Impact of water saturation on diffusion coefficients determined by constant volume diffusion method

In recent decades, carbon dioxide (CO₂) injection has become a promising technique for enhanced oil recovery (EOR). To design and model the CO₂-EOR process in a petroleum reservoir, knowledge of the molecular diffusion of CO₂ in oil is required. In this research, the constant volume diffusion (CVD) method is utilized to determine the diffusion coefficient of CO₂ in a core saturated with synthetic oil. In addition, the effect of the water saturation level in the core on the resulted diffusion coefficients is examined. This method includes an oil-saturated core, which is in direct contact with a CO₂ chamber. CVD tests were conducted at initial pressure and temperature of 410 psi and 64°C for water saturation of 0, 39, and 53%. CO₂ is injected from the top of the core and oil is produced from the bottom. Once the CVD initial condition is established, the injector and producer valves are shut-in and the CVD test starts. In the CVD test, the pressure drop data are collected and used to tune the diffusion coefficients using a compositional reservoir simulation model equipped with a developed equation of state. History matching of the pressure decay data is conducted to determine the tuned diffusion coefficients. Experimental results show that the existence of water in the core causes a smaller pressure drop in the system. We find that diffusion coefficients decrease linearly with increasing water saturation in the core operating at the same initial pressure and the system temperature. The presence of water in the core reduces the rate of CO₂ diffusion into the synthetic oil. Finally, we present an empirical relationship of the diffusion coefficient and the irreducible water saturation at the proposed lab conditions.