Estimating oil reserves of fields with oil/water transition zones

The amount of recoverable oil in an oil/water transition zone depends on the distribution of oil saturation as a function of depth and on the relationship between initial and final oil saturation in the transition zone (S-oitz and S-ortz), as well as the volumetric extent of the zone. Conventionally, it is assumed that S-ortz is constant and equal to the residual oil saturation in the oil column (S-or) above the transition zone. However, limited data in the literature show that residual oil saturation depends on initial oil saturation as described by the trapped-oil relationship. Thus, S-ortz should be a function of S-oitz. The purpose of this paper is to present 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. 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.