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Abstract The impact of an operational failure during a coiled tubing (CT) intervention is typically more severe than that of other failures because of the nature of the activity. Failure of the tubing or any component of the well intervention process in a live well scenario can compromise well control and/or the safety of personnel. Statistics on causes of CT operational failures (OFs) indicate that a majority of these failures can be attributed to human error. Incorrect actions, or the lack of action, are very difficult to predict and therefore a major challenge to control. Running CT in and out of the well involves a high degree of human interaction and human fatigue, and short periods of inattention during this process are not uncommon.
During such activities, inattention can lead to actions that damage, kink or part the CT, with potentially disastrous results. Other causes for OFs include unintentional tensile overloads, overpressuring, runaways and other such events.
An electric over-ride device, developed for installation in the hydraulic circuitry of a CT unit, allows setting of limits on all pertinent operating parameters of the injector head. Setting equipment limits for weight, velocity and pressure gives the operator an extra set of eyes, greatly increasing operational safety and efficiency of the treatment.
This paper discusses OFs caused by human error and presents case histories that contributed to the conclusion of which parameters require control. The over-ride device used in the control process is discussed in technical detail, and case histories demonstrate the impact of its use on overall safety and service quality in the CT industry.
Introduction CT material failures have been an industry focus for some time. Comprehensive research in the failure mechanisms of the tubing steel has made the behavior of low-carbon steel fairly well known, and this behavior is well documented. CT failures are typically very serious, but they are not the only failures to consider. Total system improvement through better service quality is obtainable through an investigation of all failures associated with CT well interventions, including those caused by equipment failure or human error.
A database of companywide, in-house service quality statistics on OFs is used to identify problem areas. Data are drawn from worldwide CT operations for 2000 that represent a cross section of CT activites in the industry; shallow to deep land operations, arctic operations, offshore platform operations and deepwater work.
Coiled Tubing Failures
OFs that do not lead to injuries are not systematically or consistently tracked and reported across the industry. Unlike safety statistics, which are readily available in a standard format, service quality statistics are still very much organization specific and therefore do not allow for industry benchmarks.
Schlumberger has defined and adapted mandatory service quality indicators for all product lines. These are grouped in the following categories:Severity of the OF based on nonproductive time (NPT) and financial loss
Catastrophic operational failure (COF): NPT >48 hr and/or loss >$500,000
Major operational failure (MOF): 12 hr
A New Approach to Monitor Tubing Limits
Zheng, Andrew S. (Schlumberger) | van Adrichem, Willem P. (Schlumberger)
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. Introduction 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. 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. 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.
Coiled Tubing Failure Statistics Used to Develop Tubing Performance Indicators
van Adrichem, Willem P. (Schlumberger Dowell)
This paper was prepared for presentation at the 1999 SPE/ICoTA Coiled Tubing Roundtable held in Houston, Texas, 25–26 May 1999.
Using Stainless Steel Coiled Tubing in a Novel Application
van Adrichem, Willem P. (Schlumberger Dowell) | Keen, Kelly (Schlumberger Dowell) | Henquet, Henri (IDGS / UGE)
Abstract Stainless steel coiled tubing (CT) was used for the first time in Spain in 1997, to convey a burner as part of an underground Coal Gasification feasibility project by Underground Gasification Europe (UGE). Minimizing the risk of combustion associated with contact between pure oxygen and carbon steeldictated the requirement of stainless steel coiled tubing for this project. Additionally, stainless steel CT was required to minimize the corrosive aggressiveness of pure oxygen. The 1.75" 316 L stainless steel coiled tubing used for this project, was milled in a continuous length in Switzerland and tested for its low cycle fatigue properties in Houston. Two stainless steel control lines were installed inside the coiled tubing to initiate the burning process, together with a thermocouple to monitor the downhole temperature during the gasification. This paper discusses the role that stainless steel coiled tubing played in this underground coal gasification project as well as the development and testing of all the components. Background Coal has been a known source of energy for centuries. Although high in energy content, coal usage has been in steady decline since the mid 1960's. The main disadvantages of coal have always been that mining is a labor intensive and potentially dangerous occupation and that the residues (ash and other components) have to be disposed of after the carbon has burned from the coal. In the seventies, methods were developed to extract the energy from the coal, through an above ground process. This process still left developers with the task to mine the coal, but coal did not have to be shipped to consumers anymore, which simplified residue disposal. The concept of gasifying the coal underground therefore is very attractive. This would enable the 'energy content' of the coal to be brought to surface, while leaving the residue underground. In addition to disposal advantages, it could also be used to exploit coal deposits which are not accessible by conventional methods, like the known coal reserves below the North Sea.
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- North America > United States > Texas (0.47)