Summary.
The variation of the rheological properties of Daqing waxy crudes with their thermal history and their time effect are discussed. Rheological properties of Daqing crude were determined in the laboratory with rotational viscometers and U-tube devices. The feasibility of using the proper heat treatment to improve the low-temperature flow property of Daqing crude was studied. Field tests on three long-distance heated-crude pipelines showed good agreement with laboratory measurement. A mathemat-ical model for determination of the optimum operation parameter for pipe-line transportation is also given.
Introduction
An extensive study of the rheological characteristics of crude oil is indispensable for lowering the energy consumption and ensuring safety and cost effectiveness in pipeline transportation of waxy crudes. The relative amount and molecular distribution of wax, resin, and asphaltene in a high-pour-point waxy crude directly affect the rheological properties of crude oil. Phase changes are observed under various thermal conditions. At an elevated temperature, wax dissolves in crude oil to form a homogeneous fluid. When the temperature is lowered, wax crystals separate out with adsorbed resin and asphaltene, and crude oil turns from a liquid to a suspension. With further temperature reduction, a continuous phase of wax crystal lattice is formed, while liquid hydrocarbon becomes a dispersed phase. Crude oil then loses flowability, displaying con-ditional freezing. This phase change causes crude oil to deviate from Newtonian behavior and to exhibit shear thinning, thixotropy, and structural strength. With continuous lowering of the temperature, waxy crude can be changed rheologically from a Newtonian fluid into pseudoplastic, thixotropic/pseudoplastic, and thixotropic/ yield-pseudoplastic fluids successively. This change in rheological types can be attributed to the thermal and shear histories, as well as to the composition of crude oil. This paper focuses on the rheological properties of Daqing crude oil on the basis of experimental work.
Experimental Procedure
The wax content in Daqing crude oil is as high as 25%. The freezing point was taken as the temperature of conditional freezing when crude oil lost flowability. The apparent viscosity was determined with an RV-II viscometer. The yield stress was determined with a U-tube device as a measure of its resistance to restart of flow after freezing. Consistent sampling and testing procedures were prescribed to guarantee data reproducibility. First, the sample was heated to 80 degrees C [176 degrees F] and kept at rest at room temperature for 48 hours. Next, the sample was transferred to the U-tube device at 50 degrees C [122 degrees F], which was higher than the wax-crystallization temperature, to minimize the effect on wax crystal shearing. It was then heated to the heat-treatment temperature specified in each experiment. Finally, the sample was cooled to the testing tem-perature at a rate of 0.5 to 1 degrees C/min [0.9 to 1.8 degrees F/m] under static conditions. The test was aimed at studying the effect of thermal conditions on the rheological properties of crude oil.
Effect of Thermal History
Wax crystals dissolve completely or partially in crude oil upon heating. Conversely, upon cooling, wax crystallizes out with adsorbed resin and asphaltene. resulting in a change in the rheological characteristics. We discuss this change from the data obtained for specified cooling procedure and different heat-treatment temperatures.
Daqing crude was most sensitive to its thermal history in the heat-treatment temperature range of 45 to 55 degrees C [113 to 131 degreeF] (see Figs. 1 and 2, and Table 1). In the range of 45 to 53 degrees C [113 to 127 degrees F], the freezing point, apparent viscosity, and yield stress tended to increase with rising temperature. In the range of 53 to 55 degrees C [127 to 131 degrees F], the freezing point, apparent viscosity, and yield stress fell drastically as the temperature rose. When the crude was heated to a temperature higher than 55 degrees C [131 degrees F], the rheological properties remained basically the same, exhibiting good low-temperature flow behavior. This phenomenon was very specific for Daqing crude compared with the crudes from other fields in China. The unique behavior of Daqing crude oil may be accounted for by the high content of low-molecular-weight paraffin and a medium wax-to-resin ratio.
The characterizing temperature that marks a change in the rheoiogical structure is different at different heat-treatment temperatures. Waxy crude experiences a successive process of crystallization, crystal growth, and interlinking of crystals into a lattice during the cooling-down process. Rheologically, this process can be classified by the characterizing temperatures (crystallization temperature, Tc, abnormal temperature. Ta, and lost-flow temperature, Tlf). At T greater than Ta, the crude oil is a Newtonian fluid. and its flow behavior is described by = . At Tlf less than T less than Ta, crude oil is a pseudoplastic or thixotropic/pseudoplastic fluid, and its flow behavior is described by =K when a dynamic equilibrium is reached; i.e., the shear stress will not change with time. At T less than Tlf, the crude oil is a thixotropic/yield-pseudoplastic fluid, and its rheological characteristics can be described by = a + K at dynamic equilibrium (see Table 2).
For a, given heat-treatment temperature, viscosity-vs.-temperature curves at various shear rates are obtained as shown in Fig. 3. Fig. 4 shows that the viscosity-vs.-temperature curves at various heat-treatment temperatures for a given shear rate can be classified into two groups: the curves at temperatures below 53 degrees C [127 degrees F] and those above degrees C [131 degrees F]. Fig. 5 shows curves of viscosity vs, shear rate at different heat-treatment temperatures. For fluids exhibiting non-Newtonian behavior, the curve with a heat-treatment temperature below 53 degrees C [127 degrees F] are sensi-tive to shear thinning, while the others have a more gentle trend, as shown in Fig. 5. However, in the region of Newtonian behavior-i.e., at a testing temperature above 42 degrees C [108 degrees F]-the viscosity of crude oil tends to be the same, no matter what the heat-treatment temperature is, as shown in Fig. 4 and Table 3.
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