Phase Behavior And Rheological Property Of The CO₂-Cosolvent-Polymer Systems

Supercritical carbon dioxide?SC-CO₂?technology is a hot research topic in unconventional oil and gas development.CO₂ is difficult to apply in unconventional oil and gas development since its low viscosity and stable property.Therefore,it is the key to promote the development of SC-CO₂ technology to explore the thickener used for CO₂ and to discuss the mechanism and influencing factors between thickener and CO₂.The systems of CO₂,cosolvent and polymer were presented based on the thermodynamic and rheological properties of these three components.A new phase behavior and rheological property measuring device for high pressure CO₂ system was designed and developed.The effects of temperature,pressure and composition on the phase behavior and rheological property of the systems were discussed,and its mechanism was analyzed.Meanwhile,high-precision prediction or correlation models for phase behavior and rheological property of the systems were established.The data of this subject provide a large amount of basic data for the study of phase behavior and rheology of CO₂ and polymer systems.The subject also provide theoretical and practical basis for SC-CO₂technology.A novel high pressure phase equilibrium apparatus was designed and developed based on the visual synthesis measurement method.In this apparatus,the automatic control system was used to adjust temperature,pressure and volume accurately.the photosensitive measurement system was fixed in the apparatus,which can accurately measure the phase behavior of various systems in a wide range of temperature and pressure.Polyvinyl acetate?PVAc?was used as tackifying polymer,and ethanol,acetic acid and ethyl acetate were used as cosolvents.the phase transition pressure of CO₂+PVAc binary systems and CO₂+cosolvent+PVAc ternary systems were measured by the high pressure phase equilibrium apparatus under certain temperature and composition.The phase transition pressure of binary systems increases with the increase of temperature,increases with the increase of polymer molecular weight,and increases first and then remains unchanged with the increase of polymer concentration.The phase transition pressure of CO₂+PVAc systems can be reduced by adding the three kinds of cosolvents,among which ethanol is the most obvious,acetic acid is the second,and ethyl acetate is the weakest.The phase behavior prediction model of CO₂+cosolvent+PVAc systems are established by using PC-SAFT equation of state.The binary interaction parameters of the systems are established by literature and experimental results in this paper.The model can predict the experimental results accurately.A novel high pressure capillary viscometer was designed and developed based on the capillary viscosity measurement method.Various control systems are used to adjust temperature,pressure,flow rate and differentical pressure accurately.The pressure of compressible fluid is adjusted by the intermediate containers and back pressure device,which can accurately measure the rheological properties of various systems in a wide range of temperature,pressure and shear rate.The viscosities of CO₂ and cosolvent unitary systems,CO₂+cosolvent binary systems,CO₂+PVAc binary systems and CO₂+cosolvent+PVAc ternary systems under certain temperature and pressure were measured by the high pressure capillary viscometer.When the pressure is above 15 MPa,the viscosity of various systems decreases with the increase of temperature,and increases with the increase of pressure.The addition of cosolvent and PVAc can increase the viscosity of CO₂.The viscosity increases with the increase of cosolvent concentration,polymer concentration and polymer molecular weight.Ethanol and acetic acid have almost the same viscosity-increasing effect and are superior to ethyl acetate.When the concentration of ethanol or acetic acid is 28 wt%,and PVAc is 4 wt%,the viscosity of these ternary systems can reach 6.6-8.2 times of pure CO₂.The empirical model is used to correlate the viscosity of pure CO₂ and pure cosolvent,and the semiempirical correlation model of polymer melt is established.At the same time,the semiempirical and empirical model and mixing rules are used to correlate the viscosity of the mixture systems.The characteristic parameters of pure components,binary interaction parameters and adjustable parameters of the mixture systems are established.The modified model can predict the experimental results of viscosity well.