Electrical Characterization Of Asphaltenes Association Behaviors From Different Heavy Oil

Asphaltene is the most polar compound in petroleum,which has the highest content of heteroatoms.The study of its associative and polar properties,as well as aggregation scale,is of great significance to heavy oil production and processing.In this thesis,the n-heptane asphaltenes were separated from the raw materials,the Haishang heavy oil(HS),Yousha bitumen(YS)and Sichuan mixed residuum(SC),as the research samples.The average dipole moment of asphaltenes were investigated through dilute solution method to characterize their polarity.Meanwhile,all of these asphaltenes have been measured by ~1H-NMR spectrometry from which the average structural parameters can be calculated.Thus the associativity of asphaltenes can be explained.The results showed that the polarity of HS asphaltene was the strongest while the SC asphaltene was the weakest,which had no obvious relationship with their average molecular weight.As the polarity decreased,the ratio of H/C atom increased,and the aromatic carbon ratio decreased.Combined with elemental analysis,it can be concluded that following with the decrease of the content of heteroatom,polarity and the association decreased.The influences of applied electric field frequency,solvent,concentration and temperature on the electrical properties of asphaltenes were investigated by means of electrical method.The results showed that in the low-frequency section,DC conductivity was the main mechanism of the charge transfer,and the conductivity depended on the polarity of the solution,the particle concentration and velocity,rather than delocalized?bond.The effect of low frequency dissipation was directly proportional to the applied voltage and the polarity of the solution,which could increase the dielectric constant significantly.In addition,the dielectric constant was related to the number of dipoles and the degree of polarization.DC conductivity mainly contributed to the dielectric loss at low frequency.What's more,the dielectric loss was inversely proportional to the frequency of the electric field.At high frequency,the effect of solution viscosity and dipole concentration on dielectric loss was more remarkable.The sizes of asphaltene aggregate particles were calculated by two different methods,the viscosity method and dielectric relaxation spectroscopy.In the method of viscosity,a variety of association levels occur in asphaltene solutions at high concentration,so the calculation of relative viscosity was corrected.For the same asphaltene,the intrinsic viscosity and the equivalent sphere diameter decreased with the increase of temperature.The size of YS asphaltene was between 2.37~3.14nm,while SC asphaltene was between 2.06~2.90nm.Dielectric relaxation spectroscopy had two or three distinct dipole loss peaks,corresponding to different aggregates in solution.The results showed that there was little difference between the two kinds of asphaltene sizes under different temperatures.It was likely to conclude that the determination of the dielectric loss peak position may require high accuracy for both the instrument and the experimental process.