Icy peril lurks on the ocean's bed

Hydrates, an ice-like deposit formed by natural gas, has been named as a cause of the sinking of the BP Deepwater Horizon rig. But it may also be a future source of fossil fuels.

Hydrates, an ice-like deposit formed by natural gas under extreme pressure, has been named as a cause of the explosion and sinking of the BP Deepwater Horizon rig two weeks ago. But it may also be an important future source of fossil fuels, writes Tamsin Carlisle A type of ice-like gas deposit is emerging as a major safety concern for deepwater drilling as it hampers BP's struggle to shut down an oil leak from a stricken well in the Gulf of Mexico.

Natural gas hydrates, which occur mostly in sediments at the bottom of the ocean, have been implicated in the April 20 well blowout and explosion that sank the Deepwater Horizon drilling platform. There are two main dangers: heat destabilises hydrates, sometimes with explosive results; and the high pressure found deep underwater is likely to lead to ruptured pipes and valve malfunctions, allowing plugs of hydrates to clog up the works.

"There was concern at this location for gas hydrates," says Robert Bea, an offshore risk assessment expert and professor at the University of California, Berkeley, who suspects hydrates were the root cause of the blowout at BP's Macondo well off the coast of Louisiana. "The deeper the water, the greater the pressure, which when high enough can keep hydrates stable below the sea floor." Hydrates clogging the pores in sediments below the ocean floor can form seals that trap conventional gas and oil in underlying rock layers over geological time. That can make the deepwater zones where hydrates form especially tempting targets for drilling, as oil companies move their search into deeper waters.

On Friday, another deepwater drilling platform sank, this time off the coast of Venezuela where an international consortium is developing the country's biggest gas discovery. The cause of that accident is not known. Prof Bea, who has extensive access to BP documents on the US incident, says drillers encountered an unexpected "kick" of gas pressure a month before the blowout, suggesting that hot fluids from deep underground had released gas from hydrate crystals in the well bore. The gas kick was powerful enough to temporarily suspend drilling as a precaution.

Hydrates later stymied BP's efforts to use a 100-tonne steel and concrete box to trap the estimated 5,000 barrels of oil a day that was leaking from the Macondo well. The problem was the formation of "jelly-like" hydrate crystals near the top of the container, which blocked the pipe through which the oil would have been pumped to the surface. Now BP is using a fleet of 12 mini robot submarines to try to thread a pipe directly into the wellhead under 1,500 metres of water. The juncture would need to be perfectly sealed to prevent water from entering the pipe and allowing more hydrate crystals to form as the oil is pumped to a tanker at the surface.

"It's never been done before in water that deep," Doug Suttles, the chief operating officer for BP, said on Friday. As a back-up, the company is preparing a smaller container known as a "top hat" for lowering over the wellhead. In theory, the smaller structure could be kept free of hydrates by injecting methanol to act as antifreeze. Halliburton, the oilfield services firm that was responsible for cementing the Macondo well to help convert it from exploration to oil production, last year gave a presentation highlighting the risks from hydrates for deepwater jobs.

"Gas flow may occur after a cement job in the deepwater environment that contains major hydrate zones," Halliburton said. "Destabilisation of hydrates after the cement job is confirmed by downhole cameras." But the sheer concentrations of gas trapped in hydrates, which leads to their explosive behaviour when the crystals fall apart, also means they could be an important future source of fossil fuel.

BP is investigating the possibility at a project in Alaska, where temperatures are low enough for hydrates to form onshore in the permafrost beneath the Arctic tundra. Japan and India, which both lack sufficient conventional oil and gas resources to fuel their large economies, are also assessing prospects for gas production from hydrates in their offshore waters, as are established northern gas exporters such as Russia and Canada.

The recent Gulf of Mexico tragedy may seem very far away from the Middle East, which still has the biggest concentration of the world's conventional oil and gas resources. Yet the planet's biggest gas hydrate deposits may lie deep in the Arabian Sea, according to recent joint Indian and US government field studies. @Email:[e-mail]