1.4 Gas Hydrates in Pipelines As discussed before, gas hydrates can form in pipelines causing serious
operational and safety problems. They can form under many normal
operations, such as:
•
Existence of Expansion and Cooling Effect: For many fluids,
sudden expansion will result in cooling (Joule-Thomson effect). This
could happen after control valves and chokes and may result in gas
hydrate formation.
Also in the vertical movement of fluids, where the pressure reduces,
gas will come out of solution and gain its heat of vaporisation from the
surrounding fluid – again resulting in a reduction in temperature.
•
Start-up and Shutdowns: Generally, in subsea pipelines the sea
water temperature is much lower than the reservoir fluid temperature.
During start-up, the initial pipeline temperature is very close, if not
equal, to the seawater temperature. Therefore the introduction of
reservoir fluid to the cold pipeline may result in the formation of gas
hydrates as the reservoir fluid cools.
During shut-downs, as the fluid flow is reduced and finally stopped, the
fluid temperature will drop. This can also result in gas hydrate
formation and pipeline blockage.
•
Well Clean-up and Testing in Offshore Wells: Generally, the well
column is filled with water or drilling fluid and corrosion inhibitors. With
time, the temperature inside the well tubing approaches that of
seawater in offshore wells.
Thus, during cleaning operations, the reservoir fluids come into contact
with cold water which could result in gas hydrate formation and tubing
blockage. Similarly, during testing the fluid temperature can drop,
resulting in gas hydrate formation.
