Abstract:

The molecular diffusion is an important factor to affect mass transfer phenomenon during a gas injection process.It is one of key parameters to determine the miscibility effectiveness of injected gas and oil system.Most of current commercial reservoir simulators for the phase behavior calculation are only based on the assumption that oil and injected gas system reaches equilibrium immediately once they contact each other.There is no diffusion phenomenon occurring between the two phases.Authors of this paper present the constant volume diffusion research for the fluid system which is of high concentration of CO₂，CH₄，N₂ and the crude oil from surface separator.The study is to aim at the investigation of diffusing mechanism between oil and injected gas under the condition of high temperature and pressure.The results show that the oil and gas do not reach balance while contacting，it is related to the component diffusion velocity.The diffusion coefficient is not a constant，it increases as pressure decreases.Once the system reaches equilibrium，the diffusion coefficient will not change.The liquid properties are not the same because of different position.This research provides a new approach for phase study and valuable information for optimizing the gas injection process.

Key words: gas injection, CO₂, equilibrium, molecular diffusion, diffusion coefficient