Energy & Fuels 2001, 15, 1013-1020
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Dissolution of Solid Deposits and Asphaltenes Isolated from Crude Oil Production Facilities L. Carbognani* Department of Analytical Chemistry, PDVSA-Intevep, P.O. Box 76343, Caracas-1070A, Venezuela Received January 24, 2001. Revised Manuscript Received June 8, 2001
Solid deposits that precipitate in oil production facilities are extremely complex materials. The most insoluble asphaltene fractions, inorganic species, very large alkanes, and insoluble hydrocarbon species often contribute to their formation. In this work, five dissolution techniques were explored to investigate their dissolution in organic solvents. Three of the approaches were based on common laboratory hardware designed for temperature and mixing control. The fourth was suited for operation at high pressure and temperature, enabling us to investigate the effect of these parameters on the dissolution process. The last approach was suited to carry out kinetic dissolution studies, using gravimetric and spectrophotometric quantitation techniques. A practical dissolution technique was selected among the five described above in order to perform routine analysis. Deposits solubility mappings are described, which combine solubility of solids in solvents of known aromatic contents plus the solids composition. Analysis of the information provided by these maps allows us to understand the behavior of each sample, as well as the selection of the appropriate treatment for deposit removal. Temperature was found to be the physical parameter that influences the most the dissolution phenomenon. Aromatic hydrocarbon content was a key chemical property of solvent mixtures that enhance their dissolving power, when applied to very insoluble materials. High asphaltene fraction aromaticity was observed to impair their solubility, being the determined dissolution kinetics inversely proportional to this property. Also, high asphaltene aromaticity values were found to correlate with high density and low H/C ratios.
Introduction The first known report in solid deposition for Venezuelan oil production facilities occurred during the seventies.1 Unfortunately, nowadays solid deposition is a widely spread problem that decreases the revenue of the petroleum industry. Rough estimates suggest that 20% of Venezuela’s daily production of light and medium crudes can suffer from the influence of this problem. Extensive research has shown that simultaneous causes influence the solid generation process, among which the following are deemed the most important:2-4(1) the solvent power of the maltene fraction of the crudes; (2) the aromaticity and aromatic condensation of the asphaltenes: (3) minerals coproduction; (4) operational variables such as production rate, gas separation, pressure, and temperature gradients. To cope with this complex problem, a general preventive and corrective scheme has been developed and is currently implemented in Venezuelan oil production facilities.5,6 Prevention is based on the determination of PVT properties * E-mail:
[email protected]. (1) Nierode, D. E. Internal report EPR.21PS: 82, VC.40.82. Exxon, 1982. (2) Carbognani, L.; Espidel, J. Visio´ n Tecnol. 1995, 3, 35-42. (3) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13, 351-358. (4) Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes and Asphalts, 2. Developments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science, B. V.: Amsterdam, 2000; Chapter 13, pp 335-362. (5) Rivas, O. Visio´ n Tecnol. 1995, 2, 4-17.
of crudes, asphaltene continuous monitoring, and phase behavior forecasting by means of a predictive model that considers the existence of multiple phases.5 The corrective actions rely on three key facts: (1) the physicochemical characterization of available deposits,2-5 (2) identification of the proper asphaltene precipitation inhibitors,6 and (3) redissolution of the solids, if their organic nature allows the use of organic solvents. The last aspect will be covered in greater detail during this work. When dealing with solubility aspects for oil fractions, most of the available information is focused on asphaltenes rather than real complex-solids such as those formed within production facilities. Two general areas of research are found in the literature. The first one is focused on fundamental aspects on solubility and flocculation.7-14 The second area is of a practical nature and it is devoted to the identification of means suitable (6) Carbognani, L.; Contreras, E.; Guimerans, E.; Leo´n, O.; Flores, E.; Moya, S. Proceedings SPE International Symposium on Oilfield Chemistry, Houston, TX, Feb. 13-16, 2001 (paper SPE 64993). (7) Hotier, G.; Robin, M. Rev. Inst. Fr. Pet. 1983, 38, 101-120. (8) Hirschberg, G. A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. JSPE 1984, 6, 283-293 (paper 17376). (9) Fuhr, B. J.; Klein, L. L.; Komishke, B. D.; Reichert, C.; Ridley, R. K. Proceedings UNITAR 4th International Conference on Heavy Crude and Tar Sand, Alberta, Canada, 1989, 2, 637-646. (10) Selucky, M. L.; Fuhr, B. J.; Frakman, Z. G. Fuel 1995, 74, 8991. (11) Cimino, R.; Correra, S.; DelBianco, A.; Lockhardt, T. P. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter 3, pp 97-130. (12) Rogel, E. Energy Fuels 1997, 11, 920-925.
10.1021/ef0100146 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/26/2001
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to improve the dissolution power of the solvent media, either by combination of diverse solvents15-19 or by solvent modification with the addition of synergic chemicals.20-27 Asphaltene dissolution is complicated by other facts that are not well understood. Reportedly, hysteresis phenomena have been observed during dissolution experiments, showing that solubility is larger during the precipitation step and lower during the redissolution step.28-30 Another complication pertains to the effects exerted by mineral matter and insoluble hydrocarbon fractions that contribute to the composition of field deposits. Some findings in this direction have been reported,3 but detailed studies on this issue are not easily found in the opened literature. Leaving mineral effects on one side, a suggestion for successful redissolution has been proposed.19 According to this proposal, the formulations that inhibit the flocculation tendency of asphaltenes, are the best alternatives suggested for redissolution of the precipitated solids. Asphaltene dissolution studies can be grouped into three sets, depending on the presence/absence of an accompanying porous medium. The first set pertains to the displacement studies carried out with asphaltenes precipitated over synthetic or natural porous media. Some of these studies showed that clays favor the deposition of organic species on the surface of the porous medium.31,32 It has been found that the amount of precipitated materials is not predicted by static experi(13) Wiehe, I. A.; Liang, K. S. Fluid Phase Equil. 1996, 117, 201210. (14) Wiehe, I. A.; Kennedy, R. J. Energy Fuels 2000, 14, 56-59. (15) Trbovich, M. G.; King, G. E. Proceedings SPE International Symposium on Oilfield Chemistry, Anaheim, CA, Feb. 20-22, 1991, pp 393-400 (paper SPE 21038). (16) Deo, M. D.; Hwang, J.; Hanson, F. V. Fuel Process. Technol. 1993, 34, 217-228. (17) DelBianco, A.; Stroppa, F.; Bertero, L. Proceedings International Symposium Oilfield Chemistry, San Antonio, TX, Feb. 14-17, 1995, pp 493-498 (paper SPE 28992). (18) Jamalauddin, A. K. M.; Nazarko, T. W.; Sils, S.; Fuhr, B. J. Proceedings UNITAR 6th International Conference Heavy Crude and Tar Sand; Meyer, R. F., Ed.; 1995, 2, 579-586. (19) Minssieux, L. Proceedings Formation Damage and Control Conference, Lafayette, LO, Feb. 18-19, 1996, pp 289-301 (paper SPE 39447). (20) Schantz, S. S.; Stephenson, W. K. Proceedings SPE 66th Annual Technical Conference and Exhibition, Dallas, TX, Oct. 6-9, 1991, pp 243-249 (paper SPE 22783). (21) Chang, C. L.; Fogler, H. S. Proceedings International Symposium Oilfield Chemistry, New Orleans, LA, Mar. 2-5, 1993, pp 339349 (paper SPE 25185). (22) Piro, G.; Rabaioli, M. R.; Barberis Canonico, L. Proceedings International Symposium Formation Damage Control, Lafayette, LO, Feb. 7-10, 1994, pp 1-2 (paper SPE 27386). (23) Barberis Canonico, L.; DelBianco, A.; Piro, G.; Stroppa, F.; Carniani, C.; Mazzolini, E. I. Proceedings 5th International Symposium Chemistry in the Oil Industry, Ambleside, U.K., Apr. 12-14, 1994, pp 220-233. (24) Bouts, M. N.; Wiersma, R. J.; Muijs, H. M.; Samuel, A. J. J. Pet. Technol. 1995, 47, 782-787. (25) Persukarome, P.; Chang, C.; Fogler, H. S. Ind. Eng. Chem. Res. 1997, 36, 3960-3967. (26) Nalwaya, V.; Tangtayakom, V.; Piumsomboon, P.; Fogler, S. Ind. Eng. Chem. Res. 1999, 38, 964-972. (27) Mohamed, R. S.; Loh, W.; Ramos, M A. C. S.; Delgado, C. C.; Almeida, V. R. Pet. Sci., Technol. 1999, 17, 877-896. (28) Andersen, S. I. Fuel Sci. Technol. Int. 1992, 10, 1743-1749. (29) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12, 1551-1577. (30) Andersen, S. I.; Stenby, E. H. Fuel Sci. Technol. Int. 1996, 14, 261-287. (31) Minssieux, L. Proceedings SPE International Sympoisum Oilfield Chemistry, Houston, TX, Feb. 18-21, 1997, pp 401-419 (paper SPE 37250). (32) Minssieux, L.; Nabzar, L.; Chauveteau, G.; Longeron, D.; Bensalem, R. Rev. Int. Fr. Pet. 1998, 53, 313-327.
Carbognani
ments. Larger quantities have been found during dynamic experiments compared to what is expected from adsorption isotherms.33 Effective chemical mixtures have been described for dynamic damage removal from reservoir rocks.34 Long-term precipitation inhibition has been claimed by chemical modification of the reservoir zone located right in front of the production tubing perforations.20 The second area of investigation comprises studies in which comparison was made between asphaltenes dissolution from bulk experiments against their displacement from porous media. Adsorbed asphaltenes proved to be harder to redissolve when compared with their bulk redissolution.23 The chemical nature of the solids was observed to influence the solubilization process,23 and mixtures of aromatic solvents enriched with alcohols and amides were observed to improve their solubility.19 The third set of publications on solubility topics comprises works devoted to the study of asphaltene bulk dissolution. Systematic studies on asphaltenes dissolution have been carried out. In these works, in-line sample cells provided with porous membranes for solids retention were described. Quantitation of dissolved fractions was achieved spectrophotometrically.25,26 Other authors describe studies based on gravimetric approaches as well as other criteria of measurement to be described in the ensuing text. Pyrolysis detection was used for chromatographic determinations which require a very small amount of sample.10 Liquid-solid Soxhlet extractors were used to facilitate the transfer of organics into solution, which were then spectrophotometrically quantified.22 Supercritical technology,16 ultrasonic energy,28-30 and addition of cosolvents15 have been described as techniques that enhance solids solubilization. Unusual equipments have been described in order to study solid dissolution kinetics. One such system relies on video monitoring of the effluents from a glass micromodel.21 Another system uses a thermostated rotating device which is removed from the system and weighted as a function of time.35 One system recently described, resembles the former one, but operates continuously on a microscale level relying on a sample probe coupled to a microbalance.36 In the present work, dissolution of asphaltenes and solid deposits sampled from oil production tubings was evaluated using different experimental techniques. Temperature and pressure effects on solubilization were investigated as well. Gravimetric and spectrophotometric approaches were compared for quantitative analysis. The general objectives of the work are (1) to get an understanding of the influence of solvent and deposit composition on redissolution, and (2) to develop a fast, (33) Piro, G.; Barberis Canonico, L.; Galbariggi, G.; Bertero, L.; Carniani, C. Proceedings European Formation Damage Conference, The Hague, The Netherlands, May 15-16, 1995, pp 317-327 (paper SPE 30109). (34) Ohen, H. A.; Moreno, T.; Marcano, D.; Acosta, A.; Mengual, R.; Gil. J.; Velasquez, A.; Daneshjou, D.; Leontaritis, K. Proceedings SPE European Formation Damage Conference, The Hague, The Netherlands, May 31-June 01, 1999, pp 133-159 (paper SPE 54772). (35) Bernadiner, M. G. Proceedings SPE International Symposium Oilfield Chemistry, New Orleans, LA, Mar. 6-9, 1993, pp 421-428 (paper SPE 25192). (36) Kleinitz, W.; Andersen, S. I. Proceedings 4th International Symposium on Thermodynamic of Heavy Oils and Asphaltenes, Copenhague, Denmark, Aug. 27-31, 2000 (paper 23).
Dissolution of Solid Deposits and Asphaltenes
reliable, and easy-to-perform routine protocol that ranks solvent formulations according to solvent power. Experimental Section Samples. Solid deposits and light-medium crude oils came either from the western Maracaibo basin or from the eastern Venezuela basin. These geographical locations have been presented elsewhere.37 One asphaltene sample was isolated from an extra-heavy crude from the Orinoco Oil Belt, i.e., Cerro Negro 550 °C+ vacuum residue asphaltene, formerly studied in another work.38 The deposits were characterized following the general isolation and analysis scheme already published.3,4,37 Prior to dissolution experiments, the deposits were dried for at least 2 days inside a vacuum oven kept at 80 °C and 130 mmHg. The dried solids were ground and homogenized in an aghata mortar. Sticky samples were homogenized by thorough mixing with the aid of spatulas. Asphaltenes isolation by n-heptane precipitation and crude oil characterization protocols have already been described in detail.3,4,37 Solvents and Commercial Asphaltene Inhibitors. Laboratory solvents were “distilled in glass” quality (suited for HPLC). They were used as received and were provided either from Burdick&Jackson, Merck, Fisher, or Baker. Kerosene and atmospheric gas oil were commercial distillates produced in PDVSA refineries located in Venezuela. Distillation ranges determined by a standard procedure39 were 154-238 °C and 180-320 °C, respectively. The aromatic cuts employed in the study were specialty products from petrochemical units located within the refineries. Dispersant asphaltene inhibitors employed in this work were commercial products formerly found to show high activity.6 Solid Deposit Dissolution Using a Rotary-Evaporator Technique. A sample of nearly 1 g (weighted to the nearest 0.1 mg) is placed in a 100 mL round-bottom flask, and 100 mL of the solvent to be tested is poured. The flask is immersed in the bath of a rotary-evaporator kept at 70 °C and the sample is rotated at 300 rpm for 1 h. The insoluble portion of the mixture is filtered through a preweighted filter paper (Whatman #1, medium type) placed in a jacketed filtering device thermostated at 70 °C. Vacuum is applied to the filtering device to speed the process. The filter is washed with n-hexane, dried, equilibrated to ambient humidity, and weighted, being the insoluble remains calculated by difference. Solid Deposit Dissolution by Refluxing. The sampling procedure described in the former section was repeated, and the following changes were introduced. A “U” glass union (Claissen) was attached to the flask. A reflux condenser was placed over one of the branches on the Claissen and a thermometer ((2 °C readings) was attached to the other. Temperature readings were taken for vapors in equilibrium with the boiling mixture. To guarantee this aspect, the thermometer was placed in the middle of the Claissen tube. Heating was provided with a heating mantle coupled to a variable voltage selector. The mixture was refluxed for 1 h, computed after the first drop of condensate was observed at the thermometer tip. Filtration was performed after the flask and its contents were cooled to nearly 70 °C, by externally blowing compressed air. Solid Deposit Dissolution with the Aid of Ultrasonic Energy. The sampling procedure described in the former two sections was repeated, but mixing of the contents of the flask (37) Carbognani, L.; Espidel, J. Proceedings UNITAR 6th International Conference on Heavy Crude and Tar Sand, Houston, TX, Meyer, R. F., Ed., Feb. 12-17, 1995, pp 551-560. (38) Carbognani, L.; Espidel, J.; Carbognani, N.; Albujas, L.; Rosquete, M.; Parra, L.; Mota, J.; Espidel, A.; Querales, N. Pet. Sci. Technol. 2000, 18, 671-699. (39) Standard Method ASTM D-2887-97a. Ann. Book ASTM Stand. Petroleum Products, Lubricants and Fossil Fuels 2000, 05-02, p 193.
Energy & Fuels, Vol. 15, No. 5, 2001 1015 was induced with an ultrasonic bath. A Cole-Parmer model 8853 equipment was used. According to the manufacturer, the bath operates at a frequency of 55 kHz. Water was the fluid employed in the system. Temperature was set at 70 °C, requiring the use of an additional 150 W thermoregulating heater to attain the set temperature. Ultrasonic energy was applied only for a period of 30 min, on the basis of experiments revealing that the dissolution of the material reaches an asymptotic plateau after this period of time. Solid Deposits Dissolution with Variable Pressure and Temperatures. A high-pressure, high-temperature supercritical fluid extractor from ISCO was used for these experiments. A model SFX-10 extractor was employed, designed to operate at 150 °C and 10 000 psig. Solvents were delivered by a syringe pump operating at the constant pressure mode. The pump was an ISCO model 260D, with a maximum operating range of 7500 psig. Extraction cells also from ISCO, with 0.5 mL capacity and 2 µm fritted terminals were used. A 0.2 g sample (weighted to the nearest 0.1 mg) was used for each extraction sequence. Temperature values investigated were fixed at 25, 70, and 140 °C. Pressures were set at predetermined values of 500, 2500, and 5000 psig. Ten milliliter samples of eluted fractions were collected inside tared 20 mL vials, provided with volume marks. Once the cell was pressurized at the selected temperature, an equilibrium period of 1 min was maintained before the cell was opened. To be able to collect 10 mL fractions, the cell filling-emptying process must be repeated between 2 and 40 times, depending on the pressure and temperature employed in each series of experiments (at high pressure and temperature, the cell empties very rapidly). Depending on the former parameters, the experimental sequence was completed within collection periods spanning from 10 min to 3.5 h. For highly soluble samples, at least three fractions were collected. Difficult to dissolve samples left some insoluble remains inside the cell, even after many fractions have been eluted (g5). For these cases, the insoluble remains were quantitatively transferred to a tared vial by repeated washing the cell with n-heptane and with the aid of ultrasonic energy. Solvents were distilled in a micro-rotaevaporator provided with nitrogen flushing. Collected fractions were brought to constant weight inside a vacuum oven kept at 80 °C and 130 mmHg. Material balances were determined for all cases. Dissolution Kinetics. Liquid chromatography (HPLC) components were assembled into a system suited for monitoring asphaltene and deposits dissolution. The sample cell was an stainless steel column (4 mm ID, 5 cm long.) provided with stainless steel fritted ends (2 µm pores according to the manufacturer). Samples in the amount of 60 ( 10 mg of asphaltenes or 85 ( 15 mg of solid deposits (weighted to the nearest 0.1 mg) were placed inside the cell. A Waters model 590 EF pump was used to deliver solvent at a rate of 0.5 mL/ min. Before the cell, a pressure gauge was inserted in a “T” connector. The effluent from the column was directed to a Milton Roy model Spectromonitor 3100 UV-Vis HPLC detector, set at 800 nm and the lowest sensitivity (2 absorbance units). The signal from the detector was registered with a linear paper chart recorder. Eluted fraction areas were computed by cutting and weighting the chromatographic traces in order to correlate with gravimetric determined fraction distributions. The eluate from the detector was collected in tared 20 mL vials (see previous section). Workup for collected fractions was the same as described in the previous section. Again, material balances were calculated for all cases. As described in the previous section, part of the samples resisted the dissolution process and remained inside the column. To recover all solid material leftovers in a quantitative way, chloroform was used as solvent. Repeated washings with warm solvent (50 °C) plus ultrasonication, allowed us to recover the material thoroughly adhered to the column walls. Hydrocarbon Group-Type Distributions for Solvents.
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Carbognani
Table 1. Composition of Solid Depositsa deposit
inorg.
sat.
arom.
res.
asph.
insol. org
M4-1 S2 F1-3I P1-3 D T9 D TJU
2b
15.4 17.4 16.8 7.1 15.2 18.6
15.1 7.5 17.8 8.8 14.2 22.0
8.7 4.4 7.7 3.9 5.7 8.1
58.7 68.8 54.8 45.9 13.1 35.9
0.0 0.0 2.7 ca. 2b 50.2 14.9
ca. ca. 1b 0.2 32.3 1.5 0.6
a Composition was determined by a published characterization scheme.4 Values are expressed in wt %, on a dried basis. Components described are inorganics, saturates, aromatics, resins, asphaltenes, and insoluble organics. b Approximate values (irreproducible results).
The relative alkane/aromatic fraction distribution for the used solvents was determined following published methodologies. Either % vol or wt % were determined, being interconverted by density values measured at 25 °C by means of pycnometry. For % vol determinations, an HPLC methodology was followed.40 Results, expressed in wt %, were obtained following a standard methodology.41 Pycnometric Determination of Asphaltene and Solvent Densities. Densities of ground asphaltene samples were determined at 25 °C by means of pycnometry. Distilled water was used for calibration purposes. However, n-heptane was used as displacing fluid for asphaltene samples. A sample in the amount of 0.50 ( 0.05 g (weighted to the nearest 0.1 mg) was introduced in a calibrated 10 mL glass pycnometer. Approximately 9 mL of solvent was added and the mixture was sonicated for 2 min to release air bubbles trapped inside the solid. The pycnometer was filled with solvent and thermostated by immersion in a water bath whose temperature was controlled with a precision of (1 °C. Reported results were average of three determinations. The values were meaningful to the second decimal figure (RSD% less than 1). Solvent densities were determined with the calibrated pycnometer as well. Samples were poured inside the pycnometer body and thermostated inside the bath before weighting.
Results and Discussion Atmospheric Pressure Dissolution Techniques. Evaluation of simple dissolution techniques was carried out in order to select the most convenient one for routine purposes. The techniques investigated comprised dissolution by refluxing, dissolution with the aid of temperature plus ultrasonic energy, and solubilization with the aid of temperature plus agitation. Two organic deposits (mineral contents < 2 wt %, see M4-1 and S2 in Table 1) were selected to carry out the comparisons. Dissolution results were plotted in Figure 1. According to the results, the observed parameter that governs the dissolution of the two samples was the aromatic content of the solvent mixture. Once the content surpassed ca. 70% vol, the whole organic components including asphaltenes were dissolved. These results agree with others formerly published.3,11,17,42 Despite the fact that deposit S2 contains more asphaltenes than deposit M4-1 (Table 1), from the results shown on Figure 1 it appears easier to dissolve. Generally, structural differences for asphaltenes can explain this behavior. In this regard, aromaticity, hydrogen deficiency, and density will be (40) Carbognani, L. J. Chromatogr. A 1994, 663, 11-26. (41) Standard Method ASTM D-5186. Ann. Book ASTM Standards 1992, 05-03, 855-857. (42) Carbognani, L.; Espidel, Y.; Izquierdo, A. Proceeds. ISCOP’95. 1st International Symposium Colloid Chemistry in Oil Production. Asphaltenes and Wax Deposition. Rio de Janeiro, Brazil, Nov. 26-29, 1995, pp 40-43.
Figure 1. Dissolution of solid deposits M4-1 and S2 with diverse laboratory techniques. Ratio of solvent to sample was 100 mL/g for all cases. Solvent mixtures are described in % vol units. AGO/T describe atmospheric gas-oil/toluene volumetric ratios. The points showing kerosenes overlap with modified kerosenes containing 200 ppm of asphaltene inhibitor DA1.
discussed in the following sections. However, for the two specific cases compared (S2 and M4-1) no explanation has been found since their chemical properties are alike. Despite the fact that the use of ultrasonic energy has been described,28,29 and it is generally recommended for breaking aggregates,43 its advantages were not obvious when compared with the results achieved by using rotary-evaporator mixing during this work. With ultrasonication, it took about 30 min to dissolve solid samples. This time span proved to be larger than expected. Probably, using systems with adjustable frequencies can optimize this parameter. However, the worst outcome from ultrasonic energy was not the experimental time required, but impaired filtration caused by fine particles generated during the ultrasonic treatment. The results from Figure 1 show other interesting features. The highest dissolutions were achieved when the refluxing technique was employed. The higher temperatures attainable during refluxing can explain this fact (Table 2). However, the decision was made not to follow the refluxing technique, since fumes generation (cracking under air atmosphere) was evidenced when high-boiling-temperature solvents were employed to (43) Sonochemistry: The Use of Ultrasound in Chemistry; Mason, T. J., Ed.; Royal Society of Chemistry: Cambridge, 1990.
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Energy & Fuels, Vol. 15, No. 5, 2001 1017
Table 2. Solvent Bulk Temperature in Refluxing Experiments solvent
temp (°C)a
toluene kerosene AGO/toluene: 1/1 volb AGO/toluene: 8/2 volb atmospheric gas oil aromatic cut
104 176 122 148 234 278
a Laboratory atmospheric pressure was 0.9 bar. b AGO: atmospheric gas oil.
Table 3. Repeatability of Deposits Dissolution with the Proposed Routine Technique wt % insoluble (triplicate)
m ( sa
deposit
solvent
M4-1
toluene kerosene
2.7 60.0
2.4 60.8
2.5 60.5
2.5 ( 0.1 60.4 ( 0.3
S2
toluene kerosene
0.8 67.3
0.7 67.5
0.6 68.5
0.7 ( 0.1 67.8 ( 0.5
a
m ( s: average ( standard deviation
carry out the experiments. Another interesting result shown in Figure 1 refers to the inefficiency displayed by a polymeric asphaltene dispersant additive, when added to modify the kerosene solvent properties. This inefficiency suggests that dispersants operate exclusively at the preventive level, inhibiting aggregation and precipitation. Once the solids have been formed and adhered to production facilities, the additives are not suited to achieve their redissolution. This topic will be further confirmed in an ensuing section, considering higher dopant amounts as well as higher P,T conditions. It is to be said that these results do not seem to agree with those found in previous studies that reportedly showed that asphaltenes dissolve in alkane solvents modified with sulfonic dispersant additives.21,25,26 However, there are many plausible explanations to account for these differences. The divergences could be ascribed to the diverse chemical functionality of the addtitives, to solvent differences (heptane vs kerosene), or to samples composition (complex deposits vs crude oil asphaltenes). From the results already discussed, the best alternative for routine determination of deposit solubility was the one based on the rotary evaporator. The repeatability of the determination is good, as illustrated with the results included in Table 3. This technique has been used during the past two years by the author in order to identify cost-effective treatments for each specific deposit.6 A tradeoff is reached after carefully balancing three parameters: (1) solvent dissolution efficiency (increases with aromatic content), (2) solvent cost (increases with aromatic content), and (3) ambient constraints (increase with aromatic content). Compositional maps, such as the one shown in Figure 2, help to take the best compromise on these aspects. High-Pressure and High-Temperature Dissolution of Solid Deposits. One important aspect to consider regarding deposit redissolution is the fact that field precipitates occurred under high pressure (700011 000 psig) and high temperature (100-150 °C). These conditions are commonly found inside unstable crude oil producing tubings. Consequently, it is most interesting to get a preliminary idea on the effects exerted by these parameters into the redissolution of deposits. This
Figure 2. Solubility map for the deposit P1-3. Described components are: Inorg. and Insol. Org.: inorganics and insoluble organics; Asph: asphaltenes; Malt: maltenes; V&H: volatile and humidity. The amount of each component is calculated from the difference of cut points on the composition axis. Commercial xylene containing 7% vol saturates was used for mixtures preparation.
Figure 3. Dissolution of deposit M4-1 with toluene under variable pressure and temperature conditions. A commercial high P, T extractor was used. Results plotted show the amount of sample dissolved in 40 mL of solvent.
will be illustrated with deposit M4-1, being the material used in the experiments conducted on a commercial extractor suited for maximum operation at conditions of 150 °C and 7500 psig. Figure 3 shows the results obtained for the chosen 3 × 3 (P,T) experimental matrix. At the lower temperatures (25 and 70 °C), a slight increase of solubility is exerted by pressure. However, this effect is not evidenced at 140 °C. The clear outcome of the experimental array, shown in Figure 3, is that pressure does not influence redissolution to a noticeable degree, temperature being the important parameter. This can be further emphasized by the results shown in Figure 4, carried out with the same deposit under a common preset pressure of 500 psig. It appeared that redissolution is linear under 70 °C, but exponential at 140 °C. This is an important result to bear in mind, since it could help for deep wells whose temperature is over 100 °C. For these cases, a tradeoff between aromatic content of the solvent and temperature can save expenditures in solvent treatments by using minor amounts of expensive aromatic cuts. It is interesting to consider that in the temperature range between 50 and 65 °C, reportedly, some sort of intermolecular interaction
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Figure 4. Dissolution of deposit M4-1 with toluene under a constant pressure of 500 psig. A commercial high P, T extractor was used.
vanished from oil aggregates, and that around 150-300 °C the polar aggregate sizes decrease.44,45 Most probably, these facts help to explain the increased dissolution found during this study when temperature was increased from 70 to 140 °C. As mentioned in the former section, experiments were conducted to shed some light on the possibility of enhancing the dissolution power of cheap industrial solvents by means of additivation with asphaltene dispersant additives. The idea came from previous reports that described this enhancement as happening when asphaltenes were dissolved by using either alkane21,25,26 or aromatic23 solvents doped with dispersant additives. To illustrate these aspects, solubility of deposit M4-1 was tested varying additive type, additive concentration, pressure, and temperature. Kerosene was selected to carry out these experiments since its aromatic content resembles atmospheric gasoils used for field applications and its less viscous nature favors laboratory work. Results are presented in Figure 5. From these, it is to be concluded that neither high pressure, nor high temperature, nor large amounts of diverse dispersant additives allowed increasing the solvent power of the kerosene for deposit dissolution. As formerly mentioned, solubilization is mainly controlled by the aromatic content of the solvent mixture. The kerosene used for this study contained 18% vol of aromatics, value deemed too low compared with the useful range (>70%) known from previous studies.3,11,17,42 Despite the fact that an increase in solubility parameters has been reported to depend on P and T,46 this effect appear to be negligible on the basis of the results shown in Figure 5. From the known composition of the deposit, it appears that kerosene was only able to dissolve maltene fractions. Maltenes represent 39 wt % of the solid (Table 1), a value that grossly compares with the average dissolution found from the experiments shown in Figure 5 (43.5 wt %). Asphaltenes and Solid Deposit Dissolution Kinetics. Assessment of Physicochemical Parameters Affecting Solubility. Other authors have pre(44) Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Energy Fuels 1995, 9, 168-176. (45) Thiyagarajan, P.; Hunt, J.; Winans, R. E.; Anderson, K. B.; Miller, J. T. Energy Fuels 1995, 9, 829-833. (46) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983.
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Figure 5. Dissolution of deposit M4-1 in kerosene with varying P, T conditions and additive concentrations. A commercial high P, T extractor was employed. Pressure values are expressed in psig. Two active asphaltene inhibitors were tested, identified as DA1 and DA2 (DA stands for dispersant additives). Additive concentrations (wt %/vol) are shown in the entry for each experiment.
viously studied dissolution kinetic behavior for asphaltene extracted from crude oils.23,25,26 However, real samples from field deposits exert a more complex behavior, because they selectively include the most refractory portions of the asphaltenes, as well as being further modified by the coprecipitation with reservoir minerals.3,4 Deposit and asphaltene dissolution kinetics were evaluated during this study in order to gain knowledge on possible compositional effects influencing the solubility process. The experiments were conducted with an assembled HPLC hardware. The approach was recently followed for studying the dissolution of complex paraffin-asphaltene composite materials.47 Toluene was used for most of the experiments. Ambient temperature and UV-vis detection (800 nm) were selected for the experiments. The elution process was observed to be discontinuous, with sudden peaks appearing randomly. In particular, for deposits these peaks were grossly observed to coincide with pressure pulses, suggesting forced elution of particles through the column retaining frit. This behavior was not observed for asphaltenes, even though some of their fractions remained insoluble under the selected experimental conditions. These findings suggest that probably the differences could be ascribed to the inorganic fractions present in the deposit. Having observed that elution is not a continuous process, selectivity of component elution was consequently suspected. This question was deemed important to answer, since the easiest way to quantitate the eluates from kinetic dissolution studies is by integration of the spectrophotometric signal used to monitor the elution process. The approach has been successfully exploited for asphaltene samples.23,25 To answer the posed question, the ratios of (chromatographic area/ determined mass) for each eluted fraction derived from four samples were calculated and plotted in Figure 6. The first aspect to observe is that the last eluted fraction from each sequence displays a large detector response compared with its collected mass. With high probability, (47) Garcı´a, M. C.; Carbognani, L. Energy Fuels, accepted.
Dissolution of Solid Deposits and Asphaltenes
Figure 6. Ratio of chromatographic area to mass for collected fractions in kinetic studies for four samples. HPLC hardware was used. Three deposits (D) and one asphaltene (A) were analyzed. Toluene was used as diluent, with one exception (kerosene). Numbers describe sequentially collected fractions for each sample. W stand for “whole” sample and was calculated as the balanced response for the combined fractions.
this behavior was a consequence of the sluggish return of the detector signal to baseline, caused by the very slow eluting components. A second aspect to be considered is the existence of small differences in each eluted fraction signal/mass ratio, and the fact that they differ from the calculated one for the whole eluted fraction. Even more important than the previous observation is the fact that analyzed eluted fractions displayed a wide array of ratios, spanning from 1 to 90. There is an obvious explanation that accounts for the behavior of the most deviating analyzed sample (deposit M4-1 eluted with kerosene). In this case, as discussed before, kerosene was not able to dissolve asphaltene components and the eluted ones did not show noticeable response at the selected spectrophotometric wavelength. However, no explanations can be found for the diverse ratios displayed by the other three samples, which covered a range from 15 to 50. The clear consequence from all these findings is that spectrophotometric detector signals do not appear to be useful for quantitative kinetic analysis of the complex solid deposits. Dissolution kinetics from asphaltenes of varying nature was next compared, aiming at the understanding of structural aspects that can influence the solubilization process. Asphaltenes isolated from field deposits, from operationally stable and unstable crude oils were included in this part of the study. As can be observed in Figure 7, solubilization follows the known operational stability of the crudes. Those isolated from stable crudes (CN and HA) were dissolved at a faster rate compared to those arising from one unstable crude (C9). The slowest soluble ones proved to be those isolated from deposits (D T9 and D S2). Hydrogen content,3,13,48 molecular mass,48 aromaticity,2-4,37,48-50 and aromatic condensation2-4,37,50 have been suggested as the main parameters responsible for oil fractions unstability. Aromaticity for the asphaltene samples employed for the formerly discussed kinetic studies, was determined by published procedures.37 When asphaltene aromaticities were plotted against solubilized amounts (Figure (48) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12, 51-74.
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Figure 7. Dissolution kinetics for asphaltenes isolated from operationally stable/unstable oils and from solid deposits. HPLC hardware was used. Toluene was the eluent and the experiments were carried out at 25 °C. Stable samples: CN and HA; Unstable: C9 (Numbers after the asphaltene key describe the distillation cut of the crude from which these were precipitated). D stand for deposit isolated asphaltenes.
Figure 8. Correlation between aromaticity and dissolution kinetics for asphaltenes of diverse nature. Toluene at 25 °C was used for the experiments. Amount of solubilized fractions was computed for 45 mL toluene eluted. Asphaltene samples were already described in Figure 7. Aromaticity was determined by 1H NMR.
8), solubilization was found to depend inversely with the determined aromaticity. Further correlations were established among aromaticity, atomic H/C ratio, and density (Figure 9). Density was observed to correlate directly with aromaticity and inversely with H/C. All these findings support previous studies that reportedly proposed selective partition of asphaltenic fractions during the precipitation process.3,4,49,50 The fractions precipitating during field operations appear to be the most aromatic, the most dense, and the most insoluble. These findings explain why highly aromatic solvents are mandatory in order to achieve their redissolution. Experiments were conducted in order to get preliminary insights on similarities or differences existing among solubility of diverse crude oil fractions. The results found with the F1-3I crude, its deposit, and asphaltene fractions isolated from both of them illustrate the general tendencies found with other samples (49) Fuhr, B. J.; Cathrea, C.; Coates, L.; Kaira, H.; Majed, A. I. Fuel 1991, 70, 1293-1297. (50) Leo´n, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6-10.
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Figure 9. Correlation between aromaticity, density, and atomic H/C ratio for asphaltenes of diverse nature. Aromaticity was determined by 1H NMR. Density values are expressed in g/mL. Tie lines were included only to facilitate the observation of tendencies.
Figure 10. Dissolution kinetics for F1-3I crude and derived fractions. Tested asphaltenes were isolated either from the original crude or from the solid deposit sampled in the production tubing. HPLC hardware was used. Toluene was the eluting solvent and the experiments were carried out at 25 °C.
(Figure 10). Results found with toluene as solvent reveal that whole deposits initially dissolve at similar and often at faster rates compared with crude oil asphaltenes. However, at a certain time during the dissolution experiment, this trend is reversed as can be observed on Figure 10. It is estimated that two reasons can account for these findings. First, during early elution, the maltene components of the deposit help the dissolution process. Second, at the end of the experiment, dissolution becomes more difficult for the deposit since the remaining organic materials are asphaltenes. These particular asphaltenes by themselves proved to be very insoluble, as can be seen on the same figure. These results are worth considering if solubility is to be evaluated on asphaltenes isolated from produced crudes, instead of asphaltenes from field deposits or the whole solid. To be able to achieve field deposits redissolution without prior knowledge about their composition, aromatic contents of solvent mixtures must be increased to achieve complete asphaltene removal. However, aromatic components increase the treatment cost and
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toxicity. For these reasons, availability and testing of deposits is recommended to be able to make the best decision. A final aspect is worth being mentioned concerning kinetic studies carried out following the described HPLC methodology. Mass balances were calculated for the fractions eluted and recovered during the dissolution studies of 5 solid deposits and 9 asphaltenes. Average recovery was 102.7 wt % and the calculated standard deviation was 3.4. Unexpected recoveries larger than 100% have been formerly observed for asphaltene and pitch fractions. Two causes have been assessed to affect these results, namely solvent occlusion and oxidation induced during fractions isolation.38 All the kinetic results reported during the present work suffered from this type of problem and have been normalized taking into account the recovery of fractions for each dissolution sequence. Conclusions Complex deposits and their corresponding asphaltene fractions are harder to dissolve in comparison with asphaltenes isolated from crude oils. High aromaticity of solutes was assessed to decrease their solubilization kinetics. This high aromaticity was found to directly correlate with a low H/C ratio and high density. Aromatic content of the dissolving media was a key chemical property that increases their solvent power toward highly insoluble components. Chemical additives effective to avoid asphaltene precipitation failed to show any improvements of solvent power to effect redissolution of solids already formed. Pressure was found to play a minor role for the solubilization of solid deposits. Temperature was assessed to be the physical variable that exerts the major role on solubilization. From the five dissolution techniques investigated, a simple approach relying on thermal procedures proved to be the best alternative. Ultrasonic mixing, refluxing, and high pressures were worthless to try since no significant improvements were derived from their use. Routine dissolution studies carried out with field samples, allowed building solubility maps that included solvent properties as well as solid composition. These plots also allow us to rationalize on sample behavior and to propose the best treatment for solid removal on oil producing facilities. Deposit dissolution kinetic studies show that the solubilization process is discontinuous and selective. One attempt to quantitate solubilized amounts by visible spectrophotometry failed due to the selective nature of the process. A gravimetric quantitation approach successfully handled this aspect. However, mass balances greater than 100% continue to pose unresolved problems that affect the preparative isolation of oil polar fractions. Acknowledgment. The author thanks PDVSAIntevep for funding and permission to publish this work. Helpful discussions and suggestions from Drs. Rosalvina Guimerans, Olga Leo´n and Marı´a del Carmen Garcı´a are appreciated. The detailed review of Drs. Ce´sar Ovalles and Youssef Espidel is appreciated for the improvement of the original manuscript. EF0100146
