Abstract
Unloading gas wells is one of the most common applications for coiled tubing (CT). Despite the large number of jobs completed, fundamental questions about the optimum gas lift rate, run-in-hole speed and how much nitrogen is required remain. This is because the unloading process is not steady state, and the commonly used CT computer simulations can only model steady state flow.
This paper describes transient software that has been developed and used to determine the nitrogen volume and cleaning time required when optimizing the process of liquid unloading from a gas well with CT.
Based on experimental test results with a full scale flow loop, a critical gas velocity model was developed. This model determines how much liquid can be lifted for a given gas rate under varying operating conditions. It also determines the minimum gas rate required for complete liquid unloading. Several examples are presented which illustrate the transient characteristics of the liquid unloading process. A few field cases illustrate the benefits of using the transient model and some problems with conventional design methods.