Evaluation Of Viscoelastic-thixotropic Model Based On Stress Loading

Viscoelasticity and thixotropy are an important rheological properties of waxy crude and its emulsion.It is the basic data for numerical calculation of shutdown and restart of oil pipeline and guarantee of pipeline safety operation.In this paper,the viscoelasticity and thixotropy of waxy crude and its emulsion under different stress loading conditions were studied,and the existing viscoelastic-thixotropic model was evaluated.Under constant shear stress,stress step rise,and linear increase and decrease of stress loading conditions,the effect of temperature,water content and stress on the shear rate of waxy crude oil and its emulsion were studied.The thixotropy of waxy crude emulsion gels was analyzed with the hysteresis area as the characteristic quantity,and the effect of water content on the structure recovery of emulsion gels was summarized.The model parameters of three loading modes were compared.Based on the experimental data of waxy crude and its emulsion near gel point in stress loading modes,the existing viscoelastic-thixotropic model was used to describe the viscoelastic and thixotropic properties of waxy crude and its emulsion.The results show that the higher the water content,the worse the model fitting effect.The higher the temperature,the better the fitting degree of the model.Under constant shear stress loading,the fitting effect of Teng Houxing Ⅳ and Dullaert model are the best.Teng Houxing Ⅱ and Ⅲ are relatively less applicable.Under stress step rise,linear increase and decrease of stress loading conditions,the fitting effect of Teng Houxing Ⅰ and Teng Houxing Ⅱ are the best.Teng Houxing Ⅳ and Dullaert model are not suitable for describing viscoelastic thixotropic properties of high water content emulsion gels on stress step rise loading mode.Zhu model is not suitable for the stress hysteresis loop loading mode.The structural parameters at the intersection point of the upper and the downlink lines of hysteresis loop are obtained by models.A better fitting effect of Teng Houxing Ⅰ and Ⅱ have similar structural parameters on that point,which can accurately reflect the structural state of waxy crude oil and its emulsion.