document

Summary

Flow-reversal studies provide insights into mixing mechanisms in flow through porous media. In these studies, the direction of flow is reversed after the solute slug has penetrated into the medium (but not exited) to a predetermined distance. We simulated the effect of flow reversal on mixing in 2D porous media using two different approaches. In the first approach, we perform direct numerical simulation of a solute-slug transport (by solving Navier-Stokes and convection/diffusion equations) in a surrogate pore space. This approach allows a direct visualization of mixing in simple flow geometries. The effect of flow reversal on mixing is investigated for several diffusion coefficients, penetration depths, and flow geometries. The second approach uses particle tracking to simulate the effect of flow reversal at larger length scales. This approach is free of numerical dispersion, can be used in the absence of diffusion, and has no limits on the size of the simulation. It is, however, limited to layered-media flow.

The simulation studies presented in this paper explain the mechanism of mixing and the origin of the irreversibility of dispersion in flow through porous media. We also explain several experimental observations on flow-reversal tests found in the literature.

Mixing in porous media takes place because of interaction between convective spreading and molecular diffusion. The converging/diverging paths and flow around impervious sand grains cause the solute front to stretch and split. In this process, the area of contact between the solute slug and the resident fluid increases by an order of magnitude and diffusion becomes an effective mixing mechanism. This local mixing, caused by diffusion, is irreversible.

For purely convective transport, solute particles retrace their path back to the inlet upon flow reversal. Convective spreading gets canceled, and echo dispersion is 0. Diffusion, even though small in magnitude, is responsible for local mixing and making dispersion in porous media irreversible. Thus, it is important to include the effect of diffusion when analyzing miscible displacements in porous media.