The Development of High-Strength Coiled Tubing with Improved Fatigue Performance and H2S Resistance

Abstract

In the recent years coiled tubing (CT) has become an integral part of many well completion programs. The trend is to higher-strength CT, larger tube diameters and higher pumping pressures while employing relatively small CT spools to satisfy transportation logistics. Coiled tubing is now routinely used at stress and strain levels that significantly exceed prior experience. These conditions have identified limitations within currently available CT products. Today there is a need for higher-strength CT grades with better resistance to severe environments and better fatigue performance in both the tube body and bias weld.A detailed study of the metallurgical basis of each production step was performed in order to understand the limitations of the current CT technology and manufacturing process. This understanding, coupled with the goal of developing new CT grades that satisfy new market demands, has resulted in a complete redesign of the CT technology and manufacturing process. Extensive trials have been performed, and a prototype manufacturing facility has been constructed to produce new tubing grades. Comprehensive mechanical testing, metallurgical evaluation, low-cycle fatigue testing, and sour (wet H 2 S) exposure testing of new 110 ksi and 125 ksi grades produced by the new manufacturing process were conducted. The fatigue testing was performed over a broad range of bending strain and hoop stress that anticipates severe operating conditions. Sour exposure testing included static, sulfide stress cracking (SSC) and hydrogen induced cracking (HIC) testing, as well as dynamic, low-cycle fatigue testing after sour exposure.Laboratory and field testing of commercial-size strings has shown the fatigue life of the new CT grades exceeds the fatigue life of currently available high-strength grades. Additionally, the bias-weld fatigue life is improved significantly, both in general and relative to the base-tube fatigue life. SSC resistance of the new CT grades is considerably better than the SSC resistance of the conventional CT grades with the same strength, making it possible to increase the tubing material strength by about 20 ksi to 30 ksi without deteriorating the sour resistance.The paper presents comprehensive information on the new CT grades, their performance in various environments under a wide range of loading conditions, as well as the qualification of the new tubing grades for field use. This allows CT operators and users to select more suitable CT grades for their applications. These new tubing grades also can enable field operations with coiled tubing that cannot be conducted safely and reliably with the currently available tubing grades.

Experimental Study of Microstructures in Bias Weld of Coiled Tubing Steel Strip With Multi-Frequency Eddy Current Testing

“…bias weld reduces the overall fatigue performance of coiled tubing, so the overall fatigue performance of the coiled tubing significantly depends on the fatigue performance of the bias weld [4].…”