B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

Gas Hydrate Water Content 
Gas hydrates have more than 85 mole% water in their structure. The number 
of water molecules in each unit cell of various gas hydrate structure is 
constant. However, the cage occupancy could be different resulting in various 
mole fractions for water. 
Depending on how and where they are formed, and the hydrate-former, the 
gas hydrate can look very much like ice or snow, see figure 5: 
Figure 5: Naturally Occurring Gas Hydrate 
(http://www.unep.org/Documents.Multilingual/Default.asp?DocumentID
=646&ArticleID=6728&l=en) 
Hydrate Purity 
Gas hydrates exclude all impurities from their structure. Gas hydrates are 
formed only from water and guest molecules, excluding almost any other water 
impurities from their structure. 
This means if gas hydrates are formed in saline water, salt concentration in the 
remaining water will increase. Therefore, if gas hydrates are dissociated the 
salinity of the water (resulted from gas hydrate dissociation) will be very low.
This property has previously been examined for the design of gas hydrate 
desalination plants. However, it is not possible to achieve very pure water, as 
some salt and saline water is trapped between the gas hydrate crystals. There 
are suggestions that gas hydrates could be used for wastewater treatments in 
the pulp industry. Also the change in chlorine ion concentration has been used 
to measure the amount of gas hydrates in marine sediments. 

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TOPIC 1: Gas Hydrates 
 
 
 
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NIVERSITY B41OA December 2018 v3 
Hydrate Abundance 
Large quantities of gas hydrates exist in nature. The amount of gas hydrates in 
marine sediments is believed to be two orders of magnitude higher that in 
permafrost regions. In general it is estimated that the amount carbon in gas 
hydrates is twice that of total fossil fuels. Massive amounts of fuel could 
potentially be obtained – given the right economic and technological 
circumstances.