Hydrates are crystallized, compact, porous and rather light mass, similar to compressed snow. They are made of water, hydrocarbons, H2S and CO2.
Unlike ice, hydrates have an unusual characteristic: they form at a temperature that is above water freezing point . For instance, they can form at 20 °C at particular pressures.
When they are exposed to air, they dissolve chugging and fizzing because of the gas that is more or less slowly freed according to the surrounding temperature. When they are lit in the air, they can slowly and completely
burn, until they leave a small residue of water. This does not represent the quantity of water they actually contain, as part of the water content is lost through evaporation. Hydrates form quite easily by simple contact of gas
and water and the formation is related to the conditions of temperature and pressure, according to the law of equilibrium.
They can quite frequently form in lines that gather gas from the various wells and transport it to the central treatment station where it undergoes dehydration.
This formation of hydrates can partially or totally obstruct the lines, limiting or hindering the transportation of gas
Hydrates can form only if the following conditions occur:
a) presence of H2O in the liquid state
b) presence of hydrocarbons
c) turbulence (created by curves, collisions etc)
d) for a given pressure, only if the temperature is lower than a certain value.
The researcher Donald L. Katz experimented the mixtures of gas with different compositions to determine the formation curves of hydrates.
read more about Gas Dehydration
Methods to prevent the formation of hydrates
In order to reduce the possible formation of hydrates in a line that transports natural gas it is essential that:
a) there is no free water;
b) the temperature is higher than that required for hydrates formation.
If one of these conditions cannot be removed, it is necessary to use other systems to prevent the formation of hydrates:
a) the use of inhibitors (anti-freezing agents), i.e. substances capable of
decreasing the hydrate formation temperature.
b) Water removal from the gas
The method for hydrates elimination depends on the nature of the obstruction.
Partial clogging: can be located when the pressure and the gas flow in the conducts vary. This is due to a partial obstruction in the conduct.
In this case, the operator should check if a hydrate prevention system has been installed upstream the hydrate clog (i.e. a heater or an inhibitor injection system) and if it functions properly..
The interventions that can follow are:
– temperature increasing of the outlet gas released by the heater;
– Increasing of the inhibitor rate.
– The inhibitors injections upstream the choke.
Complete clogging: The complete clogging with hydrates provokes a total production shutdown, so the methods adopted in case of partial clogging are no longer valid. In this case, decompression will be the proper method
The decompression must be carefully carried out, since it can be dangerous.
First of all, the pressure upstream the choke has to be reduced by balancing the pressures upstream & downstream the choke itself. This is done in order to avoid that a higher ΔP and the hydrates clog detachment can have evident consequences on the pipeline.
Once the pressure is balanced, the contemporary released upstream and downstream depressurisation must reach the atmospheric pressure.
At this point the clog melts on its own, having absorbed the molecules of gas at the pressure level in which the hydrate had formed.
From the nature of hydrates and their formation it can be stated that, correct gas treatment must be carried out so that such dangerous formations are avoided.