In November 1984 Phillips Petroleum Company discovered subsidence of the seabed overlying the Ekofisk oil reservoirs offshore Norway. This phenomenon is the result of the compaction of the porous chalk reservoirs and the transmission of this compaction through the overburden to the seafloor. This paper describes the geological and reservoir related aspects of subsidence including the mechanism leading to reservoir compaction and its effect on reservoir performance.
The compaction of the Ekofisk reservoirs is shown to be a result of pore pressure depletion. Although some of the compaction is elastic, the bulk is due to plastic deformation (pore collapse) of high porosity chalk. Reservoir compaction is shown to cause subsidence of the seabed through deformation of the overlying sediments. Reservoir compaction has also, had a pressure maintenance effect on the reservoir. Thusfar, no loss in reservoir productivity has been observed. Based on analogy with the neighboring West Ekofisk Field, no loss in reservoir productivity is anticipated, at least for the near term.
The Ekofisk Field is located in the Central Graben in the southern part of the Norwegian sector of the North Sea (Figure 1). It is the largest of six fractured chalk fields operated by Phillips Petroleum Company Norway on behalf of the Phillips Norway Group.
Water depth in the area is about 235 feet (72 meters), and frequent storms during fall and winter make the environment rather hostile.
In 1963 the Phillips Norway Group started seismic surveys in the Norwegian sector of the North Sea. Exploration drilling commencing in 1967 led to the discovery of the Cod gas condensate sandstone reservoir in 1968 and the Ekofisk chalk field late in 1969. Over the next three years, five additional chalk fields were discovered in what is now referred to as the Greater Ekofisk Area.
Early production started in July 1971, and since then the field development has evolved through various phases with a peak production rate of 349,000 barrels (55,500 m3) per day reached in 1976.
Today the Ekofisk Complex is the processing center for all production from the Ekofisk Area fields. Figures 2 and 3 show the size and location for all the fields in the area.
Ekofisk is also the transportation hub for all gas produced from oil and gas condensate fields in the Norwegian sector of the North Sea. At Ekofisk, gas from Ula, Valhall, the Statpipe system and the seven Ekofisk Area fields enters the Norpipe gas pipeline to Emden in West Germany (Figure 1). From Germany the gas is further distributed to France, Belgium and the Netherlands.
Oil from Ula, Valhall and the seven Ekofisk Area Fields is also transported to Ekofisk. At Ekofisk, the oil is combined with NGL' s separated from produced gas and shipped to Teesside in England via the Norpipe oil pipeline (Figure 1).