Rheology of Waxy Crude Oils

Abstract

Abstract Crude oils from China, Australasia and many other parts of the world, containing waxy crystals and colloidal asphaltenes, possess distinct non-Newtonian flow properties which depend strongly on the shear and thermal history and result in special difficulties being encountered in all types of viscometers. Experimental techniques are discussed which allow reproducible equilibrium data to be obtained in the concentric cylinder and the cone and plate viscometers. The shear history dependence of the flow properties, which is shown to occur in both Australasian and Chinese crudes, alters the conventional design and scale-up assumptions. Flow in a capillary or pipeline is shown to be a composite effect of the range of shear rates that occurs across the radius of the pipe. An identical composite effect must occur for the scale-up assumption to remain valid. Pipeline flow can be determined from laboratory data by modified design techniques which also allow the quantitative assessment of oil treatment methods. A lower limit to pipeline operation, due to the shear history effect, is shown to exist, which has an effect on the assessment of oil treatment methods and on the operation of declining oil-fields. Introduction The viscosity of a crude oil is one of its most important physical properties. Investments of millions of dollars are committed to the transport of crude oil in pipelines designed on the basis of this single number. The return on the investment may disappear if the flowrate in a given pipeline is not as high as anticipated due to a lack of understanding of the factors affecting the oil's viscosity. Crude oils are often regarded as simple fluids with the pressure drop (shear stress) directly proportional to the flowrate (shear rate) in simple streamline flows, that is, the oil is said to be "Newtonian". Although the flowability of oil may vary from the consistency of water to that of road tar, the same simple assumption of Newtonian viscosity is often made. This is reflected in the instruments commonly used in the oil industry for measuring viscosity, for example flow cup viscometers, conventional U-tube viscometers and falling ball viscometers, where the design and operation of the instrument is based on this Newtonian viscosity assumption. At sufficiently high temperatures, crude oils are indeed Newtonian. However, as the temperature is reduced from well-head to ambient conditions, two classes of components, wax crystals and colloidal asphaltene particles can lead to non-Newtonian characteristics which dominate the flow properties of many oils and give rise to serious errors in both the conventional viscometers mentioned above and in the more sophisticated rotational and capillary viscometers. The presence of wax crystals leads to a wide range of non-Newtonian characteristics including yield stress, pseudoplasticity (shear thinning), time dependency, but most importantly, a dependence of the flow properties on the prior treatment received by the oil, that is, the shear and thermal history. The consequence of this history is that for a waxy or viscous oil, a wide range of viscosities are possible at a given temperature and rate of shear by altering the shear and thermal history. A joint project between the University of Melbourne and the ICI Australia Research Group in Melbourne has sought to establish a rigorous rheological approach to the measurement of the flow properties of waxy crude oils and to develop design procedures appropriate to the rheological characteristics so obtained. Most Australian crude oils contain high proportions of low molecular weight waxes (up to 25% nC16 - nC30) and are pipelined at temperatures well below the pour point. P. 803^