74
3 Gas Hydrates
to form in the presence of a THI than in the absence of one. As a first approximation, the temperature depression for clathrate hydrate formation may be considered
to be similar to the temperature depression of the freezing point of ice by a solute.
However, Nielsen and Bucklin derived an equation which indicated that the depression temperature of a clathrate hydrate is always 30–40% less than that of ice [1, 71],
which is detrimental to the thermodynamic inhibition efficacy of THIs.
The inhibition capability of alcohols diminishes with the increasing molecular
weight because the hydrophobic moiety of an alcohol increases with the molecular
weight. Since methanol has by far the smallest molecular weight among alcohols it
will be the most effective THI at a given mass concentration (that corresponds to a
much higher molar concentration than the other alcohols).
A glycol is more hydrophilic than an alcohol because it has a higher fraction of
–OH groups than an alcohol of the same molecular weight. Among glycols, monoethylene glycol (MEG) is more hydrophilic than the higher molecular weight counterparts of di-ethylene glycol (DEG) or tri-ethylene glycol (TEG). In addition, since
MEG has by far the smallest molecular weight among glycols, it will be the most
effective THI at a given mass concentration (that corresponds to a much higher molar
concentration than the other glycols).
For these reasons, methanol and MEG have been the most common THIs. The
amount of thermodynamic inhibition required in a given field operation depends on
the temperature and the pressure that are encountered by the production fluid. The
amount of required THIs increases as the temperature becomes lower and the pressure
becomes higher inside the flow lines. Thus, large quantities of THIs are required for
an aging field that produces progressively more water. Logistics of supplying large
quantities of THIs to remote locations is expensive and hence requires as much
regeneration and recycling of THIs as possible. Then, a regeneration unit will be
required on the production site. This requirement poses an additional challenge when
the space is limited (e.g., an offshore platform).
3.3.2 Kinetic Hydrate Inhibitors (KHI)
The above challenges and limitations of THIs led to the development of kinetic
hydrate inhibitors (KHI) since the late 1980s [72]. The basic idea behind KHIs
is that it may not be essential to prevent the formation of clathrate hydrates for
the flow assurance of oil and natural gas pipelines as long as one can prevent the
nucleation of clathrate hydrates from developing into a more serious problem of
pipeline blockage by arresting the growth into a hydrate plug. Formation of a hydrate
plug from an initially hydrate-free state requires several kinetic steps of development.
One of such several kinetic steps that could be arrested is the crystal growth of
clathrate hydrate after its nucleation. Another kinetic step that could be arrested is
agglomeration or deposition of clathrate hydrate polycrystals or particles after their
nucleation and crystal growth. Arresting of any such kinetic step would suffice to
meet the requirement of flow assurance. The value of this approach comes from
3 Gas Hydrates
to form in the presence of a THI than in the absence of one. As a first approximation, the temperature depression for clathrate hydrate formation may be considered
to be similar to the temperature depression of the freezing point of ice by a solute.
However, Nielsen and Bucklin derived an equation which indicated that the depression temperature of a clathrate hydrate is always 30–40% less than that of ice [1, 71],
which is detrimental to the thermodynamic inhibition efficacy of THIs.
The inhibition capability of alcohols diminishes with the increasing molecular
weight because the hydrophobic moiety of an alcohol increases with the molecular
weight. Since methanol has by far the smallest molecular weight among alcohols it
will be the most effective THI at a given mass concentration (that corresponds to a
much higher molar concentration than the other alcohols).
A glycol is more hydrophilic than an alcohol because it has a higher fraction of
–OH groups than an alcohol of the same molecular weight. Among glycols, monoethylene glycol (MEG) is more hydrophilic than the higher molecular weight counterparts of di-ethylene glycol (DEG) or tri-ethylene glycol (TEG). In addition, since
MEG has by far the smallest molecular weight among glycols, it will be the most
effective THI at a given mass concentration (that corresponds to a much higher molar
concentration than the other glycols).
For these reasons, methanol and MEG have been the most common THIs. The
amount of thermodynamic inhibition required in a given field operation depends on
the temperature and the pressure that are encountered by the production fluid. The
amount of required THIs increases as the temperature becomes lower and the pressure
becomes higher inside the flow lines. Thus, large quantities of THIs are required for
an aging field that produces progressively more water. Logistics of supplying large
quantities of THIs to remote locations is expensive and hence requires as much
regeneration and recycling of THIs as possible. Then, a regeneration unit will be
required on the production site. This requirement poses an additional challenge when
the space is limited (e.g., an offshore platform).
3.3.2 Kinetic Hydrate Inhibitors (KHI)
The above challenges and limitations of THIs led to the development of kinetic
hydrate inhibitors (KHI) since the late 1980s [72]. The basic idea behind KHIs
is that it may not be essential to prevent the formation of clathrate hydrates for
the flow assurance of oil and natural gas pipelines as long as one can prevent the
nucleation of clathrate hydrates from developing into a more serious problem of
pipeline blockage by arresting the growth into a hydrate plug. Formation of a hydrate
plug from an initially hydrate-free state requires several kinetic steps of development.
One of such several kinetic steps that could be arrested is the crystal growth of
clathrate hydrate after its nucleation. Another kinetic step that could be arrested is
agglomeration or deposition of clathrate hydrate polycrystals or particles after their
nucleation and crystal growth. Arresting of any such kinetic step would suffice to
meet the requirement of flow assurance. The value of this approach comes from