Reservoir Aspects of Ekofisk Subsidence

Abstract

In Nov. 1984, Phillips Petroleum Co. 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 geolgoic- 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 results from 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. Thus far, no loss in reservoir productivity has been observed. On the basis of analogy with the neighboring West Ekofisk field, no loss in reservoir productivity is anticipated, at least for the near future.