Shell has in the past 10 years acquired time-lapse data over 8 fields in the Gulf of Mexico (GoM). Direct cost for acquisition and processing for the total effort was $24 million. Total combined cost saving of the entire time-lapse effort is also $24 million. Most of the savings were realized in a single field. In addition, recoverable volumes were increased by 10 mmboe in one field and a contribution was made to mature another 3.5 mmboe in another field.<br>The observed time-lapse data was different from the expectation. The most important mismatch is the observed insensitivity of the reservoir impedance to depletion. For all surveys the expectation was that depleting reservoirs would show an easily detectable increase in impedance. It has now become clear that this increase is significantly smaller and hence harder to detect. The acquisition of future surveys has to better duplicate the base survey as a direct result of the need to detect subtle pressure depletion effects. Successful practices from the North Sea turned out to be inadequate in the GoM. On the positive side, saturation changes have invariably been larger than expected. Nearly all business impact from time-lapse seismic on the 8 fields has come from the interpretation of these saturation effects.


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