136
5 Nucleation of Gas Hydrates
When water freezes, dissolved gases (air) are excluded from the ice lattice. Some
may be trapped between the grains of polycrystalline ice as fine bubbles which
may be released from the trapped spots when the ice was subsequently melted [87].
Others may form an interfacial gaseous state because the heterogeneous nucleation
of a bubble on a sloid wall is always easier than the homogenous nucleation of a
bubble in water [80]. When such a sample is subsequently warmed, the melting
generally takes place from the inner wall of a container because the wall should be
the warmest. Importantly, the solubility of air is low in the warmer part of the water
(next to the walls) than in the colder part (near the center of the container). Then the
fine bubbles that have been trapped between the grains of polycrystalline ice will not
immediately dissolve back into water when released, especially for those near the
walls. A simple estimate shows that it will take many hours for the air to dissolve
back into the degassed water [88] and the lifetime of interfacial gaseous states are
surprisingly long [89]. Plus, warming of a wall has been known to induce interfacial
nanobubbles [90]. In short, (1) the air in interfacial gaseous states would be the last
to disappear among all the air bubbles in the container and (2) it will take many hours
for them to do so.
If such a sample were subsequently exposed to pressurized guest gas, the guest gas
would promptly diffuse through the interfacial gaseous states made of air (the guest
diffusion is expected to be faster in air than in water) and stabilize these interfacial
gaseous states. That Takeya et al. indeed found that the nucleation rate of CO 2 hydrate
decreased (1) when their non-treated water was degassed or (2) when their melted
water was heated to 298 K for 1 h, is in fact consistent with the interfacial gaseous
states hypothesis [62].
5.4 Effects of Electrolytes
5.4.1 Introduction
An aging oil field or gas field produces progressively more water with time as the
reservoir pressure falls. Such oilfield waters are generally not fresh but mineralized.
Oilfield waters can contain much higher concentrations of minerals than seawater
does, as high as up to 10 times (for comparison, typical salt concentrations of seawater
is 3.5 wt%). Dissolved cations commonly found in oilfield waters are Na
+ , Ca
2+ ,
Mg
2+ , K
+ , Ba
+ , Li
+ , Fe
2+ , Sr
2 and common anions are Cl
– , SO 4
2– , HCO 3
– , CO 3
2– ,
NO 3
– , Br
– , I
– , BO 3
2– , S 2
– . Such oilfield water has different compositions than other
brine, even those in the immediate vicinity of that field. Drilling operations, especially
offshore, also encounter clathrate hydrate issues in the presence of salts. For these
reasons, nucleation of clathrate hydrates in electrolyte solutions is of great practical
interest in oil and gas industries.
Salts are thermodynamic hydrate inhibitors (THIs) because salts are highly soluble
in water. The colligative effect of lowering of water activities through the entropy of
5 Nucleation of Gas Hydrates
When water freezes, dissolved gases (air) are excluded from the ice lattice. Some
may be trapped between the grains of polycrystalline ice as fine bubbles which
may be released from the trapped spots when the ice was subsequently melted [87].
Others may form an interfacial gaseous state because the heterogeneous nucleation
of a bubble on a sloid wall is always easier than the homogenous nucleation of a
bubble in water [80]. When such a sample is subsequently warmed, the melting
generally takes place from the inner wall of a container because the wall should be
the warmest. Importantly, the solubility of air is low in the warmer part of the water
(next to the walls) than in the colder part (near the center of the container). Then the
fine bubbles that have been trapped between the grains of polycrystalline ice will not
immediately dissolve back into water when released, especially for those near the
walls. A simple estimate shows that it will take many hours for the air to dissolve
back into the degassed water [88] and the lifetime of interfacial gaseous states are
surprisingly long [89]. Plus, warming of a wall has been known to induce interfacial
nanobubbles [90]. In short, (1) the air in interfacial gaseous states would be the last
to disappear among all the air bubbles in the container and (2) it will take many hours
for them to do so.
If such a sample were subsequently exposed to pressurized guest gas, the guest gas
would promptly diffuse through the interfacial gaseous states made of air (the guest
diffusion is expected to be faster in air than in water) and stabilize these interfacial
gaseous states. That Takeya et al. indeed found that the nucleation rate of CO 2 hydrate
decreased (1) when their non-treated water was degassed or (2) when their melted
water was heated to 298 K for 1 h, is in fact consistent with the interfacial gaseous
states hypothesis [62].
5.4 Effects of Electrolytes
5.4.1 Introduction
An aging oil field or gas field produces progressively more water with time as the
reservoir pressure falls. Such oilfield waters are generally not fresh but mineralized.
Oilfield waters can contain much higher concentrations of minerals than seawater
does, as high as up to 10 times (for comparison, typical salt concentrations of seawater
is 3.5 wt%). Dissolved cations commonly found in oilfield waters are Na
+ , Ca
2+ ,
Mg
2+ , K
+ , Ba
+ , Li
+ , Fe
2+ , Sr
2 and common anions are Cl
– , SO 4
2– , HCO 3
– , CO 3
2– ,
NO 3
– , Br
– , I
– , BO 3
2– , S 2
– . Such oilfield water has different compositions than other
brine, even those in the immediate vicinity of that field. Drilling operations, especially
offshore, also encounter clathrate hydrate issues in the presence of salts. For these
reasons, nucleation of clathrate hydrates in electrolyte solutions is of great practical
interest in oil and gas industries.
Salts are thermodynamic hydrate inhibitors (THIs) because salts are highly soluble
in water. The colligative effect of lowering of water activities through the entropy of