Study On Stress And Strain Response Of Gelled Waxy Crude Oil In Yield Process

When the temperature of gelled waxy crude oil decreases below that of the gelation temperature,a gelled structure with complex rheological properties such as viscoelasticity,yielding or thixotropy will be formed.It is meaningful for the restart-up of crude oil pipeline to investigate the stress and strain response and develop quantitative characterization methods for the crude oil yielding.However,the study on the yield process of waxy crude oil is still at the qualitative level,and the influence of historical factors on the yielding behaviors of waxy crude oil is always blurred.In this context,this paper carried out the following research:Firstly,the isochronous stress-strain curve proved that the creep process of gelled waxy crude oil is a non-linear process,featuring that the larger the creep stress and the longer the creep time,then the more evident the non-linear process of the creep process.Then,based on theory of fractional calculus and theory of damage mechanics,a non-linear damage creep model used to describe creep behavior of gelled waxy crude oil has been established.The model not only has a good effect in the early stage of creep,but also can simulate the whole creep process,including accelerated creep.This model has solid mechanical basis,clear physical meaning and can described the creep curve precisely.The average relative deviation is less than 5%.At last,by means of controlled shear rate loading,small amplitude oscillatory shear and wax crystal microscopic observation,the systematic study on the influence of historical factors on the gelled waxy crude oil proved: Decreasing cooling rate or testing temperature,or extending the time of constant temperature could change the both structure and morphology of wax crystal,thereby changing the yielding properties of crude oil,which in return could increase the yield stress and decrease the yield strain.In the case of a certain amount of wax precipitation,the increase of wax crystal size contributes to the formation of a more powerful gelled structure.Compared with static cooling,the yielding strain would add an order of magnitude after dynamic cooling.As the shear rate increases,the yielding stress would firstly decrease and then increase.So,it can be concluded from the study that some certain level of medium-speed sheering imposed on the oil sample during the cooling process could improve the low-temperature fluidity of gelled waxy crude oil at different levels.