Mining "Ice That Burns"

Newly discovered methane hydrate reserves deep in the ocean show promise for mining.

Trapped in molecular cages resembling ice, at the bottom of the ocean and in terrestrial permafrost all over the world, is a supply of natural gas that, by conservative estimates, is equivalent to twice the amount of energy contained in all other fossil fuels remaining in the earth's crust. The question has been whether or not this enormous reserve of energy, known as methane hydrates, existed in nature in a form that was worth pursuing, and whether or not the technology existed to harvest it.

Dig deep: A drilling rig on the North Slope of Alaska that helped the USGS estimate the amount of natural gas that could be recovered from the area. Credit: United States Geological Survey

Last Friday, the United States Geological Survey (USGS) announced the discovery of suitable conditions for mining methane hydrates 1,000 meters beneath the seabed in the Gulf of Mexico. Together with Chevron and the U.S. Department of Energy, the USGS discovered the reserve of hydrates in high concentrations in 15-to-30-meter-thick beds of sand--conditions very much like terrestrial methane hydrate reserves, which have already yielded commercially useful flow rates. These deposits are substantially different from the gas hydrates that have previously been discovered in U.S. coastal waters, which exist in relatively shallow waters at the surface of the seabed and have become a concern for climate scientists because of their potential to melt rapidly and release large quantities of methane into the atmosphere.

In the spring of 2008, a joint Canadian-Japanese expedition in Mallik in the Northwest Territories, Canada, established that methane hydrates could be harvested by using a water pump to depressurize a well already drilled into the reserve. This involved lowering the pressure by pumping out the water that naturally accumulates in the well. Crucially, it required only 10 to 15 percent of the energy represented by the gas that flowed out of the well, making it a much more viable approach than earlier methods used to harvest hydrates, which involved melting them with warm water. Standard oil and gas drilling equipment was used to reenter an old well drilled to a depth of 3,500 feet and then "refurbish" it by casing the entire well with lengths of steel tubing that cemented into place in order to prevent it from collapsing.

Hydrates require both cold temperatures and high pressure to form; eliminating either condition frees the gas from its icy cage, but past attempts to do this by heating the hydrates proved prohibitively difficult. The Canadian-Japanese expedition successfully produced up to 4,000 cubic meters of gas a day during a six-day trial in 2008 using depressurization.

"I think [the Gulf of Mexico find] and Mallik are two revolutionary events," says Timothy Collett, a geologist with the USGS and one of the world's foremost authorities on gas hydrates.

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While no one believes that all of the world's methane hydrates will be recoverable, the scale of global reserves has been described by the U.S. Department of Energy as "staggering." They occur anywhere that water, methane, low temperatures, and high pressure co-occur--in other words, in the 23 percent of the world's land area covered by permafrost and at the bottom of the ocean, particularly the continental shelf.

Increased interest in naturally occurring methane hydrates has been driven by the desire for energy independence from the Middle East and Russia and by the need to find energy sources with less of a potential impact on the climate than coal. (Natural gas produces half as much carbon as coal per unit of energy.) This is reflected by an exponential growth in the number of scientific papers published on the subject per year, according to Carolyn Koh, codirector of the Center for Hydrate Research at the Colorado School of Mines. More than a dozen expeditions designed to harvest or sample terrestrial and marine hydrate reserves have been launched since 2001, not only in the United States and Canada, but also in Japan, Korea, China, and India, according to Collett.