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Complex Nature of Separated Solid Phases from Crude Oils L. Carbognani,*,† M. Orea,† and M. Fonseca‡ Analysis and Evaluation Department and Reservoirs Department, PDVSA-INTEVEP, Apdo. 76343, Caracas-1070A Venezuela Received September 24, 1998. Revised Manuscript Received December 10, 1998
Precipitated solid phases from oil wells and storage tanks have hampered oil operations and decreased revenues in Venezuelan facilities during recent decades. Physicochemical characterization of oils and separated solids was undertaken 6 years ago to understand the phenomenon and contribute to its prediction under production conditions. A multidisciplinary approach was followed, adopting diverse gravimetric, spectrometric, and chromatographic procedures. The purpose of this contribution is to summarize some of the most relevant findings from this research. Emphasis was focused more on the analytical aspects rather than on the engineering ones. Three groups of variables were assessed simultaneously to help in solving the problem. These were (I) the intrinsic nature of crudes, (II) mineral and porous matter effects, and (III) operational variables.
Introduction Solid precipitation during oil production, transportation, and storage in Venezuelan facilities was first reported during the 1970s. The problem became severe during the present decade, and substantial efforts were undertaken in order to understand, predict, or eventually achieve better control over the problem. A multidisciplinary systematic approach described elsewhere1-3 was found to be successful in handling such a difficult task and, therefore, was followed in this instance as well. Preventive and corrective tools were simultaneously developed in order to handle problems arising at diverse stages. Prevention was based on a multiphase predictive model, and correction was commonly achieved by a careful selection and injection of mixtures of dispersant additives and solvents. Examples and field results in this regard can be found in previous publications.3-4 Physicochemical characterization of crude oils was initially required as an input for the predictive model. A very detailed and extensive characterization scheme was developed for this purpose.1 A restricted set of information (SARA hydrocarbon group-type analysis) was selected from this scheme, making it possible to establish comparisons among stable and unstable crudes. These comparisons shed some light on the causes of †
Analysis and Evaluation Department. Reservoirs Department. (1) Rivas, O. Vision Tecnol. 1995, 2 (2), 1. (2) Izquierdo, A.; Rivas, O. Proceedings of the International Symposium on Oilfield Chemistry, Houston, TX, Feb. 18-21, 1997; Paper SPE 37251. (3) Carbognani, L.; Fonseca, M.; Izquierdo, A.; Leo´n, O. Proceedings of the 9th International Oilfield Chemistry Symposium, Geilo, Norway, March 22-25, 1998; paper 12. (4) Salazar, R.; Blohm, N.; Molina, R. Proceedings of the 1st International Symposium Colloid Chemistry Oil Production, Asphaltenes and Wax Deposition, Rio, Brazil, Nov. 26-29, 1995; p 168. ‡
crude oil stability. The most interesting outcome was the fact that unstable crudes showed large amounts of alkane components mixed with highly aromatic and condensed asphaltenes.5,6 Also notable was the fact that unstable crudes typically proved to be medium or light oils, low in asphaltic components, and commonly produced from very deep reservoirs such as those studied by other authors.7 Crude oil characterization8,9 and sludge analysis related to refining operations10 are commonly carried out in the oil industry. Also, studies on waxy deposits can be frequently found in many articles.11-18 Engineering factors affecting wax deposition have been exten(5) Carbognani, L.; Espidel, Y. Proceedings of the 6th UNITAR International Conference on Heavy Crude and Tar Sands, Houston, TX, Feb. 12-17, 1995; p 551. (6) Carbognani, L.; Espidel, J. Vision Tecnol. 1995, 3 (1), 35. (7) DeBoer, R. B.; Leerloyer, K.; Eigner, M. R. P.; VanBergen, A. R. D. Paper SPE 24987, European Pet. Conf. Cannes, Fr. 16-18 Nov. 1992, 259. (8) Speight, J. G. The Chemistry and Technology of Petroleum; 2nd Ed, Marcell Dekker: N. Y., Basel, Hong Kong 1991. (9) Altgelt, K. H.; Boduszynski. M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcell Dekker: Basel, 1994. (10) Mushrush, G. B.; Speight, J. G. Petroleum Products: Instability and Incompatibility; Speight, J. G., Ed.; Applied Energy Technology Series; Taylor & Francis: Washington, D.C., 1995. (11) Graves, R. H.; Tuggle, R. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1968, 13 (8), 46. (12) Alex, R. F.; Fuhr, B. J.; Rawluk, M.; Kaira, H. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1991, 36 (2), 237. (13) Thomson, J. S.; Grigsby, R. D.; Doughty, D. A.; Woodward, P. W.; Giles, H. N. Proceedings of the 4th International Conference Stability and Handling of Liquid Fuels, Orlando, FL, Nov. 19-22, 1991; p 65. (14) Aquino Neto, F. R.; Nobrega Cardoso, J. N.; dosSantos Pereira, A.; Zupo Fernandez, M. C.; Caetano, C. A.; Castro Machado, A. L. J. High Resol. Chromatogr. 1994, 17 (4), 259. (15) Philp, R. P. J. High Resol. Chromatorgr. 1994, 17 (6), 398. (16) Agrawal, K. M.; Joshi, G. C. Fuel Sci. Technol. Int. 1994, 12 (7,8), 1113. (17) Abu Fazal, S.; Zarapkar, S. S.; Joshi, G. C. Fuel Sci. Technol. Int. 1995, 13 (7), 881.
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sively covered in recent studies.19-25 On the other hand, during the initial stages of this work, very few reports were found dealing with the characterization of solid deposits from oil production and transportation operations. A brief account appeared concerning some of Mexico’s experiences.26 Most of the relevant literature which can be helpful was focused on the close relation between mineral matter and organic compounds, the interaction responsible for the insolubility of organic matter, and emulsion generation during heavy oil production from tar sands.27-37 The availability of many so-called asphaltenic solid deposits (due to its black appeareance) and some waxy materials from Venezuelan facilities prompted us to devise a systematic characterization plan. Mineral and porous matter were found to influence deposit formation. Available historical production records allowed correlation of operational parameters such as flow and temperature to solids formation and also the realization that the observed phenomena were cumulative in nature, occurring at a trace level. This paper presents some of the latest findings in this regard and discussed them in a comprehensive way in order to illustrate the complex and synergistic causes identified for solid precipitation during oil production and storage. The intrinsic nature of crude oils, mineral-porous matter, and operational variables were the three groups of synergistic factors that were observed to play a role in the studied phenomena. (18) Abu Fazal, S.; Zarapkar, S. S.; Joshi, G. C. Fuel Sci. Technol. Int. 1995, 13 (10), 1239. (19) Misra, S.; Baruah, S.; Singh, K. Pap. SPE Prod. Facil. 1995, 50. (20) Brown, T. S.; Niesen, V. G.; Erickson, D. D. 68th Annual Technology Conference and Exhibition, Houston, TX, Oct. 5-6, 1993; Paper SPE 26548, p 353. (21) Khan, H. U.; Handoo, J.; Agrawal, K. M.; Joshi, G. C. Indian J. Technol. 1993, 31 (10), 697. (22) Hamouda, A. A.; Davidsen, S. International Symposium on Oilfield Chemistry, San Antonio, TX, Feb. 14-17, 1995; Paper SPE 28966, p 213. (23) Hsu, J. J. C.; Santamaria, M. M.; Brubaker, J. P. 69th Annual Technical Conference and Exhibition, New Orleans, LA, Sept. 25-28, 1994, Paper SPE 28480, p 179. (24) Erickson, D. D.; Niesen, V. G.; Brown, T. S. 68th Annual Technical Conference and Exhibition, Houston, TX, Oct. 3-6, 1993; Paper SPE 26604, p 933. (25) Hsu, J. J. C.; Santamaria, M. M.; Brubaker, J. P. 26th Annual OTC, Houston, TX, May 2-5, 1994; Paper OTC 7573, p 565. (26) Escobedo, J.; Mansoori, G. A. Proceedings II LAPEC, Caracas, Venezuela, March, 1992; Paper SPE 23696. (27) Ignasiak, T.; Kotlyar, L.; Sanman, N.; Montgomery, D. S.; Strausz, O. P. Fuel 1983, 63 (3), 363. (28) Mikula, R. J.; Axelson, D. E.; Sheeran, D. Fuel Sci. Technol. Int. 1993, 11 (12), 1695. (29) Kotlyar, L. S.; Ripmeester, J. A.; Sparks, B. D.; Kodama, H. Proceedings of the 4th UNITAR International Conference on Heavy Oil and Tar Sands, Alberta, Canada, 1988; Paper 16. (30) Sengupta, S.; Hall, E. S.; Tollefson, E. L. Proceedings of the 4th UNITAR International Conference on Heavy Oil and Tar Sands, Alberta, Canada, 1988; Paper 103. (31) Kotlyar, L. S.; Ripmeester, J. A.; Sparks, B. D.; Woods, J. R. Proceedings of the 5th UNITAR International Conference on Heavy Oil and Tar Sands, Caracas, Venezuela, 1991; p 97. (32) Kotlyar, L. S.; Sparks, B. D.; Kodama, H.; Gratam-Bellew, P. E. Energy Fuels 1988, 2, 589. (33) Kotlyar, L. S.; Ripmeester, J. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1988, 33 (2), 253. (34) Kotlyar, L. S.; Ripmeester, J. A.; Sparks, B. D.; Woods, J. R. Fuel 1988, 67 (11), 1529. (35) Majid, A.; Sparks, B. D.; Ripmeester, J. A. Fuel Sci. Technol. Int. 1993, 11 (2), 279. (36) Kotlyar, L. S.; Ripmeester, J. A.; Sparks, B. D.; Montgomery, D. S. Fuel 1988, 67 (6), 808. (37) Axelson, D. E.; Mikula, R. J.; Potoczny, Z. M. Fuel Sci. Technol. Int. 1989, 7 (5-6), 569.
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Experimental Section Samples. The geographical origin of the crudes and solid deposits was previously described.5 Separation Scheme. Details on Analytical Procedures. These topics have already been covered in detail.5 However, some modifications have been introduced and new methodologies developed and/or adopted. These will be described in the following paragraphs. Liquid-Solid Extraction of Organic Compounds in Solid Deposits. Dried deposits were sequentially extracted with (I) a mixture of CH2Cl2-MeOH (90-10) by vol), (II) a mixture of toluene-2-PrOH (90-10 by vol). Extracts were mixed and filtered, and the total soluble organics were recovered after solvent removal. This solvent sequence replaced the original one (pure CH2Cl2),5 since organic materials were observed to remain trapped in that case. Methanol helped for removal of very polar adsorbed compounds. Toluene was selected due to the higher operation temperature attainable, well-suited for dissolution of very large alkanes, if present. SARA Separation of Waxy Deposits. “Waxy” Asphaltene Isolation. Most of the samples were deasphalted according to a standard procedure.38 Maltenes were then separated into SARA group types following a routine methodology.39 However, asphaltenes isolated from bottom sediments proved to be mixtures of asphaltenes and very large alkane components. Isolation of pure asphaltenes in these cases, was possible by re-extraction of the original precipitate with isooctane. Serendipity dictated the selection of this effective solvent. At the time of these experiments, a paper was read on the use of isooctane as an asphaltene precipitant.40 SAR separation of waxy maltenes was achieved by a hightemperature liquid chromatography methodology already described.41 Silica gel and n-heptane at temperatures g60 °C were employed to meet this purpose. Alkane Chromatographic Subfractionation over Solid Adsorbents. Crude M12 was stored inside a barrel at ambient conditions for 4 years. During this period of time, a bottom sediment formed which was then separated by pouring off the supernatant crude. This sediment was separated into grouptype SARA fractions as described previously, and the saturates fraction was submitted to further separation using columns packed with diverse adsorbents. Fractions were sequentially eluted and collected following a step temperature gradient. Fractions described in the present work were isolated at 73 ( 2 °C. Chosen adsorbents were a synthetic attapulgite, silica gel (60 Å pores, according to the manufacturer), nonporous glass beads, and a solid nC7-asphaltene precipitated from Cerro Negro heavy oil (from Venezuela’s Orinoco oil belt). Procedures and materials have already been covered in complete detail.41 Molecular Mass Distributions (MMDs) of very Large Alkane Mixtures. MMDs of waxy alkane mixtures were obtained by size-exclusion chromatography (SEC), performed with silica colums operating at high temperature. The procedure has been already published.42 Infrared Spectrophotometry (IR). IR spectra were run on a Nicolet 20 SXB instrument. KBr compressed pellets were prepared with ca. 2 wt % sample to obtain the spectra. The analysis protocol was applied to organic materials and also to the materials remaining after ashing deposits at 400 °C under air for a period of 48 h. 13C Nuclear Magnetic Resonance. Spectra were obtained with a Brucker ACP-400 instrument at a frequency of 100.614 (38) Standard IP 143. Asphaltenes Precipitation with Normal Heptane; Standards for Petroleum and Its Products; Institute of Petroleum: London, 1981; Vol. 1. (39) Carbognani, L.; Izquierdo, A. J. Chromatogr. 1989, 484, 399. (40) Reynolds, J. G. Fuel Sci. Technol. Int. 1987, 5 (5), 593. (41) Carbognani, L.; Orea, M. Pet. Sci. Technol. 1999, submitted for publication. (42) Carbognani, L. J. Chromatogr. A 1997, 788, 63.
Nature of Separated Solid Phases from Crude Oils MHz. To get quantitative analysis, samples were dissolved in a 0.18 M CrIII acetylacetonate solution in CD2Cl2. An inversegated decoupled technique was adopted for supression of the Overhauser effect. Chemical shifts were referenced to tetramethylsilane. Fe2O3 Enrichment of Solid Sorbents. Silica gel Woelm (32-63 µm), synthetic clay (Attapulgite pellets, ca. 1-2 mm, Engelhardt), and natural clays from Venezuelan quarries (Montmorillonite and Kaolinite) were enriched with Fe2O3. An aqueous solution of FeCl3 was poured over the solid, and oxidation of the halide was forced inside an oven maintained at 100 °C, with a continuous air stream. Oxidation was carried out over a period of 2 weeks. A calculation was performed in order to leave an additional 3 wt % of Fe2O3 over the enriched solid. The iron content of the original adsorbents was determined by inductively coupled plasma atomic emission (ICP-AES), after samples were mineralized by HCl-HF microwaveassisted treatment in a CEM MDS 2100 instrument. ICP-AES measurements were performed after appropriate dilutions were prepared, employing an ARL model 3410 spectrometer set at 238 and/or 259 nm lines. Measured iron contents were 1.20, 0.36, and 0.12 wt % for the original Attapulgite, Montmorillonite and Kaolinite, respectively. The metal was not detected for the case of silica gel. These results permitted an estimate of the total iron content of Fe2O3 enriched minerals (wt %): Attapulgite (3.30); Montmorillonite (2.46); Kaolinite (2.22); silica gel (2.10). Insoluble Organics as a Function of Fe2O3 Content on the Adsorbents. A 20 g amount of virgin and/or Fe2O3enriched adsorbent was contacted with 50 mL of a toluene solution of crude, maintained at ambient temperature. The solutions contained ca. 2 g of crude (weighed to the nearest 0.1 mg), and the contacting time was 24 h. After this period, the adsorbent was filtered (Whatman paper no. 1) and then Soxhlet-extracted with toluene. Strongly adsorbed polar materials were recovered by changing the extractor solvent to CH2Cl2-MeOH (90-10 by vol). Solvents were removed with a rotary evaporator, and recovered polar compound were brought to a constant weight inside a vacuum oven (80 °C and 130 mmHg). Insoluble Organic Contents as a Function of Mineral Matter in Deposits. Dried deposits were extracted with solvent mixtures with varying amounts of aromatics. Atmospheric gas oil and commercial xylene were employed to prepare these solvent mixtures. Total aromatics were determined by supercritical fluid chromatography43 in the case of the gas oil and by gas chromatography in the case of xylene. An in-house normalized methodology44 was followed for the later case, based on a previous proposed method.45 These values were employed to calculate the aromatic content of the mixtures. Nearly 100 mg of the dried solid (weighted to the nearest 0.1 mg) was placed inside 20 mL vials and then extracted with 3 mL of the chosen solvent. The mixture was rotated for a period of 2 h inside the thermostated bath of a rotary evaporator maintained at 80 °C. Solids were filtered through preweighted nylon membranes (0.45 µm pores), with the aid of a vacuum. A constant weight of the remaining solids was achieved in the same way as that described in the previous section. The presence of insoluble organic materials in the remaining solids was checked by carbon microanalysis. The mineral content (and again insoluble organics, by difference) was determined by ashing as described elsewhere.5 If carbonates were detected in the insoluble fraction (X-ray difraction and/ (43) Standard D-5186. Ann. Book ASTM Stand.; 1989, 05-03, p 855. (44) Ehrmann, U.; Romero, I.; Herrera, M.; Aranguren, S. Report INT-01569, 86. Intevep, S. A. 1986. (45) Green, L. E.; Matt, E. Technical Paper No. 100; HewlettPackard: Avondale, 1982.
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Figure 1. Hydrocarbon group-type SARA distributions for stable/unstable crudes. Asphaltenes were precipitated with n-heptane. Integration of determined naphtha GC-saturatesaromatics, HPLC-SAR distributions of deasphalted 220 °C residua, and asphaltene quantitation gave the results displayed. or IR spectrophotometry), boiling in a 10% HCl aqueous solution was carried out before the C determination.
Results and Discussion Intrinsic Nature of Crudes as a Key to Stability. The first part of this discussion will be devoted to the effect of oil composition and its relation to observed stability. The criteria to differentiate between stable and unstable crudes was based on historical production records. No solids were ever found during stable crude oil production. The frequency of solid deposition for unstable crudes varies, typically spanning the range from days to months. The SARA group-type distributions for stable and unstable crudes can be observed in Figure 1. Attention is drawn to the fact that unstable crudes displayed larger amounts of saturates. On the contrary, stable ones showed an opposite behavior, being enriched in aromatics. These results allowed rationalization of the appropriate aromatic nature of the solvent phase of stable crudes. Asphaltenes were considered to be the dispersed phase of studied crude oils and resin components considered as the natural dispersants. Regarding asphaltenes, average structural parameters calculated by NMR and microanalysis permitted us to assess the differences between these components isolated from diverse crude oils. Two parameters were found to noticeably vary between asphaltenes. The first one, aromaticity (fa), is defined as the ratio of aromatic to aromatic + aliphatic carbons. The other parameter was defined as the aromatic condensation index CI/C1. CI stands for bridging or internal aromatic carbons. C1 denotes peripheric or nonbridging aromatic carbons. The results for these two asphaltene parameters were plotted in Figure 2. Aromaticity was found to be larger for asphaltenes isolated from unstable crudes. Aromatic condensation was simultaneously found to be greater for these compounds. The picture is even more dramatic when asphaltenes were isolated from solid deposits, as illustrated for the third group displayed in the same figure. Other authors have observed a similar behavior for diverse solubility classes of polar oil components.46 (46) Waller, P. R.; Williams, A.; Bartle, K. D. Fuel 1989, 68 (4), 520.
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Carbognani et al. Table 1.
13C NMR Derived Parameters for Asphaltenes from a Crude and Its Bottom Sediment
asphaltene crudea upper layerb bottom layerb
fac
aliphatic Cn/C1d
0.66 0.59 0.83
4.0 3.6 6.0
a Original M12 crude. b Four years stored at ambient conditions. Aromaticity (ratio of (104-158)/(14.7-52) + (104-158) ppm signals). d Ratio of (30.41/14.67) ppm signals. c
Dehydrogenation reportedly can be correlated with increases in aromaticity and condensation,47 as discussed recently.48 The former results were combined in order to get a stability parameter. The picture arising from this procedure is that unstable crudes intrinsically have a potential for solids precipitation, since they simultaneously possess aromatic-condensed asphaltenes prone to separate from their alkane-rich solvent phases. This was illustrated in Figure 3. One of the unstable crudes was observed to fall into the domain of the stable ones. The aromatic content for this sample was abnormally high, due to the presence of commercial xylene. The typical aromatic content for unstable crudes range betweeen 30 and 40 wt %. For the cited example, 12 wt % in excess was determined as a consequence of the solvent, a fact that helps to explain the larger value observed in Figure 3 for this unstable crude. Recent results obtained with so-called waxy asphaltenes isolated from bottom sediments of stored oil dramatically add evidence supporting the results already discussed in the preceding paragraphs. Waxy asphaltenes proved to be easy to recognize based on their sticky appeareance. Nevertheless, their IR spectrum provides an objective tool for identification of this type of material. A large doublet signal is observed at 719 and 730 cm-1 due to the presence of very large alkyl moieties of crystallized waxes. Purification of these species with the reported isooctane treatment (see
Experimental Section) allowed further characterization of the nature of the asphaltenic compounds selectively associated with the waxes. Results based on 13C NMR analysis are shown in Table 1. For this particular example, asphaltenes selectively associated with waxes were found to be extremely aromatic. Also, the average chain length for their alkyl substituents proved to be the longest of the three samples compared, as suggested by the ratio of the internal methylene signal to methyl chain ends (Cn/C1). Previous studies on waxy crudes indeed indicated that a high aromaticity and high molecular weight are characteristic for the lowest solubility and most paraffinic-solvent incompatible asphaltenes.49 The existence of the wax-asphaltene interaction was shown to affect the crude pour point and its response to paraffin crystallization inhibitors.50 With high probability, for cases involving stored oils, bottom sediments, and waxy asphaltenes, the driving force for precipitation was alkane crystallization. This can be supported by the wax/asphaltene ratios being larger than unity. For the particular case described in the previous paragraph, the ratio was found to be very large (150). In other studied cases (not shown), these ratios typically were observed to span the 4-6 range. Similar results have been found by others.17 Speculation based on these findings suggests that coprecipitated asphaltenes were probably entangled through their alkyl appendages into the wax crystals. Gravitational sedimentation could then be enhanced by the particular contribution of the more dense aromatic portion attached to the particles formed. To date, the interaction mechanism remains unknown but despite this fact the effect can be reportedly taken into account in order to improve the efficiency of commercial paraffin inhibitors.50 To summarize the findings discussed in this section, it is possible to assert that the relative abundance of hydrocarbon group types found in produced oils intrinsically affect their stability. Selective precipitation of highly aromatic asphaltene components was commonly observed when oils were paraffinic in nature. Mineral and Porous Matter Effects. Canadian investigators have found close relations between minerals and the presence of insoluble organic matter.27-37 Previous findings from our research group suggested that solid deposits can be classified into three groups, depending on the relative abundance of organic and mineral matter contents.3 Field cases have been recently studied in our laboratories, aimed at identifying optimized solvent mixtures that effect deposit removal from production tubings. Failure to identify a suitable solvent
(47) Carbognani, L.; Espidel, Y.; Izquierdo, A. Proceedings 1st International Symposium Colloid Chemistry Oil Production, Asphaltenes and Wax Deposition, Rio, Brazil, 26-29 Nov. 1995, 040. (48) Wiehe, I. A.; Liang, K. S. Fluid Phase Equil. 1996, 117, 201.
(49) Fuhr, B. J.; Cathrea, C.; Coates, L.; Kaira, H.; Majeed, A. I. Fuel 1991, 70, 1293. (50) Schuster, D. S.; Irani, C. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30 (1), 169.
Figure 2. Average molecular parameters for asphaltenes from diverse origins, calculated by 1H NMR and C,H elemental analysis (see text for definitions): (*) stable crudes; (9) unstable crudes; (O) deposits.
Figure 3. Proposed stability parameter for crude oils, relating solvent power (aromatic contents) to dispersed phase properties (asphaltenes aromaticity).
Nature of Separated Solid Phases from Crude Oils
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Figure 4. Abundance of insoluble organics as a function of mineral contents in deposits. Estimation of insoluble organic materials was performed by both ashing and C microanalysis.
mixture was observed in many cases, partially because insoluble minerals proved to be noticeably abundant. However, this study did allow us to find clear correlations between mineral matter content and the abundance of insoluble organics, as illustrated in Figure 4. The organic matter from deposits containing less than 10 wt % of minerals appeared totally soluble if the aromatic content of the solvent was greater than 80 wt %. On the other hand, if the mineral content was greater than 20 wt %, results showed that even with strong solvents (80-100 wt % aromatics), insoluble organic materials remained attached to the mineral matrix. Addition of alcohols (not shown) improved the solubility in many of these cases. Nevertheless, even in these situations, the presence of organics was almost always confirmed when the mineral content was greater that ca. 40 wt %. Recently, the pore space in minerals has been suggested as a proper site for generation of kerogen in reservoirs,51 suggesting that the results discussed appear to be controlled by the same kind of phenomena. Apart from the abundance of the minerals already discussed, the influence of their chemical nature on the selective trapping of organic compounds can a priori be envisaged. Some results obtained with deposits from the Maracaibo Basin (Western Venezuela) supported this supposition. Many reservoirs within this basin are rich in carbonate rocks. Asphaltenes isolated from solid deposits generated in wells from this area showed that indeed their carbonate nature caused selectivity toward carbonyl compounds. This is illustrated by the carbonyl signals displayed within the IR spectra shown in Figure 5. Other authors have previously found selectivity of phosphate rocks and limestones toward oxygenated compounds.52 Selectivity of carbonate rocks toward oxygen compounds also appears to be supported by results found from microanalysis carried out on Venezuelan deposits. The information presented in Figure 6 shows that asphaltenes and insoluble organic materials isolated from deposits in which carbonates were present have nearly twice the oxygen content compared to materials in which carbonates were absent. Systematic studies on insoluble organics have not been conducted at our facilitites. Nevertheless, the results from Figure (51) Bishop, A. N.; Philp, R. P. Energy Fuels 1994, 8 (6), 1494. (52) Benalioulhaj, S.; Trichet, J. Org. Geochem. 1990, 16 (4-6), 649.
Figure 5. IR spectra for asphaltenes isolated from diverse solid deposits, relating carbonyl signals to carbonate contents.
Figure 6. Measured oxygen content determined for asphaltenes and insoluble organic materials. Differentiation for asphaltenes isolated from deposits containing carbonates is shown.
6 suggest that insoluble organics appeared as oxygenrich compounds. The fact that all the studied insolubles analyzed were isolated from carbonate deposits precluded unambiguous conclusions in this regard. However, many reports have previously published similar trends concerning insoluble organic materials. Reportedly, most of characterized organic insolubles are oxygenates such as carboxylic acids, humic acids, and ketonic compounds,28-37which appeared to be the components selectively associated with minerals for the generation of insoluble composites. Another variable related to minerals whose effects on the formation of solid deposits has been explored was their iron content. The presence of this metal has been observed in almost all of the Venezuelan solid deposits
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Figure 7. Polar hydrocarbon retention on solid adsorbents with/without Fe2O3 addition. A medium crude oil with 9.2 and 8.9 wt % of resin and asphaltenes was employed throughout the experiments. Neutral hydrocarbons were recovered with toluene and polar hydrocarbons by CH2Cl2-MeOH extractions: ([) original adsorbent (no Fe2O3 added); (O) adsorbent activated with 3 wt % Fe2O3.
studied to date. Iron species reportedly enhanced organic retention over minerals containing them.8,28-32,34 Screening of iron compounds carried out at our laboratories identified Fe2O3 as the most active promoter of organic retention over solid adsorbents. Some of the results found can be observed in Figure 7. Increased polar compound retention on tested natural clays and silica gel was clearly observed. On the other hand, no apparent effect could be detected for synthetic Attapulgus clay. A possible explanation deduced from experiments not shown in this paper revealed that this oxide acts at iron levels ranging from parts per million to nearly 1wt %. The original iron content of this clay (1.20 wt %) was already larger, having surpassed the activity plateau typically found for solid adsorbents doped with the oxide. The last topic to be discussed within this section devoted to mineral effects pertains to the role exerted by their pore space and also by the observed asphaltene porosity. Recently, trapping of saturated hydrocarbons into asphaltene molecules and micelle cavities has been cited.53 Previous experiments carried out with squalane as a probe molecule have suggested the porous nature of asphaltenes.54 Some of our recent research has dealt with chromatography of alkane concentrates carried out over mineral and asphaltenes acting like solid adsorbents.41-42 In these experiments, the porosity of the solid appears to be the relevant property governing the elution of multimodal paraffin fractions. The results included in Figure 8, obtained by high-temperature SEC,42 illustrate one of these aspects. Monomodal elution for alkanes isolated from nonporous glass and polydispersity for those arising from the asphaltene were the facts enabling us to propose the porous nature of the asphaltene. Multimodal elution was found to be typical for porous solids, as shown with the clay and silica included on the figure. More details on this topic can be found elsewhere.41-42 Waxes compounding solid deposits were frequently described as multimodal in nature.14-15,55-56 The discussed findings allowed us to propose an effect exerted by the porosity of minerals and (53) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171. (54) Strausz, O. P. Personal communication; 5th Chemical Congress of North America, Symposium on Advances in the Chemistry of Asphaltenes and Related Substances, Cancu´n, Mexico, Nov. 11-17, 1997.
Figure 8. Size distributions for alkane subfractions isolated at 75 °C by n-heptane elution from diverse solid stationary phases. Original alkane concentrate was separated from a crude oil bottom sediment. Displayed size distributions were achieved by high-temperature SEC.
asphaltenes and also by temperature gradients, in relation to the observed multimodality of alkane fractions found in solid deposits. In conclusion, the findings discussed supported the fact that the mineral matter and the intrinsic porosity of organic solids play an important role in deposit formation. Despite extensive research conducted on asphaltenes,57-58 the black color commonly displayed by deposited solids very often mislead field operators, who consider them to be like asphaltene mixtures and underestimate the effects of the presence of minerals. Operational Variables Affecting Solid Deposition. Probably one of the most important findings from research conducted in our laboratories over the past 6 years was that solids deposition related to oil production, transportation, and storage is in terms of mass balance a trace phenomenon. Close interaction between the research center and field operators made this possible. A typical example is included in Figure 9, illustrating production problems faced at deep wells in eastern Venezuela. Falling oil production was observed in this case over a 1-month period which ended in well failure. Mass estimation of removed deposited materials indicated that the amount of solids occurred at the parts per million level. Similar figures have been estimated for other Venezuelan production and storage facilities. Other researchers have come to the same estimates, when conducting basic research on asphaltene precipitation. Reportedly, only some of the more polar and (55) Wavrek, D. A.; Dahdah, N. F. Proceedings SPE International Symposium of Oilfield Chemistry, San Antonio, TX, Feb. 14-17 1997; Paper SPE 28965, p 207. (56) Biao, W.; Lijian, D. Paper SPE 29954, Proceedings of the International Meeting of Petroleum Engineering, Beijing, China, Nov 14-17, 1995; p 33. (57) Yen, T. F.; Chilingarian, G. V. Asphaltenes and Asphalts, 1. Developments in Petroleum Science; Elsevier Science: Amsterdam, Lausanne, N. Y., Oxford, Shannon, Tokyo, 1994. (58) Sheu, E. Y.; Mullins, O. C. Asphaltenes. Fundamentals and Applications; Plenum Press: London, 1995.
Nature of Separated Solid Phases from Crude Oils
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Figure 9. Trace-level solid deposition in a medium crude oil produced from a deep well located in Eastern Venezuela.
larger molecules precipitate at the deposition onset.49,59-60 All of these facts imposed severe constraints for the prediction of precipitation onsets, since extremely high detectabilites are mandatory in order to cope with the real trace levels involved. Consequently, common solvent precipitation protocols49,61-62 can be regarded only to be very gross indicators of stability-compatibility. Another source of solid deposition has been identified within the parameters that control oil production. A dramatic example was previously described concerning the generation of large clastic materials formed as a consequence of excessive production rates.47 Other fieldrelated causes can be indirectly deduced from chemical analysis, a valuable tool that sheds light on many of the causes of solid precipitation. An example supporting this assertion can be seen in Figure 10. IR spectrometry of residues after two deposits were ashed easily revealed that most of the problems faced in these examples was the existence of BaSO4 leftovers. The insolubility of this salt in commonly employed organic solvents ruled out the intended use of them for deposit removal. Operational variables such as flow rate, shear, heat flux, time, and temperature have been studied concerning waxy solid deposition during oil production.20-25 The relative distribution of alkane families24,63 and pressure effects63 have equally been studied. Here we wish to describe some of our recent findings based on analytical studies currently under development. No systematic studies have been carried out to date. Nevertheless, it was deemed important to comment on these since they support some of the results already cited from the other research groups. (59) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12 (11,12), 1551. (60) Andersen, S. I.; Keul, A.; Stenby, E. Pet. Sci. Technol. 1997, 15, 611. (61) Lambert, D. C.; Holder, K. A. Proceedings of the 4th UNITAR International Conference on Heavy Crude & Tar Sands, Alberta, Canada, 1988; paper 229. (62) Pauli, A. T. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1996, 41 (4), 1276. (63) Pan, H.; Firoozabadi, A.; Fotland, P. Proceedings SPE Annual Technical Conference and Exhibitition, Denver, CO, Oct. 6-9, 1996; Paper SPE 36740.
Figure 10. IR spectra for solid deposits and their ashes. KBr compressed disks with ca. 2 wt % of sample were employed to run the spectra.
Figure 11. Size distributions of whole saturates isolated from bottom sediments of storage tanks. Saturates were isolated by high-temperature HPLC-SAR separation. Size distributions were achieved by high-temperature SEC.
The availability of deposits formed in crude oil storage tanks or laboratory containers, spanning month to year periods, allowed us to detect the influence of time, flow, and large temperature changes on the nature of deposited organics. Paraffins were the selected probes to monitor the changes occurring. From the results included in Figure 11, it can be deduced that long storage times, low flow, and low temperature (ambient, ca. 2535 °C) enhanced the multimodality of the alkane mixtures isolated from the bottom sediments of field storage tanks. Longer storage periods (years) favored
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presumably are playing simultaneous roles, mainly because sample availability depends on field facilities failure. As a concluding remark for this section, chemical analysis linked to engineering knowledge provided some clues that revealed the influence of operational parameters on some of the solid deposition cases studied. Conclusions
Figure 12. Size distributions of whole saturates isolated from bottom sediments of barrel-stored oils. Other conditions were already described in Figure 11.
the appeareance of high molecular weight modes. Multimodality appeared to also be favored by flow absence, which is typical of oil stored in barrels. Some examples can be observed in Figure 12. Again, periods of years were required for separation of bottom sediments displaying multimodal alkane mixtures. However, if sedimentation was forced by centrifugation, flocs formed after 1 month of storage already show the multimodal nature. All formerly discussed findings pointed toward an important role played by low temperature and flow absence. This was easy to rationalize on the basis that these are the parameters that mainly govern crystal growing for large alkanes. All these results were interesting and worth reporting, however, no attempts were made in order to isolate each of the variables that
Solids deposition in oil production, transportation, and storage has been found in both laboratory and field studies to be affected by three groups of variables, namely, crude oil chemical composition, mineral and porous matter, and operational parameters. Simultaneous effects from some of them made the problem difficult to understand and demanded multidisciplinary approaches to overcome uncertainties and to attempt to control deposition. The term asphaltenic deposit, based on the visual appeareance of solids, can be misleading in many cases. However, this fraction appeared to be implicated in most deposition problems. Acknowledgment. The authors thank the project managers O. Rivas, A. Izquierdo, and O. Leo´n for helpful discussions and support. Funding from the former PDVSA subsidiary companies Corpoven, Lagoven, and Maraven made these studies possible. PDVSA-Intevep is also thanked for permission to publish this paper. Analytical support from the personnel of the Analysis & Evaluation Department from PDVSA-Intevep, particularly M. Farrera and L. DeLima, is acknowledged. Involvement of Y. Espidel during the initial phase of the work was very important for obtaining the whole picture of the present work. Finally, Prof. O. Strausz is thanked for his encouragement to prepare this paper. EF9801975
