In simulating field performance of polymer and micellar flooding, it is frequently necessary to calculate the behaviour of chemical slugs which are small compared to the well spacing, and not much larger than the normal gridblock dimension. A polymer slug in a heterogeneous reservoir will then be subject to disruption from numerical dispersion, viscous crossflow and viscous fingering. The problem addressed in this paper is whether oil recovery from the slug can be simulated without an impracticable degree of grid refinement.

Analytical modelling is used to explain the process of slug disruption by viscous crossflow in a layered reservoir, and to demonstrate the close relationship between crossflow and oil recovery. Some of the concentration fronts will spread even in the absence of dispersion. Numerical simulation is used to examine the sensitivity of slug behaviour to grid refinement, dispersion, slug size and viscosity and viscous fingering.

It is shown how a coarsening of the gridblock structure leads to large qualitative changes in slug flow and the movements of polymer within the reservoir. However, it is also demonstrated that the quantity of incremental oil recovery is much less sensitive to numerical dispersion, over a wide range of likely slug properties. This allows a much more adequate calculation of polymer flood performance in very coarse grids than would be expected from the ratio of slug to gridblock size, and is most important in permitting practicable reservoir simulation of this EOR process.

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