Effect Of ScCO₂ Treatment On Macroscopical Stability And Interfacial Expansion Viscoelasticity Of Crude Oil Emulsion

At present,the scCO₂ flooding technology has been applied and popularized in many oil fields in China.Compared with the existing oil displacement technology,the stability of the crude oil emulsion produced by the scCO₂ flooding has changed significantly,and then it has an effect on the subsequent ground gathering process.In order to investigate the stability changes of crudeoil emulsion and their mechanisms after scCO₂ flooding,this paper took Changqing Crude oil and Caoqiao heavy oil and its W/O emulsion as the research object.Firstly,use of scCO₂ treatment unit simulates scCO₂ flooding formation conditions to treat the crude oil.Then,study the effect of scCO₂ treatment on crude oil physical properties through total hydrocarbon components and SARA analysis,density method,viscosity scanning and particle size analysis.Moreover,the effects of scCO₂ treatment and demulsification temperature on the macroscopic stability of crude oil emulsion were investigated by bottle test,conductivity method,viscosity scanning method,microscopic observation and particle size analysis.The influence of scCO₂ treatment,demulsification temperature and expansion frequency on the dilatational viscoelasticity of oil-water interface of Changqing crude oil/Caoqiao was systematically studied by interfacial dilation rheometer system.Finally,from the two aspects of emulsion viscosity and oil-water interfacial film strength,the reasons for the stability of crude oil emulsion before and after scCO₂ treatment were discussed.The results showed that the content of gum and asphaltene in crude oil increased,the interaction of asphaltene is enhanced and the particle size of the chimerism increased after scCO₂ treatment.As a result,the viscosity of the crude oil and the emulsion formed by the emulsified liquid increased,and the probability of droplets colliding with one another decreases in the emulsion system decreased.In the end,the macro stability of Changqing crude oil and Caoqiao heavy oil emulsion was enhanced.Through the expansion viscoelastic test of oil and water interface of crude oil,it was found that after scCO₂ treatment,the adsorption rate of active substances in the oil phase is accelerated and the dynamic interfacial tension is reduced;and the elastic modulus increased,which agreed with the enhancement of macro-stability of the emulsion system.It shows that there is no direct causal relationship between the stability of the emulsion and the interfacial tension,and it is linear with the elastic modulus of the oil-water interface.The macroscopical stability of Caoqiao heavy oil and Changqing crude oil decreased with the increasing of demulsification temperature,and the viscosity of the two emulsion systems also decreased with the increasing of temperature.However,the expansion viscoelasticity of oil-water interface is inconsistent with the change of temperature.With the increasing of temperature,the dynamic interfacial tension,expansion modulus,elastic modulus,phase angle and viscous modulus of Changqing crude oil decreased.The dynamic interfacial tension value of oil-water interface of Caoqiao heavy oil decreased,but its elastic modulus increased with the increasing of temperature,which was not consistent with the variation of macrostability with demulsification temperature.（a）For Changqing crude oil W/O emulsions,the viscosity of the emulsion is relatively small.Although increasing the emulsion breaking temperature can reduce the emulsion viscosity,the decrease is not significant.The increase of demulsification temperature leads to a significant increase in oil-water affinity and a decrease in the dynamic interfacial tension value of the oil-water interface;at the same time,the increase of demulsification temperature plays a role in desorption and reduces the concentration of active substances on the oil-water interface,making the strength of the interfacial film weakened.In conclusion,with the increase of demulsifying temperature,the viscosity of emulsion decreases and the strength of interfacial film weakens,leading to a marked deterioration of emulsion stability.（b）For the W/O emulsion of Caoqiao heavy oil,the viscosity of the emulsion and the concentration of asphaltene in the oil phase are high.The association state of asphaltene in the oil phase and the viscosity of the emulsion have a significant influence on the interfacial adsorption and emulsion stability.Increasing the temperature of demulsification not only enhances the oil-water affinity,but also leads to the dissociation of natural surface active substances such as asphaltenes.This promotes the adsorption of asphaltenes and other active substances at the oil-water interface.And this causes the decrease of oil-water interfacial tension and the increase of interfacial strength.At the same time,increasing the demulsifying temperature greatly reduced the viscosity of the emulsion,which also helped the adsorption of asphaltenes and other active substances at the oil-water interface.Although the increase of the interfacial film strength contributes to the stability of the emulsion,but the large decrease in the viscosity of the emulsion significantly increases the sedimentation and collision rate of the water droplets and further deteriorates the stability of the emulsion.In conclusion,the large decrease in emulsion viscosity dominates the stability of Caoqiao heavy oil W/O emulsions,leading to a significant deterioration of the macro stability of the emulsion.Finally,the influence of expansion frequency on the viscoelasticity of the Caoqiao crude oil-water and Changqing crude oil-water interfaces was tested.In the dynamic measurement process,the expansion modulus,elastic modulus,and viscous modulus values of the two crude oil-water interfaces all increased with the increase in the expansion frequency.And the influence of demulsification temperature and scCO₂ treatment on the viscoelasticity of crude oil-water interface in Changqing is consistent in the equilibrium state and dynamic measurement process.It shows that the two methods can obtain the change rule of expansion viscoelasticity of the oil-water interface.