Gas Hydrates

When was the last time a bacterium blew a hole in the sea floor, sank a sailboat or caused a tsunami? In all likelihood, it may not have been very long ago. Billions of bacteria live in every cubic inch of the sediment covering the oceans’ floors. These bacteria produce a wide array of waste products, including natural gas and oil. The fluids can blowout from the sea floor, leaving behind craters and bubbles streaming through the sea toward the surface. These blowouts are a very real danger to drilling rigs and their support ships. There could even be rare occasions when one of these blowouts might affect a voyaging sailboat.

It isn’t just ships and boats that are in danger. Marine geologists are beginning to recognize that natural blowouts have caused enormous sub-marine landslides in the recent past, tsunamis big enough to wash over islands and may have contributed to the largest mass extinction of life since the demise of the dinosaurs. Luckily, the news is not all bad – the same frozen combination of natural gas and water that is the source of much of this danger may contain more than a thousand times the energy of known oil and gas reserves. All due to a few bacteria.

Before investigating these bacteria, however, we need to examine the slow, continuous rain of clay, silt and organic material that falls onto the sea floor. Slow is an important word here – a deposition rate of an inch per century is considered fast. Once on the bottom, the organic material and all the free oxygen is consumed by bacteria. A few yards into sea floor sediments, the oxygen is gone, and single-celled organisms called Archea digest the rest of the organic matter. (Archea look and act much like bacteria, but are from a different, and only recently discovered, branch of life.) One of the byproducts of this activity is hydrogen sulfide, the rotten egg smell many people have discovered by digging deeply into the sand at low tide. In sediments where temperatures and pressures are just right, these anaerobic Archea continue to digest the organic material to depths of one to two miles below the sea floor. At these depths, the waste products are not just hydrogen sulfide, but methane as well. Methane is better known to most consumers as natural gas. The vast majority of natural gas used in the world is the waste product of these single-celled Archea.

Enormous reservoirs of natural gas

Once produced, the methane rarely stays put. Rock and sediment are not completely solid. They have connected pore spaces that give them a sponge-like texture. The gas flows with glacial slowness through this sponge of pores toward the sea floor. Migration of a mile or two can take millions of years. Even then, much of the gas never makes it to the surface. Instead it is trapped beneath impermeable layers of rock, much like aquifers form on land. These trapped gas deposits can grow to enormous proportions and the deposits are drilled to provide the world with natural gas.

Exploration companies are expert at finding and tapping these pockets of gas and oil. A single production rig in the North Sea or Gulf of Mexico might have a dozen different drill stems into the rock beneath it, some of which reach sideways for miles under the sea floor to tap small pockets of gas!

Here we find the first danger associated with natural gas – blowouts. If the gas is under high pressure when its trap is punctured, it can blowout the drill stem, erupting with an almost vengeful force, blowing the sand and mud from the bottom, ejecting the drill stem from the hole, even engulfing a drill rig in a roiling mass of gassy water. Suffocation becomes a real worry. So does sinking – the water/gas mixture has such a low density that ships can lose buoyancy and sink. Captain Keith McLaren, a British offshore supply vessel skipper, has watched three blowouts in his years on service ships:

“The most dramatic was out from Aberdeen in 1990 when a Sedco rig hit methane gas and was forced off their location by the tremendous upheaval. They winched themselves upwind as far as they could. When we arrived, the most violent area of the ‘boil’ had created a head on the surface of the water about one meter high and was approximately 40 to 50 meters in diameter with a further severely aerated area of several hundred meters. It seemed obvious to anyone that to enter such an area would be foolhardy as there would surely be an immediate loss of buoyancy as well as severe and treacherous turbulence, not to mention the risk of flame-out caused by a spark from a hot stack. “After a day the blow-out ended and a research vessel came out and sent down an ROV [remotely operated vehicle]. The crater on the seabed was about 30 to 40 meters deep and the same in diameter. We moved the rig and read in the Notice to Mariners later that the blowout location had been cited as leaking gas and for mariners to beware.”

Vast undersea craters

Craters, like the one described by McLaren, are common on the sea floor and many are natural events. (A wonderful book, Seabed Pockmarks and Seepages, by Martin Hovland and A.G. Judd, documents them well.) These craters can range from small pockmarks a few feet across, to significant craters, even small valleys. Most, but not all, are found in areas with significant natural gas concentrations. A sea-floor crater in almost 7,000 feet of water in the Gulf of Mexico measures a staggering 900 by 1,300 feet and more than 150 feet deep, nearly the size and depth of two Astrodomes standing side by side. The crater is recent, and may have formed within the last 100 years. Only a sudden, violent release of gas could have made the crater and its surrounding apron of debris. So what could have stoppered the gas for so long? Enter methane hydrates.

A hydrate is an icy jail, a form of water ice with a twist. Water can freeze into a cage-like structure that traps other molecules. This particular facet of water’s chemistry was discovered almost 200 years ago, but it wasn’t until the mid 1960s that a natural hydrate of water and methane was discovered. In the 1970s, scientists found methane hydrates in sea floor sediments along the continental shelves, the relatively shallow-water skirts of the continents.

How methane hydrate forms (and breaks down) is still unclear. What is well known is their structure. Water naturally forms rings of five or six molecules, but under high pressure (above roughly 50 atmospheres, or water depths of about 1,500 feet) and temperatures below 60° F, these rings can join together to form three-dimensional cages. If methane, or another molecule of the right size is around, it becomes trapped inside one of the cages. The resulting compound is an icy-looking solid, stable to temperatures far exceeding the melting point of normal water ice. Hydrates look much like regular ice and have a similar density, but they trap enormous quantities of gas. One cubic centimeter of methane hydrate holds 160 cubic centimeters of free methane gas. So much gas is held in the structure that engineers have contemplated using hydrates to transport natural gas. Methane hydrate will burn under the proper conditions, an orange-red flame dancing above apparently ordinary ice.

Stable when under pressure

On the ocean floor, methane hydrates typically form within the sediments themselves, always in water deeper than 1,500 feet. The hydrates rarely form continuous masses. Instead they occur as thin layers or small blobs within the pore spaces of the rock. As long as they are cold and at high pressure, the methane hydrates form a perfectly stable and very strong part of the ocean floor. They are impermeable to gas, so they prevent any gas generated in underlying sediments from percolating up through the sediments. Temperatures tend to increase with depth in the sediments, so the hydrate stability zone is rarely more than 1,500 to 3,000 feet thick. Below the stability zone, methane gas accumulates within the pore spaces; this layer will rarely be more than 1,500 feet thick, as well.

Clearly a lot of gas is involved in these hydrates. The total estimates vary widely, but a conservative estimate is that the total amount of methane in and trapped beneath methane hydrates on the ocean floor may exceed all known reserves of natural gas by 10,000 times. This is a staggering amount of gas. So much, that one British expert actually called gas hydrates the “end of the energy crisis.” A Japanese drilling ship is currently attempting to recover some of this gas, and at least one American company has begun planning such operations.

However, there is a danger. When hydrates destabilize, the gas they release expands rapidly to 160 times the hydrate’s volume. If a sufficiently large piece of hydrate “melts,” a catastrophic and rapid release of methane gas would follow. Another danger lies in the balloon-through-the-keyhole effect. The free methane below the hydrate layer is light, and constantly presses against the hydrate layer. As long as the hydrate is relatively strong and thick, it holds the gas in check, but if enough gas builds up, even small openings in the hydrate begin to leak. Dr. Daniel Orange, a research scientist at U.C. Santa Cruz and Head of the Marine Geosciences Division of AOA Geophysics, described it this way. “It’s rather like trying to push an inflated balloon through a keyhole. At first, the tension on the balloon prevents it from going into the keyhole. As more and more of the balloon is forced through the opening the going gets easier until, at the midway point, the balloon suddenly shoots out the hole.”

The resulting natural blowout has all the force of the one described by Captain McLaren. This could prove dangerous to more than just the ship mining the deposit. Because the hydrate is mechanically strong, it forms an important part of the structural integrity of the sea floor. If the slope of the sea floor is steep enough, the sudden weakening can cause a submarine landslide. This, in turn, could cause further destabilization of hydrates and lead to further landslides.

Large-scale collapses While this may sound fanciful, even alarmist, there is evidence that large-scale collapses and landslides have occurred in the recent (geological) past along the eastern seaboard of North America. A recent research cruise headed by Woods Hole Oceanographic Institute researchers (and supported by the National Science Foundation) found evidence for 18,000-year-old landslides (the scarps are 60 miles wide!) and more recent pockmarks more than three miles long. The largest landslide in the world lies under the water in offshore Norway. The Storegga slide was so vast, the tsunami it generated may well have washed over Iceland, no small feat for a few bubbles of methane.

The methane gas is dangerous as well. Neglecting the nuisance aspect (its release would be accompanied by release of hydrogen sulfide and other noxious gases), methane is dangerous because it is a potent greenhouse gas. Once in the atmosphere, methane has the curious property of allowing sunlight to pass through it without difficulty. This light falls on Earth’s surface, where it radiates back into the atmosphere as infrared radiation. Alas, methane absorbs this infrared radiation from the earth, trapping the energy in the atmosphere and heating it like an extra blanket on a cold winter’s night. Carbon dioxide has this property, too, but methane is ten times as efficient as carbon dioxide in trapping the heat. Although methane escapes the atmosphere rapidly, a large release of methane could raise the earth’s temperature enough to change the climate, even cause extinctions.

Fifty-five million years ago, life on earth went through the greatest extinction since the dinosaurs. Evidence from sea floor sediments throughout the Atlantic point to one cause: a massive release of hydrate-bound methane. Massive here meaning methane equivalent to 1 percent of all the carbon in all the plants and animals on Earth. Researchers from Rutgers recently described sediment cores which contain evidence of both gigantic landslides and methane-induced changes in sea life – all right at the time of mass extinction. Following the methane release, other researchers have determined that water temperatures near the poles and at the sea floor increased 8° to 16° F! What drove this sudden release of methane is still not known, but either a small temperature increase or a sea level decrease was probably the culprit. A beneficiary of all this change were the primates, who became more diverse and widespread following the warming. We may well owe our existence to the unwitting aid of countless bacteria-like organisms happily digesting methane far below the sea floor.

One cannot help but note the controversy surrounding global warming and its long-term effects. Against the backdrop of recent warming (around 1° to 2° F in the last century) is an even larger question – could these small temperature changes provoke larger changes that might have broader effects? It was, after all, a small temperature change in the deep ocean waters that apparently caused the destabilization of methane hydrates 55 million years ago. Like the balloon through the keyhole, changes that take place after a long latent period can be impossible to stop.

Larry McKenna has a Ph.D. in geology, is president of Working Knowledge Inc. and an Ocean Navigator seminar instructor.

By Ocean Navigator