In-Line Quench-and-Temper Technology Applied to CT Improves Safety and Reliability
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In unconventional resource plays across the United States, lateral well sections are being extended beyond 10,000 ft. The increased application of larger coiled tubing (CT) of 2.375-in. and 2.625-in. diameter for operations in high-pressure wells has pushed the reliability envelope for conventional CT strings. This paper discusses the advantages of the in-line quench-and-temper (Q&T) process, which enhances overall CT life and reliability by producing tubing with more-uniform microstructure throughout its entire length, increased material strength, and improved bend-fatigue performance.
Applying Q&T technology to CT has transformed well intervention in unconventional shale plays, particularly for multistage fracturing applications in 10,000-ft laterals or longer that are commonly completed with more than 80 intervals per wellbore. With some laterals 14,000 to 16,000 ft long, with 130 or more entry points, manufacturers and service companies have strived to deliver safe, cost-effective solutions for operational challenges. The implementation of technological innovations in surface equipment, downhole tools, and Q&T custom-engineered CT strings, along with refined operational practices and logistics, allows the performance of low-risk completions with over 10,000‑ft laterals on a larger scale. At the time of writing, predictions suggested that more than 90% of the US CT market would switch to Q&T products by the end of 2018.
Extended-Reach CT Design With Rapid-Taper-Strip Technology
Rapid-taper-strip technology, which features a quick increase or decrease of wall thicknesses within the CT string, is an engineered solution for CT applications in difficult well conditions and geometries. This technology provides rapid wall-thickness strip transitions of 300‑ft to 700-ft long to place specific thicknesses purposefully along the length of the string.
Rapid-taper strips combined with Q&T grades enable engineers to design unique string configurations that can achieve unprecedented well lateral reach and service life. The technology helps minimize tubing weight in the horizontal section and increase stiffness in the vertical section to avoid the onset of CT buckling inside the wells.
After its successful application in North America, the flexibility and operational benefits of this strip technology became well known. It has expanded into other applications, particularly in unconventional-resource developments where CT is subjected to extremely challenging environments.
With weight restrictions being the predominant challenge in extended-reach CT-string designs, the flexibility of rapid-taper strips allows for custom-engineered hourglass CT strings that maximize reach, overpull, and fatigue life while meeting CT surface-equipment-design constraints. An hourglass string configuration features a reduced wall thickness in the upper end of the CT string. This reduced wall provides safe overpull capacity during operations. Hourglass CT-string configuration advantages include weight optimization, reduction of frictional pressure losses, and added rigidity where needed while minimizing wall thickness elsewhere in the string to reduce overall tubing costs.
Field Performance of Q&T CT Technology
Before Q&T CT products were marketed, laboratory testing showed that gains in fatigue life of more than 50% compared with conventional CT could be expected. A fatigue retirement safety factor is applied when taking into consideration nonideal field conditions and data scatter.
In field-deployed and retired Q&T strings, the average increase in running footage after failure or voluntary retirement is approximately 45% greater than for conventional CT. When the maximum achieved running footage relative to conventional grades is considered, Q&T products retire at 70% higher running footage, an observation supported by data from the Eagle Ford, Permian, and Bakken plays.
Q&T Retirement Mechanisms
Data show that performance is greater with Q&T CT grades; however, similar failure modes occur as those seen in conventional grades. Relative to conventional products, CT voluntary retirement is higher with Q&T products, likely because of higher running footages.
An analysis of running-footage performance conducted by the authors indicates that corrosion has a noticeable effect during all stages of a string’s working life. The data seem to vary cyclically by region depending on many variables, such as availability of fresh water and attention to mitigation procedures in the field. Mechanical damage also plays a large role in retirement. However, voluntary retirement in each running-footage category also exerts a significant influence even with the relatively low consumed fatigue life.
Several forms of mechanical damage are commonly found on retired CT strings (Fig. 1). Fig. 1a shows injector damage, likely caused by chain tension and the tip of the injector block digging into the pipe. This type of damage creates a stress concentration at each indentation, which often leads to localized necking. Subsequent axial and bending loads further work-harden these areas, leading to a ductile fracture. Fig. 1b shows a classic plow mark caused by injector slippage or downhole debris. The damage induces localized spikes in hardness and cracks at the roots of the plow mark subsequently propagated by fatigue, leading to a pinhole. Fig. 1c shows an example of erosion from a perforation or from the flow cross. These localized stress risers eventually propagate, as shown in Fig. 1d, and cause tubing failure. Mechanical damage is often preventable with careful attention to detail in the field through equipment maintenance, pipe movement, and diligent service operations.
CT abrasion is a type of mechanical damage induced by high contact forces against the casing and is typically exacerbated by the high cycle vibrations from downhole agitators. An increase in hardness may yield better abrasion resistance; however, other operational methods can be effective in reducing this damage, such as relaxing the helix with short wiper trips and the use of pipe-on-pipe friction reducers. Appropriate string design on the whip end also can minimize abrasion and contact forces in the horizontal section while optimizing reach.
In data concerning abrasion-related failures in Q&T and conventional grades, an increase of failures on 0.145-in. and 0.156-in. wall tubing was evident. This may have been attributable to the historical string designs carried over from conventional materials, but a significant percentage of Q&T whip ends still suffers from similar failures, especially with a 0.156-in. wall.
Efforts to reduce this failure type by increasing tube-wall thickness at the whip end have not been successful. Increasing the wall thickness potentially can increase wall contact forces and the necessary uphole force needed to push the downhole end. Once the whip end is increased, then, typically downhole vibration tools are used to reach the target depth. The result is more abrasion in the heavier wall. Careful assessment of string design to optimize reach and minimize the use of extended-reach tools can be helpful.
Some corrosion damage is attributable to storage corrosion between jobs because of the lack of pitting uniformity around the circumference. Residual fluids settle in the bottom wraps of the tube and create ponds that are ripe for forming localized cells.
The majority of corrosion-related failures returned from the field differ from corrosion failures in which chemical inhibition techniques are underused. In the cases investigated by this study, corrosion tends to be uniform around the entire circumference. Storage-type corrosion accounts for approximately 75% of the corrosion-related failures causing early retirement.
Microbiologically induced corrosion (MIC) was identified as a common CT problem in 2012; the industry has since improved inhibition techniques to mitigate this type of corrosion. However, current practices of corrosion mitigation show varying success in both killing the bacteria and eliminating corrosion.
Fig. 2 shows cavernous pit profiles generally associated with MIC. In these types of corrosion pits, the presence of sulfur, chlorine, and other corroding elements is frequently observed. Wall loss, in this case, is approximately 13%.
Regardless of improvements in microstructure, fluids management remains mandatory. Apart from tracking the life of the CT string, a comprehensive corrosion-management program should be in place that aims to eliminate MIC. In addition to treating the fluids for bacteria, subjecting the string to a thorough purging process with a wiper ball or brush pig followed by a slug of inhibitor and nitrogen can reduce this type of damage.
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In-Line Quench-and-Temper Technology Applied to CT Improves Safety and Reliability
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19 June 2019