An introduction to natural gas hydrate/clathrate: The major organic carbon reserve of the Earth

Gas hydrates belong to a general class of inclusion compounds commonly known as clathrates. A clathrate is a compound of molecular cage structure made of host molecules encapsulating guest molecules. It is also considered a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule (Sloan, 1998; Taylor et al., 2004). Natural gas clathrates owe their existence to the ability of H2O molecules to assemble via hydrogen bonding and form polyhedral cavities as shown in Figs. 1 and 2. Natural gas hydrate is a naturally occurring ice-like solid (see Fig. 3), which is made of water molecules as the cage forming host and other molecules (mostly methane) as the guest. The guest molecules, like methane or carbon dioxide, are of an appropriate size such that they fit within cavities formed by the host material. Common clathrate compounds of interest are those formed from CO2/H2O and CH4/H2O mixtures: the former for application in carbon sequestration and the latter for methane extraction. The physical appearance of the natural gas hydrate is like other crystalline substances. At standard pressure and temperature, a methane hydrate molecule contains approximately 160 volumes of methane for each volume of water. Until recently, methane hydrates, known to scientists for almost 200 years, have remained a scientific curiosity. It was not until the 1930's that it was realized that methane hydrate was responsible for plugging natural gas pipelines, particularly those located in cold environments. For the next 40 years, a small body of researchers investigated the physics of various clathrates, including the construction of the first predictive models of their formation. A prime focus of this work was (and continues to be) the development of chemical additives and other methods to inhibit hydrate formation.