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Time-lapse seismic in Gannet A

One more lead firmly integrated

Shell U.K. Ltd., Aberdeen, Scotland

Corresponding author: Toon.A.J.Weisenborn@Shell.com

The first 20% of the full text of this article appears below.

The Gannet A field is a mature oil and gas field in the central North Sea. Its Upper Tay reservoir consists of generally high quality deepwater turbidite sandstones of Eocene age in a large, low-relief, mainly structural, partly stratigraphic trap. A 53-ft oil rim contains 190 million barrels of stocktank oil with 190 billion standard cubic feet (= Bscf) dissolved gas, overlain by a 325 Bscf gas cap. The reservoir sand has 30–33% porosity with 1/2-2 Darcy permeability. Reservoir quality deteriorates from 95–99% net-to-gross in the northwest to 40–50% in the south. The field has been developed with 11 evenly spaced horizontal oil producers, which have on average completions of about 3000 ft in the middle of the oil rim. By year-end 2002, 61 million barrels of oil had been recovered from the field. With a forecast ultimate oil recovery factor of 40%, the initial oil rim development could be considered successful and final, in particular if compared to recovery factors achieved for similar oil rims (Weber and Dronkert, 1998).

Traditional static and dynamic modeling was carried out in 1999 and 2000. The primary objective was to understand well production behavior. The inflow of a strong aquifer on the northern and southeastern sides of the field and the degradation of the reservoir quality from northwest to southeast were givens in the model. A key result of the study was recognition of the influence of a thin, field-wide siltstone bed. Without this layer, the water-cut development in the production wells could not be modeled in an acceptable way. The study furthermore led to identification of two pockets of unswept volumes of between 2.5 and 3.5 million barrels of recoverable oil. These targets appeared in all history-matched geologic realizations. Other potential targets varied in size between the realizations, from complete absence to . . . [Full Text of this Article]