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Energy & Fuels 2000, 14, 14-18
Asphaltene Precipitation from Live Oils: An Experimental Investigation of Onset Conditions and Reversibility A. Hammami* DB Robinson Research Ltd., Edmonton, Alberta, Canada T6N 1E5
C. H. Phelps and T. Monger-McClure Marathon Oil Company, Petroleum Technology Center, Littleton, Colorado
T. M. Little Marathon Oil Company, Oklahoma City, Oklahoma Received May 27, 1999. Revised Manuscript Received October 18, 1999
The behavior of asphaltene in live oils was investigated using a fully visual PVT cell equipped with a laser-based particle detection system. Through a series of isothermal pressure depletion experiments, the asphaltene stability envelope was established for several Gulf of Mexico oils. These tests were performed under pressure and temperature conditions typical of a field production system. Evidence of asphaltene precipitation above and redissolution below the saturation pressure was observed. Significantly, these tests also showed that pressure depletioninduced asphaltene precipitation is a highly reversible process. This latter observation helps to resolve a long-running debate in the literature concerning the reversibility of the asphaltene precipitation and flocculation process.
Introduction Operational problems associated with asphaltene deposition are encountered in all facets of petroleum production and/or processing. These problems are expected to increase as development moves offshore and into deep water where prevention and remediation costs rise dramatically. Asphaltenes are generally defined as the solid material precipitating from crude oil, asphalt, or bitumen upon addition of excess low-molar-mass paraffin solvent.1,2 This classical definition is somewhat arbitrary, as the amount and nature of the precipitated solids are solvent dependent.2-4 The actual chemical structure of asphaltene is difficult to define using existing analytical tools, and it remains the subject of ongoing research.6-8 The available laboratory and field data indicate that asphaltenes separated from crude oils usually comprise condensed aromatic and naphthenic * Corresponding author. (1) Long, R. B. The Concept of Asphaltenes. In Chemistry of Asphaltenes; Adv. Chem. Ser. 1981, 195, 210-227. (2) Speight, J. G.; Long. R. G.; Trowbridge, T. D. Fuel 1984, 63, 141146. (3) Fuhr, B. J.; Cathrea, C.; Coates, L.; Kalra, H.; Majeed, A. I. Fuel 1991, 70, 1293-1297. (4) Hammami, A.; Chang-Yen, D.; Nighswander, J. A.; Stange, E. Fuel Sci. Technol. Int. 1995, 13 (9), 1167-1184. (5) Andersen, S. I. Fuel Sci. Technol. Int. 1994, 12 (1), 51-74. (6) Starusz, O. P.; Mojelski, T. W.; Lown, E. M. Fuel 1992, 71, 13551364. (7) Espinat, D. SPE 25187 presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, LA, 1993, March 2-5. (8) Speight, J. G.; Long. R. G. Fuel Sci. Technol. Int. 1996, 14 (1,2), 1-12.
molecules of molar masses ranging from several hundred to several thousand grams per mole. They also consist of polar molecules containing much of the heteroatom (N, S, O) and metal (Ni and V) content of heavy oils. Such an extensive range of molar mass distribution and composition suggests that asphaltenes may be partly dissolved and partly suspended/peptized by resins in the crude oil.9 A vital step toward formulating adequate mathematical description of this complex problem is an increased understanding of the main factors influencing asphaltene behavior and the mechanisms of precipitation/deposition. Field experience10,11 and experimental observations3-5,12,13 indicate that asphaltene stability is dependent on factors including the composition, pressure, and temperature of the oil. The effect of composition and, by inference, pressure on asphaltene precipitation is generally believed to be stronger than the effect of temperature. However, there still exists some disagreement in the literature regarding this point.5,7,13 Asphaltene precipitation can occur in-situ during miscible gas or other solvent injection operations. The (9) Leontaritis, K. J.; Mansoori, G. A. SPE 16258 presented at the SPE Symposium on Oilfield Chemistry, San Antonio, TX, 1987, February 4-6. (10) de Boer, R. B.; Leerlooyer, K.; Eigner, M. R. P.; van Bergen, A. R. D. SPE Production and Facilities 1995, 2, 55-61. (11) Kokal, S. L.; Sayegh, S. G. SPE 29787 presented at the SPE Middle East Oil Show, Bahrain, 1995, March 11-14. (12) Fotland, P. Fuel Sci. Technol. Int. 1996, 14 (1, 2), 313-325. (13) Wang, J. X.; Brower, K. R.; Buckley, J. S. SPE 50745 presented at the SPE International Symposium on Oilfield Chemistry, Houston, TX, 1999, February 16-19.
10.1021/ef990104z CCC: $19.00 © 2000 American Chemical Society Published on Web 12/15/1999
Asphaltene Precipitation from Live Oils
addition of light, paraffinic compounds shifts the solubility of the asphaltene component in the bulk oil. According to the colloidal model of asphaltene behavior, resin molecules respond to the addition of light hydrocarbons by desorbing from the surface of the asphaltenes in an attempt to reestablish thermodynamic equilibrium. Desorption of the peptizing resins forces the asphaltene micelles to agglomerate in order to reduce their overall surface free energy. If agglomeration proceeds to such an extent that the Brownian forces of suspension are overcome by gravity, asphaltene particles will separate as a discrete solid phase.14 Pressure depletion alone can destabilize asphaltenes and is likely the major reason for asphaltene deposition in wellbore tubulars. Such deposits are often found at tubing locations corresponding to the saturation pressures of the crude oils.10-12 These observations are consistent with various experimental studies showing that the bulk of asphaltene precipitation from undersaturated crude oils occurs close to bubble-point pressures.10,11 This phenomenon is rationalized in terms of the reduced solubility of asphaltenes in the lower density crude created by depressurization. That is, the difference in molar volume between the asphaltene and the bulk oil is a maximum at the bubble point leading to a minimum in asphaltene solubility. Below the bubble point, the volatile hydrocarbons evaporate from the liquid as a gas phase thereby changing the molar volume of the liquid phase and reestablishing some of its lost asphaltene solubility. That is, the light ends and asphaltenes compete for solvency in the crude oil so the loss of light ends implies better solubility of the asphaltene components.10,11,14 The de Boer Plot. de Boer et al.10 compared the properties of some crude oils from asphaltene problemprone fields to oils from fields operating problem-free. All of these oils were from either Kuwait or the North Sea. In general, problems were encountered with light crudes that are high in C1-C3 (>37 mol %), relatively low in C7+ fraction (
