Research On The Diffusion Of CO₂ In Multi-phase Liquid System

Research of CO₂ diffusion in liquid plays a vital role in CO₂-flooding,CO₂ capture and storage and it controls the diffusion velocity,CO₂ diffusion and amount and the diffusion time.In most cases,both oil and water phases should be taken into consideration when considering the diffusion of CO₂ in reservoirs.However,many researchers nowadays ignored the research of CO₂ diffusion in water phase and only the diffusion of CO₂ in oil phase is included.Firstly,in this thesis,the pressure decay models of CO₂ diffusion in pure liquid phase were established.By fitting the experimental data with the models,the diffusion coefficients of CO₂ in pure liquids were calculated and the impact factors were analyzed based on the results.Secondly,based on the front position of CO₂ when diffusing in water-oil liquid system,two diffusion stages were separated and two models were created,which are analytical model ignoring the oil swelling effect and numerical model considering the oil swelling.Conducting the diffusion experiments of CO₂ diffusion in oil-water liquid system under different conditions to solve the diffusion coefficients and to analyze the impact factors.The results indicate that the relative error of two models is within 5%.Also,based on the results,the pressure and temperature have a strong impact on the diffusion coefficients of CO₂ in oil and water phases,the oil viscosity of oil phase has only impact on diffusion coefficient of CO₂ in oil phase but not on water phase.And the percentage of oil phase and water phase have no impact on the diffusion coefficients.Additionally,based on the simulation results,the concentration of CO₂ in oil phase is much greater than that in oil phase,and the addition of water phase increases the diffusion amount of CO₂ into liquid phase and the swelling amount of oil phase.Lastly,the simulations of CO₂ diffusion in porous media saturated by oil and water phases were carried out based on the established models of CO₂ diffusion in oil-water liquid phase.The impact of water saturation and the pores position saturated by water phase were analyzed based on the simulation results.The results show that CO₂ will preferentially diffuses into the pores saturated by oil phase.There is a huge concentration gap between oil phase and water phase due to the difference of diffusion coefficients in water phase and oil phase.With the increasing of water saturation,the amount of CO₂ diffusion into porous media will decrease.And the outlet diffusion time will increase.If the pores which connecting the porous media were occupied by water phase,the amount of CO₂ diffusion into porous media will decrease greatly.In contrast,if the water didn't block the pores effectively,the changes of CO₂ diffusion amount and the outlet diffusion time are pretty small.Researches of this thesis complete the basic theory of CO₂ diffusion in liquid phase and can guide the parameters design and optimization of CO₂ diffusion amount and injection rate.At the same time,it laid the foundation for the further development of CO₂ diffusion research in multiphase fluids.