Formation of gas hydrates requires thermodynamic conditions like high

Formation of gas hydrates requires thermodynamic conditions
like high pressure and low temperature. It is due to this reason, they are
naturally found in deep sea sediments or permafrost regions. Over 90 hydrate
occurrence sites (also known as hydrate reservoirs) have been identified
worldwide. Figure below shows global reserves of gas hydrates spread across the
world, identified using BSR (Bottom Simulating Reflector) technique or by the
sample of its core. Three sites where gas production has been attempted are
also seen in the map.

Figure 1.2: World map of natural gas hydrate reservoir sites.
Adapted from 3.

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Currently it is estimated that these hydrate sites could
source between 1015 to 1017 m3 of methane gas
recoverable at STP. Even the most conservative estimate of the total
recoverable energy from methane hydrates, is two times the energy contents of
the total fossil fuel reserves recoverable by conventional methods4. Apart from its abundance,
methane gas is also a relatively cleaner source of energy. Therefore naturally
occurring methane hydrates are good fit for the present day energy scenario and
finding efficient and safe methods for producing methane gas from it is
important commercially.

1.1      
Methods of Gas Recovery from Hydrate Reservoirs

Gas recovery from hydrate reservoirs can be broadly
classified into two classes of schemes. First scheme is about gas recovery by
causing dissociation of the hydrates. Second scheme is gas exchange method in
which parent gas molecules are swapped with the injected gas at their
interstitial sites.

Dissociation of hydrates requires shifting the
thermodynamic equilibrium existing between three phase system of water, hydrate
and gas. Three commonly known methods to do so are: (1) depressurization, (2)
thermal stimulation and (3) inhibitor injection. Figure 1.3 is the schematic
representation of each of these first class of schemes.

Figure 1.3: Phase equilibrium diagram of CH4
hydrate with different production methods of first class of schemes.

1.1.1       
Depressurization

In depressurization method, pressure within the hydrate
stability zone is decreased leading to decomposition of the hydrate structure
and as a result the trapped methane gas is released. Depressurization is done
by removing the reservoir’s existing free gas and water through production
well. As hydrate dissociation is endothermic in nature, a decrease in the
temperature of reservoir is likely to be seen as depressurization progresses.

1.1.2       
Thermal Stimulation

In thermal stimulation technique the reservoir conditions
are moved out of hydrate stable region by a source of heat that is provided
either directly by injection of any heated fluid or indirectly by sonic or electric
means. Thermal stimulation method is regarded as expensive means for decomposing
hydrates as a large fraction of the supplied heat is lost to the porous media
containing hydrate.

1.1.3       
Inhibitor Injection

Chemical inhibitors like salts, alcohols and glycols are
used to shift the pressure temperature equilibrium conditions of the reservoirs
leading to dissociation of the hydrates. This method is also considered to be
expensive because of the cost of the chemicals. Moreover, inhibitor injection
method requires highly permeable hydrate bearing sediment in order to allow
injection of fluid.