A novel hybrid finite-infinite diffusion model for determining CO2 diffusion coefficient in oil-saturated porous media: Applications for enhanced oil recovery and geological carbon storage

CO2 diffusion in reservoirs is vital for enhanced oil recovery and CO2 geological storage. This study employed pressure-decay experiments with a modified CO2 diffusion cell in oil-saturated porous media with permeabilities of 10 mD, 50 mD, and 500 mD. Nuclear magnetic resonance tests were conducted to measure the effective diffusion distance. A novel method, combining infinite-acting and finite-acting models, was introduced to estimate Henry's constant and diffusion coefficient. Subsequently, the impact of CO2 diffusion on oil recovery and geological storage was evaluated across various permeability conditions. Results indicate that Henry's constant and diffusion coefficient are on the orders of 105 Pa⋅m3/kg and10−8 m2/s, respectively. As permeability decreases, the solubility and diffusion of CO2 in oil-saturated porous media are restricted, leading to significant differences in the CO2-rich zone, even when the CO2 diffusion fronts appear similar. Moreover, according to Nuclear magnetic resonance signal characteristics along the core, the effective diffusion distance was categorized into three areas: the enrichment area, transition area, and sweep area. The enrichment area, with a critical concentration of 20 kg/m3, significantly contributes to oil recovery. However, the effective diffusion distance is constrained in low-permeability reservoirs, while the total diffusion distance is insensitive across different permeabilities. Additionally, CO2 diffusion shows considerable potential in high-permeability reservoirs. Finally, the parameters estimated using the proposed diffusion model align closely with literature values, confirming its applicability to both gas/liquid-saturated porous media and gas/liquid systems.