Total restarted production at its huge Elgin gas field in the North Sea on Saturday, almost a year after a major gas leak which cost the company millions of euros in lost output and even knocked UK economic growth.

Total said on Monday full details of what went wrong at the G4 well would not be released for the next few weeks as it is still awaiting results of technical and scientific analysis.

The company's G4 well in the Elgin field, which spewed gas for more than seven weeks after the leak, will return to production of 60,000 barrels of oil equivalent per day (boepd) by the end of the month, half its pre-leak level, two Total executives said.

"We can expect that after 11 months of outage, the restart will take place in a series of restarts and stoppages," Yves-Louis Darricarrere, Total's head of exploration and production, told Reuters in a telephone interview.

The field, located 240 km off the east coast of Scotland and operated by Total which has a 46.2% stake, is expected to return to its pre-accident production level in 2015 and reach between 130,000 and 140,000 boepd in 2016, a higher level than forecast before the accident, the executives said.

"The accident has not diminished the reserves, so what we didn't produce during the time interval (2012-2015) will mean more production for 2016, 2017 and 2018," Darricarrere said.

Elgin and the nearby Franklin field, which started production in 2001, accounted for nearly 3% of Britain's output and the shutdown registered on Britain's ailing economy, as falling oil and gas production knocked 0.2 percentage points off gross domestic product last year.

Total, which is one of Britain's biggest investors, also said more stringent safety rules at the field meant it would kill a minimum of 10 wells, out of a total of 19, over a three-year period, by injecting some 800 metres of cement per well.

The production increase will be achieved by the development of two big projects: phase two of West Franklin and a revamp of Elgin, expected to be finalised this summer, Darricarrere said.

"The Elgin Franklin field has produced 700 million barrels of oil equivalent and there is still 500 million to produce," the executive said.

'Humility lesson'

News of the gas leak in March last year revived memories of BP's Deepwater Horizon disaster and had an initially severe impact on Total shares, which fell almost a fifth in the two months after the initial leak.

The stock has since recovered to trade just €2 below the €41 seen before the accident, but is still valued at only 7.3 times 12-month forward earnings, a 20% discount to its peers globally and a 10% discount to BP and Shell .

Darricarrere said the cost of the closure for Total had reached 1 million pounds ($1.5 million) a day in lost revenue - suggesting a total loss in revenue between March 25, 2012, and March 11, 2013 of $526 million.

The company said almost all of its repair bill had been reimbursed by insurers.

Total insisted the accident, which forced the evacuation of 238 workers, was unique to the G4 well and that such an accident could not occur in the other Elgin Franklin wells, where bromide was not used in the same way.

Patrice de Vivies, chairman of Total Holdings UK, said the accident was a first in the history of the oil and gas industry, but declined to disclose the details of what made it such a special case.

He cited pipe corrosion due to a chemical reaction between bromide in the drilling fluid and grease in the pipework in a highly pressurised and heated environment.

But de Vivies acknowledged the equipment was commonly used in the sector and those circumstances alone were not sufficient to fully explain the accident.

De Vivies initially said the group needed the go-ahead of Britain's Health and Safety Executive (HSE) before it could give a full public explanation but the regulator later on Monday said it was up to Total to disclose information.

The HSE continues to investigate the Elgin incident.

"We have nothing to hide," De Vivies said in an e-mailed statement. "The causes of the accident have been identified. Research is still ongoing in several laboratories and we are awaiting their conclusions to communicate all the technical and scientific facts."

De Vivies said the accident had been a jolt for the fifth-largest western oil group, which had used the Elgin Franklin field, one of Britain's largest, to showcase its technological prowess.

"This is of course a humility lesson," De Vivies said. "We were using the best technology there and the best teams worldwide so it was a shock that it happened to us."