Methane from Gas Hydrates
339
Region 4: Deep-water gas hydrates. Ninety-five percent of earth’s gas hydrates
are at depths >3000  ft. Even with an increase in the ocean temperature,
they are likely to stay stable over thousands of years. They also occur deep
within the sediments, and the released methane will remain in the sediments, and if they move upward, they will form new hydrates or consumed
by oxidation within water [117–180] (Kennett, 2012, pers. comm.).
Region 5: Seafloor gas hydrate mounds. At some marine seeps such as the Gulf
of Mexico, massive relatively pure gas hydrate occurs in seafloor mounds.
While seafloor gas hydrate mounds and shallow subsea floor gas hydrate
constitute only a trace amount of the global gas hydrate inventory, they can
dissociate rapidly due to the expulsion of warm fluids from the seafloor and
release significant amount of methane to the atmosphere.
Based on the analysis of these five regions, a general consensus [117–180] (Kennett,
2012, pers. comm.) is that catastrophic widespread dissociation of methane gas
hydrates will not be triggered by continued climate warming at a contemporary
rate (0.2°C per decade) over a timescale of few hundred years. In spite of this conclusion, there has been an enormous interest in studying methane release from
hydrates to the atmosphere and its effect on environment. The vast literature [117–180]
(Kennett, 2012, pers. comm.) is cited here to demonstrate the significant interest on
the subject.
12.5 PrOdUCtiOn OF methane FrOm
Gas hydrate reserVOirs
Hydrates are known to occur at temperatures <295 K and pressure >3000 kPa. The
dissociation of these hydrates occurs as
CH 4 ⋅ 6 H O s → CH g + 6 2
2 ( )
4 ( )
H O l
( )
(12.1)
with enthalpy  = 10–20  kcal/mol of gas dissociated [1–16] (Harrison, 2010, pers.
comm.). This reaction requires an external energy source to propagate along the
right-hand side [1–16] (Harrison, 2010, pers. comm.).
In conventional gas reservoirs, natural gas migrates to the recovery point via
pressure gradients. For these reservoirs, the recovery rate is a function of the formation permeability and pressure gradients established between the reservoir and
the extraction well(s). Production of methane from hydrate-bearing deposits requires
additional energy to dissociate the crystalline water lattice that forms the gas hydrate
structure. A variety of methods have been proposed for producing natural gas from
hydrate deposits: (1) thermal stimulation, where the temperature is increased above
the hydrate stability region; (2) depressurization, where the pressure is decreased
below the hydrate stability region; (3) chemical injection of inhibitors, where the temperature and pressure conditions for hydrate stability are shifted; (4) CO 2 or mixed
CO 2 and N 2 exchange, where CO 2 and N 2 replace CH 4 in the hydrate structure; and
(5) enhanced gas hydrate recovery (EGHR) methods, where two-phase emulsion (of
CO 2 and water) and other solution injection techniques are used to replace methane