Royal Dutch Shell PLC’s Nigerian subsidiary completed 4D seismic acquisition over deepwater Bonga oil and gas field off Nigeria in an effort to maximize ultimate recovery from the field.
The effort, which the company described as the first 4D survey in Nigeria’s deepwater region, started with a baseline 3D seismic survey in 2000. That information guided the siting and drilling of early wells and other relevant activities.
Shell started production from the field in more than 1,000 m of water in late 2005 and was to quickly ramp up output to 225,000 b/d of 30° gravity oil and 150 MMcfd of gas (OGJ, Dec. 12, 2005, Newsletter). The field, now with 16 subsea oil producing and water injection wells, covers 60 sq km about 120 km offshore.
The most recent work, carried out by a Lagos contractor, took 76 days and involved one marine seismic vessel and three support vessels. The data, collected after 2 years of production, are expected after further processing and interpretation to reveal how reservoir conditions and fluid content have changed and result in better placement of subsequent wells, Shell said.
Marine 4D seismic data acquisition operations, especially those involving close passes of fixed installations, are technologically complex and involve several high-risk activities including operating air guns and towing long streamers with close passes to fixed installations, Shell said.
The four vessels with 100 people recorded 133,000 man-hr without a lost time injury, and 40% of the onboard personnel were Nigerians. Shell staff used the opportunity to gain valuable deepwater 4D experience.
Other firsts for Bonga field include one of the world’s largest floating production, storage, and offloading vessels at 2 million bbl capacity, the first iconel clad steel catenary risers on an FPSO, and the world largest and most sophisticated FPSO mooring buoy 23 m in diameter, 12 m high, and weighing 870 tonnes.
