Study On The Thixotropic Characteristics And Model Of Waxy Crude

Thixotropy is a typical non-Newtonian property about the apparent viscosity which would change with the load size and loading time, and it's an important branch of rheology. Waxy crude would present thixotropy when its temperature is low enough, and the thixotropy of crude oil has an important influence on the restart process of shutdown pipeline, so it has been a hot issue of rheological study. However, the thixotropy of waxy crude has been the difficulty of rheological study as also, because the thixotropy of low temperature waxy crude does not exist alone, the thixotropic research should consider the effect of other rheological behaviors, and due to the sensitivity of thixotropic structure to the temperature and shear conditions, the thixotropic study also need to consider the effect of history. Thixotropic model is the most important part of thixotropic study, the current thixotropic model of crude oil mainly based on the method of structural parameter, and generally these model has a high accuracy, but these model often has more parameters which would cause difficulty in the calculation, and these model can not interpret the thixotropic characteristics caused by the temperature, or the thixotropic characteristics when the load is stopped. In addition, there is no evaluation method of thixotropy influential factors, and the research about the influential factors is less.The thixotropy of low-temperature waxy crude oil was studied with the viewpoint of capacity in this paper, the content included thixotropic experiment and thixotropic theory. In the thixotropic experiment part, the thixotropic characteristics and influential factors of JiDong waxy crude were studied with the use of strain-controlled rheometer ARES-G2. The thixotropic characteristics include the characteristics of apparent viscosity and thixotropic structure, both of them would change with the tempreture, shear rate and time, and the characteristics of hysteresis loop which would change with the variation of load times. Moreover, the influences of thermostatic time, temperature history and shear history on the thixotropic characteristics of JiDong waxy crude were obtained, an evaluation method based on the step-changed shear rate was proposed in the study process, and the damage extent and recovery extent of apparent viscosity were used in the evaluation method, the results indicated the more stable of the internal structure, the damage extent of the apparent viscosity would be greater, and the recovery extent of the apparent viscosity would be smaller. In addition, the influences of loading rate and maximum shear rate on the hysteresis loop were studied.In the thixotropic theory part, the thixotropic method based on the viewpoint of capacity and the thixotropic model based on the potential function were proposed. Structure damage potential function and structure recovery potential function were proposed in the thixotropic method, and the relations of the two potential functions were used to explain the thixotropic characteristics of waxy crude. Moreover, the structure damage potential function was divided into two parts on the basis of thixotropic characteristics of waxy crude, through the establishment of the balance relations of the two potential functions and the relations of the potential function and shear stress, the thixotropic method on the viewpoint of capacity can be used to application. In addition, a two-grade thixotopic model of rate type was proposed on the basis of the characteristics of the two potential functions, and the comparision of the two-grade thixotropic model, one-grade thixotropic model and Houska model indicated the two-grade thixotropic model has a highest accuracy. Additionally, the mathematic relations of the two potential functions when the load was stopped and when the temperature was changed were studied in this paper.