Experimental study on CO2 diffusion in bulk n-decane and n-decane saturated porous media using micro-CT

Molecular diffusion has been considered to be an underlying mechanism for many oil recovery processes. Reliable estimation of the molecular diffusion coefficient as a transport property is therefore important for CO2-enhanced oil recovery. In the present work, the dynamic processes of CO2 diffusion in bulk n-decane and n-decane saturated porous media were investigated using the micro-focus X-ray CT (micro-CT) scanning technique. CO2 diffusion was visually and quantitatively analyzed by interpreting the CO2 concentration with grayscale CT images. Next, local CO2 diffusion coefficients, varying with time and position, were calculated from concentration profiles based on Fick's second law. The results showed that the local diffusion coefficients in bulk n-decane demonstrate an exponential function of diffusion distance and time. The total diffusion coefficients in bulk oil under pressure from 1 to 6 MPa and temperature at 29 °C and 35 °C were calculated. The results showed that the initial pressure has a strong influence on the diffusion coefficient, i.e., high CO2 initial pressure leads to high CO2 diffusivity in oil. Experiment results in n-decane saturated porous media showed that the CO2 local diffusion coefficient decreases gradually along the diffusion path with time until reaching a stable state. The total diffusion coefficients in n-decane saturated porous media were smaller than those in bulk oil under the same pressure and temperature conditions because the diffusion path is more complicated than in bulk oil. It is demonstrated that the pathways of porous media impede CO2 mass transfer and decrease the diffusion coefficient.