341
Methane from Gas Hydrates
Moridis et al. [197,201,202] and Moridis [203] numerically simulated the effect
of depressurization at Mallik site assuming 0.03°C/m temperature gradient in the
hydrate-bearing formation. The simulation showed a vertical drop in temperature
in response to depressurization and hydrate dissociation. This temperature drop can
be reversed by the injection of warmer water in the well, which provides the needed
energy to sustain hydrate dissociation in the depressurized system. The simulation
also indicated that, when steam or hot methane gas was injected from a second well,
natural gas production was superior in terms of the ratios of produced gas to water
and fraction of produced methane from hydrates.
Several other simulation studies showed that hydrate dissociation rates and
associated gas productions are controlled by the far-field reservoir pressure and
temperature, via energy supplied by natural gas conveyed from the far field to the
dissociation front [203–212]. Few studies have reported experimental data of gas
recovery by depressurization [194,195]. While depressurization is a viable option
because of thermal self-regulation of gas hydrates, the method results in slow production rates. Sustained production requires a heat source, which at the Messoyakha
field is supplied by thermal conduction and convection in the dissociation zone. This
heat transfer ultimately controls the production rate.
There are three important mechanisms involved in the depressurization of the
gas hydrates: (1) kinetics of dissociation, (2) conductive heat transfer, and (3) convective flow of fluids like gas and water. A significant theoretical work that uses a
three-dimensional model of a porous media and simulates the exact conditions of a
reservoir with regard to all the mechanisms involved has been reported [201–214].
However, to this date, conclusions of such analysis are only based on certain assumptions, whose validity needs to be experimentally verified. Often a two-well system
involving a combination of depressurization at the production well and a thermal
input (by hot fluid injection) at the injection well appears to be better than a single
vertical system [190,192–200].
12.5.3 inhiBiTor injeCTion
Sung et al. [214], Kawamura et al. [215], and Li et al. [216,217] showed that the thermodynamic inhibitors lower the hydrate formation temperature, which can result in
hydrate dissociation when injected into a gas hydrate-bearing formation. The most
important thermodynamic organic inhibitors are methanol, monoethylene glycol
(MEG), and diethylene glycol (DEG) commonly referred to as glycol [218–222].
Dissolved salts such as NaCl, CaCl 2 , KCl, and NaBr can also be inhibitors [191].
While gas hydrate inhibitors are an effective methodology for preventing hydrate
formation in engineering applications, their use in the production of NGHs is restrictive due to environmental impact, prohibitive costs, and thermal self-regulation of
gas hydrates. Of the inhibitors examined, methanol and glycols are the most successful ones [221]. The principles by which alcohol, glycols, and salts inhibit hydrates are
the same. However, salts have corrosion problems, and they cannot be easily vaporized due to their low vapor pressures.
In adding inhibitors, besides temperature and pressure conditions, composition
and amount of inhibitors are important. The inhibitor must be at or below its water
Methane from Gas Hydrates
Moridis et al. [197,201,202] and Moridis [203] numerically simulated the effect
of depressurization at Mallik site assuming 0.03°C/m temperature gradient in the
hydrate-bearing formation. The simulation showed a vertical drop in temperature
in response to depressurization and hydrate dissociation. This temperature drop can
be reversed by the injection of warmer water in the well, which provides the needed
energy to sustain hydrate dissociation in the depressurized system. The simulation
also indicated that, when steam or hot methane gas was injected from a second well,
natural gas production was superior in terms of the ratios of produced gas to water
and fraction of produced methane from hydrates.
Several other simulation studies showed that hydrate dissociation rates and
associated gas productions are controlled by the far-field reservoir pressure and
temperature, via energy supplied by natural gas conveyed from the far field to the
dissociation front [203–212]. Few studies have reported experimental data of gas
recovery by depressurization [194,195]. While depressurization is a viable option
because of thermal self-regulation of gas hydrates, the method results in slow production rates. Sustained production requires a heat source, which at the Messoyakha
field is supplied by thermal conduction and convection in the dissociation zone. This
heat transfer ultimately controls the production rate.
There are three important mechanisms involved in the depressurization of the
gas hydrates: (1) kinetics of dissociation, (2) conductive heat transfer, and (3) convective flow of fluids like gas and water. A significant theoretical work that uses a
three-dimensional model of a porous media and simulates the exact conditions of a
reservoir with regard to all the mechanisms involved has been reported [201–214].
However, to this date, conclusions of such analysis are only based on certain assumptions, whose validity needs to be experimentally verified. Often a two-well system
involving a combination of depressurization at the production well and a thermal
input (by hot fluid injection) at the injection well appears to be better than a single
vertical system [190,192–200].
12.5.3 inhiBiTor injeCTion
Sung et al. [214], Kawamura et al. [215], and Li et al. [216,217] showed that the thermodynamic inhibitors lower the hydrate formation temperature, which can result in
hydrate dissociation when injected into a gas hydrate-bearing formation. The most
important thermodynamic organic inhibitors are methanol, monoethylene glycol
(MEG), and diethylene glycol (DEG) commonly referred to as glycol [218–222].
Dissolved salts such as NaCl, CaCl 2 , KCl, and NaBr can also be inhibitors [191].
While gas hydrate inhibitors are an effective methodology for preventing hydrate
formation in engineering applications, their use in the production of NGHs is restrictive due to environmental impact, prohibitive costs, and thermal self-regulation of
gas hydrates. Of the inhibitors examined, methanol and glycols are the most successful ones [221]. The principles by which alcohol, glycols, and salts inhibit hydrates are
the same. However, salts have corrosion problems, and they cannot be easily vaporized due to their low vapor pressures.
In adding inhibitors, besides temperature and pressure conditions, composition
and amount of inhibitors are important. The inhibitor must be at or below its water