Viscosity of Crude Oils

The measurement of a crude oil s viscosity at different temperatures is particularly important for the calculation of pressure drop in pipelines and refinery piping systems, as well as for the specification of pumps and exchangers. 

The change in viscosity with temperature is not the same for all crudes. 

The viscosity of a paraffinic crude increases rapidly with decreasing temperature on the other hand, for the naphthenic crudes, the increase in viscosity is more gradual. 

The viscosity is determined by measuring the time it takes for a crude to flow through a capillary tube of a given length at a precise temperature. This is called the kinematic viscosity, expressed in mm /s. It is defined by the standards, NF T 60-100 or ASTM D 445. Viscosity can also be determined by measuring the time it takes for the oil to flow through a calibrated orifice standard ASTM D 88. It is expressed in Saybolt seconds (SSU). 

Some conversion tables for the different units are used and standardized (ASTM D 2161). 

Several methods have been published to predict viscosity of liquids (Table 1.4) however, most of them are applied to reduced range of temperature and pressure and for pure components. To predict viscosity of heavy (10 API 20) and extra-heavy ( 10°API) crude oils, the only reported approach is the modified Egbogah-Jack s equations (De Ghetto et al, 1995). 

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Dilatational Elasticities and Viscosities of Crude Oil at 1 mHz with Polydimethylsiloxanes (PDMS) [300]... 

Fig. 3.6.1 Correlation of relaxation with viscosity of crude oils and viscosity standards [14].

Fig. SI shows experimentally determined relationships between the viscosity of crude oil and the oil recovery factor [32]. The curves show that, as an EOR method, in situ combustion is more effective than water flooding. In situ burning is especially suitable for deposits that contmn heavy oil. With the application of the counterflow variant, the oil recovery factor increases up to S0%, and with the direct flow variant, up to 70% and higher [28]. During production of heavy crude the pores of reservoir rock in the unheated part of the oil-bearing bed often plug up. To remove these plugs, an oxidizer must be injected into the petroleum reservoir under high pressure.

All nonpolar substances, depending on viscosity, may be subdivided into immobile and mobile. Under normal conditions, dynamic viscosity of crude oils, as a rule, does not exceed 200 sP, and in reservoir conditions is within the range of 0.4-1,000 sP. At viscosity of more than 1,000 sP these liquids are incapable of migrating relative rocks. 

VI Graifer, GA Lazarev, Ml Leont ev. Effects of various factors on the viscosity of crude oil and water emulsions. Int Chem Eng 15 274—276, 1975. 

Most chemical agents used for demulsification are preferentially oil-soluble blends consisting of HMW polymers. These blends commonly consist of (1) floc-culants (large, slow acting polymers) (2) coalescers (LMW polyethers) (3) wetting agents and (4) sol-vents/cosolvents. Some chemical structures of demulsifiers used for breaking crude oil emulsions have been listed by Jones et al. (42). Much work has been carried out in order to identify and understand the mechanisms behind chemical demulsification. Fiocco (43) concluded that the inter facial viscosity was kept at a low level when demulsifiers were present. Later on it was realized that the interfacial shear viscosity of crude oil emulsions does not have to be very low in order to ensure accelerated water separation (44). 

The viscosities of crude oil found in these sands are typically high because of low temperature and recovery from these sands will require enhanced oil recovery (EOR) technology. Based on the EOR screening, miscible and thermal processes are considered to be suitable for recovery of crude from West Sak sands, while only thermal processes are considered to be applicable for recovery of Ugnu tar sands. The unconsolidated and friable sands also pose problems in well completion and production. 

Interfacial shear viscosity measurements were made on the viscous traction shear viscometer developed by Wasan et al. (25) for determining interfacial viscosities of crude oil-aqueous systems. 

A stable foam possesses both a high surface dilatational viscosity and elasticity (21). In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an insoluble, one-component layer does not depend on its thickness. This effect has been verified (22). Silicone antifoams reduce both the surface dilatational elasticity and viscosity of crude oils as illustrated in Table 2. The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. silicone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

It is known that the composition of oil depends on the source. This is found from the fact that the composition of oil from the Middle East is different from that found in Mexico or North Sea or elsewhere. The most significant component is the asphaltene content. Asphaltenes give rise to higher viscosity and other characteristics. As known from fluid flow through porous media, higher viscosity gives rise to a slow movement in the pores. The viscosity of crude oil in a reservoir may vary between about 0.1 and 100,000 mPa s, whereas the viscosity of water lies between 0.5 and 1.2 mPa s, depending on temperature and salinity (Ahmed, 2011 Birdi, 1999). 

For instance, plastics and metals develop brittleness and lose their flexibility at low Arctic temperatures (Dutta 1988, Singh 2013). As another example, low Arctic temperature together with waves, currents, and winds increase the viscosity of crude oils drastically through some weathering processes such as emulsification and evaporation (Brandvik Faksness 2009, Fingas 2011, Fitt-Rasmussen et al. 2012, Nordvik 1995). Pumping of highly viscous oils may lead to mechanical failures in pumps and connections (Fig. 4) (NOAA 2010). 

KM CDR [Probable derivation Kansai Mitsubishi Carbon Dioxide Recovery] A process for capturing carbon dioxide from flue gas and using it for enhanced oil recovery. The carbon dioxide is captured in a proprietary hindered amine using a packed column. It is used under supercritical conditions to reduce the viscosity of crude oil and thus enhance its recovery. Developed in 2004 by Mitsubishi Heavy Industries, Kansai Electric Power Co., and Shell EP International. Now operated in India and Bahrain. 

People usually want to know in advance which crude oils should be mixed and in which volumetric ratios, in order to achieve the desired value of viscosity. Thus the prediction of viscosity of crude oil blends becomes a mathematical issue rather than experimental. That is done with the help of mixing rules. A total of 26 mixing rules are reported in the literature (Centeno et al., 2011 and references cited therein), which can be classified according to the number and type of parameters involved for calculation of viscosity, as well as the experimental information required (Tables 1.5 and 1.6). 

FIGURE 1.10 Comparison of calculated and experimental viscosity of crude oil blends, (o) Experimental, (—) REFUTAS method, (-------) Binary interaction parameter. 

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