4D FWI using towed-streamer data: A case study near Laverda oil field

Abstract

Time-lapse (4D) seismic surveys are devised to detect subsurface changes resulting from hydrocarbon production and fluid injection. Full-waveform inversion (FWI) of time-lapse seismic data has been reported to provide high-resolution estimates of 4D changes. However, successful applications of 4D FWI on field data have only been seen in a few surveys, either with high repeatability or with large velocity changes. We propose a new workflow to tackle some challenges in 4D FWI, such as cycle-skipping and amplitude mismatch between modeled synthetic data and recorded field data and water-layer variations between baseline and monitor surveys. We applied this approach to a 4D towed-streamer survey in the Exmouth basin, Western Australia. The resulting 4D signals from the direct FWI velocity difference accurately detected softening and hardening effects with changes as low as 1.5% in the reservoir and matched with production history. In addition, the 4D noise was significantly reduced below the reservoirs when the respective baseline and monitor velocity models from 4D FWI were used to migrate baseline and monitor data.