Study On The Yield Behavior And Pipeline Restart Process Of Gelled Crude Oil

It is important and complicated to avoid and deal with the gelation of waxy crude oil for long-distance pipeline, considering the poor low-temperature flow properties of domestic crude oil due to its features of high wax content, high gel point and high viscosity. The research on the oil start-up properties is necessary and important to direct the management and operation of waxy crude oil pipeline.By virtue of rheometer and laboratory experimental loop, the mechanical behavior and pipeline restart process of gelled crude oil were studied according to the viscoelasticity theory. Firstly, the flow rate controlled and pressure controlled pipeline restart processes of gelled crude oil were respectively simulated with the experimental loop. The test results showed that, the propagation speed of the pressure signal along the pipeline was dependent on oil properties, loading conditions and propagation distance, etc. The damping action of gelled crude oil was so strong that the pressure signal moved much more slowly than that of pressure wave. Furthermore, the propagation of the yield front for gelled crude oil was analyzed by monitoring the differential pressure of each pipe section between two pressure sensors. The moving speed of the yield front was dependent on oil properties and loading conditons, and it propagated later than the pressure signal. The moving speed of the yield front was the decisive factor for calculating the pipeline restart duration, and the pipeline restart operation would not be successful until the yield front reached the end of the piepline. Secondly, by virtue of the ultrasonic displacement transducer mounted above the surge tank, the compressibility of gelled crude oil was measured, and the oil flow rates for pressure controlled start-up tests were monitored, which contributed to understanding the relationship between pipeline restart and the changes of pressure and flow rate. The pipeline restart process was accompanied with deformation, yield and thixotropic flow of gelled crude oil, under the actions of both pressures and shear stresses. The pressure distribution and flow rate variation in the pipeline restart process were mainly dependent upon the thixotropy of gelled crude oil, under high start-up pressures; while, the pressure distribution and flow rate variation in the pipeline restart process were mainly dependent upon the creep behavior of gelled crude oil, under low start-up pressures. Thirdly, the effects of oil temperature, static temperature drop, shut down duration and start-up flow rate on the pipeline restart properties of gelled crude oil were experimentally studied with the test loop. The support vector regression method, which was a common and efficient technique for nonlinear multivariate analysis, was applied to the test data, and a new formula with improved precision was obtained to predict the start-up yield stresses of gelled crude oil under different conditions. Besides, by using Anton Paar Physica MCR301 rheometer, the mechanical responses of gelled crude oil in the start-up tests were studied under different loading conditions. A new start-up constitutive equation was obtained on the basis of Maxwell model and nonlinear viscoplastic model, and an empirical formula was put forward by dimensionless analysis, to describe the mechanical behavior of gelled crude oil before structural failure. The physical meaning of shear strain measured by the coaxial cylinder apparatus was analyzed as well.