The Effect Of Selective Producing Intervals On Secondary Recovery Efficiency In A Layered Reservoir
Authors: Seba Richard D. 
Published: December 2007

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Abstract
The effect of partial completion in stratified reservoirs with crossflow between layers was investigated, using a resistance net-work model. The model simulated linear steady-state flow for two and three-layered systems of different thicknesses and permeability values. The relationship between sweep efficiency and completion interval for variation in layer thickness and permeability is analyzed in detail for the two-layered cases studied substantiate the conclusions reached on the basis of results from the two-layered systems.
The investigations indicates that an improvement in vertical sweep efficiency by partial penetration or selective completion of a st ratified reservoir can only be achieved if the completion is entirely in the zone or zones of lower permeability. The method can be most advantageously applied to formations having a large permeability contrast with a thick layer or layers of low permeability. The maximum improvement in sweep efficiency is achieved by a completion in about 30 percent of the zone of lowest permeability and displaced the maximum possible distance from the boundary between the layers. The advantages of this method of completion are reduced by greater well spacing and thin layers of low permeability.

Introduction
An important problem facing the petroleum industry today is finding new methods and procedures which may result in increased recoveries of oil from depleted and semi-depleted petroleum reservoirs. One type of petroleum reservoir which deserves considerable attention is the interconnected stratified reservoir that displays a marked variation in permeability. The stratified reservoir can be defined as one that exhibits lateral continuity throughout or a portion thereof with zones or strata of different permeability within it. In many cases these zones are not separated by a shale or other impermeable barrier, but they are in complete contact with each other. In such reservoirs there is a movement of fluid between the layers of different permeability, so-called "crossflow", which has a definite effect upon the behavior of this type of reservoir and the recoveries that may be obtained from it.
One reason for studying this type of reservoir is the frequency with which it is encountered. It is natural for this type of reservoir to be quite common due to the process of deposition that formed the sedimentary rock strata. Most producing horizons were deposited over a considerable length of time and under a marked variation of conditions. These factors could account for a wide variation of particle size as the formation was deposited, as well as different degrees of cementation of it, both of which have a great influence upon the permeability of the formation. Also the concept of layers of different permeability in contact with each other is consistent with geological considerations, because the deposition of particles generally occurred on a flat, nearly horizontal surface and continued for some length of time in this manner. Thus, as conditions changed with time, the character of the deposits changed, with a pronounced variation in the vertical direction. Since the deposition in many cases was not a continuous process but an intermittent one, each layer could be nearly homogeneous within itself but quite different from the layer above it or below it.
Another reason for studying this type of reservoir is that in many cases only a small percentage of the oil is recovered from it due to differential depletion or water breakthrough.