Flow Assurance (2)

Gas Hydrate

 

Gas hydrate is crystalline solids wherein guest (generally gas) molecules are trapped in cages formed from hydrogen bonded water molecules (host).  They are formed as a result of physical combination of water and gas molecules, i.e. methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfide. 

The necessary conditions are:

– Presence of water or ice

– Gas molecules C₁, C₂, C₃, n-C₄, i-C₄, CO₂, N₂, H₂S

– Suitable temperature and pressure conditions

Figure below shows typical of hydrate curve. Left side is region where hydrate can be formed. When system pressure and temperature is within this region, water and gas will form hydrate. Right side is non-hydrate region. When system pressure and temperature is within this region, water and gas will not form hydrate.

Fig 1. Hydrate curve

Some scenarios which hydrate can occur are:

o   gas expansion or cooling effect

o   startup and shutdown

Hydrate can easily form at downstream of choke valve when system pressure and temperature decreases and it reaches right side of hydrate curve.

 

There are some methods to avoid hydrate problems, i.e.:

1.       Water removal (dehydration)

2.       Increasing/maintaining temperature

– Insulation

– Heating

3.       Decreasing pressure

4.       Injection of thermodynamic inhibitors

– Methanol, ethanol, glycols

5.       Using Low Dosage Hydrate Inhibitors

– Kinetic hydrate inhibitors (KHI)

– Anti-Agglomerants (AA)

6.       Various combinations of the above

 

Source :

Boyun Guo, Shanhong Song, Jacob Chacko, Ali Ghalambor, Offshore Pipelines, Gulf Professional Publishing, Oxford, 2005

E. Dendy Sloan Jr., Natural Gas Hydrates, SPE, Colorado School of Mines, 1991.

Bahman Tohidi, Gas Hydrates: Friend or Foe?, SPE, Heriot-Watt University, Edinburgh, UK.