LONDON/PARIS (Reuters) - A corrosive drilling fluid that triggered the North Sea's worst gas leak in 20 years could threaten similar deep-sea wells across the world, and operator Total has already warned Shell that its nearby Shearwater field may be at risk.
The corrosive fluids implicated in the leak at Total's Elgin field, such as calcium bromide, are commonly used in such deep-sea wells, and experts fear a recurrence as operators, under pressure to offset declining output from conventional reservoirs, turn to deeper, hotter and higher pressure fields.
"Bromide brines have been used in thousands of wells since their introduction in the 1980's," John Downs, a chemical engineer who runs his own consultancy group, told Reuters. "An extensive well repair program may be needed if the stress corrosion cracking caused by bromide brine in Elgin is also happening elsewhere."
So worried is Total about recurrence in the Elgin field itself, it has plans to kill at least 10 other wells in the complex as well as the one that leaked, which could cost more than 1.5 billion pounds ($2.34 billion) to replace.
The North Sea is host to the highest number of high-pressure, high-temperature (HPHT) reservoirs of any mature oil and gas producing basin.
Total UK chairman Patrice de Vivies told Reuters he had cooperated particularly closely with Royal Dutch/Shell when sharing information on the causes of the leak.
"With Shell we have shared even more as they have a neighboring field, Shearwater, meaning they potentially have, perhaps, not identical, but similar problems," de Vivies said.
Like Elgin, Shell's Shearwater field is fed by a HPHT reservoir where temperatures can reach 140 degrees Celsius.
Shell declined to comment.
At peak output, Elgin and Shearwater account for more than a tenth of British gas production. Elgin restarted this year.
Any signs of another Elgin may warrant wider investigations into the effects of fluids used in HPHT wells, potentially leading to the killing of more wells and upsetting plans to squeeze more from the North Sea.
The recent revival in the North Sea's fortunes hinges on investment in HPHT fields. They are also common off the coast of Brazil and in the Gulf of Mexico. The industry has had to adapt the materials and fluids it uses to cope with the treacherous conditions.
Total's own investigation into the causes of the leak remain incomplete, though the field resumed output in March, with the blessing of Britain's Health and Safety Executive (HSE).
"We hope that the laboratory will be in a position by the end of the year to reproduce the phenomenon," de Vivies said.
Drilling engineers, equipment manufacturers and chemical experts say the long-term consequences of exposing well casings to bromide-based fluids are poorly understood, and some corrosive bromide fluids have already been banned.
"It's an operational well, and there are hundreds of thousands of those out there, and any one of them at any time potentially could suffer from the sort of problem Elgin has apparently had, so we need to know about it as an industry," said Liane Smith, founder of well integrity specialist Intetech, recently acquired by Wood Group.
The G4 well in the Elgin field leaked for a month and a half, creating a huge cloud of flammable gas above the platform about 150 miles east of Aberdeen in Scotland.
The leak pushed up gas prices and cut UK supply by 7 percent. An air and sea exclusion zone was imposed as personnel were evacuated from the area. Had the gas cloud caught fire, the results would have been devastating.
Total has said the leak was caused by a corrosive reaction between calcium bromide used to complete the well and grease in the pipework, which under high pressure cracked the piping. The French firm described it as a "unique event".
Calcium bromide is not generally regarded as corrosive and is still widely used in the North Sea, but Reuters uncovered a number of historical cases in which chemically related fluids known as halides, such as zinc bromide and calcium chloride, have corroded pipework.
Halide fluids, known as brines, can react with oxygen, carbon dioxide or hydrogen to form a corrosive substance, particularly in deep wells such as Elgin where temperatures can reach 200 degrees Celsius.
"Bromide belongs to the same group as chloride, and so there is always the risk of halide cracking of susceptible steels. If the steels used are susceptible to chloride cracking, they will also be at risk of bromide cracking," said Paul Rostron, professor of corrosion chemistry at the Petroleum Institute in the United Arab Emirates.
In 1999 piping in a well on the HPHT Erskine field, operated at the time by Texaco and now by Chevron, cracked after calcium chloride reacted with air to corrode the steel.
Zinc bromide is also known to be increasingly corrosive at high temperatures and its use is banned in the North Sea because it is regarded as dangerous.
"Zinc bromide and other bromides are highly corrosive to any form of steel it comes into contact with," said consultant Downs.
Total has explained how gas got into the Elgin well, but the company has stayed silent on how it breached three further well walls after its initial point of entry.
Industry insiders want to see a wider discussion in petroleum journals and at conferences. If a scenario has been discovered in which calcium bromide becomes dangerously corrosive, the information must be shared, they said.
"They need to tell us what we need to keep out of our wells so that we don't stupidly go and do the same thing again," a corrosion engineer told Reuters. He declined to be named lest his comments affect his relationship with clients.
Total says it has shared the information with operators.
"Before we can publish scientific papers we want to reproduce what happened in a laboratory," Total's de Vivies said.
Total Chief Executive Christophe de Margerie said: "The final conclusion hasn't yet been released because it is not yet over, and there are talks to understand everything that happened and everything that could happen in the future."
Britain's HSE and Total have conducted separate investigations into what happened at Elgin.
The HSE investigation will soon be sent to authorities in Scotland where a decision will be taken on whether to prosecute Total over the incident. Until such time, the HSE investigation cannot be released, a spokesman from the watchdog said.
While the rest of the industry waits to find out what caused the leak, Total is taking pre-emptive action.
"In the light of the lessons drawn from the G4 accident, we have redefined the technical requirements our wells must meet, and the consequence is that a minimum of 10 wells will have to be killed," Yves-Louis Darricarrere, head of E&P at Total, told Reuters in March.
The process will take about three years to complete, and the replacement cost of each well is about 140 million pounds, said a source with direct knowledge of the abandonment procedure.
"Total took a very conservative approach to the remaining Elgin wells," said the source.
Additional reporting by Michel Rose in Paris; Editing by Andrew Callus and Will Waterman