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

Carbon dioxide miscible flooding technology,a kind of tertiary oil recovery technology,has broad prospects in enhancing oil recovery and greenhouse gas geological storage.Especially in enhancing oil recovery of tight reservoirs after water flooding,carbon dioxide miscible flooding technology has attracted extensive attention in the industry.In this context,the experimental study on miscibility and diffusion characteristics of CO₂/oil/water system was carried out.In this paper,two important parameters in carbon dioxide miscible flooding,the minimum miscible pressure and the diffusion coefficient between phases are studied.In the study of the minimum miscibility pressure,the influence of water on the minimum miscibility pressure of oil-water system is mainly studied.Firstly,a new method,Dynamic Volume Measurement(ODVM),was used to measure the minimum miscibility pressure of CO₂/paraffin system and CO₂/paraffin/water system,CO₂/C₁₆H₃₄ system and CO₂/C₁₆H₃₄/water system at different temperatures.Then the results of three-phase experiment are compared with those of two-phase experiment,and the miscibility characteristics near the minimum miscibility pressure of three-phase experiment and two-phase experiment are emphasized.The effects of temperature,pressure and the presence of water phase on the minimum miscibility pressure of oil and gas system are analyzed.For the study of diffusion coefficient,dynamic pendent drop volume analysis(DPDVA)is used and an assumption of equal volume of oil and gas is introduced innovatively.The diffusion coefficient of CO₂/paraffin system in miscible phase was calculated.The experiment result compared with experiential formula results,and the diffusion characteristics between oil and gas under miscible phase condition are analyzed.The research conclusions are as follows:(1)For the CO₂/C₁₆H₃₄/water system,the minimum miscibility pressures at 27.1?,40.3?,55.4?and 70?are 6.2MPa,7.6MPa,10.2MPa and 13MPa respectively.The minimum miscibility pressures of the CO₂/C₁₆H₃₄ system at 26?,40.3?,55.4?and 70?are 6.6MPa,8.2MPa,11MPa and 13.6MPa respectively.It is found that the minimum miscibility pressure of the CO₂/oil/water system is proportional to the temperature,and the linear relationship between the two is good.The water phase reduces the minimum miscibility pressure of CO₂/C₁₆H₃₄ system,and the minimum miscibility pressure point moves forward.(2)For the CO₂/paraffin/water system,the minimum miscibility pressures at 19.5?,40.6?and 61.2?are 5.8MPa,8.2MPa and 10.2MPa respectively.The minimum miscibility pressures of the CO₂/paraffin system at 21.5?,40.6?and 61.2?are 6.4MPa,9MPa and 11.8 MPa respectively.The results of CO₂/paraffin/water system are the same as that of CO₂/C₁₆H₃₄/water system.The minimum miscibility pressure of CO₂/paraffin/water system increases with increasing temperature.The water phase reduces the minimum miscibility pressure of CO₂/paraffin system,and the minimum miscibility pressure point moves forward.(3)The diffusion coefficients of the CO₂/paraffin miscible system at 40.6?,61.2?and 81.2?were measured by using the assumption of constant volume mixing.It is found that under the same temperature conditions,heavy hydrocarbons are continuously extracted and participate in the miscible process,and the diffusion coefficient of CO₂/paraffin system increases exponentially with the increase of pressure.When the pressure is the same,the diffusion coefficient of the system decreases with the increase of temperature and occurs in the high temperature region,so the influence of temperature on the diffusion coefficient will be weakened.