By Oleg Vukmanovic and Karolin Schaps
LONDON (Reuters) - Signs of trouble aboard a North Sea drilling platform where a natural gas leak has triggered fears of a massive explosion began in a plugged well a month ago, operator Total said on Friday.
The leak began five days ago and remains unchanged at the Elgin platform off Scotland's east coast, where all 238 workers were evacuated and a two-mile exclusion zone has been set up for safety reasons.
"On February 25 we started observing irregular pressure on the plug on the G4 well on the Elgin field," Total's UK Managing Director Philippe Guys told a news conference in Aberdeen.
He said the company responded within days to regulate the pressure in the well, which had been plugged in February 2011.
"On March 25 we observed a sudden pressure increase followed by an escape of mud and gas."
Drilling of a relief well could begin in 7-10 days should the company decide that is the best way to tackle the leak, Guys said.
The use of heavy mud to block is another possible approach, one which is faster but far more dangerous.
Relief drilling would take six months and require boring through 4 kilometers of rock with painstaking precision in order to intercept the gas pocket, one engineer said.
A team of international experts is advising Total on how to plug the leak but the company has not made a final decision.
The company earlier said it hoped the leak would run itself dry as reservoir pressure dropped, removing the need to bore a hole that could take six months and cost up to $3 billion.
Total believes a section of well pipe 4,000 meters below the seabed - subject to high pressure and extreme heat - had become porous, the UK energy department said.
FLARE STILL BURNING
Total is also still to decide how to extinguish a flare only 100 meters away from where the gas is leaking which was lit as part of the evacuation of the platform to relieve pressure in the troubled well.
Water could be dropped from a helicopter or nitrogen sprayed to starve the flame of oxygen, the UK energy department said.
Two fire-fighting vessels are stationed near the platform in case of a blast, with the flare posing the threat of igniting gas leaking out of the platform, the department said.
But it added aerial surveillance has suggested the flame has reduced in size, offering hope for a safe resolution to the crisis.
"Total has assured the government the platform is designed so the flare is located in a position where the prevailing wind blows the gas release away from it," the department said.
Total has dismissed the risk of a blast at the platform while one engineering consultant has warned Elgin could become "an explosion waiting to happen".
UK Energy Minister Charles Hendry said the potential permanent shutdown of the rig would be decided by the operator and regulators though identifying improvements to safety would be an integral outcome of this incident.
Union officials have called the frequency of safety lapses in the British sector of the North Sea's energy business intolerable.
The worst North Sea accident occurred 24 years ago with the Piper Alpha platform fire, when 167 people were killed in the world's deadliest offshore oil disaster.
The UK government said Total's response to the incident had been very effective and that the platform was evacuated within three and a half hours.
The platform sits in waters less than 100 meters deep and 240 km (150 miles) off the east coast of Scotland.
Total as well as UK authorities have described the expected environmental impact from the plume of gas and a spreading sheen of light oil on the water as "minimal", although environmental experts said the gas would be poisonous at close quarters.
An explosion aboard the Deepwater Horizon platform in the Gulf of Mexico which killed 11 workers and ruptured BP's Macondo well unleashed millions of barrels of oil. BP struck a deal estimated at $7.8 billion with businesses and individuals suing over the spill.
To help Total in its response the UK government said it had convened a crisis committee of energy ministry, healthy and safety, marine and coastguard officials.
The firm's shares have fallen around 7 percent since the incident, wiping off around 7 billion euros ($10.62 billion) off its market value.
Shell's nearby Shearwater platform, also a high-pressure, high-temperature well like Total's Elgin, was shut down on Tuesday for "precautionary measures".
($1 = 0.7532 euros)
(Additional reporting by Henning Gloystein; editing by James Jukwey and Jason Neely)