Research On Structural Shrinkage And Start-up Process Of Gelled Crude Oil

Through the laboratory experimental loop, some experimental and theoretical researches were carried out on the recovery of gel structure after shutdown of waxy crude oil pipeline and the compression and yield process of gelled crude oil during startup of pipeline under constant flow rate. The article mainly consists of three parts: the structural shrinkage property of waxy crude oil after shutdown at constant temperature; a simulation method to calculate the start-up yield stress under constant flow rate is proposed; research on the calculation method of compression coefficient of gelled crude oil.After shutdown, there exists the phenomenon of volume shrinkage during the recovery of gel structure, the lower the shutdown temperature, the greater the shrinkage, and this is another important property which distinguish waxy crude oil from Newtonian fluid; the volume shrinkage is the fundamental reason for the pressure drop through pipeline during the coalescence and growth of wax crystal precipitated from crude oil. During the experiment, the pressure drop rapidly after shutdown firstly, and then” jump back”, finally, the pressure remain unchanged at a certain value, which depends on temperature and the component of waxy crude oil. The pressure through pipeline after shutdown is the result of the degree of structural shrinkage of waxy crude oil combined with the saturated vapor pressure(SVP) at the shutdown temperature.Based on the propagation of yield front during start-up of gelled crude oil pipeline, a simulation method to calculate the start-up yield stress of gelled crude oil is proposed. The result is irrelevant with the length of pipeline, and can reflect the real strength of gel structure; Homotopy algorithm is applied to solve the ternary nonlinear equations, which can avoid misconvergence due to the improper initial value selection; With regard to the coefficient of thixotropic time a, its value is relevant with the thixotropy of crude oil and the propagation of yield front. The method in this article can serve as a reference when solve the start-up yield stress under other conditions. Based on the definition of start-up yield stress, its value can be obtained directly through curve fitting according to least square method(LSM).Based on two assumptions and the real restart process of gelled crude oil pipeline, a calculation method is proposed through the mass conservation relationship between pipeline filling quantity and the quantity of crude oil pumped into the pipeline to solve the compression coefficient of gelled crude oil under constant flow rate, the solving result can be applied in the computational simulation of start-up of gelled crude oil pipeline. Before flow, gelled crude oil will undergo the deformation process of plastic-elastic-plastic-plastic, and its compressibility will also change during yield process. For simplicity, the compressibility of crude oil before yield is considered constant, therefore, the compressibility of crude oil in the process of yield would be a study emphasis in future research. Besides, under the influence of the startup property of pump, certain time is needed for the growth of flow rate from zero to metered flow, and this would bring error to the result of compression coefficient and need to be amended in future work.