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.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
17 July 2008
- Manuscript approved:
25 July 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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