Experimental Study On Miscibility And Diffusion Characteristics Of CO₂/Oil Phase System

CO₂ miscible flooding is one of the most ideal schemes in oil recovery practice.Accurate prediction of minimum miscible pressure（MMP）in CO₂/oil systems is critical for CO₂ enhanced oil recovery（CO₂-EOR）and carbon geological storage applications.At the same time,the determination of CO₂ and oil phase diffusion coefficient is of great significance for guiding the enhanced CO₂ recovery and the field practice of greenhouse gas geological storage.Aiming at the miscible characteristics of CO₂/oil phase system,a new method,namely the measurement of oil drop volume change（ODVM）method,was used to determine the minimum CO₂/oil phase miscible pressure based on the volume shrinkage behavior of oil droplets.The minimum miscible pressure of CO₂/n-cetane(n-C₁₆H₃₄)and CO₂/n-octadecane(n-C₁₈H₃₈)systems was determined.The minimum miscible pressure of CO₂/oil system at different temperature levels was determined based on the specific quantitative standard of oil drop volume reduction rate.The measured results are in agreement with the corresponding thermodynamic equilibrium calculations,and the advantages and application potential of this method are illustrated based on detailed comparison with the commonly used lost interfacial tension（VIT）method.Aiming at the mass transfer characteristics of CO₂/oil phase system,a new method for quantitative measurement of the diffusion coefficient of CO₂/oil immiscible binary system based on the expansion behavior of oil droplets is proposed and proved based on the dynamic volumetric measurement of oil droplets（DPDVA）.The diffusion coefficients of CO₂/oil systems were determined using 5 oil samples of n-decane,n-dodecane,n-tetradecane,n-cetane and liquid paraffin as oil phases.The method is verified based on the good agreement between the measured results of pure and synthetic oils and those reported in the literature.The diffusion characteristics of various CO₂/oil systems were studied in the pressure range of 2-10 MPa and the temperature range of 26.9-82.1℃.The measurements clearly show that increased system pressure leads to a stronger diffusion process between CO₂ and the oil phase,and that the effect of temperature on the diffusion process is further complicated by the synergistic effect of CO₂ density and oil viscosity.In view of the influence of the presence of water phase on the mass transfer characteristics,the diffusion coefficient of CO₂/oil/water system was measured by DPDVA method.The measured oil phases were n-decane,n-dodecane,n-tetradecane,n-cetane and liquid paraffin.The measured pressure range is 2-7 Mpa and the temperature range is 39.6-81.5℃.The measurement results show that the diffusion coefficient of immiscible ternary two-phase system decreases significantly compared with that of immiscible two-phase system.The diffusion rate increases significantly with increasing system pressure,while the effect of temperature on the diffusion process is further complicated by the synergistic effect of CO₂ density,oil viscosity,and water solubility in CO₂.