work fruitfully in the presence of water. The viscosity is reduced around 57.7% without water, and the reduction of viscosity is gradually decreasing with the addition of water and it is reduced to around 10% with addition of 20 wt % water into the reaction mixture. It is explained that in the absence of water, sulfur compounds and disulfides are decomposed to H2S and lower molecular weight thiols. However, such decomposition is limited in the presence of water. It is also stated that the concentration of surface SO42- of the catalyst may be reduced when water is present. The ionic liquids and metal ion modified ionic liquids have also been reported for use for the viscosity reduction of heavy oils.28,29 The different catalysts, as stated above, mineral, water-soluble, water-insoluble, dispersed, solid (26) Chheda, B. D.; Banavall, R. M.; Mazza, G. Recovery and transportation of heavy crude oils. U.S. Patent 6,402,934, June 11, 2002. (27) Ping, J.; Qingbiao, L.; Mei, H.; Daohua, S.; Lishan, J.; Weiping, F. Chem. Eng. China 2008, 2 (2), 186. (28) Fan, Z.-x.; Wang, T.-f.; He, Y.-h. J. Fuel Chem. Technol. 2009, 37 (6), 690. (29) Fan, H.f.; Li, Z.-b.; Liang, T. J. Fuel Chem. Technol. 2007, 35 (1), 32.
(23) Wei, L.; Hua, Z. J.; Hua, Q. J. J. Fuel Chem. Technol. 2007, 35 (2), 176. (24) Chen, Y.; Wang, Y.; Wu, C.; Xia, F. Energy Fuels 2008, 22, 1502. (25) Wang, Y.; Chen, Y.; He, J.; Li., P.; Yang, C. Energy Fuels 2010, 24, 1502.
2814
2815
VR = viscosity reduction, AR = asphaltene reduction, AMWR = average molecular weight reduction, SR = sulfur reduction.
280 200 Liaohe extra heavy oil heavy oil
a
Nano Ni catalyst and methylcyclohexane aromatic sulfonic iron, aromatic bis-Schiff base, keggin K3PMo12O40, naphthenic acidic iron 24 24
mineral (China) Fe(II), Co(II), Ni(II), Mo(II), Al(II), tetralin Nano keggin heteropoly acid salt K3PMo12O40 oil soluble molybdenum oleate, FeSO4, NiSO4, iron naphthenate, nickel naphthenate 10-25 3 160-280 160-260 200-280 280 Liaohe heavy oil Liaohe heavy oil heavy oil Liaohe heavy oil
6.4 6-7
24 240
(30) Liu, Y.; Fan, H. Energy Fuels 2002, 16, 842. (31) Strusz, O. P.; Mojeisky, T. W.; Payzant, J. D.; Olah, G. A.; Surya Prakash, G. K. Energy Fuels 1999, 13, 558.
12-48 24-240 24 48
mineral (China) VO , Ni , Fe
2þ
P (MPa) feed
From the above discussion, it is noted that the catalysts indeed work for the reduction of viscosity and hence these catalysts can be used to improve oil production. The laboratory
heavy oil
4. Oil Field Test
T (°C)
reaction conditions
reaction time (h)
catalyst
3þ
(catalyst)
Table 2. Activities of Different Catalysts Used for Aquathermolysisa
The use of various hydrogen donors like cylcohexane, methylcyclohexane, and tetralin during aquathermolysis is reported to have extra benefits for viscosity reduction.14,16,18,23,30,31 It is believed that at aquathermolysis conditions, hydrogen donors produce hydrogen which takes part in upgrading of heavy crude oils. The use of tetralin during upgrading of Hamaca (Venezuelan) extra heavy oil was investigated by Ovalles et al.14 In a typical experiment, 150 g of Hamaca sand, 15 g of crude, 15 g of tetralin, and 15 g of water are heated in a batch reactor at 280 °C for 24 h. The viscosity of crude oil is gradually decreasing with the addition of tetralin, and it is reduced to 2900 cP from 9870 upon addition of 50% w/w of hydrogen donor. The presence of naphthalene in the final products suggests that tetralin is converted to naphthalene via an intermediate 1,2-dihydronaphthalene and hydrogen. This hydrogen takes part for upgrading of heavy crude oil at 280 °C. The use of tetralin during thermal cracking is also very helpful for reduction of coke formation.16 The coke formation of vacuum residue with and without tetralin has been investigated, and it is found that the presence of tetralin during thermal cracking drastically reduces the coke formation from 3.6 to 0.3 wt %. It has also been investigated that with increasing the amount of tetralin, in general, the coke formation is less. This observation is found up to 10 wt % of tetralin. Methylcyclohaxene (MCH) is also employed as a hydrogen donor during hydrocracking of Alberta’s heavy oil.31 It is believed that under hydrocracking conditions in the presence of superacid HF.BF3, MCH is oligomerized and produces conjugated alkenes. Wei et al.23 uses cylcohexane, methylcyclohexane, and tetralin as solvent for the preparation of the nano-nickel catalyst. The activity of this catalyst has been discussed earlier. It is mentioned that these solvents can also act as a hydrogen donor at aquathermolysis conditions. Methylcylcohexane is converted to toluene and hydrogen at reaction temperature. This hydrogen improves the H/C ratio of the upgraded crude oil. A synergic effect of the hydrogen donor with metal has been reported.18 At 240 °C and 72 h of reaction, the viscosity is reduced to 40 and 60% in the presence of only tetralin and Fe, respectively. However, it is reduced to 90% when both tetralin and Fe are present in the reaction system. Therefore, the amount of hydrogen donor in the presence of Fe(II) catalyst has been studied and the results showed that with an increasing amount of hydrogen donor the viscosity reduction increases. The maximum viscosity is reduced to 90% with addition of 15 mL of tetralin and 0.02 M of Fe(II). However, the use of more than 15 mL of tetralin does not improve the viscosity significantly.
23 24
12
VR = 3.8 (no additive), VR = 13.4 (water), VR = 25.6 (water þ mineral), VR = 76.4 (water þ catalyst), VR = 86.3 (water þ mineral þ catalysts) VR = 25.8 (240 °C), VR = 36.9 (280 °C), AMWR = 19.1 (280 °C) VR = 90, SR = 67 VR = 92, AR = 2.6 VR = 90, AR = 3.8 (no catalyst), AR = ∼7.6(NiSO4), AR = ∼9.2 (FeSO4), AR = 13 (Ni-naphthenate), AR = 14.9 (Fe-naphthenate) VR = 98, SR = 49, AR = 16 VR = 93, AR = 4.22
3. Use of Hydrogen Donor
15 18 19 22
ref
super acids, etc., have been used by different investigators for the reduction of viscosity as well as improvement of oil qualities. However, the efficiency for the viscosity reduction is not the same for all catalysts. The activities of these catalysts have been compared in Table 2.
results (%)
: DOI:10.1021/ef100230k
2þ
Energy Fuels 2010, 24, 2809–2816
Energy Fuels 2010, 24, 2809–2816
: DOI:10.1021/ef100230k
Table 3. Comparison of Viscosity Reduction between Laboratory and Oil Field Tests viscosity reduction (%) ref
catalyst
lab
oil field
difference of viscosity reduction (lab - oil field)
16 18 24
water-soluble Fe(II) with tetralin oil soluble molybdenum oleate dispersed aromatic sulfonic iron
90 90 93
80 78 82
10 12 11
test results show that with the use of catalysts, whether watersoluble or oil-soluble, the viscosity is reduced around 70-90% compared with the original heavy crude oil depending on the type of catalyst and temperature used. However, there is no guarantee that these laboratory catalysts can also equally work in the real oil field. Therefore, a few researchers have also tested their catalysts in the oil fields. Five different oil fields are selected for the real oil field test (Liaohe Oilfield 2001).18 At first, a little steam is injected to heat up the oil reservoir. Then the catalyst is injected at 0.2 kmol per ton of steam and hydrogen donor at 0.1 m3 per ton steam. The catalyst Fe(II) is used with the hydrogen donor or separately. A maximum 80% of viscosity reduction is noted after 14 days. The recovery of oil is increased significantly on average 828 tons per well. The properties of the recovery oil have also been improved. Liaohe oil fields, Qi-40 and Qi-108 blocks, have been chosen for the catalyst test by Wen et al.20 Some steam is injected to heat up the oil reservoir. The catalyst solution and the rest of the steam are then injected to the reservoir and then oil field is closed for 7-10 days. For their experiment, an oil-soluble molybdenum oleate organic catalyst was used. The viscosity is reduced by a maximum of 78.2%. The results also show that the oxygen, sulfur, and nitrogen contents are lower than the untreated oil. The cycle decline rate of heavy oil production is also improved. The catalytic aquathermolysis indeed helps to reduce viscosity and improve oil production as reported by Chen et al.24 The viscosities of the two oil fields G61012 and G6606 are reduced by 79.66 and 82.25%, respectively, due to catalytic aquathermolysis. The results are after 14 days of reaction. It is worth mentioning that in this case, aromatic sulfonic iron was used as the catalyst. It is also noted that oil production increases by 189 and 217 ton, respectively, for these two oil fields. The results of viscosity reduction obtained in laboratory experiments and the field test have been compared in Table 3.
The comparison is not exact because the viscosity reduction depends on several factors. In some cases, the crude oil properties of the laboratory test and the oil field test are not the same. In this table, three catalysts, one is water-soluble, one is oil-soluble, and another is dispersed catalysts, have been compared. All three catalysts show slightly higher activity in laboratory tests. The differences between the laboratory and field tests for all these three cases are more or less the same (10-12%). Although there are slight differences between the laboratory and oil field tests, the results show that indeed the use of catalyst, in whatever form, has a synergic effect on the viscosity reduction of heavy oil and hence the catalytic aquathermolysis could be used effectively for oil recovery. 5. Concluding Remarks Aquathermolysis is widely used for the exploration of heavy crude oil. It is noticed that a catalyst substantially reduces the viscosity and hence improves the flow properties of the heavy oils. In general, the catalysts employed for aquathermolysis are mineral, water-soluble, oil-soluble, and dispersed catalyst. The viscosity reduction achieved by these catalysts is in the order of mineral < water-soluble < oil-soluble < dispersed catalyst. It has also been found that during aquathermolysis, the content of saturates and aromatics increases while the amount of asphaltene and resin decreases. The use of different hydrogen donors in aquathermolysis also improves the quality of the heavy crude oil. The most commonly used hydrogen donor is tetralin. Moreover, when tetralin is used with a catalyst, the viscosity is reduced more effectively. The use of catalysts in real oil fields indicates that the catalysts can substantially reduce the viscosity, and hence the catalytic aquathermolysis process can be used effectively for exploration of heavy crude oils.
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