How Frozen Fuel Works

Frozen fuel is the catchy name for a family of substances known as gas hydrates. The gas in question is natural gas, a mixture of hydrocarbons, such as methane, propane, butane and pentane. Of these, methane is by far the most common component and one of the most-studied compounds in chemistry.

Like all hydrocarbons, methane contains only two elements -- carbon and hydrogen. It is an example of a saturated hydrocarbon, or a molecule composed entirely of single bonds and therefore the maximum number of hydrogen atoms allowed. The general formula for saturated hydrocarbons is C_nH_2n+2. Methane only has one carbon atom, so its chemical formula is CH₄. Chemists describe this shape as a tetrahedron.

Methane is a colorless, odorless, combustible gas produced by bacterial decomposition of plant and animal matter. It forms in a process shared by all fossil fuels. First, marine plants and animals die and fall to the seafloor. Next, mud and other seafloor sediments cover the decomposing organisms. The sediments put a great deal of pressure on the organic matter and begin to compress it. This compression, combined with high temperatures, breaks down the carbon bonds in the organic matter, transforming it into oil and natural gas.

Generally, this methane -- what geologists describe as "conventional" methane -- is located beneath the Earth's surface. To get to it, workers must drill through rock and sediment and tap into the methane deposits to release the gas. Then they pump it to the surface, where it's transported through pipes across the country.

Methane can also form unconventionally if the sediments producing it are located about 1,640 feet (500 meters) below the ocean surface. The near-freezing temperatures and high pressure of these conditions causes the methane to become encased in ice. The methane doesn't bond chemically with the water. Instead, each tetrahedral methane molecule sits inside a crystalline shell made of ice. This unique substance is known as methane hydrate, and as soon as it reaches warmer temperatures and lower pressures, the ice melts away, leaving behind pure methane.

Geologists discovered naturally occurring methane hydrate only recently, but chemists have known about it for years, as we'll see in the next section.

The history of gas hydrates can be traced back to Humphrey Davy, a chemist from Cornwall, England, who identified chlorine as an element in 1810.

Davy and his assistant, Michael Faraday, continued to work with chlorine throughout the early 1800s, mixing the green gas with water and cooling the mixture to low temperatures.

It's very likely that Davy observed the strange solid that resulted as chlorine atoms became encased in ice crystals, but Faraday gets official credit for the discovery. In 1823, Faraday issued a report describing the strange substance and called it chlorine clathrate hydrate. Other types of clathrates, each involving a guest compound locked inside the lattice structure of a host, were soon discovered, but they remained a laboratory curiosity.

Then, in the 1930s, natural-gas miners began to complain of an icelike material clogging pipelines exposed to cold temperatures. Scientists determined that this material was not pure ice, but ice wrapped around methane. They wasted no time trying to find ways to prevent hydrates from forming and turned primarily to chemicals, such as methanol or monoethylene glycol. Since then, mining companies have added these materials to their natural-gas pipelines to inhibit hydrate formation.

In the 1960s, scientists discovered that methane hydrate, or "solid natural gas," existed in the Messoyakha gas field in western Siberia. This was significant because naturally occurring gas hydrates had never been found before. Geologists and chemists arrived in the vast basin and began to study the conditions in which the hydrates were forming. They found that sub-permafrost sediments were rich in hydrates and began to look for similar deposits in other high-latitude regions. Soon, another team of researchers found methane hydrate in sediments buried deep below the North Slope of Alaska.

Based on these early findings, the U.S. Geological Survey (USGS) and the Department of Energy National Energy Technology Laboratory conducted extensive research between 1982 and 1992, revealing that methane hydrate deposits could be found in offshore sediments as well. Suddenly, what had once been a curiosity and an industrial nuisance looked like it might be a significant resource. In the mid-1990s, Japan and India took the lead in methane hydrate research, with the goal of finding more deposits and developing ways to extract the trapped methane economically. Scientists have since discovered methane hydrate deposits in numerous locations, including the Mackenzie River delta in Canada and the Nankai Trough off the coast of Japan.

Up next, we'll consider the impact methane hydrate could have on the world's energy supply.

Once scientists began looking for methane hydrate deposits, they weren't disappointed. They found them beneath Arctic permafrost and  beneath the seafloor, especially in areas where one tectonic plate slides over another. These regions are known as subduction zones because the edge of one plate moves beneath another. For example, off the coast of Washington and Oregon, the Juan de Fuca plate is sliding underneath the North American plate. Like a piece of wood being drawn across the blade of a plane, the sediments, including hydrates, of the Juan de Fuca plate are removed by the rocky crust of the North American plate. This creates a ridge of hydrates that runs parallel to the coast.

Hydrate deposits have also been found in regions where large ocean currents meet. Blake Ridge is a formation located off the coast of South Carolina, in water ranging from 6,562 to 15,748 feet (2,000 to 4,800 meters) deep. Geologists believe the ridge formed during the Oligocene epoch, about 33.7 to 23.8 million years ago. The Greenland Sea opened up during this time, allowing huge amounts of cold, dense water to flow south along the Atlantic coast. As this cold water ran headlong into warm water being carried northward on the Gulf Stream, the currents slowed down and dropped large amounts of sediment. Organic material buried in these sediments eventually gave rise to a large amount of methane hydrate.

How much of this frozen fuel exists at Blake Ridge and other sites around the world? Some estimates put the amount of methane locked away in hydrates at anywhere from 100,000 trillion to 300,000,000 trillion cubic feet (2,832 trillion to 8,495,054 trillion cubic meters). Compare that to the 13,000 trillion cubic feet (368 trillion cubic meters) of conventional natural gas reserves remaining on the planet, and you can understand why jaws in the scientific community have dropped [source: Collett].

Of course, finding the hydrate deposits is one thing. As we'll see in the next section, getting them out -- and doing it safely -- is another thing entirely.

The potential rewards of releasing methane from gas hydrate fields must be balanced with the risks. And the risks are significant. Let's start first with challenges facing mining companies and their workers. Most methane hydrate deposits are located in seafloor sediments. That means drilling rigs must be able to reach down through more than 1,600 feet (500 meters) of water and then, because hydrates are generally located far underground, another several thousand feet before they can begin extraction. Hydrates also tend to form along the lower margins of continental slopes, where the seabed falls away from the relatively shallow shelf toward the abyss. The roughly sloping seafloor makes it difficult to run pipeline.

Even if you can situate a rig safely, methane hydrate is unstable once it's removed from the high pressures and low temperatures of the deep sea. Methane begins to escape even as it's being transported to the surface. Unless there's a way to prevent this leakage of natural gas, extraction won't be efficient. It will be a bit like hauling up well water using a pail riddled with holes.

Believe it or not, this leakage may be the least of the worries. Many geologists suspect that gas hydrates play an important role in stabilizing the seafloor. Drilling in these oceanic deposits could destabilize the seabed, causing vast swaths of sediment to slide for miles down the continental slope. Evidence suggests that such underwater landslides have occurred in the past (see sidebar), with devastating consequences. The movement of so much sediment would certainly trigger massive tsunamis similar to those seen in the Indian Ocean tsunami of December 2004.

But perhaps the biggest concern is how methane hydrate mining could affect global warming. Scientists already know that hydrate deposits naturally release small amounts of methane. The gas works itself skyward -- either bubbling up through permafrost or ocean water -- until it's released into the atmosphere. Once methane is in the atmosphere, it becomes a greenhouse gas even more efficient than carbon dioxide at trapping solar radiation. Some experts fear that drilling in hydrate deposits could cause catastrophic releases of methane that would greatly accelerate global warming.

Does that make methane from hydrate fields off-limits? This is the question scientists from all over the world are trying to answer.

In 1997, the U.S. Department of Energy (DOE) initiated a research program that would ultimately allow commercial production of methane from gas hydrate deposits by 2015. Three years later, Congress authorized funding through the Methane Hydrate Research and Development Act of 2000. The Interagency Coordination Committee (ICC), a coalition of six government agencies, has been advancing research on several fronts. Much of what we know about the basic science of methane hydrate -- how it forms, where it forms and what role it plays, both in seafloor stabilization and global warming -- has come from the ICC's research.

Interesting ideas about how to extract the methane from hydrates efficiently are also emerging. Some experts propose a technique in which miners pump hot water down a drill hole to melt the hydrate and release the trapped methane. As the methane escapes, it is pumped to the seafloor through a companion drill hole. From there, submarine pipelines carry the natural gas ashore. Unfortunately, such pipelines would need to travel over difficult underwater terrain. One solution is to build a production facility on the seafloor so it is situated near the hydrate deposits. As methane escapes from the heated sediments, workers in the plant would refreeze the gas to form "clean" methane hydrate. Submarines would then tow the frozen fuel in huge storage tanks to shallower waters, where the methane could be extracted and transported safely and efficiently.

Is all of this necessary? Won't renewable energy sources make it a waste of time to pursue another nonrenewable fossil fuel so vigorously? Realistically, fossil fuels will still be an important component of the world's overall energy mix for decades to come. According to the Energy Information Administration (EIA), total U.S. natural gas consumption is expected to increase from about 22 trillion cubic feet (0.622 trillion cubic meters) today to about 27 trillion cubic feet (0.76 trillion cubic meters) in 2030. Global natural gas consumption is expected to increase to 182 trillion cubic feet (5.15 trillion cubic meters) over the same period [source: EIA]. Tapping into the methane locked away in hydrates will obviously play a key role in meeting that demand.

That means the frozen fuel from methane hydrate can buy more time as scientists search for alternatives to power our planet. Think of it as an important stepping-stone in our transition to cleaner, greener energy sources.