Nigerian's first 4D seismic acquisition in deepwater oil and gas operations has been achieved at SNEPCo-operated Bonga field, 120km southwest of the Niger Delta.
The 4D seismic acquisition, also known as Time Lapse, was achieved over a 76-day period during which images of the subsurface were captured with the aid of state-of-the art equipment operating from one marine seismic acquisition vessel and three support vessels staitoned in the field. The acquisiton was carried out by a company based in Lagos, and the results will enable better well placements resulting in maximization of Bonga field life-cycle production.
The first (baseline) seismic survey in the Bonga field was conducted in 2000. The the results guided the siting and drilling of wells and relevant activites prior to start-up of production in November 2005. After about two years of production, 4D survey results will show how reservioir conditions and fluid content have changed.
SNEPCo's Geophysics and Geomatics Services Manager, Abayomi Adejonwo, said, "To put it simply, we now have a picture of the Bonga subsurface and this will enable us to decide how best to drill new wells for maximum drawdown of oil. Having produced a field for two eyars, it was very improtant to get this picture if we are to achieve our aspiration of operating the field to the benefit of all stakeholders including the Nigerian Government and Shell."
Marine 4D Seismic data acquisition operations, especially those involving close-passes of fixed installations is technologically complex and involves several high-risk activities including operating air guns, towing long streamers with close passes to fixed installations. The Bonga survey utilised a total of four vessels with 100 personnel on-board and recorded 133,000 man-hours without a Lost Time Injury (LTI).
A significant aspect of the survey was the involvement of Nigerians which made up of 40% of the total personnel on-board. Following the acquisition, the seismic data will be further processed and interpreted also within Nigeria. This will provide a much-improved understanding of the ongoing reservoir drainage performance in Bonga and facilitate efforts to maximize the oil recovery from the field. Also several Shell staff have utilized the opportunity to gain valuable deepwater 4D experience.
Commented Executive Vice President Shell Africa Ann Pickard, "We're proud of the contributions of Nigerians to this important seismic activity; it is quite challenging to conduct a 4D seismic survey in water depths of 1000m and this has been executed safely through understanding the risks, meticulous planning and then sticking to the plan."
The successful 4D seismic survey adds to a growing list of firsts for Bonga since it commenced production from one of the world’s largest Floating, Production, Storage and Offloading (FPSO) vessels named after the field. Bonga's initial 16 subsea oil production and water injection wells are connected to the two million barrel capacity FPSO by production flowlines, risers and control umbilicals. Notably, Bonga was the first time that iconel clad steel catenary risers had ever been used on an FPSO anywhere in the world. Produced hydrocarbons flow through these risers from reservoirs beneath the seabed to the FPSO.
Nigerian contractors, expertise and workers continue to play key roles in the Bonga story such as subsea engineering support as well as operations and maintenance. The FPSO's mooring buoy was fabricated by Nigerdock, a Nigerian company in Lagos. Measuring 23 meters in diameter and 12 meters in height and weighing approximately 870 tonnes, Bonga's buoy was, at the time of completion, the largest most sophisticated facility of its kind in the world.
The expectation is that this invaluable experience will expand to serve the deepwater industry for many years to come.
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