As covered in Part I of "Balancing a Gas Lift System" (November 2017), it’s recommended that imbalances be expressed as a percentage of total measurement to effectively monitor for measurement and operational problems. Part II of this topic presents three of the most common measurement problems that can cause excessive "Lost and Un-Accounted For
" (LUAF) quantities in gas lift systems. When these occur, measurement error is often severe.
Compressor generated pulsation
can be one of the most difficult measurement problems to identify. The most effective way to avoid this problem is to install meters at an appropriate distance from the compressors, but this may not always be an option…especially in a gas lift system. Other measures to reduce or even eliminate pulsation include:
- Installing a smaller orifice plate to increase the differential pressure.
- Installing pulsation dampeners between the source and the meter.
- Shortening the gauge lines or using direct mount block manifolds.
Meters affected by pulsation will usually overstate the volume, although understatement is possible, and the impact of pulsation-induced errors can easily exceed 50% of the volume.
can also cause measurement problems in a gas lift system. Among the good practices that can mitigate measurement errors from free liquids at the meter are:
- Installing meter fittings with vertical taps.
- Installing gauge lines with sufficient slope.
- Installing additional liquid separation, as necessary.
Errors related to free liquids can overstate or understate volumes and can range from insignificant to severe depending upon how much liquid is present and where it’s located at the meter.
Unrepresentative gas analyses
can affect measurement accuracy in a gas lift system. A single analysis applied to both injection and return meters may introduce errors as their gas qualities can vary considerably. Accurate gas sampling can be a challenge, in part, due to contamination from free liquids. However, most streams can be accurately sampled when using the proper equipment and procedures, as discussed in GPA Standard 2166.
While more than 50 common sources of orifice measurement errors have been expressly identified, compressor generated pulsation, free liquids, and unrepresentative gas analyses are very common in gas lift systems. Whenever possible, proactive measures should be taken at start-up to minimize or eliminate these potential sources of error. And since these problems can be difficult to identify and resolve, you should consult with your measurement experts, whether in-house or third-party, for assistance.
"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
~ Arthur Conan Doyle, British author and physician