Comparison of 4D Seismic Inversion Methods for Qualitative Water Breakthrough Risk Assessment before Drilling

4D time lapse seismic is an efficient method to describe variations of reservoir properties during field production, especially pressure variations and fluid replacement. Since the seismic information does not provide directly these properties, results of preliminary seismic inversion processes are used for physical attribute estimation.

We present in this paper the case of a 4D signal encountered in an undrilled area located in a turbidite reservoir system, suggested to be horizontally drained. The 4D relative velocity change attribute was estimated using a standard processing sequence on 4D poststack data sets. It appeared difficult to interpret the result in term of effective saturation change and pressure variation effect because the attribute became null on the second monitor.

The qualitative interpretation of the 4D signal using more than two vintages was conducted gathering results of three different inversion techniques, each providing different 4D attributes.

The consensus of the different methods leads to two operational consequences. On one hand this risk assessment will allow to better define the area which might be completed. On the other hand it gives the maximum length of completion in order to be able to produce with a low water breakthrough.