| During the transportation of crude oil produced fluid from wellhead to combined station,due to the shearing action of wellbore,nozzle,valve,pump,etc.,emulsification often occurs between crude oil and produced water,forming water-in-oil emulsion.Asphaltene,as a polar component in crude oil,can adsorb on the oil-water interface and form a strong interfacial film.The existence of this interfacial film makes the droplet of dispersed phase of crude oil emulsion not easy to coalescence when colliding with each other,so it can stabilize the emulsion of crude oil.Temperature and asphaltene mass fraction,wax content and solvent polarity can change the association state of asphaltene,thus affecting the structural characteristics of the interfacial film and the stability of the emulsion.In order to explore the influence of various factors on the association state of asphaltene,and then analyze the interfacial characteristics of oil and water and the mechanism of emulsion stability by association state,this paper first extracts asphaltene from Tahe crude oil to prepare simulated oil.By changing the preparation conditions and composition of simulated oil,the effects of temperature,asphaltene mass fraction,solvent polarity and wax content on the simulation oil are analyzed.The influence of temperature,asphaltene mass fraction,wax content and solvent polarity on the dynamic adsorption behavior and structural characteristics of asphaltene was studied by suspension drop method.Finally,the macro-stability of asphaltene simulated oil-brine emulsion was tested by bottle test method,and the influence of temperature,asphaltene mass fraction,wax content and solvent polarity on the stability of the emulsion was studied.By combining the simulated oil-water interface characteristics with the simulated oil-water emulsion stability,the practical production of theoretical application will be studied.The results show that the influence of asphaltene mass fraction on the structural strength of the interfacial film is partitioned(discontinuous).When the mass fraction of asphaltene in simulated oil is low,with the increase of asphaltene mass fraction in simulated oil,the size of asphaltene particles increases,but the increase is small,and the diffusion coefficient decreases sharply with the increase of mass fraction;the interfacial expansion modulus increases rapidly,and there is no obvious fold on the interface during the process of simulated oil droplet shrinkage.When the mass fraction of asphaltene reaches a certain value,the particle size suddenly increases,which means that the association degree of asphaltene increases at a higher mass fraction.With the increase of asphaltene mass fraction,the rate of decrease of diffusion coefficient slows down,and the rising trend of interfacial dilatational modulus slows down,which indicates that the structural strength of the interfacial film does not increase linearly with the increase of asphaltene mass fraction,but the shrinkage of the droplet is still reversible.Solvent polarity enhancement will reduce the structural strength of the interfacial film.When the mass fraction of asphaltene in simulated oil is low,the particle size of asphaltene particles increases slowly,the diffusion coefficient decreases sharply with the increase of mass fraction,and the interfacial expansion modulus increases with the increase of asphaltene mass fraction.Asphaltene associates when it reaches a higher mass fraction.The stronger the polarity of solvent is,the higher the mass fraction of asphaltene.This indicates that the association degree of asphaltene decreases with the increase of solvent polarity at the same mass fraction.Because asphaltene synthesizes into large aggregates,the diffusion coefficient decreases slowly.With the increase of the proportion of Methylnaphthalene in solvents,the diffusion coefficient increases,indicating that the association degree of asphaltene decreases with the increase of solvent polarity and the interfacial expansion modulus tends to be flat with the increase of asphaltene mass fraction.Under the ratio of four kinds of methyl naphthalene and n-decane,the structural strength of the interfacial film increased with the addition of asphaltene.The stronger the polarity of the solvent and the smaller the interfacial dilatation modulus at the same asphaltene mass fraction,the stronger the structural strength of the interfacial film decreased with the increase of solvent polarity.The change of carbon number distribution and content of wax will significantly change the association state of asphaltene in simulated oil and the structural strength of the interfacial film formed by asphaltene at the oil-water interface.When the content of 50-52 and 62-64 slice paraffins in simulated oil is lower than 5 wt% and 2 wt% respectively,the average size of asphaltene particles in simulated oil increases,but the increase is not significant.The association state of asphaltene is basically unchanged,and the variation of diffusion coefficient with wax content is very small.With the increase of wax content,the interface expansion modulus increases,but the range is small.When the wax content increases to 10 wt% and 5 wt% or more,the association degree of asphaltene increases,and more and more asphaltenes associate with each other to form larger aggregates,which reduces the diffusion ability from oil phase to interface.The diffusion coefficient decreases with the increase of paraffin content in simulated oil,and the reduction is larger.The interfacial expansion modulus increases with the increase of paraffin content in simulated oil.This is influenced by two aspects.On the one hand,asphaltene can synthesize large volume particles under the action of wax.The asphaltene particles adsorbed on the oil-water interface are larger in volume,increasing the thickness of the interfacial film formed on asphaltene at the oil-water interface,enhancing the strength of the interfacial film,and on the other hand,with the increase of wax content,the adsorption on the oil-water boundary is significant.The film thickness formed by wax molecules increased further.The two aspects together enhance the structural strength of the interfacial film and increase the interfacial expansion modulus greatly. |
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