Thursday, March 29, 2012
Factbox - Total's options for stemming North Sea gas leak
By Oleg Vukmanovic
LONDON (Reuters) - The massive gas leak from Total's Elgin platform in the North Sea could last longer than initially thought after the French company on Thursday said the gas originated thousands of metres below the seabed.
The depth of the pocket leaking gas to the Elgin platform via compromised layers of piping suggests that there is more gas present rather than less, increasing the pressure on Total to drill a relief well, an engineer with knowledge of the matter said.
The company previously stated that it hoped the leak would run itself dry as reservoir pressure drops, removing the need to bore a hole that could take six months and cost up to $3 billion.
But news that the source is just 1.5 kilometres away from the main reservoir, one of the world's deepest, throws up technical challenges and suggests that the quantities of gas feeding the leak may be larger than previously thought.
"The key question is: are these reservoirs connected?" the engineer said. "As the reservoir leaks it can also depressurize, allowing gas from neighbouring reservoirs to flow in and replenish the volumes ... this process could go on for weeks or longer."
An extended leak puts pressure on Total to begin relief well drilling.
Total has raised several options aimed at plugging the leak including a relief well, which is the safest but is also slow and costly, and pumping cement into the outer layers of pipe from the wellhead platform, which is quick but extremely dangerous.
Here follows a description of the technical challenges involved in each option:
PLATFORM "WELL KILL"
"This is the dangerous option," given the toxic and explosive plumes pumping out of the wellhead platform and posing a significant risk to life, the engineer said.
A team of engineers on the platform could ignite the gas cloud if their electrical equipment or metal tools generate any sparks.
A direct platform intervention, which would take less time than boring a separate hole, involves plugging the affected well with cement pumped by workers actually on the platform.
"The relief well option is a lot safer than a direct platform intervention, which puts engineers at risk of unintentionally igniting the gas cloud, as it allows engineers to work remotely from the gas source," the engineer said.
The average well takes three months to drill and is an exacting process as it requires puncturing through kilometres of hard rock, he said.
Relief drilling would require boring through 4 kilometres of rock with painstaking mathematical precision, because it must intercept the gas pocket at exactly the right point, requiring constant alterations in course, the engineer said.
The well would require around 15 to 20 kilometres of piping in total to reach the required depth of 4 km, because several layers of piping of varying width slotted together comprise the well.
"The first thing is a 30-36 inch diameter pipe cemented into the seabed, then another layer of 20-inch wide pipe could go down a kilometre within the first layer, then you cement that in," he said.
A typical well pipe includes several such layers of casing. Total has said that the outermost layer of casing has been compromised and is responsible for transmitting gas to its Elgin platform, despite successful work undertaken a year ago to plug the main part of the pipe.
"If the leak looks like it will take weeks, Total will have to do something like a relief well ... it is not just a case of appearing to do nothing but also they may have no idea of how long the leak will last," he said.
Like BP's response to its Deepwater Horizon oil spill disaster two years ago, Total may need the relief well as a back-up plan, he said.
BP never actually used the relief well that it drilled in the Gulf of Mexico, he said.
(editing by Jane Baird)
Copyright © 2013 Reuters