Structural and time-lapse imaging through gas clouds via FWI at Eldfisk field, North Sea

SUMMARY In this study, we address the persistent challenges of structural and time-lapse imaging within the Seismically Obscured Areas (SOAs) at Eldfisk field, North Sea. Despite numerous seismic programs, imaging efforts within SOAs at structures A and B continue to be hindered, primarily due to the presence of expansive, complex gas clouds in the overburden. To address this, we apply Full-Waveform Inversion (FWI) to 3D and 4D Ocean Bottom Node (OBN) data, aiming to enhance both structural and time-lapse imaging in these areas. Starting with a legacy velocity model, FWI is used as the main tool for model building, capturing detailed velocity changes associated with gas clouds. The refined model is then input into a Q (the quality factor)-compensated Reverse-Time Migration (Q-RTM) pseudo-gather-based imaging flow to generate significantly improved structural images in the SOAs. Additionally, our application of 4D FWI successfully uncovers time-lapse velocity changes that align with the injection and production history within the SOAs, which were not observed by traditional seismic-based 4D methods that require a good image.