Abstract

In weak formations, significant pressure depletion may lead to compaction. Transmission of compaction through the rock layers overlying the depleted formation may, in turn, cause subsidence. At the Ekofisk field, a large producing oil and gas reservoir located in the Norwegian sector of the North Sea, subsidence of the seabed due to reservoir compaction led to extensive facilities modifications in the mid-1980's and has significantly contributed to the need for the on-going Ekofisk II field redevelopment project.

While reservoir compaction is the source of the volume loss that leads to subsidence, transmission of compaction through the overburden strata often is a critical governing factor on its impact. Theoretical modelling work has shown, for example, the relationship between reservoir depth or width to the transmission of compaction to subsidence. Building upon the theoretical work and utilizing available field data, a numerical study was conducted to evaluate the impact that overburden behavior at Ekofisk field has on compaction and subsidence.

For the study, available field data for validating the modelling of the overburden were reviewed, including reservoir compaction data, platform subsidence data from the center as well as the edges of the subsidence bowl, data from eight bathymetry surveys providing information on the extent and change of the subsidence bowl with time, and overburden core and log data. The results of the study shed light on the impact of overburden stiffness, the extent, nature, and development of reservoir compaction, and the compaction to subsidence transfer ratio (C/S ratio) for the field.

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