The shear rate vs. shear stress plots for different oils at 20 °C are shown in Fig. (2). The shear stress of Model oil 1# and crude oils increase with shear rate in nonlinear form and can be described by power law equations shown in Eq. (1). The shear stress of Model oil 2# increase with shear rate in linear form and can be described by Bingham model shown in Eq. (2).

(1)

(2)

Fig. (2) Rheograms of the oil samples.

Where τ is shear stress in Pa. γ is shear rate in s-1. K is consistency index. n is rheological index. τ ₀ is apparent yield stress in Pa. η is apparent viscosity in Pa.

According to the previous study [17Li, C.; Li, G. Frictional drag behavior of non-newtonian crude oil. Oil Gas Storage Trans., 1997. 17+61-14], as the flow rate increases, the friction loss of pseudoplastic fluids increases less than Newtonian fluids. This difference is attributed to the shear thinning behavior of the non-Newtonian fluids. Therefore, under the same operating condition, with the improvement of flow rate, the pressure gradient in Model oil 1# increases more severely than in the crude oil.

Fig. (3) indicates that at 20 °C,the apparent viscosity of XJ heavy oil is high at low shear rate and become lower at higher shear rate. Both XJ heavy oil and the diluted crude oils presented a shear-thinning behavior. This is due to chain type molecules in crude oils are disentangled, stretched, and reoriented parallel to the driving force with the increasing shear rates [18Ghannam, M.T.; Esmail, M.N. Rheological properties of carboxymethyl cellulose. J. Appl. Polym. Sci., 1997, 64, 289-301.
[http://dx.doi.org/10.1002/(SICI)1097-4628(19970411)64:2<289::AID-APP9>3.0.CO;2-N] ]. The shear-thinning behavior in Model oil 1# is less obvious than crude oils, partly because it is made from crude oil deprived of asphaltenes contain large molecules. The viscosity of Model oil 2# keep constant with the increasing of shear stress because it is composed of pure polydimethylsiloxane, quite different from crude oils.

Fig. (4) depicts the apparent viscosity between model oils and crude oils at the flow rate of 400 s^-1 and within the temperature range from 20 to 60 °C. The viscosity of all oil samples decreases with the temperature. The viscosity-temperature relationships of XJ heavy oil, diluted crude oils and Model oil 1# are nonlinear. The viscosity decreasing gradient become lower with the temperature. The viscosity-temperature relationship of Model oil 2# is linear and its viscosity decreasing gradient is smaller than other samples. Moreover, in the flow loop experiments, the temperatures of the pump keep increasing with the prolonging of time, leading to the temperature difference between the start and end of the experiment [1Bannwart, A.C.; Rodriguez, O.M.; de Carvalho, C.H.; Wang, I.S.; Vara, R.M. Flow patterns in heavy crude oil-water flow. J. Energy Res. Technol., 2004, 126, 184-189.
[http://dx.doi.org/10.1115/1.1789520] ]. Thus it is hard to obtain stable pressure gradient data in a short period. According to the viscosity-temperature relationships, the use of Model oil 2# instead of Model oil 1# or heavy crude oil will overcome such disadvantages and help to achieve constant pressure gradient.

Fig. (3) Apparent viscosity for the oil samples at 20°C.

Fig. (4) Viscosity behavior for the oil samples.

The state of the sample relative to solid or liquid was evaluated using viscous moduli (G”) and elastic moduli (G'). Fig. (5) shows the results of G” and G' of model oils and heavy crude oil at 25 °C.

G' of both the model oils and heavy crude oil are higher than G' within the experimental temperature range, demonstrating that the flow ability is mainly dominated by the viscidity, and rarely by the elasticity. Therefore, these samples can be considered as viscous fluids. Model oil 1# and Model oil 2# are very similar to the XJ heavy oil in respect of viscoelasticity.

Fig. (5) The elastic modulus and the viscous modulus of the oil samples at 25 °C.

Photomicrographs can reflect the microstructure of the emulsions. In this study, the emulsions were prepared by stirring oil and water at a weight ratio of 1:1. Fig. (6) shows the photomicrographs sampled from the upper part of the emulsion. According to visual observation, after experiencing the same shear history, both types of diluted crude oil have formed homogeneous emulsions, while both Model oils presented separated water phase. W/O emulsions are form in all oil-water samples. Furthermore, the water concentration and average droplet size are larger in Model oil 2# emulsion than Model oil 1# emulsion, and in Diluted crude oil 2# emulsion than Diluted crude oil 1# emulsion. According to Wegmann et al. [5Wegmann, A.; Rudolf von Rohr, P. Two phase liquid-liquid flows in pipes of small diameters. Int. J. Multiph. Flow, 2006, 32, 1017-1028.
[http://dx.doi.org/10.1016/j.ijmultiphaseflow.2006.04.001] ], it is because the higher oil-water interfacial tension will prevent the dispersed phase from building small droplets.

Bottle tests or morphology can only record emulsions at a specific point of time. Turbiscan lab can reflect real-time coalescence, sedimentation and clarification process in emulsions in a non-destructive way. The principle of this measurement is based on the variation of the droplet volume fraction (migration) or mean size (coalescence), thus resulting in the variation of backscattering and transmission signals [19Celia, C.; Trapasso, E.; Cosco, D.; Paolino, D.; Fresta, M. Turbiscan lab expert analysis of the stability of ethosomes and ultradeformable liposomes containing a bilayer fluidizing agent. Colloids Surf. B Biointerfaces, 2009, 72(1), 155-160.
[http://dx.doi.org/10.1016/j.colsurfb.2009.03.007] [PMID: 19376689] ]. In this section, Turbiscan lab is adopted to analyze the kinetics during the destabilization process.

Fig. (7) shows the destabilization process of model oil emulsions. The transmission intensity is expressed in the scale of %, referring to the detected light intensity relative to the standard sample (Latex suspension: 0.3 m - 10% and Silicone oil). The Y-axis stands for the transmission intensity, while the X-axis for the height of the glass-made sample cell. The legends at the right side indicate the aging time in the scale of day: hour: minute: second. Each test lasted for 2 h. The data is valid within the height within the height range of 3 to 38 mm, because the light will be refracted at the interface between the glass and the emulsion at the cell bottom, or between the emulsion and air at the cell top.

Fig. (6) The photo and the photomicrographs of the emulsions. (a) Model oil 1#; (b) Model oil 2#; (c) Diluted crude 1#; (d) Diluted crude 2#.

Fig. (7) Transmission profiles of the emulsification process. (a) Model oil 1#; (b) Model oil 2#.

The upper part of the sample cell (18-40 mm) proved to be W/O emulsion and the lower part(3-17 mm) O/W emulsion. The transmission intensity changing with time can reflect the destabilization process. In the model oil emulsion, the transmission changes in the water is bigger than in the oil. This is attribute to the dispersed oil droplets in the bottom water flow upward and the dispersed water droplets in the top coalesce and sediment. According to Liu et al. [20Liu, J.; Huang, X-F.; Lu, L-J.; Li, M-X.; Xu, J-C.; Deng, H-P. Turbiscan Lab^® Expert analysis of the biological demulsification of a water-in-oil emulsion by two biodemulsifiers. J. Hazard. Mater., 2011, 190(1-3), 214-221.
[http://dx.doi.org/10.1016/j.jhazmat.2011.03.028] [PMID: 21458159] ], in the initial period, coalescence play a main role in the emulsion, proved by the decreasing of transmission. As the droplets become bigger, they migrated from top to the bottom. Gradually, the water layer separated at the bottom of the sample cell. Meanwhile, the oil phase migrated to the top of the sample cell.

Fig. (8) displays the intensity of the backscattered light versus the sample height in the Diluted crude oil 2# emulsions. As it can be observed, W/O emulsion presents at the upper part(23-38mm) and O/W emulsion at the lower part(3-23 mm). Considering the backscattering signal decrease in very small extents, the diluted crude oils emulsions should be treated as stable emulsions.

Fig. (8) Backscattering profiles of the emulsification process. (a) Diluted crude 1#; (b) Diluted crude 2#.

The emulsion stability was assessed with the proportion of water separated from the total water volume. The separated water fraction was recorded after each fresh emulsion was holding for 24 h at 25 °C, as shown in Fig. (9). Observation shows that the separated water fraction from left to right are 98%(Model oil 1#), 88%(Model oil 2#), 53%(XJ heavy oil), 86%(Diluted crude oil 1#), 87%(Diluted crude oil 2#), respectively. Therefore, the emulsion stability increases in the order of Model oil 1#, Model oil 2#, Diluted crude oil 2#, Diluted crude oil 1#, XJ heavy oil. This sequence differs from the oil-water interfacial tension sequence. It indicates that the oil-water interfacial tension is not the determining factor for emulsion stability. According to the interfacial tension theory [21Murray, B.S.; Dickinson, E.; Wang, Y. Bubble stability in the presence of oil-in-water emulsion droplets: influence of surface shear versus dilatational rheology. Food Hydrocoll., 2009, 23, 1198-1208.
[http://dx.doi.org/10.1016/j.foodhyd.2008.07.015] ], the emulsion stability is the result of the dispersed droplet size and the presence of the interfacial film. The properties of the interfacial film is affected by crude oil type(asphaltic, paraffinic, etc.), composition and pH of the water, temperature, concentration of polar molecules in the oil phase, contact or aging time, the compressed degree of the adsorbed film, etc [22Jones, T.; Neustadter, E.; Whittingham, K. Water-in-crude oil emulsion stability and emulsion destabilization by chemical demulsifiers. J. Canadian Petrol. Tech., 1978, 17.
[http://dx.doi.org/10.2118/78-02-08] -24Sridhar, S.; Zhang, H-Q.; Sarica, C.; Pereyra, E.J. Experiments and Model Assessment on High-Viscosity Oil/Water Inclined Pipe Flows. In: SPE Annual Technical Conference and Exhibition; Society of Petroleum Engineers: Denver, Colorado, USA, 2011.
[http://dx.doi.org/10.2118/146448-MS] ].

Fig. (9) Photos of the emulsions after setting for 24h (a) Model oil 1#; (b) Model oil 2#; (c) XJ heavy oil; (d)Diluted crude 1#; (e) Diluted crude 2#.

Based on the droplet size variation and droplet migration, TSI (Turbiscan Stability Index) can reflect the stability of emulsions during the standing. A larger TSI indicates the lower stability of an emulsion. Fig. (10) shows the TSI profiles during the oil-water separation. The experimental results show that the two transparent model oils are very similarly stable and both are less stable than the two diluted crude oils. The two diluted crude oils are less stable than the corresponding original undiluted heavy crude oils. This phenomenon can be mainly attributed to the surface- active molecules (e.g. asphaltenes and resin) in the natural heavy crude oil. It is generally believed that asphaltene could form stable thick network at the interface, and is one of the main amphiphilic compounds responsible for the stability of water-in-oil emulsion [25Alvarez, G.; Poteau, S.; Argillier, J-F.; Langevin, D.; Salager, J-L. Heavy oil-water interfacial properties and emulsion stability: influence of dilution. Energy Fuels, 2008, 23, 294-299.
[http://dx.doi.org/10.1021/ef800545k] ]. Dilution changes the interface rigidity. Model oil contains no surfactants, so its emulsion is the least stable. The TSI tests are consistent with the bottle tests.

Fig. 10 TSI profile of dispersions.