Image courtesy Southwest Research Institute.

Perhaps the world's best-known example of a chemical phase transition occurs when water at atmospheric pressure is cooled to a temperature of 0° C (32° F). At that point, the energy in the system is no longer sufficient to keep the water molecules from bonding together. The liquid water restructures itself into a rigid solid, called ice. However, it turns out that in nature, this process is not always so clear-cut.

When pressures are suitably high, cooling water molecules will begin to form complex solid structures at temperatures significantly above the normal freezing point. Unlike ice, these structures are characterized by regular networks of large, open cavities and are therefore inherently unstable. As cooling continues, the normally compact and stable ice structure will ultimately form, unless some outside "guest" molecule of appropriate size enters the structure and supports the cavity. In nature, the most abundant guest molecule is methane (CH₄). The resulting stable, solid compound is commonly called "methane hydrate."

In the lexicon of inorganic chemistry, a “hydrate” is a stoichiometric compound (one with a fixed composition) that has water molecules as an integral part of the crystal. For such compounds, a definite formula (such as Al₂O₃3H₂O, or alumina trihydrate — a material commonly used in making ceramics) can be written. A similar formula cannot be written for methane hydrate, the best we can do is XCH₄46H₂O—with X being as much as eight, but usually less—and even this is not correct, as other guest gases may also be present. Therefore, natural gas hydrates are more appropriately grouped within a special class of non-stoichiometric compounds called clathrates.

Clathrates are compounds that consist of an inherently unstable network of host molecules characterized by regular open cavities. Guest molecules of appropriate size fill the cavities without bonding—they are held in place by Van der Waals forces. When a sufficient number of the cavities are occupied, a stable solid structure is formed.

One of the most common host molecules is water, and the water-based clathrates are commonly referred to, particularly in the oil and gas business, as "hydrates." Gas hydrates have guest molecules that exist in the gas phase at standard temperature and pressure. The most common is methane (methane hydrate), but many gases, including carbon dioxide, hydrogen sulfide, and larger hydrocarbons such as ethane and propane, can stabilize the water lattices and form a "hydrate." When only very small molecules are present (hydrogen or helium, for example), no hydrate will form because the available guests are not large enough to support, or be trapped in, the cavities.

The three types of cavities present in Structure I and II methane hydrates. Courtesy of Centre for Gas Hydrate Research at Heriot-Watt University.

Structures I and II: Two primary types of hydrate structures are known to exist commonly in nature, termed simply, structure I and structure II. These structures represent different arrangements of water molecules resulting in slightly different shapes, sizes, and assortments of cavities. Which structure forms depends on various aspects of the available guest gas. Methane preferentially forms structure I.

A unit cell (the smallest repeatable element) of a structure I hydrate consists of 46 water molecules surrounding 2 small cavities and 6 medium-sized cavities. The unit cell of structure II hydrates consists of 136 water molecules creating 16 small cavities and 8 large cavities. Both structures I and II can be stabilized by filling at least 70 percent of the cavities by a single guest gas—and are therefore known as simple hydrates.

The two types of cavities unique to Structure H methane hydrates. Courtesy of Centre for Gas Hydrate Research at Heriot-Watt University.

Structure H: In 1987, Ripmeester and others discovered a third hydrate structure (structure H) that requires the cooperation of two guest gases (one large and one small) to be stable. A unit cell of this new "double hydrate" consists of 34 water molecules producing 3 small cavities, 12 slightly larger cavities, and 1 relatively huge cavity. It is this large cavity that allows structure H hydrates to incorporate large molecules (such as butane and larger hydrocarbons), given the presence of other smaller help gases to fill and support the remaining smaller cavities. Structure H hydrates are rare, but are known to exist in the Gulf of Mexico, where supplies of thermogenically-produced heavy hydrocarbons are common. It is probable that other, more exotic, hydrate structures remain to be discovered.

Gas Storage Capacity: The structure of methane hydrate compresses methane molecules into a very dense and compact arrangement. When dissociated at normal surface temperatures and pressures, a 1 ft³ block of solid methane hydrate with 100 percent void occupancy by methane will release roughly 164 standard cubic feet of methane (however, occupancy typically ranges from 70 percent to 90 percent). This gives methane hydrate an energy content of roughly 184,000 btu/ft³. This value lies in between the values for methane gas (1,150 btu/ft³) and liquefied natural gas (LNG: 430,000 btu/ft³).