Abstract: Most of researchers claim that supercritical CO₂ diffusion is one of the key considerations for CO₂ huff-puff EOR in tight reservoirs. In this paper, a matrix-fracture model was designed and applied to measuring the diffusion coefficient of supercritical CO₂ in saturated oil cores using pressure decay method. The influence of reservoir conditions such as pressure and core petroproperties on the diffusion coefficient and concentration distribution of supercritical CO₂ was comprehensively investigated, and a method predicting CO₂ concentration field and diffusion front was established. The experimental results showed that the diffusion coefficient of supercritical CO₂ in the tight core (0.06 mD) is in the order of 10^-12 m²/s. The diffusion coefficient was found to increase with the increase of the initial gas injection pressure before finally leveling off after a certain pressure. The maximum diffusion coefficient occured around the critical pressure point. In addition, the diffusion coefficient increased with the increase of matrix permeability and porosity, whereas it decreased with the increase of core tortuosity. After 900 days, diffusion in tight reservoirs, the diffusion front only advanced 0.095 m. Therefore, it is rational to neglect the diffusing effect of CO₂ in the field operation cycle of CO₂ injection in tight reservoirs. In the later stage of diffusion, the concentration gradient of CO₂ became low leading to gradual decrease of diffusion.

Key words: supercritical CO₂, tight oil reservoir, CO₂ injection, pressure decay method, diffusion coefficient, diffusion front