The Elgin, Franklin and West Franklin fields are HP/HT fields located in the Central North Sea. The production of these fields is through natural depletion. Each field has an equivalent reservoir thickness and share a similar overburden. However, each field shows a different 4D behavior in the overburden while the fields have undergone similar pressure depletions. A new geophysical interpretation shows that structural differences and the thickness of the underburden underlying the reservoirs are different for each field. These differences were used to update the 3D geomechanical model previously performed in 2013. At stress initialization of this updated 3D geomechanical model, the underlying salt, either salt weld or thick salt sheet, creates stress perturbations on salt/rock interface. Then, the 3D model shows a reservoir compaction that vertically stretches into the overburden differently. This stretching varies according to the underburden thickness and the presence and shape of salt below the reservoir. The results of the geomechanical model accounting for the underburden thickness and the presence of salt below the reservoirs explains the differences between the overburden 4D signals for Elgin, West Franklin and Franklin.

1. Introduction

There are several field cases showing that productioninduced depletion trigger geomechanical changes in the reservoir and its overburden (De Gennaro et al., 2008b, Hatchell and Bourne., 2005a, Onaisi et al, 2015, Schutjens et al., 2018). The geomechanical changes arise from the reservoir depletion leading to more compressive effective stresses in the reservoir causing the reservoir to compact. As a result, the reservoir compaction triggers more changes in the surrounding rocks, causing for instance the overburden to stretch vertically. The magnitude of the changes occurring in the reservoir and its overburden vary from one field to another depending on various parameters such as the size and shape of the reservoir, the amount of depletion in the reservoir, the mechanical properties of the reservoir rocks and also of the overburden and underburden and the presence of faults. These geomechanical changes are often studied by monitoring the reservoir compaction, the subsidence at seabed, microseismic events or changes between seismic surveys, i.e. 4D seismic monitoring.

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