An offshore reservoir monitoring system based on fiber optic sensing of seabed strains

One of the problems affecting mature fields in the North Sea is seabed subsidence due to reservoir depletion. Seabed subsidence can directly affect integrity of production facilities. Severe subsidence, as in the case of the Ekofisk field, can lead to platform sinking below the sea level and, subsequently, to very high repair costs or possibly to a complete platform replacement.

Subsidence is an issue also in the Tyra field, and it will become a problem in other fields as depletion is taking-off. Fluid injection is a widely used method to boost production and/or maintain reservoir pressure in order to mitigate compaction and subsidence. Both reservoir depletion and fluid injection operations induce seabed deformations.

The deformation pattern potentially holds useful information about production efficiency and reservoir management, which could be captured by careful and continuous monitoring of seabed strains. Current technologies for monitoring offshore seabed deformations only provide point or line readings. The idea that this sprint project explores is achieving nearly full-field and continuous monitoring of seabed surface deformations by means of distributed fiber optic sensors.

The objective of the study was to theoretically assess whether current fiber optic sensing technology is sensitive enough to detect production-induced seabed strains originating at a 2000 m deep reservoir.