Time-lapse (4D) analysis of legacy seismic data presents unique challenges, as neither the acquisition nor processing is designed for seismic monitoring. Two legacy seismic data sets from the Lena Field, Gulf of Mexico, are analysed for time-lapse effects. The analysis involves post-stack processing of the legacy seismic data including cross equalization and residual migration, and the definition of a new suite of 4D seismic attributes. These new attributes are used in both processing and interpretation. The time-lapse differences are interpreted using forward modeling and production data. The 4D difference anomaly is interpreted to be the result of gas cap expansion. The identification of potentially bypassed oil based on this interpretation may affect future drilling decisions.
Seismic monitoring (time-lapse or 4D seismic) has the potential to significantly increase recovery in existing and new fields. However, there are many issues associated with the application of time-lapse seismic data. Two of the most significant are repeatability of the seismic data in the non-reservoir portion of the data volume and the robustness and credibility of the seismic difference anomaly within the reservoir (Ross et al. 1996, 1997).
While future field developments should benefit from seismic acquisition designed for time-lapse monitoring, the portfolio of current seismic monitoring opportunities for most companies consists 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 very different in terms of acquisition and processing parameters. In addition, the seismic acquisition is rarely timed to optimally map reservoir changes or impact development decisions.
Seismic repeatability is sufficient for time-lapse interpretation if the seismic differences in the region of interest are substantially greater than the differences outside the region of interest. 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 reservoir changes but repeatability and interpretability can only be determined by the analysis of multiple seismic surveys.
The main goal of this study is to understand the magnitude ofthe processing effort required to obtain reliable time-lapse differences. The reliability of the seismic difference is measured by repeatability in the seismic volume and the reconciliation of the time-lapse anomaly with geologic and production data.
The Lena Field (Mississippi Canyon Block 281) is located south of the modern Mississippi delta in 1,000 feet of water. The field is situated on the western flank of a salt dome within a fault-bounded intraslope basin. Hydrocarbon production is from six Pliocene-age sands.
The B80 reservoir is located about 10,000 feet below SL at about 3 seconds seismic TWT. The interval is interpreted as a low-stand fan systems tract representing deposition in distributary lobes composed of amalgamated and channelized turbidities. The updip limit of the sands lies about 2,000 feet west of the salt flank and the reservoir thickens basin-ward to the west.
