Accurate Measurements of Seabed Subsidence Above Norwegian Gas Fields

Abstract

Summary Field-wide seafloor subsidence has been monitored at ten hydrocarbon fields offshore Norway. Subsidence provides lateral information on compressibility and pressure depletion in the reservoirs. This has been used to calibrate geomechanical and reservoir flow models, to map reservoir and aquifer compaction and identify poorly drained compartments. Water pressure measurements have given survey-internal precision of 2–5 mm. Tidal variations in the seafloor pressure have been carefully measured and corrected, which requires a spatial grid of reference sensors. A segment of the Midgard gas field showed significantly less subsidence than expected. This was interpreted as being caused by less depletion. A new well was drilled based on the subsidence data, a reinterpretation of the structural model, and a revised flow model. Almost virgin pressure was found. The well has since been the best producer on the field and paid back the cost of data acquisition by at least two orders of magnitude.