Time-lapse wave-equation migration velocity analysis

Summary

Time-lapse analysis of 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 analysis is performed in the time domain assuming a constant baseline model; however, this procedure becomes difficult when the subsurface is significantly altered by production/injection and large time anomalies and complex 4D coda are recorded. We argue that a more robust 4D analysis procedure in these situations requires iterative wave-equation depth imaging and time-lapse velocity analysis.

Wave-equation depth migration requires accurate knowledge of the velocity field usually obtained by one of two ways. First, data-space methods are where recorded data are matched to those calculated through a background velocity model. Differences between the two datasets are used in tomographic backprojections to generate velocity model updates. Alternatively, imagespace methods are where discrepancies between migrated images (non-flat gathers) are backprojected to estimate velocity model updates. These types of approaches are termed migration velocity analysis (MVA).

This abstract focuses on extending 3D wave-equation MVA (WEMVA) approaches to time-lapse velocity analysis. We discuss the differences between 3D and 4D WEMVA inversion goals, and how we leverage the locality of 4D image perturbations to provide highresolution velocity model updates. We demonstrate the utility of 4D WEMVA analysis in a synthetic CO₂ geosequestration experiment by successfully inverting for a velocity perturbation corresponding to a thin layer (<20m) of injected CO₂ in a typical North Sea reservoir.

Key words::

• seismic
• prestack
• inversion
• 4D
• and velocity