Research On The Rheological Law Of Crude Oil Transported By Qingha Pipeline Under The Condition Of Mixing Russian Oil

With the decrease of large oilfield production and the increase of imported crude oil,it has become a common transportation method to mix crude oil to ensure that the pipeline is in a state of normal transportation and prevent entering unstable working areas.The Daqing-Harbin oil pipeline in northeastern China adopts mixed transportation,and the medium to be transported is the mixed crude oil of Daqing and Russia.The chemical compositions of Daqing crude oil and Russian crude oil are quite different.After mixed,the basic physical properties and rheological properties of the mixed oil are not a simple additive model,and it is difficult to accurately estimate,which makes the Qingha oil pipeline have certain obstacles in the design of the operation scheme.Therefore,the macro-rheological properties and wax crystal microstructure characteristics of the mixed crude oil with different mixing ratios were systematically studied in this paper by rheological measurement technology,conventional polarizing microscopic measurement technology and innovatively constructed reho-polarized microscopy in-situ sync measurement technology.The main research contents are as follows:Firstly,by testing the gel point,wax precipitation characteristics and wax dissolution characteristics of crude oil mixed with 10%?20%?30% and 40% of Russian oil,and combined actual working condition,the appropriate mixing ratio was selected to carry out the follow-up rheological study.The test found that with the increasing ratio of Russian oil,the gel point of crude oil decreased from 30°C to 17°C,the wax appearance temperature decreased from 47.46°C to 43.51°C,the wax dissolution temperature decreased from 65.4°C to 63.1°C,and the basic physical parameters changed nonlinearly with the increase of the mixing ratio.Under the comprehensive consideration and comparison combined with the actual working condition,the chosen mixing ratios are 20% and 30%,and the measurement of Daqing oil is added to compare and analyze the rheological properties of the three.Secondly,using the rheological measurement method,the shear-step test,the yield characteristic test and the creep/recovery characteristic test were carried out on the Daqing oil and the Daqing oil mixed with 20% and 30% Russian oil.It is found that with the increase of the mixing ratio,the rheology of Daqing crude oil has been significantly improved,its viscosity gradually decreases,the abnormal point is gradually reduced and the range from the abnormal point to the gel point gradually expanded,and the maximum difference was 12°C.In addition,by comparing the rheological properties near the gel point of crude oils with different mixing ratios,it can be seen that in the range from the abnormal point to the gel point,the crude oil with a mixing ratio of 20% has the highest apparent viscosity and the strongest shear thinning due to the large amount of wax precipitation,and near the gel point,the yield stress of this ratio is the largest,the highest is 56.06 Pa,the strain generated in the creep phase is small,the constant stress range is wide,and the constant stress required to reach instantaneous yield is as high as 81 Pa.Then,the influence of different initial cooling temperatures and different cooling rates on the rheological properties of crude oil with different mixing ratios was analyzed.It is found that there are different initial cooling temperatures that make crude oil rheology worse and better,and with the mixing of Russian oil,the different initial cooling temperatures that make the crude oil rheology worse and better do not change.Furthermore,under dynamic cooling,the apparent viscosity measured in real-time decreases with the increase of cooling rate.However,under static cooling,the apparent viscosity and gel structure strength of crude oil increased with the increase of cooling rate after a period of time at constant temperature.Besides,through comparison,it is found that the influence of different initial cooling temperatures on the rheology of crude oil is greater than that of different cooling rates,and near the gel point,the crude oil with a mixing ratio of 20% has a large response to the change of thermal history factors,when the initial cooling temperature is 60°C,its apparent viscosity and gel structure strength increase significantly,and the yield stress is as high as 190.11 Pa.Moreover,the conventional polarizing microscopy imaging system was used to photograph and observe the changes of wax crystal morphology and structure of crude oil with different mixing ratios and crude oil with different mixing ratios at different initial cooling temperatures and different cooling rates.The essential differences of wax crystals formed by different mixing ratios of crude oil under different influencing factors were identified by wax crystal microscopic images and wax crystal characteristic parameters,and further elucidate the macroscopic rheology of crude oil from the microscopic point of view.Besides,the motion of wax crystals in crude oil with different mixing ratios in the flow field was observed and photographed by the constructed rheo-polarized microscopy in-situ sync measurement technology.The wax crystal dynamic behavior of crude oil was compared and analyzed by corresponding characteristic parameters combined with rheological experimental results.It is found that at the same temperature,the number of wax crystals in Daqing crude oil is larger,the size and fractal dimension are larger,the spacing between wax crystals is smaller,the average velocity in the flow field is smaller,and the resistance of wax crystals to the flow of crude oil is large,resulting in poor rheology.However,when near the respective gel points,the crude oil with a mixing ratio of 20% and 30% has wax crystals of larger size and complex structure formed after sufficient growth due to the low gel point,as a result,the average velocity of wax crystals in the flow field becomes smaller,and the resistance to the flow of crude oil becomes larger,resulting in the deterioration of the rheological properties of the two is higher than that of Daqing crude oil.Besides,reducing the initial cooling temperature and increasing the cooling rate will gradually grow a large number of fine wax crystals in the crude oil,thus forming a dense wax crystal structure and deteriorating the rheological properties of the crude oil,but the effect of initial cooling temperature on the wax crystal morphology is greater than that of cooling rate.Finally,the corresponding formula fitting is carried out using the results of the gel point test and rheological test of crude oil with different mixing ratios.Through the corresponding fitting formula,the gel point of crude oil with different mixing ratios,the yield value in the selected measurement temperature range and the deformation of the gel structure under different constant stress can be calculated to provide some basic data and theoretical basis for the formulation of the pipeline operation plan.