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

1.2 Where do Hydrates Form? 
Gas hydrates can form anywhere if the conditions for their formation have 
been met, some examples are as follows: 
1. In oil and gas pipelines, where gas molecules are provided by the 
hydrocarbon phase(s). Water may come from condensed water or 
produced water. Pressure difference is required for fluid transportation, 
and the system temperature could be low if the ambient temperature is 
low (e.g., subsea transfer lines) or due to sudden expansion. 
2. In processing facilities, such as NGL plants (where the inlet gas is 
cooled to remove heavier hydrocarbons), or after control valves. 
3. In heat exchangers where fluid temperature is reduced. 
4. In marine sediments and permafrost regions. In marine sediments the 
hydrostatic pressure combined with low water temperature provides 
favourable conditions for gas hydrate formation – the gases being 
released by biogenic and thermogenic activity (see figure 3). 
Reminder-1: Since the maximum density of liquid water occurs at 4
o
C, 
this water will sink to the sea bottom, maintaining a low temperature 
condition near to the sea floor. 

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TOPIC 1: Gas Hydrates 
 
 
 
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Reminder-2: Micro-organisms at the top few hundred metres of marine 
sediments consume organic materials within the sediments and 
produce various gases, mainly methane. The resulting gas is termed as 
biogenic gas. 
Reminder-3: Organic materials transferred to higher depths are 
decomposed due to high pressure and temperature conditions. The 
resulting hydrocarbons migrate upward due to lower density, compared 
with that of water in the porous media. This type of hydrocarbon is 
called thermogenic. 
5. In permafrost regions the ice overburden pressure and pore water 
pressure (inside the sediments where temperature is higher than ice 
melting point), combined with low temperature environment, provide 
favourable conditions for gas hydrate formation. 
The geographic location of gas hydrate deposits are shown in Figure 3: 
Figure 3: Locations of Identified Hydrate Deposits (adapted from Koh 
and Sloan, 2008) 
6. In offshore drilling, the drilling fluid pressure together with low 
temperature conditions in deepwater and arctic drilling provides the 
necessary conditions for gas hydrate formation. 
Reminder-4: If a gas kick occurs then the gas influx must be circulated 
out of the drill string. If conditions are favourable, then this can lead to 
hydrate formation. 

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TOPIC 1: Gas Hydrates 
 
 
 
8 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3