An Improved Method for Estimating Oil Reserves in Oil/Water Transition Zones

Abstract

Abstract 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. Details of the experimental work may be found in Christiansen et al.,1 which discusses in depth the appropriate rock-fluid properties for characterizing transition zones. In this paper, the effect of the trapped oil relationship on reserves determination is shown in two ways: first, with an analytical model that shows the maximum possible incremental benefit of including the trapped oil relationship; and second, with an extended black oil simulation that incorporates the effects of relative permeabilities on reserves determination.