EFFORTS to plug the gas leaking from Total's Elgin rig in the North Sea could stretch over many months, experts warn, as the French oil company comes under renewed fire for its handling of the accident.

Total is pursuing two options aimed at preventing 200,000 cubic metres of methane from spewing out of the rig every day. One, known as "top kill", is to block the well by smothering it with heavy mud, and the other is to drill relief wells.

Although top kill could be achieved within a matter of weeks, according to Dr Simon Boxall, an oceanographer from the University of Southampton, there was only a "slim chance" it would work. He pointed out that it had failed at BP's 2010 blow-out in the Gulf of Mexico, and that the gas leaking from Elgin was at similarly high pressure.

It was more likely that Total would have to revert to digging relief wells in from the side to try and reduce the pressure, he argued. "But that's a huge task and will take from six months to a year," Boxall said.

There was an ever-present risk of an explosion, particularly in still weather conditions, while poor weather could cause serious delays. "It's a risky operation," he told the Sunday Herald. "It will be expensive and take a while."

It took 74 days to fix a similar leak at the Montara oil rig north of Western Australia in 2009. Remedial work triggered an explosion and fire in the last few days of the leak.

It took five attempts to fix the leak because it was "a little like threading a needle kilometres underground", said Dr Gilly Llewellyn, the conservation director of the environmental group WWF in Australia. "This will not be over any time soon."

The Elgin leak started on March 25, and is said to be costing Total over £1.5 million a day in lost production. Some 238 workers were evacuated from the rig, and large air and sea exclusion zones are imposed around it.

It took three days for Total to issue a statement about the risks of an explosion from the flare that burned for a week after the leak. Since then the company has faced criticism from trades unions, experts and environmentalists for lacking transparency and cutting corners.

Professor Andrew Watterson, an occupational and environmental health expert from the University of Stirling, argued that Total put "optimising productivity" before safety and environmental performance. "Environmental and worker safety would do much better if Total prioritised the environment and safety above productivity," he said.

"Total profess to use sophisticated technologies in oil and gas exploration. In the North Sea it would seem they are not working well on dealing with the basics that can affect safety and the environment so seriously."

On Thursday, Total staff returned to the rig for the first time by helicopter, with specialists from the Texas company, Wild Well Control. Over four hours, they conducted a preliminary survey in preparation for attempts to plug the leak.

On Friday, Total said the survey had confirmed that its plans could proceed. Gas was not polluting key parts of the rig, and observations "seem to suggest that the gas leak rate has decreased during the last few days".

The Scottish Environment Secretary, Richard Lochhead, also met with Total's managing director, Philippe Guys, in Aberdeen to be briefed on progress. The environmental impact of the leak was "minimal", the minister said.

Nevertheless the marine research vessel, Alba na Mara, is this weekend sampling fish, sediment and water from the affected area. The results of its analysis will be completed in the next few days.

Environmental groups dispute that the impact is minimal. "Such assurances don't ring true when we know that there's 100 or more tonnes of methane escaping every day," said Dr Richard Dixon from WWF Scotland. "Methane is a very potent climate change gas."