Unlocking subsalt reservoir time-lapse insights with 4D elastic FWI imaging in Atlantis Field

Abstract 4D seismic data have played a critical role in understanding the hydrocarbon habitat, guiding field development, and the targeting and sequencing of injectors and producers in Atlantis Field. In the past two decades, relentless seismic imaging efforts have been made in the 4D seismic study in Atlantis Field, leading to tremendous progress since the acquisition of the first ocean-bottom node survey in 2005. More recent imaging advancements have come primarily from the successful application of acoustic and elastic full-waveform inversion (FWI) and have resulted in significant improvements to the complex salt canopy velocity model and, consequently, improved illumination of subsalt areas of the field. However, the challenge of capturing the elusive 4D signals underneath the complex salt canopy has persisted due to the reliance on conventional imaging methods such as reverse time migration, with the weak imbalanced subsalt illumination being a significant bottleneck. This has resulted in poor subsalt 3D images and even poorer 4D images, with signals lost in the noisy background. In recent years, FWI imaging, especially elastic FWI imaging, has proven to be a superior method for subsalt imaging, with greatly improved illumination, signal-to-noise ratio (S/N), and event focusing over conventional imaging methods. Extending elastic FWI imaging from 3D to 4D, therefore, was the next logical step, although the feasibility had not been tested previously in Atlantis Field. In this study, we present the results of the first application of elastic time-lag FWI (E-TLFWI) imaging to Atlantis 4D data sets. The results showed major uplift in subsalt 4D signals with significantly improved 4D S/N and more accurate and reliable 4D responses. The observed subsalt 4D signals are supported by well data and are in line with subsalt production history, highlighting the potential of 4D E-TLFWI imaging to revolutionize 4D imaging in complex subsalt environments.