| Most of the crude oil produced in China is high in wax.Due to the presence of wax,the rheology of crude oil has become very complicated,bringing many problems to the safe transportation of pipelines.For oil pipelines,due to maintenance and accidents of the equipment,it is usually necessary to stop the transportation.After the waxy crude oil pipeline was stopped,the rheology of the crude oil deteriorated due to continuous cooling of the crude oil.When the structural strength of the crude oil is higher than the starting pressure that the pump can provide or the pressure that the pipeline can withstand,a serious coagulation accident can occur.Thixotropy is one of the complex rheological properties of low-temperature wax crude oil,the understanding and description of thixotropy can provide basic data for the numerical simulation and computation of pipeline restart.The rheological properties such as the yield behavior and thixotropy of low-temperature waxy crude oil are studied by controlling the loading shear rate,for example,the loading of shear rate increases and decreases linearly.Based on the loading of shear rate increasing and then decreasing linearly,the effects of experimental temperature and change rate of shear rate on the yield stress and yield time of low-temperature wax crude oil were studied.Based on the experimental data and the determination standard of the waxy crude oil yield point,the yield point of the experimental oil sample under the loading conditions set forth in this paper was obtained;The thixotropy of low-temperature waxy crude oil was studied,and the effects of different test temperature and change rate of shear rate on the thixotropy of waxy crude oil under the conditions of no pre-shearing and pre-shearing applied after standing at constant temperature were analyzed and compared.Based on this,to analyze experimental data,to calculate hysteresis loop area,to synthesize experimental data and to explain the effects of test temperature and change rate of shear rate on the thixotropy of crude oil gel system,such as the area and the direction of movement of hysteresis loops,and the structural recovery characteristics after standing at constant temperature for different periods of time,and the concept of structural recovery degree was proposed;under the experimental conditions of coaxial cylinder measurement system,especially at lower test temperature conditions,there will be obvious wall slip phenomenon,through the experimental data,and the experimental data was used to further explain the effect of test temperature and change rate of shear rate on the wall slip phenomenon. |
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