By Oleg Vukmanovic and Gwladys Fouche
LONDON/OSLO, March 27 (Reuters) - A cloud of explosive natural gas boiling out of the North Sea from a leak at Total's abandoned Elgin platform forced wider evacuations off the Scottish coast on Tuesday as the French firm warned it may take six months to halt the flow.
Dubbed "the well from hell" by a Norwegian environmentalist who said the high pressure of the undersea reservoirs in the field made it especially hard to shut off, a plume of gas was visible over the platform, officials said, and a sheen of oil, also produced from the rig, was spreading over the water.
Officials imposed an air and sea exclusion zone around the platform, which had been pumping 9 million cubic metres of gas per day or three percent of Britain's natural gas output and lies some 150 miles (240 km) east of the city of Aberdeen.
A senior Total manager said the firm was looking at two main options - drilling a relief well, which could take six months, or the faster - potentially riskier - alternative of sending in engineers to "kill" the leak affecting a platform that also accounts for some 5.5 percent of Britain's total oil production.
But, Total manager David Hainsworth added: "The well itself could die on its own. This is the dream option."
Otherwise, "There are two options for intervening," said Hainsworth, who is health, safety and environment manager at Total Exploration and Production UK Ltd. "One is drilling a relief well which could take about six months. The other is a platform intervention to kill the well," he told Reuters.
"This would be a faster option," he added, saying a decision on how to tackle the problem would be taken in the coming days.
"We are exploring all the options and we are looking at what-if scenarios."
As well as flying in 10 to 20 specialist engineers, Total has enlisted the services of Wild Well Control, which was heavily involved in efforts to cap BP's Deepwater Horizon oil spill in the Gulf of Mexico in 2010, the worst in U.S. history.
Total's shares dived about five percent in an otherwise flat market on Tuesday as it scrambled to present a strategy to deal with the production, safety and environmental issues arising from the rupture of what it called a non-producing gas reservoir where engineers had done some work on a plug a few weeks ago.
As well as gas, Total said Elgin also produces 60,000 barrels per day (bpd) of light crude oil, exported via the BP-operated Forties pipeline system. Britain's total output last month was 1.09 million bpd.
Britain's energy minister Charles Hendry played down the extent of leaking oil, which is a light form known as condensate, spreading over the surface: "Some tonnes of condensate have escaped," he told Reuters. "The size of the sheen is one-sixteenth of the size of an Olympic swimming pool."
"Any leak we take very seriously and I think the right measures have been taken What we've identified, procedures appear to have been followed properly."
Shipping was ordered to come no closer than two miles from the Elgin platform and aircraft no nearer than three miles if they flew lower than 4,000 feet - effectively shutting out helicopters but not affecting airline traffic.
Shell cut staffing at two of its nearby facilities, the Shearwater production platform, which continued pumping oil, and the Noble Hans Deul exploration rig, which suspended drilling.
Total also said that it had suspended drilling development work at its West Franklin site.
Elgin's gas flows through the SEAL pipeline to the Bacton terminal. British gas prices for this week extended gains, rising close to four percent to 57.7 pence per therm.
Hainsworth said that some weeks ago Total engineers had decided to pump in mud to piping on a gas reservoir that had been plugged about a year ago. This recent operation appeared to have been resulted in the escape of gas: "We believe the leak is coming out of the outer casing of the well," Hainsworth said.
"WELL FROM HELL"
British officials said the gas, containing poisonous hydrogen sulphide - familiar from the smell of rotten eggs - should disperse in the atmosphere. But it poses a risk to anyone close to the source, making capping the well complex.
Poison in the gas could also threaten fish and other marine life nearby, although the rate at which it dissipates in air and water meant it was not a significant threat to people on land.
Environmental campaigners have been critical.
"This is the well from hell," said the activist, Frederic Hauge, head of Bellona, a leading Norwegian group that closely monitors the oil industry. "This problem is out of control."
Platform staff had struggled for 14 hours to contain the leak before having to evacuate early on Monday, said Hauge citing anonymous sources involved in the incident whom he said Bellona had spoken to. "They saw the sea bubbling with gas under the platform," he said. "This is quite shocking.
"This situation is only going to get bigger and bigger."
Total removed all workers aboard the Elgin rig on Sunday, shutting down oil and gas production and reporting no injuries.
Care had been taken to avoid any ignition of the gas.
The leaking Elgin reservoir lies above the main production reservoir, Hainsworth said. The latter lies some 6,000 metres, or nearly four miles, beneath the seabed.
Shell described as a "prudent precautionary measure" the removal of some workers from its two platforms nearby.