An offshore reservoir monitoring system based on fibre-optic distributed sensing of seabed strains

One of the problems affecting mature hydrocarbon fields, e.g., Ekofisk, Tyra, and Dan in the North Sea, is seabed subsidence due to reservoir depletion. 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 might induce seabed deformations. The deformation pattern potentially holds useful information about production efficiency and reservoir management, which could be captured by careful monitoring of seabed strains. Therefore, the idea suggested herein was performing near full-field and continuous monitoring of seabed deformations by means of distributed fibre-optic strain sensing. The objective of the study was to theoretically calculate and assess whether current technologies (i.e., off-the-shelf optical interrogators) are accurate and sensitive enough to detect production-induced seabed strains originating at a 2 km-deep reservoir. The analysis indicates that depletion-induced subsidence is potentially measurable with seabed distributed fibre-optic strain sensing. However, the operation of an injector-producer array induces seabed strains that are too small to be detected with current capabilities.