When a producing well’s output approaches an economically impractical level, producers often turn to more specialized operations to extend the well’s life. One of the more common techniques is a gas lift system in which natural gas is injected back into the well. The gas is typically measured prior to injection and the combined production and injection gas stream (referred to as return gas) is subsequently measured. The "formation production" is generally determined by calculating the difference between the return gas and injection gas volumes. The sales gas is then measured at the point of custody transfer and any fuel consumption used for compression or by other equipment is assessed. The total formation production volume less the sales gas and consumed fuel is the basis for performing system balances.
However, there is a common reporting problem in balancing a gas lift system with this approach. Even in the absence of other measurement problems, an imbalance is often an unusually large percentage of the formation production volume which is determined by taking the difference between two relatively large numbers: the injection volume and the return volume. When this occurs, stating the imbalance as a percentage of formation production is significantly less useful, as demonstrated by the following:
In this case history summary, the uncertainty compared to the calculated formation production is 38%; as a percentage of the total measured volume, however, the uncertainty is only 2%.
The issue of expressing the imbalance as a percentage of the formation production cannot be resolved since this is inherent in the formation production calculation method itself. The uncertainty percentage is better understood when expressed in terms of the total measured volume. Otherwise, normal variances in formation production uncertainty may mask actual measurement or operational issues.
Measurement problems that are commonly experienced in gas lift systems will be covered in Part II of this Statement of the Month topic in January 2018. In the meantime, if you believe that a measurement problem exists in your system, be sure to consult with your measurement experts, whether in-house or third-party, for a thorough analysis and final resolution.
"We are what we repeatedly do. Excellence, then, is not an act, but a habit."
~ Aristotle, Ancient Greek Philosopher and Scientist