A confident approach to exploration and development has paved the way for the first of several barn-burning fields offshore Angola.

Block 17 offshore Angola seems to be a classic overachiever. Operator TotalFinaElf E&P Angola, with partners Esso Angola, BP, Statoil and Norsk Hydro, has chalked up 10 discoveries in the block. Of these, Girassol is the closest to coming online, with first production slated for the end of this year. It's also a textbook example of what can be achieved when a company takes its time, does its homework, applies its best technical people and trusts its instincts.
TotalFinaElf's predecessor Elf Aquitaine became interested in Block 17 in the early 1990s as part of an overall push to develop its presence in deep water. Elf already was participating in Angola's conventional offshore in blocks O (10%), 2 (27.5%) and 3 (operator 50%), as well as other projects offshore West Africa, so it had a fairly sound working knowledge of the region, said Antoine Quesnoit, a TotalFinaElf project manager on Block 17. "We had good familiarity with the zone and its potential," Quesnoit said.
Initially the company had access only to speculative 2-D seismic issued to all companies participating in the bid round. Quesnoit said the company's strong regional geologic knowledge and experience enhanced the interpretation of this data.
Blocks 16 and 17 were the first deepwater tracts to be offered to the industry, and in the bidding round Elf secured operatorship of Block 17 in September 1991. The production-sharing agreement was ratified a year later.
The discovery well was drilled in 1996 in water depths of 4,600 ft (1,400 m), and the decision to develop the field was based on only two wells. "Our geoscientists convinced our partners that they could take the risk," said Jean-Luc Vermeulen, president of exploration and production for TotalFinaElf. Added Quesnoit, "The confidence gained regarding the economics of the development permitted us to make the decision to sanction the project quickly."
In the initial stages of the exploration campaign on Block 17, TotalFinaElf undertook a further survey using 3-D seismic, and after project sanctioning it acquired and interpreted further high-resolution 3-D seismic throughout the entire development phase. Vermeulen said the quality of the data was such that it provided almost direct reference to sedimentology.
During this phase, TotalFinaElf used specially adapted geological tools and employed them in a three-phase process. The first phase uses the seismic data to produce a geological scheme from which a geologic model is created. This allows the team to arrive at a clearer estimation of the oil in place.
Then the static data is introduced into a dynamic simulation model, which is the basic tool of development optimization. "This phase assists in the selection of the location and the number of wells to be employed to drain the reservoir," Quesnoit said. "On the basis of this data we are in a position to launch screening studies of several potential development concepts for the field."
During this stage, several options are evaluated, he said, involving wet and dry tree schemes and their corresponding field architectures - for instance, a floating production, storage and offloading (FPSO) vessel or tension-leg platform. The team compares the proposed schemes on the basis of technological and economical issues as well as health, safety and environmental considerations. Based on these comparisons, the best-case plan is recommended as a model for development.
At Girassol the final plan was to connect an all-subsea scheme to an FPSO. Several design competitions were launched with contractors to optimize the design of the different parts of the project, particularly the umbilicals, flow lines and riser system. "The selection of the innovative riser-tower concept proposed by Stolt Offshore and Bouygues Offshore was a product of this process," Quesnoit said.
In June the Stolt/Bouygues alliance known as the Alto Mar Girassol Joint Venture installed the first riser tower at the field. The 4,270-ft (1,300-m) tower was transported from the fabrication yard by a subsurface tow. The bottom of the tower was lowered to the sea floor, bringing it to a vertical position. It then was secured to its anchored base. Within a month the installation of the remaining two riser towers had been completed.
Each riser tower will house six production and injection lines plus gas lift or service lines, and each is anchored to the seafloor by a suction pile and held in place by a 130-ft (40-m) steel buoyancy tank 165 ft (50 m) below the sea surface. They will be connected to the FPSO through flexible jumpers and to the flow lines by jumper spools.
The Girassol FPSO is the world's largest, with a 2 million-bbl storage capacity. It arrived at the field in July and was linked by flexible flow lines to the top of the riser towers. Each flow line consists of a 30-in. outer steel tube containing two 8-in. production lines enclosed in syntactic foam specially developed to insulate the flow lines from the high seabed pressure and low temperatures. During installation, each flow line was towed more than 130 miles (220 km) on the seabed to reach the location.
Onboard the FPSO, water-injection capacity will be 390,000 b/d to 400,000 b/d, and gas-injection facilities will operate at up to 280 MMcf/d, with compression at about 8 MMcf/d. No gas will be flared - all of it belongs to Sonangol, Angola's national oil company, under the contractual terms. Later a gas line will be installed to link to an onshore liquefied natural gas plant.
The development plan calls for 39 subsea production and injection wells tied back in bundles via the riser towers to the FPSO. Oil will be exported through an offloading buoy to visiting tankers. Of these wells, 23 will be oil-producers, 14 will be water-injectors, and two will be gas-injectors. Water injection will be applied throughout the life of the field to maintain pressure.
Individual wells are expected to produce at 20,000 b/d to 30,000 b/d, and a collective rate of 220,000 b/d is feasible. The plan is to attain plateau production of 200,000 b/d 6 months after start-up and maintain that level for at least 4 years.
A second Girassol development phase, to be completed by mid-2003, will determine future production rates. This phase will involve additional umbilicals and flow lines, and the remaining 15 of the 39 wells will be drilled.
Girassol is an ambitious project, but it's just one of many anticipated developments on Block 17. Already bids are out on the Dalia field. The design competition for the umbilicals, flow lines and risers and the FPSO basic engineering are nearing completion, with the invitation to tender expected to be issued during the latter half of this year for award in early 2002. Subsea production systems bids are under evaluation.
Dalia's FPSO will be able to produce 225,000 b/d of Miocene oil. The topsides composition and weight will be different than the Girassol FPSO because the oil is significantly heavier than Girassol's Oligocene oil.
Dalia's base case for development involves 68 subsea wells - 35 producers, 30 water-injectors and up to three gas-injectors. However, reservoir management is still open, and the optimum number of wells needed at first oil, scheduled for 2005, is under review.
Girassol and Dalia will become poles around which the remainder of the Block 17 discoveries will be developed. TotalFinaElf has completed conceptual engineering studies, and the selected scheme is to connect all Oligocene discoveries - Jasmim, Rosa, Cravo and Lirio - to the Girassol FPSO and all Miocene discoveries - Tulipa, Perpetua, Camelia and Orquidea - to the Dalia FPSO. The company is working to define the sequence, timing and methodology for connecting the remaining discoveries.