West Africa is only one region where french producer Elf is working. Elf's Pierre Delaittre reeled off a list of deepwater areas where his company was building up experience, either as an operator or block participant. These included new very deepwater blocks in China's Pearl River area, the Timor Sea, and the Azeri Caspian. As well as operating 20 wells offshore West Africa, including Girassol, the region's deepest discovery to date at 1,400 meters, Elf is also a partner in most of Shell's deepwater West African wells. In effect, Elf is involved in half the region's exploration effort, as well as operating production from the deepest waters tackled so far, Congo's Nkossa in 170 meters water depth.
As part of the operator combine preparing for prospects such as Girassol, Elf had been working on a new floating production, drilling, storage, and offloading system, and a deep draft semisubmersible supporting dry xmas trees. It had also studied proven solutions in the Gulf of Mexico such as TLPs. But there were still difficulties in adapting these concepts to West Africa.
A typical North Sea TLP producing 180,000 b/d with 25,000-ton topsides would be designed for 106 Kton displacement, he said. In the Gulf of Mexico, that displacement could be reduced to 66 Kt, while in West Africa, where wave heights reach on average seven meters compared to 23 meters in the North Sea, displacement could be lowered farther to 45 Kt.
But new designs of mini-TLPs are now available that are half the size of a conventional semi. These can achieve the same production levels, yet with only one-tenth the displacement of an equivalent operating unit in the North Sea.
Another issue for Delaittre in West Africa was production risers. "In deepwater, we believe steel catenary risers have a future. Here we are working with Shell, the leaders in this technology. We believe Coflexip flexibles can also be used in deepwater, but the price is unattractive, so we have to work with metal risers."
Flow assurance is another 1,000m-plus concern for Elf in the Gulf of Guinea. With seabed temperatures of just 2-3degC, there are attendant problems of asphaltene build-up, wax and solids deposition in flowlines. One solution might be multiphase pumping or alternately deepwater artifical lift. This could be achieved by injecting energy into the riser base, obviating the need for costly well intervention to maintain the equipment.
He concluded that in 1970, a typical West African offshore development needed 10 wells to produce 10,000 b/d economically. Today the target was one to two deepwater wells for the same number of barrels. The solution, he said, lay in the use of intelligent wells and multilaterals, transferring hydrocarbons between the formation and the bottom hole as part of a drive to increase productivity.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.
