4-D constrained depth conversion for reservoir compaction estimation; application to Ekofisk Field

Ekofisk Field (in the Norweigian sector of the North sea) is a remarkable example of a strongly compacted chalk reservoir. Since the start of production in 1971, the water bottom has subsided by over 8 m as a result of reservoir compaction (Hermansen et al.). The field-wide waterflood, begun in 1989, was expected to halt pressure depletion and reservoir compaction. After a few years of waterflooding, Ekofisk reservoir pressure began increasing, but compaction and subsidence continued. Laboratory studies on chalk cores indicated that high-porosity, oil-filled chalk compacts strongly when flushed with seawater. These studies, along with reservoir flow simulation and geomechanic modeling, led to a model of a water-weakening compaction (Sylte et al.). Ongoing efforts deployed to monitor the reservoir compaction and seabed subsidence include twice-a-year compaction logging in a monitoring well, daily GPS measurements at the platforms, and periodic bathymetry surveys (nine surveys from 1970 to 1999).