Study On The Yield Law Of Gelled Waxy Crude Oil

Yield characteristic is an important rheological behavior of waxy crude oil,which can greatly influence pipeline's shutdown and restart.Traditional study on yield properties of waxy crude oil,the yielding phenomena under constant shear rate and constant shear stress are divided and cannot combine together.In this paper,rheological experiments are conducted on rheometer and with two loading mode of controlled shear rate and shear stress,the yield characteristics of Nanyang waxy crude oil under different shear loading methods are analyzed.Under the constant shear rate loading experiments,yield stress?the maximum shear stress in stress-strain curve?of Nanyang waxy crude oil decreased with the decrease of the shear rate,while yield time increased with the decrease of the shear rate.However,yield strain is first decreases and then increase as the shear rate decreases,also that the trends in yield strain show the"V"font as the shear rate.Under the constant shear stress loading experiments,When the stress is less than the critical value,after a long time,the crude oil will not yield;when the shear stress is greater than the critical value,the crude oil is yielded.The yield time decreased with the increase of shear stress.Comparison the yield characteristics under constant shear rate and constant shear stress,Discovered for the first time,the yield stress of Nanyang waxy oil which just happened to yield under constant shear stress experiments is very close to the yield stress under constant low shear rate 0.0001s^-1 experiments.Therefore,the yield phenomenon?yield stress?of waxy crude oil at constant shear rate and constant shear stress is consistent,the yield phenomenon both can be unified.At present,yield stress of waxy crude oil is mainly determined by actual measured flow curves according to some criteria.In this paper,firstly,rheological experiments are conducted on two stress-control rheometers with different types,and the results show that under the same shear loading conditions,the actual response characteristics of different rheometersin the initial stages of testing are different and the rheological curvesvarygreatly.Yield response characteristics of waxy crude oil under two stress-control rheometers are analyzed and results show that the yield stress and yield strain differs a lot,which indicates that the yield characteristics of waxy crude oil is strongly associated with the actual response characteristics of rheometer after loading.Therefore,the determination of waxy crude oil yield stress must consider the actual loading condition in the process of testing.Therefore,based on the concept of ultimate strength of material in solid mechanics,the maximum value of elastic stress obtained from the viscoelastic-thixotropic model is defined as the yield stress.The newly proposed yield stress,with solid physical meaning,can consider the actual loading condition of rheometer in testingprocess.