USGS finds giant gas hydrate deposits on North Slope

Buried beneath Alaska's North Slope are about 85.4 trillion cubic feet of technically recoverable natural gas hydrates, according to a new U.S. Geological Survey assessment. That would be a significant source of energy to add to the U.S. energy mix — enough natural gas to heat 100 million homes for 10 years, Secretary of the Interior Dirk Kempthorne announced Wednesday.

Natural gas hydrates are mixes of ice and methane gas in which the gas molecules are trapped within molecular structures, or "cages," of ice. They exist within a limited range of temperatures and pressures: beneath the permafrost of northern Alaska and the Canadian Arctic, for example, or under the seafloor in the deep ocean.

Natural gas hydrates are the largest known resource of natural gas in the world and have long been considered as a potential "unconventional" source of natural gas, USGS Director Mark Myers said at a press conference Wednesday. However, over the past 25 years USGS has worked with other federal agencies, including the Department of Energy and the Bureau of Land Management, as well as universities and companies like BP and ConocoPhillips, to understand hydrate geology and better assess the size and distribution of the hydrates on the North Slope.

As a result, the agencies assessed not only previously undiscovered reserves of the hydrates, but also estimated how much of them are technically recoverable — in other words, how much of the hydrates can be extracted using existing technology and infrastructure for more conventional oil and gas extraction.

Conventional oil and gas extraction already takes the possible existence of gas hydrates into account, Myers said. When pressures decrease or temperatures increase, gas hydrates can become unstable, and the methane is released. Unlike shallow gas hydrates located just under the seafloor, which can destabilize as the oceans warm, deeper pockets of gas hydrates aren't likely to be affected by climate change. However, drilling can destabilize them, so companies must take steps to ensure that they remain stable.

Lingering questions remain about how economical the resource might be, however, Myers said. Long-term well production tests, which may take six months to a year to complete, should help determine whether the modeled rates of natural gas production from the hydrates will match reality. And a natural gas pipeline in the North Slope will also be essential for further development. The current assessment also uses proven methods of extraction, which include depressurizing the hydrates. But other potential extraction methods could include increasing the temperature, or even replacing the methane extracted with carbon dioxide, which would then be sequestered under the seafloor, and might impact the economic viability of the hydrates as an energy resource.

Also, the assessment found that while most of the hydrates — 96 percent — are located in federally or state-held lands, or in already-leased areas of the North Slope, about 4 percent of the hydrates are in protected regions such as the Arctic National Wildlife Refuge.

The find has "huge potential and is truly significant," Kempthorne said. "Today we move the story of natural gas hydrates from the realm of science and speculation to the realm of actual and useful."

Map of gas hydrates on Alaska's North Slope
U.S. Geological Survey

 

Carolyn Gramling
Wednesday, November 12, 2008 - 10:00