VII Brazilian Conference on Rheology

VII Brazilian Conference on Rheology–BCR 2015 Curitiba, PR, Brazil, July 05-08

Influence of Thermal Treatments on the Yield Stress of Waxy Crude Oil Emulsions Ângela Camila Pinto Duncke ([email protected]) Carla Napoli Barbato ([email protected]) Thiago Oliveira Marinho ([email protected]) Márcio Nele ([email protected]) Departamento de Engenharia Química, Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro, Brazil - CEP: 21941-909 Gizele Batalha Freitas ([email protected]) Programa de Engenharia Química, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco G, sala 115, Cidade Universitária, Rio de Janeiro, RJ, CEP 21941-972. Márcia Cristina Khalil de Oliveira ([email protected]) Centro de Pesquisa Leopoldo Américo Miguez de Mello (CENPES), Petrobras, Cidade Universitária, Rio de Janeiro, RJ, Brazil, Cep: 21941-598 1 INTRODUCTION The thermal treatment of waxy oil strongly influences the rheological properties of waxy crude oil emulsions, such as viscosity, yield stress and elastic and viscous modulus (Zhao et al., 2012; Lin et al., 2011; Tinsley et al, 2009). Therefore, it is vital to develop a laboratory procedure for thermal treatment of waxy crude oil emulsions which accounts for the reality of field environments and allows reproducible yield stress estimations. The structure formed by wax precipitation is strongly influenced by cooling process. Initial and final cooling temperatures, cooling rates, shear history and aging time at low temperatures are some of the most relevant factors (Lin et al., 2011; Oh and Deo, 2011;Venkatesan et al.; 2005). The aim of this study is evaluate the influence of heat treatment procedure on waxy crude oil emulsion yield stress 2 MATERIALS AND METHODS 2.1 W/O Emulsion Preparation Three waxy crude oils (A, B and C) supplied by PETROBRAS were tested. Two heat treatment methods of crude oil were used (Figure 1). In method (I) waxy oils were heated to 60°C for 1 hour. After heating step, emulsion preparation was carried out at 60°C. In method (II) waxy oils were heated at 60°C for 1 hour, and then cooled to 45°C at cooling rate of 0.7°C/min. The emulsion was prepared at 45°C. Water-in-oil (W/O) emulsions were prepared using synthetic brine consisting of 5.0 wt% NaCl in milli-Q water at aqueous volume fractions of 0.50. The emulsification process was performed using a Polytron PT 3100 homogenizer at 8,000 rpm for three minutes.

Figure 1 –Waxy crude oil heat treatment methods.

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2.2 Yield Stress Rheological tests were carried out in an AR-G2 rheometer from TA Instruments. Temperature control was provided by Peltier elements. The sample was loaded into rheometer using a syringe. Parallel plate geometry was used (D = 40 mm). The procedure employed to measure the waxy crude oil emulsion yield stress consisted of five steps: 1st Step: Emulsion conditioning in the rheometer at 45°C or 60°C. 2nd Step: Sample cooling to promote wax precipitation and gelled waxy crude oil network formation. 3rd Step: Gap reduction 4th Step: Sample conditioning at 4ºC to favor precipitation of wax crystals. 5th Step: Yield stress determination of the W/O emulsions, prepared with waxy crude oil. The method chosen to measure the yield stress, in the last step of the procedure (Figure 1), was the stress amplitude sweep because of its simplicity and the short time required to evaluate this rheological property. The yield stress was defined as the point where the storage modulus is equal to the loss modulus. The experiments were carried out with the frequency equal to 1 Hz, and the shear stress range was 0.1−1000 Pa. 2.3 Optical Microscopy Waxy crude oils (A, B and C) heated at 60°C for 1 hour were observed at optical microscopy before and after the thermal treatment. An inverted optical microscope Axiovert 40 MAT from Carl Zeiss, equipped with Axiocam MRC camera was used. 3 RESULTS AND DISCUSSIONS Figure 2 and 3 show images of waxy oils (A, B and C) at room temperature before (Figure 2) and after (Figure 3) heating at 60°C for 1 hour, observed at a polarized reflected light optical microscopy. It was verified the presence of free wax and small agglomerates of paraffin before the heating process (Figure 2). After heating, the amount of paraffin was reduced, however without complete wax solubilization. Thus, the condition of heat treatment (T = 60°C and t = 60 min) proved to be not adequate for erasing the thermal history.

(B) (A) (C) Figure 2 – Images of waxy crude oil at (A) A; at; (B) B and (C) C at room temperature before heating at 60°C for 1 hour.

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(A) (B) (C) Figure 3 – Images of waxy crude oil at (A) A; at; (B) B and (C) C after heating at 60°C for 1 hour. Table 1 shows the yield stress values of waxy crude oil emulsions prepared with the waxy oils A, B and C. It was verified for emulsions prepared with waxy oils A and C, that the values of yield stress are very different when the oils are heated by methods I and II .This result was not the same for the emulsion prepared with waxy oil B. The elastic and viscous modulus and yield stress for emulsions A and C increased with the decrease of the temperature that emulsions was prepared. This temperature decrease favored paraffin precipitation and contributed to the increase of yield stress. Table 1 – Values of G’, G” and yield stress of waxy crude oil emulsion (W/O). Method

Waxy Crude Oil

I

Yield Stress (Pa)

G’ (Pa)

G” (Pa)

0

12.19 ± 0.84

14.76 ± 1.34

163.9 ± 25.6

1,175.00 ± 397.15

130.74 ± 67.71

5.4 ± 0.7

519.93 ± 31.25

321.32 ± 18.43

6.2 ± 0.7

563.94 ± 20.16

293.92 ± 10.13

144 ± 21.6

2,620.2 ± 318.94

262.99 ± 28.10

298.6 ± 24.3

4,048.12 ± 664.40

278.99 ± 26.99

A II I B II I C II 4 CONCLUSIONS The heating of waxy crude oil at 60°C for 1 hour is not suitable to erase the oil thermal history. At this condition, it was observed the presence of precipitated paraffin. If the waxy crude oil were treated at 80°C for 2 hours instead, the oil thermal history could be erased and values of yield stress would be less. The A and C waxy oils emulsions were characterized by higher values of yield stress when prepared by method II than method I. Results have shown that the yield stress of emulsions A and C increased with the decreasing of the emulsion temperature preparation. These results confirm the importance of waxy crude oil heating procedure to determine unequivocally the yield stress of waxy crude oil emulsions. 5 ACKNOWLEDGEMENTS The authors wish to thank to Petrobras S.A., CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and FAPERJ (Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro).

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6 REFERENCES Lin, M.; Li, C.; Yang, F. Ma, Y. 2011. Isothermal Structure Development of Qinghai Waxy Crude Oil Afther Static and Dynamic Cooling. Journal of Petrouleum Science and Engineering. V. 77; p.351-358. Oh, K. e Deo, M. D. 2011. Yield Behavior of Gelled Waxy Oil in Water – in – Oil Emulsion at Temperatures Below Ice Formation. Fuel. V. 90, p. 2113-2117. Paso, K.; Kallevik, H., Sjöblom, J. 2009 Measurement of Wax Appearance Temperature Using Near Infraread (NIR) Scattering Energy and Fuels. V. 23. p.4988-4994. Tinsley, J. F.; Jahnke, J. P., Dettman, h. D., Home, R. K. P. 2009. Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology. Energy & Fuels. V. 23, p. 2056-2064. Venkatesan, R., Nagarajan, N. R., Paso, K., Yi, Y. B., Sastry, A. M., fogler, H. S. 2005. The Strenght of Paraffin Gels Formed Under Static and Flow Conditions. Chemical Engineering Science. v. 60, p. 3587-3598. Visintin, R. F. G., Lapasin, R., Vignati, E., D’Antona, P., Lockhart, T. P. 2005. Rheological Behavior and Structural Interpretation of Waxy Crude Oil Gels. Langmuir. V. 21, p. 6240-6249. Zhao, Y.; Kumar, L.; Paso, K. Safieva, J. Sariman, M.; Sjoblom, J. 2012. Gelation Behavior of Model Wax Oil and Crude Oil Systems and Yield Stress Development. Energy and Fuels. V. 26, p.6323-6331.

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