What we have learnt about fluid substitution and velocity pressure dependence on Early Cretaceous carbonate rocks from the Middle East?

Fluid substitution and pore pressure changes are the main factors affecting time-lapse 4D seismic. Fluid changes are often predicted by Gassmann, which works generally well in clastic rocks. However, the applicability of Gassmann in carbonates is not well understood yet. Changes in pore pressure due to production might become important under certain conditions of reservoir production and injection. In addition, to be able to identify accurately these fluid and/or pressure changes in a time-lapse 4D seismic, uncertainty of the seismic signature should also be evaluated. Therefore, it is important to determine and quantify the effect of these factors to have a full understanding of the changes in the 4D seismic data. In this paper, we present a summary of preliminary results of different studies of pressure changes, fluid substitution, and seismic uncertainty analysis in a carbonate reservoir and surrounding facies from two different producing oil fields in the Middle East.