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

Methane Gas Hydrates 
Methane is the main gas in the natural gas hydrate. Methane is a greenhouse 
gas 21 times stronger than CO
2
. Dissociation of natural gas hydrates could 
release vast quantities of methane to the atmosphere. 
It is believed that during glacial ages where the sea levels were reduced by 
around 100 metres (due to the transfer of water as ice cap to the poles), gas 
hydrates were dissociated. 
The resulting increase in the concentration of greenhouse gases had a 
buffering effect, reducing the duration of glacial ages. Also, methane released 
from marine gas hydrates is believed to be responsible for the extinction of 
95% of life at the end Permian period some 250 million years ago. 
Hydrate Formation and Disassociation 
Gas hydrates result from the physical combination of water and gas. The 
formation and dissociation of gas hydrate are purely physical processes. 
Hydrate Trapped Gas Composition 
The composition of gas trapped in gas hydrates is different from that of the 
vapour phase. In general gas hydrate formation favours light hydrocarbons, 
i.e., increasing their concentration in their structures. This property of gas 
hydrates has been suggested for purification and distillation of gases. 
1.3 Gas Hydrate Structure 
There are three known gas hydrate structures, namely, structure-I (sI), 
structure-II (sII), and structure-H (sH): 
1. Gas hydrate structure-I is formed from light compounds, such as CH
4
and CO
2
. This structure is the dominant structure in natural gas 
hydrates. 
2. Structure-II is formed from intermediate molecules, such as C
3
H
8
,
i-C
4
H
10
, C
6
H
6
, etc. It is the stable structure in the majority of systems 
encountered in oil and gas industries. 

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3. Structure-H was discovered in 1987. It needs very large molecules 
such as C
6
H
12
, C
7
H
14
(together with small molecules as help gases). 
It is very unlikely that structure-H hydrates are formed in natural 
environments. 
Each gas hydrate structure is composed of various cavities (cages) as shown 
in Figure 6A. Structure-I has two types of cavities, commonly referred to as 
small and large. Small cavities in sI have 12 pentagonal faces (5
12
) and large 
cavities have 12 pentagonal faces plus two hexagonal faces (5
12
6
2
). 
If the 5
12
cavities are joined together by additional water molecules, the voids 
that are created are the 5
12
6
2
cavities. The crystallographic unit cell of 
structure-I has 2 small cavities and 6 large cavities made up from a total of 46 
water molecules. Therefore the maximum number of gas molecules that can 
be trapped in structure-I is 8 gas molecules per unit cell. The unit cell is a cube 
with approximately 12 Å sides. 
Gas hydrate structure-II, see Figure 6B, also has two types of cavities, i.e., 
small and large. Its small cavity is the same as that of structure-I, i.e., 5
12
. 
However, its large cavity has 12 pentagonal faces and 4 hexagonal faces 
(5
12
6
4
). 
If the large cavities are joined together by sharing of a hexagonal face, the 
voids that remain are the 5
12
cavities. The unit cell of structure-II is a cube with 
approximately 17.3 Å sides, with 16 small cavities and 8 large cavities made 
up from a total of 136 water molecules. Therefore, a maximum number of 24 
gas molecules, per unit cell, may be trapped in structure-II. 
Figure 6A: Gas Hydrate Structure-I (sI) 

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The medium cavity has 3 tetragonal faces, 6 pentagonal faces and 3 
hexagonal faces (4
3
5
6
6
3
). The large cavity has 12 pentagonal faces and 8 
hexagonal faces (5
12
6
8
). The unit cell is hexagonal (a=12.26, c=10.17) with 3 
small, 2 medium, and one large cavity made from of a total of 34 water 
molecules. 
Figure 6B: Gas Hydrate Structure-I (sI) Compared to Structure-2 (sII) 
Structure-H has three types of cavities. They are small, medium and large. 
The small cavities are 5
12
is the same as the other two structures, see Figure 
6C. 
Figure 6C: Gas Hydrate Structure-I (sI) Compared to Structure-II (sII) 
and Structure-H (sH) 

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The specifications of the three gas hydrate structures are summarised in 
Table-1 and their crystallographic structures, and construction from water 
cavities, are all shown together in Figure 6C. 
Table 1: Structural Properties of Structure-I, Structure-II and Structure-
H Gas Hydrates 
Prior to the discovery of structure-H hydrates, and heavy hydrate formers, it 
was believed that n-butane was the heaviest hydrocarbon that can form gas 
hydrates. 
However, in 1987, it was suggested that compounds like benzene, 
cyclopentane, cyclohexane and neopentane could be trapped in the large 
cavity of structure-II type gas hydrates. Also, the discovery of structure-H 
hydrates suggested that molecules as large as methylcyclohexane can form 
gas hydrates (Ripmeester et al., 1987). 
The above discoveries sparked numerous research programmes to quantify 
the hydrate characteristics of the newly discovered hydrate formers and more 
importantly their impact on the hydrate phase boundary of real reservoir fluids.
The results showed that structure-H is very unlikely to be the stable structure 
in real systems. Therefore, the exclusion of structure-H heavy hydrate formers 
would not introduce any major error in the calculations. However, structure-II 
heavy hydrate formers should be taken into account for a more reliable phase 
boundary prediction in real reservoir fluids. 

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