An important feature of the miscible fluid displacement is the mixing of the solvent and oil. This mixing occurs on the microscopic scale. The mixing has positive and negative effects on the miscible process. It could exert a considerable damping effect on the growth of viscous and gravity fingers as an advantage of the dispersion, on the other hand mixing of the solvent with oil in a reservoir decreases the effective strength of the solvent, which can have a detrimental effect on miscibility and recovery efficiency.
Mixing in porous media originates from a complex interaction of molecular diffusion and convective spreading.
The diffusion process is governed by the diffusion coefficient. An effective diffusion coefficient is defined for porous media, which has a dependency on the medium properties.
Convection spreading could play a major role in accelerating the dilution process by increasing diffusivity and expanding the interface area between the two fluids. The dispersion coefficients are representative of both convective spreading and molecular diffusion lumped together.
It is important to know the actual origin of pore-scale mixing but dispersion coefficients describe only the apparent mixing of the medium and do not provide any information about the individual roles played by mechanical dispersion and molecular diffusion over a range of flow conditions and pore geometries at different parts of the porous medium.
Based on the sub-pore scale modeling method, a special program has been developed to simultaneously solve the Navier-Stokes, continuity, and general mass transfer equations directly on an image of the porous medium to investigate the mass transfer process considering all medium geometrical properties. The program is capable of predicting the effective diffusion and
dispersion coefficients for a porous medium under different conditions using just fluid properties and an image of the medium.
To study the heterogeneity effect, a pattern generator was developed to generate different patterns of porous media with different geometrical properties.
A glass micromodel was built and used to experimentally investigate the miscible displacement process. An image analysis technique was developed to evaluate the results and extract the concentration profiles from the images taken during the test.