Simplified Performance Calculation for Pattern Waterfloods in Stratified Reservoirs
Authors: Hiatt William N. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
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Abstract
A simplified method is developed to calculate the performance of pattern waterfloods in stratified reservoirs. The method is applicable to reservoirs in which prior gas saturation has been developed as a prior gas saturation has been developed as a result of pressure depletion. The method also accounts for the effects on volumetric coverage that result both from permeability stratification and from variations in pattern sweep efficiency. A previously developed layer system analysis is combined with existing correlations of pattern sweep vs injected volume to give volumetric coverage; this is used in conjunction with an oil-water-gas displacement factor analysis, based on estimated or experimentally determined fluid saturation changes caused by waterflooding, to give the development and progress of the oil bank through the partially progress of the oil bank through the partially depleted reservoir. Oil, water, free gas production, and recovery of oil and free gas in production, and recovery of oil and free gas in terms of percent displaceable are computed as a function of displaceable volumes injected.
In principle, calculations can be carried out for any degree of permeability stratification or for any reservoir water-oil mobility ratio or displacement factor.
A comparison is presented of the observed performance of a pilot waterflood in a performance of a pilot waterflood in a California reservoir to that calculated by the methods described; and, after consideration of certain extraneous factors, rather good agreement between observed and calculated performance is shown to exist. performance is shown to exist

Introduction
To the author's knowledge, there are only three published methods of waterflood performance calculation that account for the performance calculation that account for the effects on volumetric coverage of both permeability stratification and variation [with permeability stratification and variation [with injection] in pattern sweep efficiency that also account for the effect on performance resulting from the formation and progress of the oil bank through the partially depleted reservoir. A comprehensive method of calculating volumetric sweep for miscible floods in layered formations has been developed, but there is no published application to waterfloods. However, the above methods involve a relatively large volume of calculations, and hence require the use of electronic computers for their efficient application.
In the method presented herein, the permeability stratification effect is accounted permeability stratification effect is accounted for by a simplified procedure presented in a previously published layer analysis, based on previously published layer analysis, based on a material balance on the injected fluid, and volumetric coverage is obtained by combining this analysis with available correlations of pattern efficiency with injected volume. pattern efficiency with injected volume.