The amount of recoverable oil in an oil-water transition zone depends on the distribution of oil saturation as a function of depth, and the relationship between initial oil saturation in the transition zone (Soitz) and residual oil saturation in the transition zone (Sortz). Traditionally, it is assumed that Sortz is the same as the residual oil saturation in the oil column (Sor) above the transition zone. However, data in the literature show that residual oil saturation depends on initial oil saturation. So, residual oil saturation in the transition zone (Sortz) should be a function of initial oil saturation in the transition zone (Soitz). The relationship between Sortz and Soitz is referred to as a trapped oil relationship.

The purpose of this paper is to show recent experimental corroboration of the trapped oil relationship and to demonstrate the impact of the trapped oil relationship on reserves determination in oil-water transition zones.

First, fundamental issues for appropriate selection of rock-fluid properties for characterizing transition zones are addressed. Graphical illustrations are presented to compare and contrast conventional methods for characterizing transition zones with an improved characterization method that requires additional rock-fluid property measurements.

Then, results from laboratory studies of recovery from transition zones are used to demonstrate features of the improved characterization method. These results show that oil in the transition zone is much more mobile and recoverable than is assumed by the conventional approach.

For these laboratory studies, unconsolidated packs of sand, glass beads, and plastic beads were used. The media were varied to observe the effects of wettability and grain size on transition zone properties. It is believed that the variety of media qualitatively represent what would be observed for actual reservoir formations. Local saturations in the transition zones were correlated to gray level of video images of the surface of the packs. The laboratory methods for unconsolidated media could be extended in the future to reservoir rock (having much lower permeabilities) with centrifuge technology.

Finally, the effect of the trapped oil relationship on reserves determination is shown with an extended black oil simulation that incorporates the effects of relative permeabilities on reserves determination.

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