Diffusion coefficients of supercritical CO2 in oil-saturated cores under low permeability reservoir conditions

CO2 diffusion in oil-saturated porous media with low permeability is of great importance for the project design, risk assessment, and performance forecast of carbon capture and storage (CCS) or enhanced oil recovery (EOR). This paper developed a method to determine CO2 diffusion in oil-saturated cores under low permeability reservoir conditions. Core, crude oil and experimental parameters were taken from the representative low permeability reservoir. In the solution of the mathematical model, oil saturation was introduced in to diffusion equation, an oil-phase swelling caused by gas dissolution was considered, but a water-phase swelling was not, which is in agreement with the actual diffusion situation. The error caused by the state equation of carbon dioxide was eliminated, improving the calculation accuracy of the diffusion coefficient. The effects of pressure (6.490–29.940 MPa), temperature (70–150 °C), oil saturation (0–63.58%) and permeability (8.62–985.06 mD) on the diffusion coefficient of supercritical CO2 in low-permeability reservoirs were studied. The order of the diffusion coefficient is from 10−10 to 10−9 m2/s. The results show that with an increase in pressure and temperature, the CO2 diffusion coefficient in the porous media saturated with oil firstly increases significantly and then the rate of increase gradually slowed down. The CO2 diffusion coefficient increases greatly with the oil saturation in porous media. The CO2 diffusion coefficient first increases greatly with permeability, and when the permeability of the core is greater than approximately 100 mD, it remains almost stable. The experimental results can provide theoretical support for CO2 transport in porous media.