Overlying salt causes casing collapse
Corrib North lies in 350 meters of water in the Slyne Trough (block 18/20), 70 km from Achill Island off the western Irish coast. Prior to the discovery in 1996, only three wells had been drilled in this north-south trending basin, two of which were dry. The third, by Enterprise, generated oil shows, but was in much shallower water.
According to UK analysts Wood Mackenzie, gas was then discovered within the Corrib North structure while targeting an oil play in the Middle Jurassic. This well, drilled to a true vertical depth (TVD) of 3,750 meters, encountered a promising gas column in the deeper Triassic Sherwood Sandstone formation. However, the overlying salt layer contributed to collapse of the casing, preventing a test from being run.
Following plugging of this well, 3D seismic was shot over the area in preparation for the appraisal well this year, spudded in June just north-east of the discovery and drilled to a TVD of 3,692 meters. This latest well yielded a stabilized flow rate of 63 MMcf/d through a 2-in. choke with a wellhead pressure of 1,306 psi.
Wood Mackenzie estimates recoverable gas at around 1.2 tcf, with several potentially analagous gas prospects in Enterprise's acreage and adjoining acreage operated by Statoil (a partner in Corrib North). The gas is sweet, with little condensate or carbon dioxide, and the reservoir is thought to be geologically simple with little faulting apparent. Deep-lying salt is the main problem in terms of interpretation. That factor has dampened original reserves expectations, but the field still looks large enough to encourage development in the near term, via subsea wells tied back to a wellhead platform, or possibly directly to the shore.
Studies for a direct 70 km link to the coast will focus on potential for liquids drop-out caused by rapid cooling of the gas in the undersea pipeline. This could lead to large-scale slugging and hydrate formation, says Wood Mackenzie, although the effects could be subdued by stable operation of the pipeline and injection of chemicals at the subsea wellheads, allied to advances in subsea separation techniques. At this stage, the analysts estimate a package costing IR£500 million will be needed for the field development, associated onshore plant and pipeline feeding into the main grid on the Irish east coast.
Onstream by 2002The project could be onstream by 2002, due to Ireland's rapidly increasing gas requirements - forecast to double by 2010 from the current 320 MMcf/d. Marathon's Kinsale Head/Ballycotton fields off southern Ireland are the only indigenous offshore producers, with Kinsale Head due to shut down in 2004.
Other supplies come through subsea pipelines between south-west Scotland and northern Ireland, and through the Irish Sea from England's west coast to a reception point north of Dublin. If Irish power generation needs do not grow as fast as predicted, some of Corrib's gas could be exported to the UK via the Irish Sea interconnector.
Protests can be expected from environmental groups, however, particularly following the findings of a new survey by the Atlantic Frontier Environmental Network. This group has commissioned a survey of 32,000 sq km to date of the North Atlantic seafloor, as E & P activity intensifies in the region.
This year's work on the biology, chemistry, and geology north of Shetland and west of Scotland's Western isle revealed an area of 50 sq km in the prospective northern Rockall Trough (90 miles north-west of Cape Wrath) characterized by a pattern of mounds up to 5 meters high and 100 meters in diameter, at depths of up to 1,100 meters. Sea floor photographs suggest that the mounds and associated "tails" represent a variety of marine life, including fish, sponges, and giant protozoans. There is evidence also of similar marine organisms in Irish waters.
Another survey, conducted by Greenpeace, found the Atlantic Frontier region to be rich in cetaceans, including rare species of whales and more common dolphins. Greenpeace, which has been campaigning against exploration in the Atlantic Margin, now wants the area set aside as a World Heritage site.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.