Structural Characteristics And Interfacial Properties Of Asphaltenes Based On N-alkanes

To improve oil recovery efficiency,a lot of research has been done on crude oil and its composition,but the polar components(resins,asphaltenes)in crude oil are less studied.As an important polar component in crude oil,asphaltenes have a certain influence on oil recovery efficiency.The lower the oil-water interfacial tension(IFT)is,the higher the oil recovery efficiency will be.Moreover,the structure of asphaltenes obtained by different solvents and different physical and chemical conditions may be different,and the asphaltenes obtained are mixtures,resulting in the diversity of the structure.Therefore,this paper focused on the structure and interfacial properties of asphaltenes.The asphaltenes and their subfractions in Daqing crude oil were divided into research objects.The structure of asphaltenes and their subfractions were characterized by modern physical research methods,and the adsorption mechanism of asphaltenes at the oil-water interface was discussed by Spinning Drop Tensionmetry1.The Daqing crude oil was separated by n-heptane and n-hexane to obtain n-heptane asphaltene(P-A)and n-hexane asphaltene(X-A),and the mixed solvent(toluene/n-hexane or toluene/n-heptane)was used to separate P-A and X-A respectively,and the corresponding six subfractions(P-A1～P-A6 and X-A1～X-A6)were obtained respectively.The asphaltenes and their subfractions were characterized by Fourier transform infrared(FTIR)spectrometer,Nuclear magnetic resonance(NMR)spectrometer,CHNSO elemental analyzer(EA),Gel permeation spectrometer(GPC),and synchronous fluorescence(SF)spectrometer,and the differences among the subfractions were compared.The glass transition temperatures of P-A and X-A were compared by differential scanning calorimetry(DSC).The structural element parameters of P-A and X-A were calculated by the improved B-L method,and the structural characteristics of P-A and X-A were compared.The effects of X-A,P-A,and the subfractions of P-A 、 X-A on the oil-water interfacial tension(IFT)were evaluated.The results showed that X-A was similar to P-A in terms of composition,elemental content,functional groups,and aromaticity.However,X-A was mixed with n-hexane insoluble but n-heptane soluble components(resins and the transition components of resins turn to asphaltenes),so the content and polarity of X-A were greater than that of P-A.The ability of X-A to reduce IFT of the oil/water interface was stronger than that of P-A.For the subfractions,whether X-A or P-A,the content of C=O in the A2 was the highest,so it was speculated that this subfraction had the largest polarity,it had the strongest ability to reduce the IFT of oil-water.Both P-A and X-A mainly contain 3 aromatic rings,and a small amount of 5-ring or more viscous cyclic aromatic hydrocarbons were present.Both P-A and X-A were altitudinal condensation.Moreover,X-A was a highly condensed aromatic polymer with the regular arrangement,short alkyl chain,low rigidity,and more naphthenic rings;P-A was a high condensation polymer with poor arrangement,long alkyl chain,large rigidity,more aromatic rings but slightly less condensation degree than X-A.2.The influence of water-oil phase composition on the performance of asphaltenes at the oil-water interface was studied by Spinning Drop Tensionmetry;The adsorption mechanism of asphaltenes at the oil-water interface was investigated by combining the short time dynamic interfacial tension equation,the long time dynamic interfacial tension equation and the Ward-Tordai equation.The effect of asphaltenes concentration on the properties of asphaltenes at the oil-water interface and the distribution of asphaltenes with different concentrations at the interface were compared.The results showed that asphaltenes,as components with strong polarity in crude oil,have a certain effect on the oil-water interface properties.The addition of asphaltenes to the oil phase alone can reduce IFT.When 2,5-dimethyl-4-tetratetrabenzene sulfonate(p-S14-4)was added to the aqueous phase and asphaltenes to the oil phase,the positive synergistic effect between asphaltenes and p-S14-4 at the interface enhanced its ability to reduce IFT.In the p-S14-4+Na Cl water-toluene oil phase system,the adsorption of asphaltenes was not completely controlled by diffusion,and the adsorption mechanism at the interface can be divided into three regions.Region I: the adsorption of asphaltenes at the interface was controlled by surfactant diffusion.Region II: due to the asphaltene-asphaltene interaction,the asphaltene aggregates particle size increased,which limited the diffusion rate to a certain extent,and the asphaltenes slowly diffused and adsorbed toward the interface.Region III: The interaction of asphaltene-asphaltene was broken,the interaction of surfactant-asphaltene and water-asphaltene enhanced.The asphaltene aggregates depolymerized,the particle size decreased,and the asphaltenes quickly adsorbed to the interface.The adsorption process was controlled by the depolymerization rate of asphaltene aggregates.In a certain concentration range,the smaller the asphaltenes concentration is,the larger the asphaltenes coverage area at the interface will be;the diffusion rate of asphaltenes to the oil-water interface was faster,and the amount of adsorption on the interface increased,the IFT decreased.