Recent technological innovations in seismic imaging, directional drilling and hydraulic fracturing and other completion techniques have made development of unconventional hydrocarbon resources economically feasible. Coiled tubing is an integral component in completing most unconventional wells. However, the life of coiled tubing used in such applications has been negatively affected by higher well pressures often coupled with larger pipe diameters relative to wall thicknesses ("D/T Ratios"), as well as sinusoidal and helical buckling unique to horizontal completions. Such stresses frequently manifest in bias welds present in certain portions of the coiled tubing string, affecting both fatigue life and potentially operational safety.
It has long been understood that higher wall thicknesses improve the fatigue performance of coiled tubing, including bias welds. However, higher wall thicknesses throughout the length of the tubing may increase the weight of the coiled tubing strings to unacceptable levels. Technology now exists to provide coiled tubing strings with greater wall thicknesses at the bias welds while maintaining the thinner nominal wall thickness over the majority of the body of the tubing. This provides the ultimate in design flexibility for engineering a coiled tubing string with optimized reach, weight, overpull, fatigue life and safety.
This paper presents data demonstrating the improved performance of coiled tubing with increased localized wall thickness at the bias welds, and outlines the string design flexibility offered by this technology for engineering the next generation of coiled tubing.