Study On Viscoelastic-thixotropic Behaviors Of Gelled Waxy Crude

Our country is rich in condensable,highly viscous,waxy crude oil.Due to planned maintenance of the equipment or unexpected accidents,etc.,the oil pipeline is inevitably stopped.After stopping,because the temperature of the oil in the pipe is higher than the temperature of the surrounding environment,the oil heats down.When the oil temperature falls below the wax precipitation temperature,the wax gradually crystallizes and the temperature of the oil product further decreases.The precipitated wax crystal forms a three-dimensional network space structure,which binds the liquid oil,and the oil product changes from a sol state to a gel state.And it exhibits complex time-dependent rheological properties including viscoelastic and thixotropic.One of the problems in the flow safety protection of waxy crude oil pipelines is the restart after shutdown.For this reason,it is necessary to carry out numerical simulation calculations of the process of stopping and restarting waxy crude oil pipelines.The viscoelastic-thixotropic properties of waxy crude oils in gelled state are important basic data for this numerical simulation calculation.Therefore,it is necessary to accurately describe the viscoelastic-thixotropic properties of gelled waxy crude oils.The main research content of this paper is as follows:First of all,with the aid of the Haake RS150 controlled stress rheometer produced by Germany,the SYD-510 petroleum product freezing point meter produced by Shanghai Changji Geological Instrument Co.,Ltd.,and the TA2000/MDSC2910 modulated differential scanning calorimeter manufactured by the American TA company.Experimental equipment for four different physical properties of gelled waxy crude oil,based on the temperature near the freezing point,the shear rate step,shear rate hysteresis loop,constant shear rate,shear stress step and shear stress hysteresis Rheology experiments with five typical loading modes for loopback.Secondly,based on the principle of mechanical comparison,a viscoelastic-thixotropic model containing nine unknown parameters was established based on the characteristics of "viscoelastic-yield-thixotropic" of waxy crude oil.The complete and accurate description of the rheological response of the wax-loaded crude oil loading process using a model is realized.The model has the characteristics of high precision,relatively few parameters,clear physical meaning,and strong adaptability.Finally,the Simulink module in Matlab was used to simulate the modeling,and the experimental data was used to verify the model by genetic algorithm.The validation of the model includes the experiment of shear rate step,shear rate hysteresis loop,constant shear rate test,shear stress step and shear stress hysteresis loop test near the freezing point of waxy crude oil with four different physical properties.For the data fitting,the average relative deviations of the model fitting values and measured values for the five kinds of tests were 4.6%,6.8%,4.2%,2.7%,and 5.2%,respectively.