|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||10 Years of WAG Injection in Lower Brent at the Gullfaks Field|
|Authors||Rune Instefjord, Annick Chazy Todnem, Statoil|
European Petroleum Conference, 29-31 October 2002, Aberdeen, United Kingdom
|Copyright||Copyright 2002, Society of Petroleum Engineers Inc.|
The Gullfaks field is a large oil field in the Norwegian sector of the North Sea. Production started in 1986 with water flooding as the primary recovery method. A WAG injection pilot in lower Brent well A-11 was initiated in March 1991. The well is located south in the GFA-area.
A simulation model was built and history-matched until January 1993 for the whole pilot area, including upper Brent. Predictions to the end of year 2005 indicated an increased oil recovery of 0.3 mill. Sm 3 in addition to some accelerated production. Most of the predicted increased oil recovery was however reached during 1993.
The WAG injection has continued in A-11 but with fewer gas periods the last years. The updated estimate of increased oil recovery from WAG injection in A-11 up to year 2001 is 2 mill. Sm .
The paper will also give a short evaluation of WAG injection in two other lower Brent injectors at Gullfaks with a joint estimated increased oil recovery of 0.7 mill Sm .
The Gullfaks Field1, located offshore Norway (Fig. 1), has a STOOIP of 582 Millions Sm of oil and expected recoverable reserves of 325 Millions Sm corresponding to a recovery factor of 56%. The field currently produces 30 000 Sm /d of oil mainly from three Jurassic reservoirs, the Brent Group and the Cook- and Statfjord Formations. The production started in 1986 and the field has to date produced 290 Millions Sm of oil. The field produces from three separate Condeep platforms, Gullfaks A, B and C.
The main reservoir is the Brent Group (Fig. 2 ) containing 74% of the STOOIP. The Brent Group consists of the Rannoch, Etive, Ness and Tarbert Formations. The permeability is in the Darcy range except for the lower part of the Rannoch Formation and some Ness sands, which have permeabilities down to 10 mD.
The Ness Formation (N-3, N-2 & N-1) and the lower part of the Tarbert Formation (T-1) consist of sandstone layers with interbedded calcite/chalk and shale layers. There is no vertical communication between the layers. The upper part of the Tarbert Formation (T-3, T-2) has full vertical communication except a partly sealing shale within T-2. There is vertical communication between the Etive and Rannoch Formations.
The structural geology of the field is very complex. There are numerous faults, both large faults, observable on the seismic, and small sub-seismic faults. Both types of faults are in some cases sealing and in other cases non-sealing. This has a great influence on the flow pattern across the field.
The main drainage strategy is water injection. The field has though some aquifer support from west and north. A gas injection facility was originally installed at one platform, GFA. The field has a gas sale agreement and the gas is transported through a pipeline to Kårstø. Because of restricted capacity of the pipeline, gas had to be injected into the reservoir. In late 1980's gas was injected into a producer in the Cook Formation, mainly for storage in the reservoir but also for pressure support. In periods without gas injection, the well was produced after some weeks with shut-in to allow for segregation in the reservoir. The production rate from the well increased after the injection periods and the result was a much higher oil recovery from the area than expected. Therefore new areas for gas injection were investigated, which would allow for a more active use of the gas for increased oil recovery. The Gullfaks gas sale contract allowed reinjecting certain volumes of produced gas with relatively small economic consequences for the Gullfaks field.
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