TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract
The Gullfaks Main Field is one of the largest oil fields in the Norwegian sector of the North Sea. Production started in 1986 and reached a peak in 1994. Today approximately 90 % of the base reserves have been produced from this mature field. Locating remaining oil reserves is therefore a challenging and important task for the future of Gullfaks. The reservoirs are comprised of middle and early Jurassic sandstones, possessing overall excellent reservoir properties. The structural setting of the field is complex, and represents one of the main factors of uncertainty concerning the drainage of the field. The paper describes how multiple time-lapse seismic (4D) have proven very accurate in identifying undrained oil pockets in the Brent Group. The importance of 4D/4C in the wells drilled is estimated to represent a reserve oil volume of approximately 4 Mill. Sm per 2004, (25 Mill. Sbbl). TTD&C (Through Tubing Drilling and Completion), being a more flexible and less expensive technology, has also been crucial in reaching smaller oil pockets on the Gullfaks Main Field. The potential offered by these technologies has been optimized by multidisciplinary teamwork on Gullfaks, thus creating synergy and flexibility. These three key elements have contributed to reverse the oil production decline, establish a new plateau, reduce costs and increase profit. Introduction The Gullfaks Main Field, located offshore Norway (Fig. 1), has a STOOIP of 581 Mill. Sm, (3655 Mill. Sbbl) of oil and expected recoverable reserves of 352 Mill. Sm, (2214 Mill. Sbbl), corresponding to a recovery factor of 61 %. Since production started in 1986 the field has produced 312 Mill. Sm of oil, (1962 Mill. Sbbl). The field produces from three separate Condeep platforms, Gullfaks A, B and C. Peak production was reached in 1994, followed by several years of production decline. However, 2001 marked a change. The production was stabilized, and a new secondary plateau was obtained from 2001 to 2004 (Fig. 2). This achievement is the result of a strong focus on utilizing technologies such as time-lapse seismic (4D), ocean bottom (4C) seismic and TTD&C (Through Tubing Drilling and Completion), and by increasing cross-disciplinary understanding and cooperation. The main reservoir is represented by the Brent Group, containing 75% of the STOOIP. The Brent Group consists of the Broom, Rannoch, Etive, Ness and Tarbert formations (Fig. 3). Their production indexes vary, as do their reservoir properties, i.e. permeability, porosity and zone thickness. The best sands have a permeability of several Darcys, while the poorer sands have permeabilities as low as 10 mD. The porosity is generally above 30 %. Vertical communication can be found within some reservoir zones, commonly in the Tarbert 3–2, Etive and Rannoch 3–1 zones, while it may be prevented by calcite, coal and shale layers in Tarbert-1, and in the Ness sands (Ness 3–1). However, this communication pattern is affected by the presence of many faults (over 300 identified), which break the impermeable layers. Large faults, observable on the seismic, and small sub-seismic faults, have various sealing potential depending on their location and juxtaposition. The flow pattern is greatly influenced by the complexity of the structural geology. As the field becomes more mature, the task of identifying the undrained oil pockets becomes more perilous, while at the same time, focus on cost benefit increases.
TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract
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