Seismic low-frequency shadows and their application to detect CO2 anomalies on time-lapse seismic data: A case study from the Sleipner Field, North Sea

Abstract

The detection and underlying mechanism of prospect-scale seismic low-frequency shadows (LFSs) has been an issue of debate. Even though the concept of LFS is widely accepted, the practical applicability of the method remains limited due to few real field case studies and little understanding of the underlying attenuation mechanism. To characterize the attenuation phenomenon responsible for the occurrence of LFS in CO2-saturated formations, we use the diffusivity and viscosity of the fluid-saturated medium to derive a complex velocity function that characterizes a high-frequency attenuation phenomenon responsible for the occurrence of LFS in a CO2-saturated formation. Synthetic seismic data sets representing pre- and post-CO2 injection scenarios are generated using 2D diffusive-viscous equations to model the LFS and understand its occurrence mechanism. Furthermore, to demonstrate the applicability of LFS in a real field, a spectral decomposition analysis of time-lapse 3D seismic data of the Sleipner Field, North Sea, is carried out using the continuous wavelet transform. The LFSs are clearly detected below the reservoir base at frequencies lower than 30 Hz in the post-CO2 injection surveys. It is shown that the seismic low-frequency shadows are not artifacts but occur due to attenuation of the high-frequency components of the propagating seismic waves in the CO2-saturated Utsira Formation. The attenuation of these frequencies is a result of the diffusivity and viscosity of the fluid-saturated medium. The LFSs are localized anomalies at the base of the formation; hence, with the present approach, these anomalies cannot be related to the migration of the CO2 plume in the Utsira Formation.