An exploration campaign of RWE Dea AG has discovered several oil fields in carbonate reservoir layers. Log analysis shows that most of the wells have a very long transition zone above Oil Water Contact (OWC). The lower part of the oil column above OWC contains significant amounts of non-producible oil. The length of this zone may falsify the economic significance of the oil column, which appears as too large.

An additional hydrocarbon contact is defined and named „Producible Oil Water Contact - POWC". The differentiation between moveable and unmoveable oil in the transition zone helps to identify and assess the producible oil in a more appropriate way. This level may be determined by using fractional flow curves. In this case the contact level is a derived value for water saturation that relates to a specific water cut at surface conditions. If dynamic reservoir modeling is available the POWC can be also determined based on 3D-simulation results, at the lowest level where oil begins to flow.

In order to comply with the industry standards for definition of OWC the methods available will be discussed and it will be verified if the newly introduced POWC fulfills common best practice and can be used as supplement.

The effect of applying the POWC on oil-in-place calculations and recovery factor is shown with an example for application in reservoirs with long transition zones. It is aimed to concentrate resource calculations to the levels with economically producible oil, in order not to artificially inflate oil-in-place volumes and keep recovery factors reasonable. Furthermore, improved contour mapping based on new POWC can help to find locations for wells with better oil productivity.

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