The role of the underburden at Elgin Franklin in the understanding of the overburden 4D signal

Elgin, Franklin, and West Franklin are high-pressure high-temperature fields located in the Central North Sea. The fields share a similar reservoir and overburden geology. During the life of the fields, they have undergone a similar depletion over time. However, each field shows a different 4D behavior in the overburden. The 4D signal on Elgin is the strongest and is located away from the wells. The Franklin 4D signal is in line with expectation as the strongest signal close to the wells. On West Franklin, the 4D signal is small and barely distinguishable from noise. To explain the differences in 4D signal between the fields, a detailed underburden salt interpretation is performed. The differences in underburden between Elgin (thick with an abundance of salt) and West Franklin (marginal underburden with proximal salt) show a strong correlation with the 4D signals of the overburden. The renewed salt interpretation is used to update a vintage geomechanical model, from which strains and stresses are computed. During the stress initialization of the geomechanical model, the underlying salt (thick sheet or salt weld) creates stress perturbations on the salt/rock interface. When the reservoir compaction is simulated, the 3D model simulates the vertical stretches differently in the overburden for the fields. The stretching varies according to the underburden thickness and presence of salt below the underburden. The results of the geomechanical modeling help explain the differences observed in the overburden 4D signal of the fields.