Investigation of concentration-dependent diffusion on frontal instabilities and mass transfer in homogeneous porous media

Mass transfer plays an important role in influencing the efficiency of miscible displacements in solvent-based processes in enhanced oil recovery. The mass transfer rate can be significantly enhanced by the appearance of frontal instabilities called viscous fingering mechanisms, which are beneficial for improving the mixing and mass transfer between the injected solvent and oil. Instead of a piston-like displacement, the interface between solvent and oil is very convoluted with intricate finger-like patterns of the less viscous solvent intruding into the highly viscous oil. This intrusion greatly increases the surface area of contact of the two fluids and results to more efficient mass transfer and mixing. Experimental measurements on the diffusion coefficients of two miscible fluids indicate that, instead of a constant diffusion coefficient (CDC), a concentration-dependent diffusion coefficient (CDDC) is more realistic. A CDDC relation in which the diffusion coefficient is exponentially proportional to concentration is adopted and its effect on the development of frontal instabilities was assessed through highly accurate nonlinear numerical simulations. The differences between the CDDC case and the widely assumed CDC case are considered, along with the enhancement of frontal instabilities on mass transfer when the CDDC at various mobility ratios and Peclet numbers. The special characteristics for the CDDC case indicated its remarkable role in miscible displacements. The relation of breakthrough time to parameters was correlated to accurately predict the breakthrough time in any CDDC scenario.