Abstract
The Gullfaks Field has been in production for eleven years and 2/3 of the estimated reserves have been recovered. The Tarbert and Ness Formations, belonging to the upper part of the Brent Group, contain 44 % of the total STOOIP.
Several 3D reservoir simulation models have been made. Due to the structural complexity and horizontal barriers it has been very difficult to history match these models, especially for the Ness Formation.
Repeated cased hole saturation logs have been collected in two wells located in the same fault block with initially oil filled upper Brent reservoirs. Standard saturation evaluation was not sufficient to understand the flooding pattern between the wells. A method making it possible to distinguish between formation and injection water even though the salinities are fairly equal has been used in this study. This has improved the understanding of the water flooding in the area. As a result of this, one of the wells (B-3) was successfully re-perforated in a thin undrained zone in the Tarbert Formation.
In 1995 a new 3D seismic survey for reservoir monitoring (4D-pilot study) was acquired over the studied area. The most pronounced 4D effects are observed at top reservoir level and at the initial oil/water contact.
One well – drilled in 1997 – confirmed an undrained area as shown by our monitoring tools and our general knowledge. In some areas there is less agreement between the 4D data and other data such as in the area around well B-3, were saturation monitoring was performed simultaneous with the acquisition of the seismic data set. Here the 4D data is not in accordance with the saturation profile in the well.
In 1996 a new seismic data set was acquired over the whole field. This data set is also interpreted for reservoir monitoring purposes. These new data together with earlier collected data will help to locate the remaining oil in the Gullfaks Field.
