In defining natural gas hydrate, it is useful to clarify the term "natural" which has two meanings. First, the term "natural" is used to indicate gas hydrate occurring naturally on Earth rather than being synthesized in the laboratory or inadvertently created during industrial transportation of petroleum gas. Second, the term "natural" indicates hydrate containing natural gas, defined in the oil industry as the gaseous phase of petroleum (Hunt, 1996), but which really includes all gases derived from naturally occurring chemical and biochemical processes. Typically, natural gas is composed of methane often accompanied by higher molecular weight hydrocarbon gases and by non-hydrocarbon gases. The
proportions of gases can vary from pure hydrocarbons to pure non-hydrocarbons, such as carbon dioxide, nitrogen, and hydrogen sulfide, all of which can form hydrates. As far as is known at present, the most widespread natural gas hydrates on Earth are those incorporating mainly methane, and these are the natural gas hydrates that are the main focus of this chapter.
Thus gas hydrate, also called gas clathrate, is a naturally occurring solid comprised of water molecules forming a rigid lattice of cages, with most cages containing a molecule of natural gas, mainly methane. Gas hydrate is essentially a water clathrate of natural gas. Three crystalline structures, I, II, and H, have been recognized in nature with structure I being the most common. In structure I, the cages are arranged in body-centered packing of the cubic crystallographic system, and the cages are large enough to include methane, ethane, and other gas molecules of similar molecular diameter, such as carbon dioxide and hydrogen sulfide. In structure II, diamond packing in the cubic system is present resulting in some cages being large enough to include not only methane and ethane, but also gas molecules as large as propane and isobutane. Structure H is least common in nature; some cages larger than in structure II are present in the hexagonal crystallographic system (Sloan, 1998). Structure I gas hydrate is emphasized in this chapter and is often referred to as methane hydrate. This gas hydrate can contain very large amounts of methane. The maximum amount is fixed by the clathrate geometry which translates to 164 volumes of methane at standard conditions to one volume of methane hydrate (Davidson et al., 1978).
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