Clathrates and methane hydrates

Clathrates (also known as gas hydrates) are solid crystals that occur when water molecules form a dodecahedron lattice around gas molecules. A wide range of gas hydrates occur naturally, but methane hydrates are the most common. Increased scientific attention has been paid to methane clathrates, both as an important factor in climate change and as a potential energy source.

Basic Principles

Methane hydrates (also known as clathrates) are lattices of solid ice enveloping (but not bonding with) methane molecules. In conditions in which temperatures are low and pressure is high, water crystallizes in a twelve-sided, three-dimensional lattice (a cage-like structure) known as a dodecahedron. Hydrates are commonly found in areas in which both water and methane are available (such as the bottom of the ocean and in polar and glacial regions).

Hydrates are in and of themselves structurally unstable, but when they envelop a guest molecule (such as that of methane and other gases and nongases), they become more sound. Still, the stability of clathrates is highly dependent on the environmental conditions in which they are located. If they are exposed to lower levels of pressure and to higher temperatures, they quickly destabilize. If brought to room temperature, for example, a clathrate will, as it melts, pop and sizzle as the methane contained therein escapes. If a clathrate is exposed to an open flame, the ice of the hydrate will burn.

There are three basic types of gas hydrates, which are differentiated by their structures. For example, methane hydrates (along with carbon dioxide hydrates) are part of the structure I category, in which forty-six water molecules are organized into lattices that contain two small cavities and six large cavities. Structure II hydrates are larger, containing multiple and smaller gas molecules such as propane and isobutane. Structure H hydrates also contain multiple gas molecules, although those molecules may be larger. Both structure II and structure H hydrates are rare, however, while structure I clathrates are found with greater frequency and in larger volumes around the world.

History

In 1810, scientists Humphrey Davy and Michael Faraday, while experimenting with chlorine and water mixtures, noticed the presence of solid material forming when the liquids were just above freezing levels. For the remainder of the nineteenth century, clathrates were simply viewed as a scientific curiosity, as researchers attempted to understand the process by which an unstable water frame could form around, but not bond with, a guest molecule.

In the early twentieth century, however, prevailing attitudes toward clathrates evolved from curiosity to irritation. By the 1930s, modern civilization had developed the technology to tap into and extract natural gas and transport it through a system of pipes. E. G. Hammerschmidt, who at the time was working for the Texoma Natural Gas Company, noticed that in cold-weather environments, the company’s pipelines were frequently clogged by frozen ice. However, the ice seemed to freeze at higher temperatures, which led Hammerschmidt to conclude that the frozen blockages were clathrates. Shortly thereafter, scientists began to study clathrates more closely in an effort to find ways to prevent them from forming in natural gas pipelines.

In the decades that followed, it became apparent that clathrates were naturally occurring compounds that could form not only in pipelines but also in the permafrost of polar regions and on the ocean floor. Scientists from around the world became interested in locating deposits of the substance, which had been dubbed solid natural gas. The United States, the Soviet Union, India, and other countries all launched efforts to find and access what were believed to be large deposits of clathrates around the world. By the end of the twentieth century, those countries, along with other nations, coordinated their efforts to study methane clathrates with two general perspectives in mind: the role clathrates play in the earth’s natural processes and the potential that methane hydrates represent as an energy source for the future.

Distribution of Clathrates

Methane hydrates are found wherever conditions are right for their stability. They are found on the ocean floor, in regions in which temperatures are just above freezing and in which pressure is high and consistent. Clathrates also are found in permafrost, the layer of soil beneath the snow and ice in polar regions. In both types of environment, the methane hydrates are found between 150 and 2,000 meters (492 to 6,560 feet) below the surface.

The volume of methane hydrates throughout the world is significant. The total amount of clathrates in the world far surpasses the volume of conventional natural gas. According to the most recent U.S. Geological Survey review of the world’s clathrate inventory, approximately 10,000 gigatons (a gigaton is the equivalent of one billion tons) of methane hydrates exist in 44 regions in which samples have been uncovered and in 113 other regions in which evidence suggests the presence of such clathrates.

A Potential Alternative Fuel Source

Scientists have become intrigued by the existence of methane hydrates in such large volume. It is therefore understandable that governments and private industry have developed an interest in pursuing clathrates as an alternative source of energy.

The challenge is extracting clathrates in a manner that is both safe and cost-effective. Governments, observing that the energy contained in methane hydrates exceeds the total volume of oil, conventional natural gas, and coal available in the world, have launched individual and intergovernmental efforts to launch the clathrate market. Japan, for example, has developed plans to tap into clathrates as an energy source by 2016. The United States has for decades invested research dollars toward this pursuit, and a growing number of other nations are developing clathrate energy programs, too.

The dangers involved in “harvesting” methane hydrates at their remote sources are both immediate and long-term. For example, drilling for clathrates on the ocean floor can destabilize the surface, creating safety risks for workers and potentially disrupting nearby fossil-fuel drilling operations. Furthermore, scientists must find ways to store and transfer methane from these clathrates once they have been harvested. When it is released naturally, methane can break down and release its carbon components (namely, carbon dioxide, a greenhouse gas) into the atmosphere. Therefore, using computer modeling and laboratory experiments, scientists are investigating ways to utilize the energy benefits of the methane clathrates while at the same time isolating carbon (a process known as carbon sequestration). This particular aspect of research on clathrates and methane hydrates remains a major challenge and, at the same time, an important key toward the successful utilization of clathrates as a fuel source.

Methane Hydrates in the

Clathrates have undergone an evolution in terms of scientific pursuits. The concept was at first a curiosity and thereafter developed into a problem to be solved. Today, methane hydrates are considered extremely important, warranting significant investment of money and research from both the private and public sectors.

Methane hydrates are also seen as key to the carbon cycle. The carbon cycle is a concept whereby one of the most critical elements for life on Earth is shared among the planet’s many different forms of life and systems. Within this framework, carbon dioxide is absorbed by plants through the air and the soil. The plants are eaten by animals, which in turn are eaten by others (including humans). The animals also exhale carbon dioxide and, when they die, decompose and release carbon back into the soil.

Clathrates add another dimension to the carbon cycle. By capturing methane, a carbon compound, clathrates maintain a balance of the earth’s methane supply. However, as stated earlier, methane hydrates are only stable under certain temperature and pressure conditions, particularly those found in permafrost and at the bottom of the ocean. If temperatures rise, the lattice can quickly destabilize, releasing its methane guest molecule. On a large scale, such events mean a rapid release of carbon into the atmosphere.

Such events have happened before in Earth’s history. For example, scientists have found plentiful evidence of clathrate involvement in changes that occurred during the late Precambrian era, hundreds of millions of years ago. Studies of Precambrian rocks show signs of rapid carbon releases in areas in which clathrates had existed. These samples suggest that in light of the significant volume of carbon released during these events, clathrates played a major role in the large-scale development of plant and animal life during that era.

Clathrates and Climate Change

Methane hydrates represent a major enticement for alternative energy enthusiasts, particularly because they exist in much greater volume than other energy fossil sources and natural gas fuel sources. Additionally, if the myriad government and privately sponsored efforts to develop ways to harvest and contain methane from these clathrates succeed, the resulting energy may be considerably cleaner than fossil fuels that produce greenhouse gases.

Separate from the issue of using clathrates as a clean energy source is the potentially significant danger that methane hydrates pose for Earth’s atmosphere. As stated earlier, when destabilized naturally, methane hydrates simply release carbon dioxide (under certain conditions, those gases are released into the air). However, in their present locations (on the ocean floor and deep within the permafrost), they remain stable under precarious temperature and pressure conditions.

If these conditions are changed even slightly, the potential exists for a major release of methane into the air and the atmosphere. For example, clathrate breakdowns in the permafrost could trigger landslides, sending sediment into the sea, contributing to rising sea levels. Geological evidence from the end of the Pleistocene epoch (nearly 12,000 years ago) shows that major changes to the slopes of the earth’s continents were caused by the release of clathrate gases.

Additionally, scientists believe that clathrate breakdowns in both the permafrost and on the ocean floor could significantly hasten global warming. This disastrous scenario stems from the notion that such a release, caused by rising temperatures in the ocean, would abruptly set off a chain reaction among the planet’s methane hydrates and, in turn, quickly raise the planet’s temperature by several degrees. Some scientists argue that such events have happened in Earth’s history, most dramatically at the end of the Permian period (about 251 million years ago). Adherents to the clathrate gun hypothesis believe that a major release of methane hydrate gas, the cause of which is unknown, triggered a mass extinction of nearly 90 percent of life on Earth.

The debate concerning the validity of the clathrate gun hypothesis continues. Many scientists believe that clathrate breakdowns were not necessarily the primary cause of the Permian extinction. Nevertheless, most researchers agree that clathrates likely contributed to some degree to that event, an event from which Earth took 30 million years to recover. In light of the severity of that event, the notion that a simple elevation of a degree or two in temperature could trigger a massive breakdown in sea- and land-based gas clathrates remains a concern among global warming experts.

Implications and Future Prospects

Clathrates, particularly methane hydrates, have evolved from a scientific curiosity to a major component in both the pursuit of alternative energy sources and the understanding of climate change. Scientists continually seek to understand the dynamics of these unusual geochemical configurations. Advances in relevant technologies, including the application of computer models and advances in thermal imaging systems, are contributing to the evolution of clathrate studies.

Although debate over the clathrate gun hypothesis continues, scientists are in agreement that methane hydrates could contribute to global warming and climate changes. However, researchers are also excited at the prospects of tapping into clathrates as an alternative energy source. To this end, scientists are pursuing technological advances to explore the safe extraction and use of methane from hydrates (including carbon sequestration practices). Their hope is to present to the modern world the use of clathrate-based energy that reduces greenhouse gas emissions and protects the planet from further warming trends.

Principal Terms

carbon cycle: a natural cycle by which carbon is absorbed by plants through the soil and air; plants in turn are eaten by animals, which exhale the carbon as carbon dioxide into the atmosphere and whose bodies decompose after death, returning carbon to the soil

carbon sequestration: a chemical process whereby carbon dioxide is removed from an energy source and isolated in a secure chamber to prevent its release into the atmosphere

clathrate gun hypothesis: a theory that states that the Permian extinction was caused by a sudden release of methane from gas clathrates

dodecahedron: a three-dimensional geometric configuration with twelve faces

guest molecule: a molecule contained inside a clathrate

lattice: a cage-like shape

permafrost: frozen soil found beneath ice and snow packs in polar regions

Bibliography

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