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

1.7 The Hydrate Stability Zone: Analytical Approach 
With all gas hydrates flow assurance techniques, it is very important to 
determine the hydrate stability zone and the effect of various production 
scenarios and inhibitor injection. As mentioned earlier, the hydrate stability 
zone depends on fluid and water compositions and it can be determined by 
experimental or predictive techniques. In this section, some of the 
experimental and predictive techniques used for predicting the hydrate stability 
zone are discussed. 
1.7.1 Hydrate Stability Zone for Pure Compounds 
Figure 19 shows the hydrate stability zone of some pure compounds. On a 
semi-log scale the hydrate stability zones are almost straight lines. Three 
different regions can be identified for most compounds, see below: 
Figure 19: The Hydrate Stability Zone for Some Compounds. 
Starting from low temperature conditions first is the Ice-Hydrate-Vapour (I-H-V) 
line, where these three phases are in equilibrium. 
Next is the lower quadruple point (Q1), where the four phases of Ice-Hydrate-
Vapour-Water (I-H-V-LW) are in equilibrium. 

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TOPIC 1: Gas Hydrates 
 
 
 
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Passing the water phase freezing point is the Water-Hydrate-Vapour (LW-H-V) 
line – the intersection of this line with the vapour pressure line results in the 
upper quadruple point (Q2) where Water-Hydrate-Vapour-Liquid Hydrocarbon 
(LW-H-V-LHC) are in equilibrium. 
Finally, we have the three phase line of Water-Hydrate-Liquid Hydrocarbon 
(LW-H-LHC). Some compounds present exception to the above general rule 
(for example methane, where there is no intersection between the LW-H-V line 
and the vapour pressure line). 
Typical hydrate stability zones for a binary or multi-component system are 
presented in Figure 20. Also shown in the figure is a typical phase envelope 
for binary or multicomponent system. 
The hydrate phase boundary is essentially similar to that of pure compounds, 
with the exception of the upper quadruple point. As shown in the figure 20, the 
upper quadruple point, where Water-Hydrate-Vapour-Liquid Hydrocarbon are 
in equilibrium, is converted into a line (corresponding to the two phase region 
inside the phase envelope). 
Figure 20: Hydrate Stability Zone – Binary & Multicomponent Systems.