Abstract

Oil-water capillary transition zone often contains a sizable portion of a field's initial oil in place, especially for those carbonate reservoirs with low matrix permeability. The field development plan and ultimate recovery may be heavily influenced by how much oil can be recovered from the transition zone. This in turn depends on a number of geological and petrophysical properties that influence the distribution of initial oil saturation (Soi) against depth, and on the rock and fluid interactions that control the residual oil saturation (Sor), capillary pressure and relative permeability characteristics as a function of initial oil saturation.

Due to the general lack of relevant experimental data and insufficient physical understanding of the characteristics of the transition zone, modeling both the static and dynamic properties of carbonate fields with large transition zones remains an ongoing challenge. In this paper, we first review the transition zone definition and the current limitations in modeling transition zones. We describe the methodology recently developed, based on extensive experimental measurements, for modeling both static and dynamic properties in capillary transition zones. We then address how to calculate initial oil saturation distribution in the carbonate fields by reconciling log and core data and taking into account the effect of reservoir wettability and its impact on petrophysical interpretations. The effects of relative permeability and imbibition capillary pressure curves on oil recovery in heterogeneous reservoirs with large transition zones are assessed. It is shown that a proper description of relative permeability and capillary pressure curves including hysteresis, based on experimental special core analysis data, has significant impact on the field performance predictions especially for heterogeneous reservoirs with transition zones.

Introduction

The reservoir interval from oil-water contact (OWC) to a level where water saturation reaches irreducible is referred to as the capillary transition zone. Figure 1 illustrates a typical capillary transition zone in a homogeneous reservoir interval within which both oil and water phases are mobile. The balance of capillary and buoyancy forces controls this so-called capillary transition during the primary drainage process of oil migrating into an initially water-filled reservoir trap.

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