Experimental Study On Phase Equilibrium And Mass Thansfer Characteristics Of CO₂/oil System

Carbon dioxide enhanced oil recovery(CO2-EOR)is the most commonly used gas flooding method,and has attracted considerable research interest as it offers not only a way to increase oil production but also a means of potentially safely sequestering large amounts of CO2(which is a greenhouse gas)in the subsurface.Two important parameters are included in the studies of CO2 miscible flooding,namely the Minimum Miscible Pressure(MMP)and the Interphase Diffusion Coefficient.Accurate and rapid determination of the MMP and Interphase Diffusion Coefficient of the oil and gas system is the basis of carbon dioxide miscible flooding process.However,the existing laboratory experimental methods have some inevitable disadvantages like timeconsuming,expensive,complicated operation and lack of quantitative standards.Therefore,this study proposes a new MMP measurement technique,named Dynamic Oil Droplet Volume Measurement(ODVM)method,for measuring the multiple-contact minimum miscibility pressure(MCMMP)and first-contact miscibility pressure(FCMP)of CO2/oil systems.Subsequently,the effect of the water phase on the two-phase system was studied by measuring and analyzing the changes of interfacial tension(IFT).The mixing process of the three-phase system(oil-gas-water)is introduced in detail,and this process is summarized into four characteristic stages according to the different phenomena.Finally,the application of ODVM method is extended to measure the diffusion coefficient of CO2/n-Hexadecane system at different temperatures and pressures.Based on the oil droplet volume variation histories,a simplified mass transfer model adapted to the oil-gas miscibility condition is proposed.The main conclusions of the study can be summarized as follows:1.In CO2/n-Hexadecane system,the IFT of oil droplets increases with the increasing temperature and decreases with the increasing pressure,as well shows a good linear relationship with the pressure changes;the MMP of CO2/n-Hexadecane system measured by ODVM method is 6.8 MPa,7.8 MPa,9.8 MPa,11.2 MPa and 15.2 MPa at the temperature of 26.9°C,37.2°C,47.7 °C,58.2°C and 79.6°C,respectively;the FCMP of CO2/n-Hexadecane system measured by ODVM method under the same temperature is 6.8 MPa,15 MPa,17 MPa,18 MPa and 21 MPa,respectively.The MMP of CO2/nHexadecane system increases with the increasing temperature,and shows a good linear relationship with the temperature changes.2.The IFT of oil droplets in CO2/Liquid paraffin system increases with the increasing temperature and decreases with the increasing pressure,as well shows a good linear relationship with the pressure changes;The MMP of CO2-Liquid paraffin system at the temperature of 21.5°C,40.6°C,61.2°C and 81.2°C is 6.4 MPa,9 MPa,11.8 MPa and 13 MPa,respectively.The FCMP of CO2 / Liquid paraffin system at the same temperature is 15 MPa,23 MPa,32 MPa and 35 MPa,respectively;the MMP of CO2 / Liquid paraffin system increases with temperature,and shows a good linear relationship with the change of temperature.3.The existence of water phase will make the IFT of oil droplet increase and make the MMP value increase by 1.1 MPa;the mixing process of three-phase system can be divided into four stages: three-phase stratification stage,two-phase mixing zone formation stage,three-phase mixing zone formation stage,three-phase mixing zone expansion and two-phase mixing zone disappearance stage.4.If CO2 is injected into the reservoir under the condition of MMP,a small diffusion coefficient may lead to the occurrence of immiscible displacement process;larger diffusion coefficient appears under lower-temperature and higher-pressure conditions in the miscible system;the diffusion coefficient value is mainly affected by density and viscosity.