FROZEN HEAT | Volume 1

1.2 WHAT ARE GAS HYDRATES?

In nature, most substances have a fixed composition of build- ing blocks. For example, in the case of methane (CH 4 ), there is always one carbon (C) atom for every four hydrogen (H) atoms, and these atoms are locked together in a fixed geo- metric structure by chemical bonds. It was initially assumed each gas-hydrate structure had a fixed ratio of gas molecules to water molecules, but this was later discovered to be incor- rect (de Forcrand 1902).

Gas hydrates are classified as clathrates. In a clathrate, the solid lattice of host molecules is physically stabilized by en- closing a sufficient, but not fixed, number of appropriately- sized guest molecules. The guest molecules reside within cages, which are open cavities within the lattice, and the sta- bility of the structure depends on the co-existence of both hosts and guests (Fig. 1.1). This combination occurs without any direct chemical bonding. Furthermore, it is stable even

Figure 1.2: Gas hydrate outcrop on the sea floor of the northern Gulf of Mexico. The hydrate has an orange hue due to the presence of small volumes of oil. This hydrate outcrop hosts pink “methane ice worms.” These worms, discovered in 1997, are generally 2–4 cm in length, and graze upon bacteria living on the hydrate (Fisher et al. , 2000). Additional ice worm descriptions are in Volume 1, Chapter 2 (Photo courtesy I. MacDonald, FSU).

FROZEN HEAT 14