| Summary: | The presence of structure H (sH) methane hydrate in natural environments, in addition to the
well-known structure-I (sI) and II (sII) hydrates, has recently been documented. Methane in the
presence of condensates (C5-C7) forms sH hydrate at lower pressure than the sI hydrate. Thus, the
occurrence of sH methane hydrate is likely to have both beneficial and negative practical
implications. On the negative side, in the presence of condensate, sH hydrate may form and plug
gas transmission pipelines at lower pressures than sI hydrate. On the other hand, sH hydrate can
be synthesized at lower pressures and exploited to store methane. The existence of natural
hydrates containing sH hydrate may also be expected in shallow offshore areas. There are at least
26 large guest molecules known as sH hydrate formers and each of them produces a sH hydrates
with different properties. The hydrate stability, the cage occupancies and the rates of hydrate
formation depend on the type of large molecule selected. Consequently, it is essential to
understand how the host and the guest molecules interact. Studies at the molecular-level are
therefore indispensable in providing information that is not obtainable from experiments or too
costly to acquire. Free energy calculations are performed to determine the relative stability
among different sH hydrate systems and the preferable cage occupancy. The latter would give
indications of how much methane gas can be stored in the hydrate. The interaction of guest
molecule inside the hydrate cage is also investigated. The results are related to the physical and
chemical properties of gas hydrates observed from the experiments or reported in the literature.
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