4D Seismic Wave-equation Depth Migration Velocity Inversion

Time-lapse analysis of 4D seismic data acquired at different stages of hydrocarbon production or fluid/gas injection has been very successful at capturing detailed reservoir changes (e.g., pressure, saturation, fluid flow). Conventional 4D seismic analysis is performed in the time-migrated image domain and assumes a fixed migration velocity model; however, this assumption is violated when the subsurface velocity is significantly altered by production/injection effects, resulting in large time-shift anomalies and complex 4D scattered wavefield coda. We argue that these situations require a more robust 4D analysis procedure that involves iterative wave-equation prestack depth migration (PSDM) and time-lapse velocity analysis. Herein, we adapt a 3D image-space wave-equation migration velocity analysis (WEMVA) approach that backprojects discrepancies (residuals) in 3D/4D migrated images to form 3D/4D velocity model updates. We discuss the differences between 3D and 4D WEMVA inversion goals, and how we localize 4D WEMVA perturbation estimates to around the reservoir zone to generate high-resolution velocity model updates. We demonstrate the benefits of 4D WEMVA analysis in a synthetic CO2 geosequestration experiment by successfully inverting for a velocity model perturbation corresponding to a thin layer (< 20m) of injected CO2 in a North Sea analogue reservoir.