Acquisition, Processing and Initial Impact of the Bonga 4D Seismic Survey
A higher-resolution pre-stack migrated version with 3D SRME was subsequently delivered. However, its interpretation is still progressing and its results will not be discussed here.
Early Interpretation Results
The intermediate time-lapse seismic data show a robust 4D signal that highlights differences around the water injection wells, as expected. However, the quality of the signal is actually better than expected, and the following conclusions can be summarised.
In general, the 4D time-lapse signal at the Bonga Main Reservoirs agrees extremely well in lateral extent with the modelled response from the dynamic reservoir simulation. Water breakthrough from production data at producing wells agrees with traceable oil–water contacts observed from the 4D seismic. The 4D result also shows that, for channelised reservoirs, the sweep may be confined to the channel axis within an individual meander belt if the injector/producer pair is also so located.
The detailed, loop-scale interpretation of Bonga field’s complex, channelised reservoirs can be refined in areas where 4D seismic illuminates details of the water sweep. Individual channel geometries and sub-seismic connectivity between geological depositional systems are evident from the 4D, and give new insights into the detailed reservoir architecture that defines Bonga. The 4D data also provide clues to subtle production anomalies that have been observed at the field, but for which definitive data did not exist earlier. This information is being used to drive integrated updates to specific reservoir static models, as well as providing both confirmation and changes to some important aspects of our future reservoir development plan and our overall field development strategy. Furthermore, the 4D data are helping to reduce uncertainty in the material balance for a number of reservoirs (see Figure 3).
Figures show details for a main Bonga reservoir.
Finally, the 4D demonstrated unambiguously that there was no formation of secondary gas caps at any of these reservoirs. The 2008 drilling schedule was revised to allow for early interpretation of the 4D intermediate results in order to mitigate concerns that one of the deeper Bonga reservoirs, which had no current injector support, might be producing below the bubble point. The scheduled well was to be drilled at the highest structural point of this reservoir, where a gas cap could potentially have been forming. However, the 4D results showed no gas cap formation; the well was subsequently optimally placed, and no gas cap was found.
Conclusions
The Bonga 4D acquisition was completed on time, within budget and with an excellent HSE performance. Early ‘intermediate’ versions of the Bonga 4D seismic volumes were delivered by Western-Geco some four months after the completion of the acquisition, and their interpretation drove important business decisions. The Bonga asset has already considered the results as of significant value and the project is a clear technical and business success.
The 4D has provided an important safety de-risking on the possibility of formation of any gas cap. It is driving the detailed placement of future producer/injector well pairs in many of the reservoirs. It has reduced material balance and volumetric uncertainties associated with estimation of ultimate recoverable reserves.
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
Figure 3: Comparison Between Pre-production, Base Seismic Amplitude Extraction (left) and the Same Amplitude Extraction After About Two Years of Production (right)
14,000 12,000 10,000 8,000 6,000 4,000
Figure 2: Extracted Amplitude Response from Vintage 2000 Field Seismic Data (left), and Pre-production Synthetic Modelled 4D Amplitude Response Showing Expected Fluid Replacement (right)
Figure 1: Bonga Field, Oml 118, Located 120km South-west of the Niger Delta (left), and Seismic Traverse Through the Bonga Field Showing a Series of Stacked Reservoirs and Discovery Well (right)
Depth (ft)
Sea bottom
Figures show a Bonga Main reservoir. Pre-production, base seismic amplitude extraction is from 2000, and the same amplitude extraction is after about two years of production from the 2008 monitor ‘intermediate’ migration. Obvious 4D fluid replacement effects are visible.
Finally, there is a richness of 4D detail in the final processed data that demands further investigation and from which a whole new level of information is emerging. Examination of detailed reservoir time shifts and amplitude versus offset responses are giving unique insights into reservoir characterisation, reservoir performance and production monitoring. n
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