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

1.8 The Hydrate Stability Zone: Experimental Approach 
Different techniques and equipment are used in measuring the hydrate stability 
zone experimentally. The two main methods in detecting hydrate formation 
and dissociation use the following approach: 
1. A visual determination using one of three operations (discussed 
below). 
2. A graphical technique. 
Three procedures are in use for hydrate point measurements (when a liquid 
water-rich phase is present) – these are as follows: 
•
Isothermal operation. 
•
Isobaric operation. 
•
Isochoric operation. 
Visual Determination of Hydrate Point 
For many years the determination of hydrate dissociation point was primarily 
based on visual observation through a sight glass. It was only after the 1940’s, 
due to safety considerations, that non-visual means of hydrate detection were 
revived. 
With the isothermal operation, the temperature of the cell is kept constant and 
hydrates are formed or dissociated by increasing/decreasing the cell pressure; 
this is achieved by the introduction/withdrawal of gas, or by changing the cell 
volume. The former cannot be used for multicomponent systems (as it 
changes the feed composition). 
With the isobaric operation, the cell pressure is kept constant by the 
introduction/withdrawal of gas, or by changing the cell volume. Hydrates are 
formed/dissociated by decreasing/increasing the cell temperature – the 
limitations are the same as isothermal operations. 
With isochoric operation the cell volume is kept constant and the temperature 
of the cell is decreased/increased for formation/dissociation of hydrates and 
the pressure is monitored constantly. It is important that at for each stepwise 
change enough retention time is given for the system to be in equilibrium. 
Graphical Determination of Hydrate Point 
The graphical method enables the hydrate phase boundary measurements at 
much higher pressures. Also, the use of the graphical method minimises the 
manpower, as all the necessary variations and recording of the system 
variables can be conducted by a computer – the collected data can even be 
analysed later for hydrate point determination. Furthermore, the method is less 
susceptive to the operator’s judgement than the visual technique. 

---

TOPIC 1: Gas Hydrates 
 
 
 
58 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
The main shortcoming of the graphical technique is that it may face some 
problems in systems without a vapour phase, in particular if the pressure 
transducer is not very accurate and/or fluctuations occur in bath temperature. 
For these cases the isochoric operation should be coupled with the visual 
observations. However, the graphical method seems to be the only reliable 
method for measuring the hydrate dissociation point for oil systems, due to 
poor visibility and possible emulsion formation in such cases. 
To determine the hydrate dissociation point, the cell is charged with water and 
the test fluid: 
•
The temperature is lowered until hydrates are formed. 
•
The temperature is then raised stepwise allowing sufficient time (at 
least 4 hours) at each step for equilibrium to be achieved. 
•
At each temperatures step below the dissociation point, some hydrates 
are decomposed and the released gas raises the pressure markedly. 
•
Once the temperature is above the hydrate dissociation point, with no 
more hydrates to decompose, the pressure rise is quite small. 
•
A plot of the pressure versus temperature will therefore give two 
distinct lines of different slopes, with the break over at the dissociation 
point. Figure 31 shows typical hydrate dissociation point measurement 
using graphical techniques. 
Figure 31: Graphical Hydrate Dissociation Point Determination 
With visual techniques, at each temperature or pressure step the system is 
checked for the presence or the absence of solid gas hydrates. According to 
some researchers who have used visual techniques, if the pressure and 
temperature of the system remain constant for 3-4 hours with a negligibly 
small quantity of hydrates present, the measured pressure and temperature 
are taken as the dissociation conditions. 

---

TOPIC 1: Gas Hydrates 
 
 
 
59 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3