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A 4D feasibility study was carried out to evaluate seismic detectability and the application of 4D technology in a green field in deepwater Nigeria. A 3D seismic survey, acquired prior to production, will serve as a baseline for future follow-up 4D survey if the production induced changes are sufficiently detectable.

The Bonga Main field, Nigeria's first deepwater discovery, was discovered by Shell Nigeria Exploration and Production Company (SNEPCo) in 1995 and is located approximately 120 km offshore at water depths of 800-1200m. First oil production started in November 2005, and Bonga has an average production capacity of 200,000 barrels/day. Oil production is from 4 main stacked turbidite reservoirs (690, 702, 710/740 and 803) using pressure maintenance via water injection through producer & injector well pairs. The Bonga Main field is covered by an initial 1993 3D seismic streamer dataset and a blockwide exploration 2000 3D baseline seismic streamer survey.To monitor changes in reservoir dynamics since start of production in Q4 2005, a 4D seismic monitor streamer survey was acquired in Q1 2008. The Bonga 4D acquisition was completed on time, within budget, and with excellent HSE performance : the data quality was excellent (NRMS ~ 12.5%) where the (2000) baseline survey could be well repeated, but was of much poorer quality in the close vicinity to the field's surface obstructions (Floating Production, Storage and Off-loading vessel -FPSO and Surface Production Manifold -SPM). The 4D results of this survey added significant value to the development of the asset by improving the understanding of reservoir connectivity and the sweep efficiency between injector and producer wells, making the project a clear technical and business success. Figure 1 shows the amplitude maps of the 702 reservoir for the 2000 baseline, 2008 monitor, and the 4D difference. The bright colors indicate the presence of oil, with the 4D difference map clearly showing where water has replaced oil. Figure 1: amplitude map of the 702 reservoir: Left: 2000 baseline survey. Middle: 2008 monitor survey. Right: 4D differenceAs indicated by the yellow polygon on the 4D difference map, the 2008 monitor contained an undershoot area, as the presence of the Bonga Main FPSO and SPM did not allow the acquisition of streamer prime lines using a single boat operation. For this undershoot, a two boat operation was used with one boat towing the streamer cable, and the other boat used as a source vessel only. The 4D data quality, comparing 2000 prime lines with the 2008 undershoot lines, in this area was not very good due to TX 75083-3836, U.S.A., fax +1-972-952-9435

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