Methane hydrate - băng cháy
WHAT do you get when you combine water and swamp gas under low temperatures and high pressures? You get a frozen latticelike substance called methane hydrate, huge amounts of which underlie our oceans and polar permafrost. This crystalline combination of a natural gas and water (known technically as a clathrate) looks remarkably like ice but burns if it meets a lit match.
Methane hydrate was discovered only a few decades ago, and little research has been done on it until recently. By some estimates, the energy locked up in methane hydrate deposits is more than twice the global reserves of all conventional gas, oil, and coal deposits combined. But no one has yet figured out how to pull out the gas inexpensively, and no one knows how much is actually recoverable. Because methane is also a greenhouse gas, release of even a small percentage of total deposits could have a serious effect on Earth's atmosphere.
Research on methane hydrate has increased in the last few years, particularly in countries such as Japan that have few native energy resources. As scientists around the world learn more about this material, new concerns surface. For example, ocean-based oil-drilling operations sometimes encounter methane hydrate deposits. As a drill spins through the hydrate, the process can cause it to dissociate. The freed gas may explode, causing the drilling crew to lose control of the well. Another concern is that unstable hydrate layers could give way beneath oil platforms or, on a larger scale, even cause tsunamis.
Lawrence Livermore's William Durham, a geophysicist, began studying methane hydrate several years ago with Laura Stern and Stephen Kirby of the U.S. Geological Survey in Menlo Park, California. With initial funding from NASA, they looked at the ices on the frigid moons of Saturn and other planets in the outer reaches of our solar system. One of these ices is methane hydrate.
Ice That Doesn't Melt
For their research, Durham, Stern, and Kirby needed good-quality samples of methane hydrate. But samples of the real thing are tough to acquire, requiring expensive drilling and elaborate schemes for core recovery and preservation. Previously developed methods for synthesizing the stuff in the laboratory generally resulted in an impure material still containing some water that had not reacted with the methane.
The Livermore-USGS team attempted an entirely new procedure. They mixed sieved granular water ice and cold, pressurized methane gas in a constant-volume reaction vessel and slowly heated it. Warming started at a temperature of 250 kelvin (K) (-10°ree;F) with a pressure of about 25 megapascals (MPa).* The reaction between methane and ice started near the normal melting point of ice at this pressure (271 K, or 29°ree;F) and continued until virtually all of the water ice had reacted with methane, forming methane hydrate.
The team studied the resulting material by x-ray diffraction and found pure methane hydrate with no more than trace amounts of water. This simple method produced precisely what they needed: low-porosity, cohesive samples with a uniformly fine grain size and random crystallographic grain orientation.
Says Durham, "In a way, we got lucky. We used the same technique we use for producing uniform water ice samples from `seed' ice. We tried adding pressurized methane gas and heating it. And it worked."
It worked, but some unexpected things happened along the way. The ice did not liquefy as it should have when its melting temperature was reached and surpassed. In fact, methane hydrate was formed over a period of 7 or 8 hours, with the temperatures inside the reaction vessel reaching 290 K (50°ree;F) before the last of the ice was consumed. Repeated experiments produced the same result: ice that did not melt (Figure 1).
A control experiment replaced the methane with neon, which does not form the cagelike latticework of gas and water molecules that is a gas hydrate. Under otherwise identical experimental conditions, the ice melted as it should. Other experiments replaced the methane with both gaseous and liquid carbon dioxide, which does form a hydrate. Here the superheating phenomenon reappeared, indicating that it is not unique to methane hydrate.
Durham and his team believe the superheating phenomenon is related to active hydrate formation. The reaction at the free ice surface somehow suppresses the formation of a runaway melt. Figure 1 shows that when the reaction ceases, melting happens immediately. The American Chemical Society was impressed enough with these rather bizarre results to give the team a cash prize and award in late 1997.
Once the team had large, pure samples they could work with, they began studying the material's physical properties and the way it forms and dissociates. This is research at its most basic. But its applications are clear when one considers that dissociation of seabed methane hydrate deposits could cost the lives of workers on an oil drilling platform.
Methane hydrate's stability curve (Figure 2) has been established for some time. If conditions fall outside that curve, the material will dissociate into its components, methane and water. Durham, Stern, and Kirby looked at how the dissociation occurs under a variety of temperature and pressure conditions outside the curve.
After the samples were created, the pressure was reduced to 0.1 MPa, the pressure at sea level. They did this in two ways: by slow cooling and depressurization and by rapid depressurization at a range of temperatures.
The compound decomposed to ice and gas as expected in all experiments except those that involved rapid depressurization at temperatures from 240 to 270 K (Figure 3). In these experiments, the team found yet another surprise. Even after the pressure drop, the methane hydrate was "preserved" as a compound for as long as 25 hours before it decomposed.
This behavior may have implications for future exploitation of the material. Preserving the mixed hydrates may be possible at an easily accessible temperature, just a few degrees below ice's melting temperature.
In another series of experiments, the team is looking at the strength of gas hydrate samples in various temperature and pressure scenarios. Results of these experiments may indicate the possible effects that stresses from gravity, tectonic activity, or human disturbance might have on gas hydrate deposits.
Thus far, the team has found that water ice and methane hydrate have about the same strength at very low temperatures of 180 K and below. But the hydrate is much stronger than ice at temperatures of 240 K and above. The most recent data indicate that methane hydrate is several times stronger than ice (Figure 4). Although methane hydrate is not as strong as rock, the data may be good news for the stability of the deposits.
More Work Ahead
Plenty of work remains to be done. The team plans to measure the molecular diffusion of gases through methane hydrate and to study special compounds that might suppress the formation of hydrates in cold pipelines. They also will do experiments to measure methane hydrate's thermal properties. Says Durham, "We already know that it is a very poor conductor of heat. If you hold a piece of it in your hand, it doesn't feel like ice at all. It almost feels like styrofoam."
A new heat exchanger installed in December at Livermore's ice physics laboratory allows Durham to heat samples from 180 to 260 K in about an hour, a process that used to take 24 hours. Durham notes, "Now we can do experiments much more quickly and thus can run a lot more experiments. Methane hydrate is a material with plenty of surprises, so there is no telling what we might discover next."
* 0 K is absolute zero. At 0.1 MPa (1 atmosphere), water freezes at 273 K and boils at 373 K.
For further information contact William B. Durham (925) 422-7046 (firstname.lastname@example.org).
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