TECHNICAL INFORMATION

Potential problems

Equipment
Produced Scale DepositionMost of the gaslift valves in common use are very reliable, in ideal conditions! They are sensitive, however, to mechanical impairment caused by solids. Solids may be naturally formed or could be debris introduced into the well from surface. They are discussed below:

Natural Scale
Scale can originate from two sources:

  • The well production
  • The gaslift gas

Well production scale formation should not adversely affect the lifting performance of a gaslift valve while in service as the gas flow should keep the injection port clean. However wireline access for valve retrieval may be hampered as scale builds-up on the fishing neck and surrounding tubing (figure right).

Tubing fluid treatments can be effective in removing the scale, but it is now more common for squeeze inhibition treatments to be carried out on the formation, thus preventing scale deposition in the well bore. The effectiveness of such treatments can last for several months depending on the problem severity. The treatment is administered by introducing the inhibitor to the tubing and squeezing it with pressure into the formation. In order to more accurately "spot" the fluid treatment, coiled tubing can be used to locate the fluid opposite the perforations from where it is squeezed into the formation.

Any scale formed by the liftgas can cause a problem in the small ports of a gaslift valve. Any scaling tendencies of the gas should have been analysed, however and appropriate inhibition added to the liftgas at surface.

Debris
One of the biggest sources of problem with gaslift valves is annulus debris.

In particular any debris around at the time of "kicking-off" the well or unloading the annulus fluid is a serious risk. Any blockage of a gaslift valve at this stage can involve an appreciable downtime period as wireline servicing is mobilized and valves are changed-out.

The costs involved in well-servicing, rig-time and lost production can amount to a staggering sum.

Debris introduced at the time of completion from the surface fluids system can quite easily be excluded by careful filtration at the time of completion or workover. Cementation debris should have been removed prior to completion by bit and scraper runs and thorough circulation.

The other main, source of debris is the externals of tubing and the internals of production casing (bottom left). By far the worst form of debris is millscale, which, when dislodged takes the form of hard flakes.

The only way to eliminate this source of potential problem is to ensure that the externals of the tubing and the internals of casing are blast cleaned to bare metal prior to installation (bottom right). This is the only safe, reliable method of ensuring complete removal of millscale. Scraper runs do not remove rolled-in millscale.

Millscale Debris

In addition to these precautions all tubing should be run through a correctly sized wiper to ensure removal of excess thread compound.