A Study Of Viscoelastic Properties And The Establishment Of A New Creep Model For Gelled Crude Oil

In this paper,the basic physical properties and the rheological properties of Changqing waxy crude oil were investigated in detail,using a combination of rheological methods and DSC(Differential Scanning Calorimeter).The formatting and destructing properties of the viscoelastic structure for Changqing waxy crude oil were researched and discussed in detail.On this base,the creep models were established by introducing damage,which can describe formatting and destructing process for gelled waxy crude oil.The main contents and conclusions are the following.(1)Two kinds of Changqing waxy crude oil are typical waxy crude oil.Their gum and asphaltene content are not high(lower than 1.3% and 0.027% respectively),density is light(lower than 0.04g/cm3),wax content is high(higher than 11%),and pour point is high(24℃ and 17 ℃ heated at 50 ℃ respectively)which would decrease obviously with heating temperature raises.They have good fluidity at room temperature.(2)The influence of cooling rate,shear stress and repeated heating on the formation characteristics of Changqing waxy crude oil was investigated.It is shown that the gel point of the waxy crude oil becomes higher and the viscoelastic structure of it becomes stronger with the cooling rate and shear stress decreasing,and reheating temperature increasing.The existence of wax solvation layer was proved by rheological experiment and calculation.By combing crystallography theory,the existence of wax solvation layer would make the concentration of internal phase higher,more liquid oil getting trapped inside,as a result,viscosity of the oil system become high.(3)The influence of stress loading method on the yielding characteristics of Changqing waxy crude oil was investigated.The results show that yielding process of gelled waxy crude oil is really a creep process under constant stress,the yielding time would decreases by increasing the shear stress,and the viscoelastic structure would not yield until the stress is higher than the critical yield stress.The yield time becomes shorter and yield stress becomes bigger as the stress loading speed is high under linear increased stress.Shear stress and its action time are the two key factors to the yield of the viscoelastic structure,the viscoelastic structure would yield in a short time under big stress or in a long time under small stress.(4)In view of the creep characteristics of the gelled waxy crude oil,a creep model was established,which can predict the increase of strain over time and stress under constant stress,according the damage described by Weibull distribution function.On the basis of this model,another function was established to describe the increase of strain over time and stress loading speed under linear increased stress.Results show that the predicted values and experimental values are in agreement and the models have good application prospect in the field of waxy crude oil.