Gas hydrates (GH) are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of ice-like crystal structures called hosts. Under suitable conditions of low temperature T and high pressure P, the hydration reaction of a gas G is described by the general equation

G + NHH2O = G • NHH2O, (1)

where NH is the hydration number. GH deposits occur in two distinctly different geographic settings: in the permafrost and in deep ocean sediments [91].

Naturally occurring hydrocarbon gas hydrates contain CH4 in overwhelming abundance. Simple CH4 hydrates concentrate methane volumetrically by a factor of 164 when compared to standard P and T conditions (STP). Such hydrates have 5.77 <NH< 7.4, with NH = 6 being the average value and NH = 5.75 the maximum one 4 1784 • Natural gas hydrates can also contain other hydrocarbons (alkanes CH2v+2, v = 2-4), but may also comprise lesser amounts of other gases (mainly CO2, H2S, or N2).

Although there has been no systematic effort to map and evaluate this resource and current estimates of the in-place amounts vary widely, the consensus is that the world­wide quantity of hydrocarbon GH is vast [80, 120, 178]. Given the magnitude of the resource, the ever-increasing global energy demand, and finite conventional fossil fuel reserves, the potential of GH as an energy source demands technical and eco­nomic evaluation. The attractiveness of GH is further enhanced by the environmental desirability of natural gas, as it is an energy resource with significantly lower carbon intensity than coal, oil, or other solid and liquid fuels.

The past decade has seen a marked acceleration in GH research and development (R&D). Among the most important developments are the increasing focus of research on gas hydrate-bearing sediments (HBSs) rather than crystalline hydrate, the improvements in tools available for sample collection and analysis, the emer­gence of robust numerical simulation capabilities, and the transition of GH resource assessment from in-place estimates to potential recoverability [8]. A fuller under­standing of the complexities of GH geological systems has emerged, including new insights into the effects of solubility, salinity and heat flow, reservoir lithology, and rates and migration pathways of both gas and H2O [151,167]. Additionally, critical data gaps, such as information on the mechanical and hydraulic properties of HBS, are being addressed. Significant inroads are also being made into our understanding of hydrate response under different production scenarios.

GH are often compared to coalbed gas, which was also considered an uneco­nomic resource in the not too distant past [21] • However, once the resource was geologically understood, the reservoir properties defined, and the production chal­lenges addressed, coalbed gas became a viable fuel in its own right and an important part of the energy mix in the United States, where it accounts for almost 10% of the natural gas production. Past experience with other unconventional energy resources shows that the evolution of GH into a producible source of energy will require a significant and sustained R&D effort. Here we discuss the current state of this effort and of the corresponding knowledge status.