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.
Operators face new technical challenges as coiled tubing applications continue to extend into high-pressure (> 10,000 psi wellhead pressure) and deep wells (> 20,000 ft). Because of the inherent risk in high-pressure, deep-well coiled tubing operations, it is of paramount important to maintain the coiled tubing's structural integrity to ensure safe operation. As a result, a real-time monitoring of coiled tubing tension and pressure limits is necessary to ensure that the tubing has adequate strength for its intended operation.
The conventional way to monitor coiled tubing tension and pressure limits in real-time is to monitor a fixed section— usually the section right below the stripper. This works well for single-wall tubing in a shallow well with low pressure. In high-pressure, deep-wells, however, tapered strings are often used. For a tapered string, it is very likely that the critical section of the tubing downhole is not at the section below the stripper, but farther down. Even for a single-wall coiled tubing string, the most critical section may not be at the section below the stripper if buckling occurs along the tubing string. In these cases, the conventional method to monitor tubing limits below the stripper is inadequate. Thus, a new way to monitor tubing limits is needed.
An adequate method to monitor tubing limits should be able to track the critical section of the tubing string downhole and allow operators to monitor this section in real-time. It is conceivable that during operation, because of different tubing makeups (such as in a tapered string) and different well completions, the critical section of the tubing may change depending on the operating depth. Thus, to identify the critical section, it is necessary to have a tubing force model and a pressure model that can calculate the distributions of tubing force and pressure difference along the tubing string.
The most popular tubing force model is the so-called soft-string model, which assumes that in term of axial force transmission along the tubing string, the tubing doesn't have any bending rigidity.1–4 On the other hand, the actual bending rigidity of tubing is used to determine the effect of buckling on the tubing force calculation. For coiled tubing applications, this soft-string model has been validated extensively.4 It has also been demonstrated that for coiled tubing involving buckling, it is most convenient to use the effective force in formulating the soft-string model for tubing force calculation.2