A New Approach to Monitor Tubing Limits

Abstract This paper presents a new approach to monitor coiled tubing tension and pressure limits during operation. The conventional way to monitor the tubing limits in real time is through monitoring the section right below the stripper. It works well for a single-wall tubing string operating in a shallow well with low wellhead pressure. For high-pressure wells, deep wells or high deviation wells, this becomes inadequate because the critical section may not be at the section below the stripper. Instead of monitoring the tubing limits at a specific section (such as the section below the stripper), the model presented herein dynamically tracks and monitors the critical section of the tubing during operation. Our approach starts with a new way to express the tension and pressure limit curve of the coiled tubing. The use of effective force to generate the limit curve facilitates the task to identify the critical section of the coiled tubing during operation. To identify the critical section, the distributions of tubing force and pressure difference along the coiled tubing downhole are calculated during the operation. This information is used to calculate the safety factor along the coiled tubing string, from which the critical section is identified. A software based on this approach has been developed, which allows operators to dynamically track and monitor the critical sections during operation. By coupling with surface measurements (such as ovality) in real time, it significantly improves operators’ ability to monitor tubing limits.