Effect Of Non-linear Growth Behavior Of Wax Crystal Flocculation On Rheological Properties Of Waxy Crude Oil

Waxy crude oil has significant characteristics of poor and complex rheology at low temperatures.In the pipeline transportation,with the decrease of temperature and dynamic shearing,wax crystals continue to nucleate,precipitate and grow to form flocculation,resulting in the deterioration of the rheological properties.To carry out the dynamic evolution of wax crystal morphology under shear and explore its effect on the rheological properties of waxy crude oil is of great significance to reveal the rheological deterioration mechanism and ensure the safe and economic transportation of pipeline.Based on the method of rheological measurement and polarizing microscope in-situ measurement technique,this paper studied the influence by Non-linear growth behavior of flocculation on rheological properties of waxy crude oil under coupling effect of cooling rate and shearing.On this basis,the dynamic growth model of wax crystal flocculation was established,and the relationship between Non-linear growth behavior and rheological properties was analyzed quantitatively.Firstly,the viscosity-temperature characteristics,shear thinning,yield characteristics and thixotropy of waxy crude oil were studied,and the corresponding mathematical model was established.The viscosity,yield stress and deformation can be calculated,and the calculation accuracy is about 95%,which lays a foundation for further rheological mechanism analysis and the influence of flocculation growth behavior on rheological properties.In addition,based on the rheological data of Daqing crude oil before and after modification,the oil transportation scheme of Nanyi oil depot is formulated.After adding the PPD,the optimal transportation temperature of the pipeline is reduced by about 8%,and the pressure is reduced by about 30%.Secondly,the Non-linear growth law of wax crystal flocculation structure during dynamic cooling was analyzed by in situ microscopic observation.The characteristic parameters of wax crystal flocculation were extracted,such as the number of particles,edge distance,average diameter and fractal dimension,characterize the nonlinear growth behavior,and reveal its variation with shear rate and temperature.It is found that with the decrease of temperature,the change rate of characteristic parameters decreases gradually,the growth of flocculation slows down,and the parameters fluctuate with the increase of shear rate.On this basis,the effects of chemical modification and crude oil composition on the Non-linear growth behavior of wax crystal flocculation were quantitatively analyzed.Compared with Daqing crude oil,the change rate of wax crystal particles in modified crude oil and Hulunbeier crude oil decreased by 30.7%and 58.9% respectively,and the particle diameter change rate increased by 43.6% and 54.5%,respectively.Furthermore,the microscopic observation results were correlated with the synchronous rheological data,and the effect of wax crystal flocculation growth behavior on the rheological properties was quantitatively analyzed.It is found that under the low speed rate,the flocculation have the max particle diameter and edge distance change rate,and have good rheological properties.Under the high shear rate,the flocculation have the max change rate of particle number and the minimum change rate of edge distance.The formed gelation structure has the max critical stress and the most significant rigid and elastic structure characteristics.Finally,on the basis of rheological experiments and in-situ microscopic observation results,the dynamic growth models of wax crystal flocculation were constructed based on population balance model,porous media and fractal theory,respectively.The practical application results show that the overall average error of the model is about 10%,which can better predict the change of the number of flocculation caused by microscopic behaviors such as collision aggregation and fragmentation in the shear flow field.Furthermore,through the comparison and combination of different models,the quantitative correlation between the dynamic growth process of wax crystal flocculation and rheological data is established,and the effect of Nonlinear growth on the rheological properties of waxy crude oil is quantitatively analyzed.In general,the research of this paper not only help to deepen the understanding of the complex rheological mechanism of waxy crude oil,but also can be used to guide the formulation of oil pipeline operation plan.The theoretical model not only has the function of mechanism explanation,but also can be used to predict the rheological properties of waxy crude oil and its modified oil with high precision,which provides a new research idea for the rheological mechanism analysis and model establishment of waxy crude oil.