|Fig. 1: (a) Small dodecahedral and (b) large tetradecahedral water cages for structure I (sI) hydrates with one "guest" molecule (methane) occupying each cavity.|
Gas hydrates (or gas clathrates) are non-stoichiometric crystalline solids comprised of hydrocarbon gases trapped within the cavities of a rigid "cage-like" lattice of water molecules. These compounds contain clusters (two or more) of gas-trapping polyhedra formed by pentagonally and hexagonally arranged hydrogen-bonded water molecules. Van der Waals interactions between the trapped (enclathrated) "guest" molecule and the surrounding water cage walls stabilize and support the individual polyhedra forming the hydrate lattice and restrict the translational motion of the guest molecule. 
Hydrate structures are classified into three categories based on the geometries of their constituent water cages: cubic structures I and II and hexagonal structure H. Each crystalline structure contains geometrically distinct water cages with different size cavities which typically accommodate only one guest molecule ranging in diameter from 0.40 - 0.90 nm.  Structure I (sI) hydrates are the most commonly encountered naturally occurring hydrate structure which encases small diameter molecules (0.40 - 0.55 nm) such as methane or ethane gas.  Structures II (sII) and H (sH) hydrates accomodate larger guest molecules, typically propane or iso-butane for sII or combinations of methane gas and nexohexane or cycloheptane for sH, but are less prevalent in nature. For sI hydrates, the unit cell consists of 46 water molecules arranged into two small dodecahedral cages ( each with twelve pentagonal faces) and six large tetradecahedral cages (each with two hexagonal and twelve pentagonal faces ) (see Fig. 1).  Assuming full occupancy, the ideal molar guest to water ratio for an sI hydrate is 1: 5.75.
|Fig. 2: Phase diagram for methane hydrate formation.|
Gas hydrates form in high pressure, low temperature environments where sufficient gas and water are present. Fig. 2 shows the phase diagram for methane hydrate formation. The hydrate formation requirements restrict the occurrence of natural gas hydrates to two types of geologic locations: i) under permafrost in the polar continental shelves and ii) in sediment beneath the ocean floor.  The blue sections in the generic curves shown in Fig. 3 illustrate regions in permafrost and oceanic sediment where the pressure and temperature conditions and the concentration of methane gas are within the hydrate formation and stability zone. These curves are based on pressure-temperature phase equilibrium data and correspond with reflection seismic data collected in these environments. [3-6] While several different models have been developed to describe the mechanisms involved in gas hydrate formation, there is a general consensus that the origin of the methane concentrated in naturally occurring hydrates is either microbial (generated by anaerobic decomposition of organic matter) or thermogenic (generated by thermal decomposition of organic matter). [4,7]
Reflection seismology and recovered core samples are primarily used to estimate methane hydrate reserves. While core samples provide direct evidence for hydrates, they are often difficult to obtain from regions with hydrate favorable conditions. [3, 8] Conversely, reflection seismology is routinely used as an indirect method to detect hydrate deposits in the Earth's subsurface.  This exploration technique monitors changes in the velocities of reflected seismic waves to indicate transitions between materials with different densities.  The locations of methane hydrate deposits are inferred by identifying bottom simulating reflectors (BSRs) on the seismic profiles.  BSRs are interpreted as the boundary between hydrate and free gas regions in the subsurface.  In general, estimates based strictly on BSRs are considered speculative since hydrate bearing sediment has been extracted from regions without BSRs and vice versa.  Therefore, estimates of the global accumulations of methane hydrates vary over three orders of magnitude (0.15 x 1015 - 3.05 x 1018 m3 of methane at STP).  However, even conservative estimates indicate that a significant amount of methane gas is concentrated in the shallow geosphere.
Despite the relative magnitude and global pervasiveness of gas hydrate deposits, the existence of naturally occurring gas hydrates was first recognized in 1965 when a Soviet oil crew located a reservoir of methane hydrates while drilling in Siberia.  Prior to this discovery, gas hydrates were only known to occur in the laboratory and within the thermodynamically favorable conditions found in oil pipelines. Since this discovery, gas hydrates have attracted interest as a potential energy resource.
|Fig. 3: Methane hydrate stability zones (blue) for (a) permafrost and (b) oceanic environment.|
One of the most intriguing properties of natural gas hydrates is their ability to concentrate and store large volumes of natural gas within their water cage cavities. When hydrates are heated or depressurized, they become unstable and dissociate into water and natural gas. Based on the bulk density of sI methane hydrates (~ 0.9 g/cm3) and assuming full occupancy of cavities, 1 m3 of methane hydrate contains 168.27 m3 of methane gas at standard temperature and pressure (STP).  A more accurate prediction of the volumetric ratio can be determined using the 90% occupancy found in naturally occurring methane hydrates by x-ray diffraction.  The molar guest to water ratio for 90% occupancy is 1:6.39 which corresponds to 155 m3 of methane gas at STP:
Two of the main challenges to utilizing methane hydrates as an energy resource are recovery and production. In order to exploit the large volumes of trapped gas within gas hydrates, hydrate-bearing sediment must first be made accessible by drilling deep wells into the oceanic and permafrost reservoirs. Following drilling, methane production can be accomplished by thermal stimulation (increasing the local temperature in the wellbore to cause the hydrate to dissociate), depressurization (lowering the pressure to force the hydrate to dissociate), or inhibitor injection (injection of inhibitors such as methanol to destabilize the hydrate).  After hydrate dissociation, the released gas must be isolated and collected.
To date, large-scale production of methane from gas hydrates has not been demonstrated due to economic and safety issues. [8,12] In order for the potential of methane hydrates to be fully recognized, improvements in hydrate technology must be achieved. Hydrate research groups in the United States and Japan expect to have made these advances by 2016. 
© 2010 Sara Harrison. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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