A significant fraction of the reservoir volume may remain unswept during a water-oil displacement. Potentially recoverable oil in a clastic reservoir may be bypassed as a consequence of poor sand connectivity or large contrasts in permeability between neighbouring sandbodies. Such conditions typically occur in fluvial deposits (poor connectivity) and channel-fill/mouthbar combinations (large permeability contrasts).

To investigate the effect of the properties of these reservoirs on waterdrive efficiency, simulations of prototype reservoirs have been carried out. All waterdrives investigated were flank waterdrives, representative of North Sea reservoirs.

The reservoirs characterised by sandbodies with different ranges of permeability could be modelled with a relatively coarse grid. The effects of sandbody geometry, well spacing and permeability contrast on the displacement efficiency were investigated. It was found that in some cases the additional recovery from an infill well, located in a poorly swept, low-permeability sand, can be as high as 7% of the STOIIP contained between the initial wells.

Modelling the complicated networks of channel sands surrounded by shales encountered in fluvial deposits requires a much finer grid with a larger number of gridblocks than modelling large sand bodies. To accommodate simulation models with a large number of gridblocks a highly simplified special-purpose simulator was developed. This simulator neglects those physical mechanisms that have only a minor effect on the displacement process considered and can handle models with more than 100 000 gridblocks easily. For these fluvial reservoirs the effects of channel density, alternative channel distributions (reservoir architecture) and of permeability contrasts between channels were investigated. It was found that at low channel densities (average nett-to-gross ratio less than 25%), oil tends to be bypassed because of poor connectivity and that the overall performance depends on the reservoir architecture. At higher channel densities, permeability contrasts can be a major cause of bypassing.

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