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

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B41OA 
Oil and Gas Processing 
Section A – Flow Assurance 
Heriot-Watt University 
Edinburgh EH14 4AS, United Kingdom 
Produced by Heriot-Watt University, 2018 

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TOPIC 1: Gas Hydrates 
 
 
 
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1.1 Introduction 
Gas hydrates are crystalline compounds formed as a result of physical 
combination of water and suitable size molecules at appropriate conditions of 
pressure and temperature. 
Neighbouring water molecules in the liquid phase are connected together in an 
approximately tetrahedral network through hydrogen bonding. Under the right 
conditions this network can be formed into cage-like structures, which are 
unstable. 
Suitably sized molecules, referred to as “guests”, can be trapped in these 
cages – thus, stabilising the whole structure and leading to the formation of 
solid gas hydrates. The formation of gas hydrates is not limited to gas 
molecules; it is known that some molecules that are liquid under ambient 
conditions (e.g., benzene, tetrahydrofuran), can also be trapped in the cavities 
made by water molecules, resulting in the formation of gas hydrates. 
Gas (or liquid) molecules in the gas hydrate structure are called guest 
molecules. Gas hydrates are similar to ice, but unlike ice they can form at 
temperatures well above ice point. Because they contain around 85% water, in 
some respects they behave very much like ice, but they do have some more 
unusual properties (Sloan and Koh, 2008). 
When they contain combustible molecules like methane they can be burned 
directly, as shown in figure 1. 
Figure 1: Direct Combustion of Methane Hydrate 
(http://blogs.discovermagazine.com/d-brief/2013/03/12/japan-
becomes-first-to-extract-gas-from-frozen-methane/#.V3uD4PkrKM8) 

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TOPIC 1: Gas Hydrates 
 
 
 
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Another interesting property of gas hydrates is the fact that unlike inorganic 
hydrates (e.g., CuSO
4
.5H
2
O) the ratio between water and gas is not constant 
(i.e., the hydration number is variable); it depends amongst other things on 
temperature, pressure and guest species. In fact this point was the reason 
behind much confusion and debate in the past. This point will be discussed in 
more detail later. 
Gas hydrates were discovered in 1810 by Sir Humphrey Davy, though some 
scientists believed that its discovery goes back to 1770s. Initially it was a 
scientific curiosity on how a combination of water and gas formed a solid 
compound at temperatures higher than 0
o
C. 
For the next century or so, several scientists worked on gas hydrates finding 
new compounds that can form gas hydrates. Some other scientists 
concentrated their research on finding the hydration number of various gas 
hydrates. 
In 1934, Hammerschmidt found that gas hydrates, and not ice, were 
responsible for gas pipeline blockage, see figure 2: 
Figure 2: A Hydrate Plug 
(http://www.itp-interpipe.com/products/subsea-production-
flowlines/heat-traced-flowlines.php) 
This discovery caused considerable amount of funds to be directed towards 
gas hydrate research and, over the following 60 years, this funded research 
into the thermodynamics of hydrates – leading eventually to methods that 
predict the conditions required for their formation. 
The main objectives were to find the crystal structures, to define the phase 
boundary for gas hydrates of different gases and their mixtures and to develop 
methods for the prediction and the prevention of hydrate formation. 
In the 1960s natural gas hydrates were discovered in permafrost regions and 
marine sediments (Makogon, 1965). Subsequently, various estimates on the 
amount of gas hydrates present in these deposits led to the consensus that 
they are considerably higher than the total conventional fossil fuels (Makogon, 

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TOPIC 1: Gas Hydrates 
 
 
 
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1988 and Klauda and Sandler, 2005). Some scientists think that gas from gas 
hydrates will play a major role in the energy supply of the world and some put 
a contribution of up to 15% within a decade. 
Today gas hydrates provide various challenges and opportunities in science 
and engineering. They are a potential hazard in deepwater drilling and provide 
an opportunity for processing, storage and transportation of oil and gas. 
It is also believed that they had important buffering effect on glacial periods 
and that they could be responsible for massive submarine landslides and 
destructive tidal waves. They also provide an interesting opportunity for the 
sequestration of CO
2
. 
The necessary conditions for gas hydrate formation are as follows: 
•
The presence of water or ice. 
•
The presence of suitably sized non-polar or slightly polar gas (or liquid) 
molecule. 
•
And suitable conditions of pressure and temperature. 
High pressure and low temperature promote gas hydrate formation. Very small 
molecules like helium cannot form gas hydrates, as they can escape through 
the faces of the cage structure. Also, very large molecules won’t form gas 
hydrates as they won’t fit in the cage. Various cages and structures involved in 
gas hydrate formation will be discussed later. 
There is no need for the presence of a gas phase for the formation of gas 
hydrates, i.e., gas hydrates can form from liquid hydrocarbon systems. It is 
believed that gas hydrates, which form in a liquid hydrocarbon phase system, 
are more transportable. 
In addition, there is no need for the presence of a water-rich phase (i.e., free 
water). Water in the form of mist – resulting from condensation – can provide 
the necessary hydrogen bonding for gas hydrate formation. 
Another important point is that gas hydrates do not need particularly low 
temperature or particularly high pressure conditions for their formation. The 
exact condition for gas hydrate formation depends on system composition 
(including exact composition of the hydrate forming mixture), the composition 
of water phase (e.g., presence of salts and/or other inhibitors), and kinetic 
factors. 
The combination of these factors can lead to gas hydrates being formed at 
surprisingly high temperature and low pressure conditions. 

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TOPIC 1: Gas Hydrates 
 
 
 
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The extent of gas hydrate formation and the resulting problems depend on the 
following factors: 
•
The amount of water and hydrate forming compounds: as one of them 
can act as limiting reactant. 
•
The composition of the fluid and water: as the hydrate free zone is a 
function of fluid composition and water activity. 
•
The system temperature and pressure conditions: the further the 
system is into the hydrate stable zone the more hydrate will form. 
•
The amount of kinetic and/or thermodynamic inhibitors: thermodynamic 
inhibitors shift the hydrate phase boundary to higher pressures or lower 
temperatures. Kinetic inhibitors, in general, increase the induction time 
for gas hydrate formation (see later discussion). 
•
The presence of natural inhibitors: some fluids have natural inhibitors 
reducing the severity of gas hydrate problems. 
•
Issues involving heat and mass transfer: a supply of gas molecules (or 
water molecules) is necessary for the growth of gas hydrate crystals. 
Also gas hydrate formation is an exothermic process and heat is 
released during hydrate formation. Any hindrance to mass and/or heat 
transfer will reduce the extent of hydrate formation. 
•
Some other important factors include the presence of growth modifiers, 
local restriction, fluid type, pipe wall characteristics, etc.