Norsk Hydro is the operator of the Oseberg Field located in blocks 30/6 and 30/9 in the Norwegian sector of the North Sea, approximately 140 km west of Bergen. The water depth of the field is approximately 100 m. The field consists of three fault blocks, the Alpha, Gamma and Alpha North Structures. The current reserve is 209 million Sm3 of oil and 92 billion Sm3 of gas and the production capacity is 240 000 BOPD. Start-up of Oseberg Field Center was 1 December 1988. The field centre consists of two platforms, one with drilling and one with processing and living quarters. Production strategy for the field is based on pressure maintenance by gas injection. In addition to associated gas, gas from the Troll Field will be injected and will act as the main drive mechanism.

In December 1985 the Oseberg partners approved the use of ‘Petrojarl 1’ for an extended reservoir test of two wells. The main objective of the testing was to gain a better understanding of reservoir dynamics and thereby optimize further development in order to increase hydrocarbon recovery.

The long-term test on Oseberg Field was performed on two wells. These were the appraisal well 30/9-TI on the Gamma Structure and the subsea production well 30/9-B-49H on the Alpha Structure. ‘Petrojarl 1’ started testing on 30/9-TI in September 1986 and the second well test was from December 1987 until June 1988.

THE EXTENDED RESERVOIR TEST

Well 30/9-TI

Three formations, Etive, Ness I and Upper Brent Well were tested from the 30/9-TI well. A brief summary of the objectives and results are as follows:

Etive Formatzon

The primary objective was to study water coning in order to optimize the locations of the future oil production wells. The second objective was to obtain information on pressure support and communication with other structures.

The test lasted 198 days Water breakthrough occurred after 59 days and a cumulative oil production of 150 000 Sm3. The test results have been used to calibrate the reservoir simulation model to give a more accurate prediction of water coning. As a result, future well locations can be further optimized to increase expected oil recovery.

Later pressure measurements in predrilled wells revealed a lack of communication between the two main structures Alpha and Gamma This has an important impact on the split in production offtake from the two structures and the means to pressure support the production from Gamma.

Ness I Formation

The primary objective was to investigate possible communication between the Ness I and the Etive Formations in order to plan an optimized management of the formations. Pressure tests performed during the 40-days test period showed that the production from the Etive long-term test had caused almost equal pressure depletion in the Etive Formation and Ness Unit I. Consequently, clear evidence of communication was obtained.

Upper Brent Formation

The primary objective of this test was to investigate the behaviour of the gas front for planning production strategy for the Oseberg Field.

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