rock

BUBBLES of methane gas rising from the seafloor off the coast of Oregon were trapped underneath dense layers of mud to form the hydrate sample shown here.

Given the vastness of the world's marine methane hydrate deposits--more than twice the carbon reserves of all other fossil fuels combined--it's not surprising that government agencies and the petroleum and natural gas industries have long been interested in harvesting this new energy supply. Reearch and development programs already exist in a number of countries, particularly Japan. But tapping into this giant energy storehouse at a reasonable cost presents enormous difficulties.

Not the least of the challenges is that marine hydrate deposits are located in ocean mud up to a kilometer below the seafloor. In addition, hydrates decompose rapidly if removed from the high pressures and low temperatures of the deep sea. Even if engineers could construct a system to bring a load of hydrate to the surface before it disappeared, extracting the methane from the matrix of mud and rock would still present a problem.

Free methane gas trapped under the hydrate layer is no easier to tap. Unlike the conditions in conventional natural gas deposits, the pressure of the overlaying water and rock is too low to expel methane at a rate sufficient to make extraction worthwhile.

Methane hydrate is not completely out of reach, however. By harnessing methods similar to those used to recover dense, viscous petroleum, engineers could pump steam or hot water down a drill hole to melt the hydrate and release more methane to escape. They could then pump the escaping methane to the surface of the seafloor through another drill hole. Ultimately, the methane would have to be brought ashore, but submarine pipelines are expensive, and on a continental slope avalanches would threaten their rigging. Mining the very hydrate that had helped stabilize the slope would exacerbate this risk.

The extent of such difficulties is reflected in the boldness of some of the mining approaches that experts in the field are discussing. For example, Timothy Collett of the U.S. Geological Survey in Denver proposed to save the cost of pipelines by liquefying the gas on ships or drilling platforms. In Collet's setup the methane would be partly burned to form hydrogen and carbon monoxide. A catalyst would then convert the mixture into a liquid hydrocarbon, which could be readily transported by ship. The downside: a 35 percent loss of energy.

In contrast, Roger Sassen of Texas A&M University envisions a production fffacility on the ocean floor, where the emerging methane would be combined with water to form hydrate uncontaminated by mud and rock. Submarines would then tow the hydrate in zeppelin-shaped storage tanks to shallower destinations where engineers could safely decompose it into water and fuel.

"We should see gas hydrate becoming a meaningful and environmentally friendly resource in the next century," Sassen says. Indeed, as the world's other energy reserves diminish, mining companies may find themselves compelled to invest in technologies for exploiting the world's last great reserve of carbon-based fuel.


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