Studies of Water-Continuous Emulsions of Heavy Crude Oils Prepared by Alkali Treatment T.H. Plegue and S.G. Frank, Ohio State U.; D.H. Fruman, Ecole Nationale Superieure Techniques Avancees; and d.L. Zakln, Ohio State U.
Summary. Pumping of heavy crudes as concentrated oil-in-water (O/W) emulsions may be a feasible pipeline transport scheme for viscous crudes. Many crude oils can be emulsified by treatment with alkali without the addition of expensive surfactants. In experimental studies of seven crudes, stable emulsions could be formed with four by alkali treatment; two could not be emulsified; and one formed unstable emulsions. High shear viscosity, particle size, and stability measurements on 60% emulsions formed with various amounts of alkali are reported. High shear viscosities below 100 mPa' s [100 cp] at 25°C [77°F] indicate that concentrated emulsions of some crudes can be transported in pipelines at concentrations of 60% and higher. Introduction Significant reserves of heavy crudes exist in the U.S., Canada, and Venezuela. 1,2 Production of these heavy crudes is expected to increase significantly in the near future as low-viscosity crudes are depleted. 3 Conventional pipelining is not suitable for transporting these heavy crudes from the reservoir to the refinery because of the high viscosities involved (lOOO to > 100 000 mPa's at 25°C [I,oooto > 100,000 cp at 77°F]). Several other transport methods have been proposed, including dilution with lighter crudes or petroleum distillates, injection of a water sheath around the crude, 4 preheating the crude and heating the pipeline, and encapsulation of the crude. Transport of viscous crudes as concentrated O/W emulsions is an alternative pipeline technique. 5-7 This method has been demonstrated on a large scale in an Indonesian pipeline 8 and in a 21km [13-mile] -long, 20-cm [7.9-in.] -diameter pipeline in California. While emulsion transport has the disadvantage of requiring de~atering of the crude after transport, it may have unique advantages in some cases. For instance, at some locations, makeup water suitable for emulsification may be more readily available at low cost than materials suitable for dilution of heavy crudes. In other cases, environmental concerns or energy costs may make heated pipelining unattractive, particularly for extremely viscous crudes. Emulsions may be transported at ambient temperatures. Water-continuous emulsions containing high concentrations of crude can have viscosities of 100 mPa' s [100 cp] or less. 5 •9 Commercial, nonionic surfactants have been used to form O/W emulsions containing as much as 85 % crude. 5.10-12 The presence of natural saponifiable acids in some crudes, however, may eliminate the need for expensive commercial surfactants. With the addition of alkali, these acids will react and form compounds that may lower interfacial tension. Emulsions suitable for pipeline transport, containing as much as 75 % crude, have been formed for some crudes by mixing them with alkaline water. 9.13 Surface-active agents in crudes have been shown to be carboxylic acids of various molecular weights and chemical structures. 14.15 Here, data are presented to show the characteristics of 60 vol % crude emulsions formed by NaOH or KOH treatment of seven heavy crudes of widely varying origin to illustrate the effects of varying the alkali/oil ratio. For the four crudes that formed stable emulsions over reasonably wide alkali ranges, the effects of NaOH or KOH addition on emulsion properties were investigated in some detail. The emulsion properties reported are apparent viscosity, mean particle diameter, and water separation after 3 days. Experimental Methods and Materials Table 1 summarizes the properties of crude oils studied. Values for specific gravity and viscosity are reported at 25°C [77°F]. Total acid numbers (TAN) were measured by titration with an alcoholic KOH solution in accordance with ASTM Method D 664-81. Copyright 1969 Society of Petroleum Engineers
SPE Production Engineering, May 1989
Demineralized double-distilled water was used for all emulsions. Reagent-grade, anhydrous NaOH or KOH in pellet form was used to prepare alkali solutions. The hydroxide was dissolved in the water phase before mixing. Emulsions were made in 45-mL batches with 60 vol % crude with a Sorvall Omni-mixer™ at a rotational speed of 9,000 rev/min for 180 seconds. Crude oils were preheated to between 65 and 95°C [149 and 203°F], depending on the crude viscosity. During homogenization, the mixing vessel was immersed in a 70°C [159°F] water bath. Relative emulsion stability was roughly measured under stagnant conditions by periodic monitoring of the volume of the water settling to the bottom of a 60-mL test tube. The height of the 45 mL of emulsion in the test tube was 100 mm [3.9 in.]. Apparent viscosities of emulsions were measured at 25°C [77°F] with a Haake Rotovisco™ viscometer with a concentric-cylinder measuring system. Inner cylinder diameter was 40.08 mm [1.6 in.] with a gap width of 0.96 mm [0.038 in.]. The maximum apparentshear rate range that could be obtained with this instrument for the emulsions studied here was 152 to 1,370 seconds - I. For the lowviscosity emulsions (which were Newtonian), this range was limited to 457 to 1,370 seconds -I. Viscosities were measured by initially increasing shear rate. Measurements were repeated after the highest shear rate was reached to check for hysteresis. Significant hysteresis was not observed for any of these samples. Apparent viscosities reported were measured at 685 seconds -I . This shear rate provided the most reliable data for all emulsions, particularly for the low-viscosity emulsions, and therefore provided the best comparison of different emulsions. Particle sizes were analyzed by the electrozone technique with an Elzone/ ADC-80™ manufactured by Particle Dynamics Inc. The measurement principle of this instrument is based on changes in conductivity caused by particles immersed in an electrolyte solution as they pass through a small orifice.
Results and Discussion Of the seven heavy crudes of varied origin, stable emulsions could be made with four of them over a range of alkali additions less than and greater than their TAN values (see Table 1). The waterseparation data in Table 1 show the hydroxide ranges in which fairly stable emulsions (less than 3-mL separation in 3 days) could be formed. With St. Lina crude, a moderately stable emulsion could be formed with only one concentration of KOH. This concentration was greater than the equivalence amount. No stable emulsions at any alkali concentration could be formed with the Vic Bilh 6 or Rospo Mare crudes. Thus, with the alkali treatment, only the Boscan, Cold Lake, Guadalupe, and Martinez crudes are potential candidates for emulsion pipeline transport. The Martinez and Cold Lake emulsions had lower viscosities than the Boscan and Guadalupe crudes. For the Cold Lake, Guadalupe, and Martinez crudes, the maximum measured apparent viscosities occurred at alkali levels close to their 181
TABLE 1-CHARACTERISTICS OF EMULSIONS OF CRUDES FORMED WITH ALKALI TREATMENT
Field Boscan
Cold Lake
Crude Viscosity at 25掳C Specific and 685 seconds -1 Location Gravity (mPa's) Venezuela 0.970 180000
Canada
0.999
80000
Martinez
California
0.953
2600
Guadalupe
California
0.994
35000
St. Lina
Canada
0.987
16000
Rospo Mare Vic Bilh 6
Italy France
0.995 0.974
16000 20000
Emulsion Mean Hydroxide Viscosity at Particle Water TAN Concentration 685 seconds -1 Diameter Separation (meq/100 9 oil) (meq/100 9 oil) (mPa路s) (mU3D) (J.'m) 1.8 1.0 4.6 31 0 2.0 3.6 97 0 4.0 3.8 64 0 8.0 31 4.1 0 1.7 1.0 24 4.4 2.5 35 3.3 0 4.0 26 4.0 0.3 5.0 4.5 23 2 2.0' 2.9 6.3 9 17 4.0' 21 2.8 0.5 7.0' 2.5 26 0 10.0' 25 2.4 0 19.0' 24 2.5 0.8 28 2.9 3.5 30.0' 1.5 46 3.9 2.2 2.0 3.1 165 4.0 3.2 69 1.1 3.0' 4.8 36 4.0' -Unstable0.9 -Unstable2.7 -Unstable-
'Emulsified with KOH.
equivalence points. For these crudes, emulsion transport in pipelines would be facilitated by alkali concentrations remote from the equivalence levels. This lower-viscosity advantage, however, must be reconciled with the reduced stability of some of the crudes at alkali levels far from their equivalence points. Mean particle diameters of the emulsions are also listed in Table 1. For the Boscan, Cold Lake, and Guadalupe crudes, the minimum particle size occurs at a hydroxide concentration close to that for the maximum in apparent viscosity. A likely explanation for the maxima in emulsion viscosities obtained for the Boscan, Cold Lake, and Guadalupe crudes and the plateau obtained with the Martinez crude is that all the available saponifiable acids are ionized at the TAN equivalence of each crude. Below the equivalence point, increasing the NaOH (or KOH) concentration increases the effective surfactant concentration in the emulsion. According to basic emulsion theory, as surfactant concentration increases, particle size can be expected to decrease while particle surface charge density is likely to increase. As surface charge densities increase, electroviscous effects and bulk viscosities increase. The reduction in particle size also will generally increase emulsion viscosities by increasing particle interactions. 16 Once the equivalence point is reached, addition of excess hydroxide will increase the emulsion's ionic strength without increasing the surfactant concentration in the system. Higher counter-ion concentrations increase screening of the particle without significantly affecting surface charge density. The charge screening supplied by the excess NaOH (or KOH) decreases electroviscous effects and, therefore, emulsion viscosities.
Review and Conclusions 1. Fairly stable (60 vol%) crude O/W emulsions were formed with only alkaline water treatment over a range of alkali concentrations for four of seven heavy crudes studied. At 60% crude volume fraction, the high-shear-rate (685 seconds-I) emulsion apparent viscosity may be as much as 10,000 times lower than the crude viscosity. 2. The apparent viscosities of crude O/W emulsions formed with alkali treatment may vary significantly, depending on the level of alkali used to form the emulsion. In general, viscosities go through maxima and particle diameters through minima at alkali levels near their TAN. 182
3. Differences in emulsion viscosities and electroviscous effects for different crudes are apparently results of differences in the natural surfactants present in the crudes. Lack of specific knowledge of the surfactant chemistry of each crude makes it impossible to predict emulsion characteristics. 4. The TAN value is useful for selecting NaOH concentrations for emulsification, but it is not adequate for quantitative prediction of emulsion viscosity or ability of a given crude to emulsify.
Acknowledgments We acknowledge support of this collaborative research under the U.S'/France Program sponsored by the Natl. Science Foundation and Ie Centre Natl. de la Recherche Scientifique, the NATO Double Jump Programme for IntI. Collaboration in Research, and Unocal. We also appreciate the contributions of crude oil from Amoco Oil Co. (St. Lina) , Shell Oil Co. (Martinez), Union Oil Co. (Guadalupe), Alberta Research Council (Cold Lake), and Ecole Nationa1e Superieure Techniques Avancees (Boscan, Vic Bilh 6, and Rospo Mare). The comments of Shlomo Magdassi of the Hebrew U., Jerusalem, are also appreciated.
References I. Schumacher, M.M.: Enhanced Recovery of Residual and Heavy Oils, Noyes Data Corp., Park Ridge, NJ (1980) 96. 2. Riva, J.P.: World Petroleum Resources and Reserves, Westview Press, Boulder, CO (1983). 3. Williams, B.: "Heavy oil steamflood projects thriving in Kern, Calif." Oil & Gas J. (Dec. 10, 1984) 12, 45-50. 4. "Oil-water line moves highly viscous crude," Oil & Gas J. (Feb. 7, 1972) 37. 5. Simon, R. and Poynter, W.G.: "Pipelining Oil/Water Mixtures," U.S. Patent No. 3,519,006 (1970). 6. Siffennan, T.R.: "Method of Transporting Viscous Hydrocarbons," U.S. Patent No. 4,265,264 (1981). 7. Marsden, S.S. and Raghavan, R.: "A System for Producing and Transporting Crude Oil as OillWater Emulsions," J.Inst. Pet. (Nov. 1973) 273-78. . 8. Lamb, M.S. and Simpson, W.C.: "Pipeline Transportation of Wax Laden Crude Oil as Water Suspensions," Proc., Sixth World Pet. Cong., Frankfurt-on-Main (1963) Sec. VII, 23. 9. Kessick, M.A.: "Pipeline Transportation of Heavy Crude Oil," U.S. Patent No. 4,343,323 (1982).
SPE Production Engineering, May 1989
10. Flock, D.L. and Steinborn, R.: "The Rheology of Heavy Crude Oils and Their Emulsions," paper 82-33-60 presented at the 1982 Annual Meeting of the Petroleum Soc. of CIM, Calgary, June 6-9. II. Pal, R., Bhattacharya, S.N., and Rhodes, E.: "Flow Behaviour of OilIn-Water Emulsions," Cdn. 1. Chern. Eng. (1986) 64,3. 12. Grosso, J.L. et al.: "Influence of Crude Oil and Surfactant Concentration in the Rheology and Flowing Properties of Crude Oil in Water Emulsions, " paper presented at the IntI. Symposium on Surfactant Solutions, Bordeaux, France, July 1984. 13. Simon, R. et al.: "Pipelining Crude Oil," U.S. Patent No. 3,487,844 (1970). 14. Jang, L.K. et al.: "Correlation of Petroleum Component Properties for Caustic Flooding," lnteifacial Phenomena in Enhanced Oil Recovery, AIChE Symposium Series (1982) 78, No. 212, 97.
SPE Production Engineering, May 1989
15. Layrisse, I., Rivas, H., and Acevedo, S.: "Isolation and Characterization of Natural Surfactants Present in Extra Heavy Crude Oils," 1. Dispersion Science and Techno/. (1984) S, No. I, 1-18. 16. Jeffrey, D.J. and Acrivos, A.: "The Rheological Properties of Suspensions of Rigid Particles," AIChE 1. (1976) 22, No.3, 417.
51 Metric Conversion Factor cp x 1.0* E-03 'Conversion factor is exact.
Pa's
SPEPE
Original SPE manuscript received for review May 13,1988. Paper (SPE 18516) accepted for publication May 27, 1988. Revised manuscript received Aug. 9, 1988.
183