Major advances in extended reach drilling into shallow pays during the past year have enabled BP Exploration to revamp its plans to develop the offshore portion of Wytch Farm oil field on the south coast of England.
Instead of building an artificial island, BP will drill about 14 wells with a reach of as much as 5 km (3.105 miles) from existing and new well sites on the coast. The wells will recover about 80% of reserves that would have been reached from an artificial island.
The lesser volume will be more than offset by substantial cost savings. By using extended reach wells, cost of offshore development will be 80 million ($146.4 million), compared with 180 million ($329.4 million) for the artificial island project.
Onshore drilling also will allay environmental fears about the effects of creating a shallow water drilling island, named Hook Island by BP.
Production start-up from the offshore portion of the field will advance 1 year to 1994.
The key element in BP's change of plans was completion of an extended reach well in a shallow reservoir by Forest Oil Corp. in the Gulf of Mexico and Unocal Corp.'s extended reach well drilled last October from Platform Irene off California in Point Pedernales field. Forest drilled its OCS-G-5518A6 from a surface location in Block 325 to a bottomhole location in Block 326, both in the Eugene Island area off Louisiana.
THE FIELD
With 300 million bbl of reserves, Wytch Farm is Europe's biggest onshore oil field. Since start-up of new facilities to produce the lower Sherwood reservoir in June 1990, production has risen to a peak of 70,000 b/d. It also produces more than 10 MMcfd of gas and 500 metric tons/day of LPG.
The field is in an environmentally sensitive area in and near Poole Harbor. Sixty-eight wells have been drilled from Furzey Island in the harbor and sites along the edge of the inland water.
About 30 million bbl are in the Jurassic Bridport reservoir that lies at 3,000 ft entirely under Poole Harbor and adjoining land. The remaining 270 million bbl are in the underlying Triassic Sherwood reservoir at 5,200 ft, of which about 100 million bbl are in the part of the structure that extends south past the mouth of the harbor into Poole Bay.
At the start of 1990 BP presented six options to develop Wytch Farm offshore reserves. Consultations with local interests came out in favor of an artificial island. In addition to local approvals, this concept requires passage of an act by the British Parliament.
EXTENDED REACH
The most productive part of Wytch Farm's offshore extension lies close to shore. When the options were presented in 1990, only 4 km extended reach wells were feasible in shallow reservoirs. Such wells would have reduced oil recovery from the offshore portion to 50%.
BP is focusing much of its drilling research effort on extended reach wells. By the time the renewed development program gets under way, wells with a reach of 6 km could be possible in the field. This would enable recovery of about 90% of the reserves that could have been reached from Hook Island.
Mike Harding, Wytch Farm manager, said when options for developing the offshore reserves were assessed 2 years ago, BP assumed improvements in technology would not extend drilling reach enough to achieve adequate recovery from onshore sites.
He said, "But drilling successes in the North Sea and other parts of the world during the past year and the knowledge we have gained from a year of full production from the Sherwood reservoir have satisfied us that we can now achieve a reach of 5 km and recover the bulk of the offshore reserves. The additional research we plan could increase drilling distances and recoveries still further."
The significance of the two U.S. extended reach wells was the shallow reservoir targets. The mood in favor of drilling from shore had been improved by BP's experience of extended reach wells in Miller and Bruce fields in the U.K. North Sea, but those had targets below 11,000 ft.
In North Rankin field on Australia's Northwest Shelf, BP had an interest in a well with more than 5 km reach at 9,840 ft. The company also was influenced by information it acquired from its interest in the world record breaking 6.06 km reach well into a target at 8,850 ft drilled by Den norkse stats oljeselskap AS in Statfjord field in the Norwegian North Sea.
Most Wytch Farm extended reach wells will have a horizontal section, and it's possible that the detailed drilling schedule will include wells that are drilled below the reservoir level, then up into the pay section. All wells will have downhole pumps.
CHOOSING THE SITE
BP has several options for onshore drillsites.
The F site on Goathorn Peninsula has three spare well slots that could be used for offshore drilling. Those wells could be used in conjunction with a new 5 acre drillsite outside the Studland nature reserve.
And there is the possibility of extending the F site to accommodate all additional wells.
The F site is the biggest, most productive drillsite in the field with five beam pump wells producing from the Bridport reservoir and 12 wells, with submersible pumps, producing from the Sherwood. They produce about 25,000 b/d.
The F site also is in a wooded area, which provides environmental screening.
It has flow lines to the main gathering center.
BP said discussions will begin with Dorset County Council planning authorities, local landowners, residents, and environmental groups, to identify the best location for the new drillsite. This will lead to preparation of detailed development plans for submission to the U.K. Department of Energy.
Experience in producing the Sherwood reservoir enabled BP to reduce estimates of the number of wells required from Hook Island to 20-21 from 40.
Experience also showed there is no need for water injection wells in the periphery of the offshore extension. The existing water injectors drilled under Poole Habor can support onshore and offshore production.
Wytch Farm oil moves through a 61 mile pipeline to a marine terminal at Hamble on Southampton Water. Gas is linked into the British Gas plc grid, and LPG moves through a rail terminal originally built to handle crude production from the Bridport reservoir that started in 1979.
BP owns a 50% interest in Wytch Farm. Its partners are ARCO British Ltd. 17.5%, Premier Oil Dorset Ltd. 12.5%, Clyde Petroleum (Dorset) Ltd. and Purbeck Exploration Ltd. 7.5% each, and Goal Petroleum plc 5%.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
