First Break - Volume 38, Issue 3, 2020

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.

In this paper we demonstrate the imaging capabilities of a newly developed 3D Gauss-Newton inversion algorithm for marine controlled source electromagnetic (CSEM) data by inverting synthetic data generated from a known salt resistivity model. We show that the high resistivity contrast between salt and background sediments can be utilized to reconstruct reliable images of the salt structure without the use of any a-priori information which could bias the outcome. Further, we re-invert a CSEM data set acquired in 2012 in the Salina basin in the Gulf of Mexico, using the same 3D Gauss-Newton inversion algorithm. The resulting resistivity model is compared to the initial salt interpretation based on seismic data. The top salt boundary in the inverted resistivity model correlates well with the initial interpretation. However, the base salt geometry, which is often difficult to map with seismic data alone, is imaged very differently. The CSEM inversion result is robust and independent of other geophysical data and therefore very valuable in a salt imaging workflow to support seismic interpretation and velocity model building.