Two 3D seismic data sets from the Lena Field, Gulf of Mexico are analyzed for time-lapse effects. The seismic analysis involves cross equalization and residual migration of the poststack seismic data, as well as full reprocessing and attribute analyses. The time-lapse differences for the B80 reservoir are compared with production data, geologic models, flow simulations, and forward seismic models. The time-lapse seismic difference anomaly is interpreted to be a region of gas invasion. Areas bypassed by the injected gas are identified from 4D seismic data as opportunities for infill drilling. Successful interpretation of this time-lapse seismic data illustrates the importance of integrating the results of modeling and simulation with seismic processing and interpretation.
Seismic monitoring (time-lapse or 4D seismic) has the potential to significantly increase recovery in existing and new fields. During production, changes in a reservoir's fluid saturation, pressure, and temperature result in changes in density and seismic velocity. Changes in density and velocity result in impedance changes which, under favorable conditions, can be detected in seismic data. As illustrated in Figure 1, an initial seismic survey is acquired before production or enhanced recovery intervention in order to determine the baseline response. Later (the time depending on the recovery process) a second survey is acquired. The difference between the two surveys can be a measure of dynamic reservoir changes.
One important issue is the significance of the seismic difference anomaly relative to non-repeatable noise. While future field developments should benefit from seismic acquisition designed for time-lapse monitoring, current seismic monitoring opportunities consist of existing fields for which one or more 3D seismic surveys have already been acquired. These legacy seismic data sets were not acquired for the purposes of seismic monitoring and are often different in terms of acquisition and processing parameters. These diffe rences can be addressed through post-stack equalization or full reprocessing of the data.
Seismic repeatability is a measure of seismic differences of two surveys over the same region where no reservoir changes occurred. The smaller the difference, the greater the repeatability. Seismic repeatability is sufficient for 4D interpretation if the seismic differences in the reservoir are substantially greater than the seismic repeatability. The smaller the change in the seismic response due to production, the greater the repeatability required of the seismic data. Seismic modeling incorporating rock physics and reservoir simulation can help estimate the magnitude of seismic changes induced from reservoir changes but repeatability and interpretability can only be determined by the analysis of multiple seismic surveys. Eastwood et al.1 have demonstrated that seismic repeatability at Lena is sufficient to image reservoir changes due to production for the B80 reservoir.
Another issue is whether 4D seismic differences can be interpreted in terms of reservoir production changes. If so, then 4D seismic can be used as a tool for reservoir surveillance or reservoir management. The reliability of the seismic difference interpretation is measured by repeatability in the seismic volume and the reconciliation of the time-lapse anomaly with geologic and production data.
