Low-frequency Model Update Using Adjustive FWI at Clair Ridge

Abstract

Summary Clair Ridge, located West of Shetland, is the new development phase for the Clair field in an area with near-surface velocity variability that causes challenges for traditional full-waveform inversion (FWI). A hard seabed with fast velocities and an inversion below, as well as shallow channelling, pock marks, and gas within the overburden, result in image distortion and severe multiple challenges. To prepare for the new ultra-dense ocean-bottom node (OBN) data set, a pre-acquisition model building strategy made use of legacy data sets to simplify the initial model and conclude the first successful FWI in this area. Adjustive FWI enabled this solution at Clair field by mitigating the requirements for standard least-squares methods that are difficult to meet in this area. Imaging with a simplified initial model and adjustive FWI delivers structural simplicity, enhanced amplitudes, and continuity, giving significant improvements from the overburden to the reservoir level. Reflection based FWI was tested and added details outside the coverage of diving-wave penetration and improved structural imaging around the reservoir. The addition of reflections also allows the velocity update to be extended deeper.