Investigation On Properties And Mechanism Of Sol-Gel Transformation For Non-Newtonian Waxy Crude Oil

Waxy crude oil is a complex mixture whose rheological properties have great influence on the exploitation, storage and pipeline transportation. Basing on the theories of physical chemistry and structure mechanics, the properties and mechanism of sol-gel transformation for non-newtonian waxy crude oil were investigated in detail, using a combination of rheological methods, DSC and optical microscopy. The main contents and conclusions are the following.The gelling properties for crude oil due to decreasing temperature were investigated.It is shown that the variation of structure parameters are distinct for different kinds of oils.The gel point of the waxy crude oil becomes higher and the strength of the gel structure increases with the decrease of cooling rate and shear stress. The existence of wax solvation layer was proved by rheological experiment and calculation.The corresponding gelling mechanism was improved by combim gexperimental phenome na and crystallography theory.The isothermal gelling properties of waxy crude oil were studied systematically. The results show that the modulus of oils after static cooling becomes higher by decreasing temperature or increasing holding time, meanwhile the loss angle decreases. For the oils after dynamic cooling or shearing under isothermal condition, the variation of modulus are similar to those of static cooled samples, but the loss angle decreases first and then increases by decreasing temperature. In addition, the loss angle also decreases first and then increases by increasing holding time at relatively lower temperatures. The isothermal structure becomes stronger with a decreasing static cooling rate.while with decreasing dynamic cooling rate, the stength of initial structure under isothermal condition increases but the strength of final structure decreases.The dependence of isothermal structure development of waxy crude oil on shear rate are also strongly influenced by temperature. At relatively high temperatures, the higher shear rates can result in stronger structure.While approaches pour point, the structure decreases first and then increases with increasing shear rate. At relatively lower temperatures, the strength of structure decreases with the increase of shear rate.The yielding properties of waxy crude oil under different measuring methods were studied.The results show that the yielding of gelled waxy crude oil under constant stress is really a creep process. The nonlinear creep model was established by introducing damage and hardening variables, which can describe the three stages of creep process for several different oils.For the shear stress loading in a linear way, a higher stress loading rate causes a higher yield stress and a shorter yielding time.For the stress loading in a stepwise way, the yield stress decreases and the yielding time increases with increasing time duration,whilewith increasing the changing rate of stress, the yield stress increases first and finally keeps constant, meanwhile the yielding time decreases. For the shear rate loading in a linear way, the shear stress-shear rate curve will appear fluctuation phenomena with a lower loading rate. As the loading rate increases, the fluctuation phenomena will decrease gradually and until disappear, while the yiled stress also increases.A new model was obtained basing on the Houska model, and it was proved to describe the thixotropic properties of waxy crude oil simply and accurately. A first theoretical model for recovery properties of waxy crude oil was established by using a structure parameter method, which was proved to fit well with the experimental data of several different waxy crude oils. Taking Qinghai oil for example, the influence of histories on the Correlation coefficient of the two models was also found.The melting properties of gelled waxy crude oil were first investigated by rheological method.The results show that the melting and gelling of waxy crude oil are not simply inverse processes.With the increase of heating rate, the gel strength increases during heating process. With the decrease of final cooling temperature, both the strength of structure during heating process and isothermal process at higher non-newtonian temperature decreases. And there is nearly no influence on the melting properties during heating process and isothermal structure development on higher temperature for isothermal holding time at final cooling temperature. The melting mechanism of gelled waxy crude oil was put forward.