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This is the most common flooding condition encountered in nature and is characterised by the following physical conditions:
| • | There is a high degree of pressure equilibrium across the reservoir section which encourages cross-flow of fluids under the influence of gravity. |
| • | The displacement occurs under strictly segregated conditions with a sharp interface between the water and oil. |
Displacement of oil by water assuming segregated flow conditions
In a segregated flow regime we assume that ahead of the interface between the displacing fluid and the oil, oil alone is flowing, in the presence of immobile water at its irreducible saturation. Behind the interface, only displacing fluid is flowing, in the presence of immobile oil at its residual saturation.
The following describes the method of calculation for segregated flow displacement, as outlined by Dake1.
Calculate the endpoint mobility ratio, from the following :
Calculate the dimensionless gravity number, from the following :
Determine if the displacement condition will be stable or unstable; the displacement will be stable if G > M - 1, and unstable if G < M - 1.
For horizontal reservoirs with no dip and stable displacement, Widbt = 1 / M, Widmax = M and Npd can be calculated, for any value of Wid between Widbt and Widmax, from the following :
For reservoirs with dip, Widbt and Widmax can be calculated from the following :
Stable displacement |
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Unstable displacement |
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and Npd can be calculated, for any value of Wid between Widbt and Widmax, from the following :
Stable displacement |
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Unstable displacement |
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where ye is the height of water at the producing well.
For completeness, the critical rate for by-passing is calculated from the following, and will be stable for dipping reservoirs at 90% of this calculated rate :
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