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

1.3.2 Simple Gas Hydrates 
If gas hydrates are formed from one type of gas, the result is called a simple 
gas hydrates. Methane, ethane and CO
2
are known to form simple structure-I 
gas hydrates. Propane, i-C
4
H
10
, benzene and cyclohexane form structure-II 
gas hydrates. Very small molecules like N
2
and O
2
are reported to form 
structure-II gas hydrates at least at very low temperature. 
Recent investigations show that some of these molecules (like N
2
) form 
structure-I gas hydrates, at least at temperatures above 0
o
C. No single 
compound is known to form structure-H simple gas hydrates, as there is only 
one large cavity per unit structure. 
1.3.3 Double Gas Hydrates 
When two or more types of gas molecules take part in gas hydrate formation, 
the result is called double gas hydrates. Binary or multi-component 
combinations of CH
4
, C
2
H
6
, CO
2
and smaller molecules generally form 
structure-I gas hydrates. 
Binary or multi-component mixtures of small molecules (such as CH
4
, C
2
H
6
, 
CO
2
and N
2
) with medium size molecules (such as C
3
H
8
, i-C
4
H
10
, n-C
4
H
10
, c-
C
5
H
10
, c-C
6
H
12
and benzene) generally will form structure-II hydrates. 
Structure-I can be the stable structure when concentration of medium size 
molecules is very low. 
Very large molecules (such as; C
7
H
14
, Adamantane) with small molecules 
(such as CH
4
, C
2
H
6
, CO
2
and N
2
) may form structure-H hydrates. In structure-
H, large cages are mainly filled with very large molecules, while small 
molecules will mainly occupy small and medium size cages. 
However, when structure-II hydrate formers are present (e.g., C
3
H
8
, i-C
4
H
10
, c-
C
5
H
10
, and benzene), structure-II is generally more stable than structure-H, 
except when the concentration of structure-H heavy hydrate formers is very 
high. 
The required concentration for a changeover from structure-II to structure-H 
has been experimentally tested for some systems. Although, the critical 
concentration is expected to be different for various systems, it is much higher 
than what we can find in natural systems. 
Recent studies have shown that the stable hydrate structure can change from 
sH to sI in some CH
4
/sH hydrate formers as system temperature and pressure 
is increased (e.g., CH
4
/i-C
5
H
12
systems). In general, it seems that structure-H 
hydrates are the least stable gas hydrate structure in real systems. 

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TOPIC 1: Gas Hydrates 
 
 
 
18 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
For most simple systems it is possible to guess the stable gas hydrate 
structures by considering the fluid composition. Also it is possible to determine 
the stable hydrate structure by calculating the hydrate dissociation point at a 
given temperature for each of the gas hydrate structures. 
The structure which corresponds to the lowest pressure is the stable hydrate 
structure for the system under investigation. Some computer programmes are 
equipped with Gibbs free energy minimisation routines, where the stable gas 
hydrate structure and the existing phases are calculated automatically. 
Obviously there would be more CPU time and a good initial guess is generally 
required to ensure convergence.