Paper Summary
One of the critical components for a successful 4D seismic project is to maximize imaging repeatability between the different seismic vintages in seismic processing by the use of same seismic processing flows and parameters, including the migration velocity model. There are examples that significant velocity changes in shallow reservoirs occur between surveys, leading to the degradation in 4D imaging repeatability and consequently uncertainties in the 4D seismic interpretation for the underlying reservoirs. A recent 4D project in West Africa is such an example where increases in gas saturation in the shallow reservoir due to production resulted in velocity changes up to 10% between baseline and monitor surveys. To mitigate this problem, we derived the needed migration velocity modifications by converting time shift data into relative velocity change estimates. The perturbed velocity model is then used to re-migrate the monitor seismic; the results are surprisingly effective. This paper will describe and discuss the observations, work processes, and the results from the application of this technique. Some possible applications of this technique to future 4D seismic projects are (1) significant localized overburden time-strain caused by pressure changes in the reservoirs, and (2) gas exsolution or gas migration.