|
Liberty seismic targets drilling corridor
Alan Bailey Petroleum News
During the National Marine Fisheries Service’s Arctic Open Water Peer Review Meeting on April 16, Bill Streever, environmental studies leader for BP Exploration (Alaska) Inc., talked about BP’s planned 2008 seismic program in the Beaufort Sea. Streever was explaining to the meeting the measures that BP is taking to avoid disturbing wildlife during the seismic shoot.
The purpose of the seismic program is the investigation of subsurface rock formations along the route of proposed drilling for the Liberty oil field, Streever said. Only a small component of the seismic surveying will target the oil field itself, he said.
The development plan for Liberty entails the drilling of six ultra-extended reach development wells from the Endicott satellite island under the Beaufort Sea to the Liberty reservoir. The drilling would involve horizontal departures of 34,000 to 44,000 feet, requiring the use of a purpose-built rig that will be the largest land rig in the world, BP has said.
But successful drilling of those record-breaking extended-reach wells will require a detailed knowledge of the nature of the rocks in the intended drilling corridor, all the way along the several miles between Endicott and the Liberty reservoir. And seismic data will be critical to that knowledge.
“You’ve got a whole lot of rock between you and that reservoir and we’ve got to get the drill string through that rock. And right now we don’t have information about that rock,” Streever said.
Water-bottom cable BP is planning a 40- to 50-day seismic program in July and August, using a water-bottom cable technique in the shallow Beaufort Sea water that lies inside the offshore barrier islands. The plan is to complete the seismic work before subsistence whale hunting starts in late August, Streever said.
Unlike a deepwater offshore seismic survey that involves a large seismic vessel towing both air guns and long streamers of surface hydrophones, a bottom-water cable survey in shallow water uses cables with geophones strung out along the seafloor to detect the reflected sound from air guns towed by one or more small boats.
The Liberty survey will use two sound-source boats to individually tow air guns, while four bowpicker fishing boats will lay the cabling that carries the geophones. Another boat will house the equipment that records the signals from the geophones, while two other boats will provide general support for the operations, Streever said.
All but one of the boats will be trucked up the Haul Road to the North Slope before the start of seismic operation, he said.
The surveying will involve laying three cables at a time, parallel to each other across part of the survey area. The sound-source boats will traverse backward and forward across the cables, shooting the air guns at intervals along paths at right angles to the cable runs. By progressively moving the cables from one location to the next after each shooting sequence, the survey team will hopscotch its way across the survey area.
Critical areas first The project design envisages collecting data from the most geologically critical areas first, to ensure that the most important data are obtained, Streever said. However, the timing of the shoot at each location within the survey area will also be predicated on the need to avoid disturbing nesting birds, he explained.
And, to avoid the harassment of marine mammals, the seismic operations will include a standard protocol for powering down or suspending activities if an animal comes too close to an active air gun. However, although the operations may encounter some seals, walrus or beluga whales, mammals are likely to be less in evidence than outside the barrier islands, Streever said.
“It’s shallow water inside the barrier islands,” he said.
Streever said that some people have asked why BP is not conducting the seismic survey on sea ice during the winter, to minimize any interaction with marine wildlife. Unfortunately, winter sea ice in the survey area is solid to the seafloor near the coastline but floats over the shallow water further offshore, Streever said. That combination of ice conditions makes the acquisition of usable seismic data extremely difficult: BP decided that the ocean-bottom cable technique presents the best approach from both a technical and an environmental perspective, Streever said.
CGGVeritas will conduct the survey for BP.
| 