Sulfide-Stress-Crack Resistance of QT-900 Coiled Tubing
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 93786, "SSC Resistance of QT-900 Coiled Tubing," by T. McCoy, Halliburton, prepared for the 2005 SPE/ICoTA Coiled Tubing Conference and Exhibition, The Woodlands, Texas, 12-13 April.
Laboratory sulfide-stress-cracking (SSC) tests were performed on specimens taken from a QT-900 coiled-tubing (CT) test string to define zones of acceptable sour service. SSC tests were performed at room temperature in a brine-fluid environment with H2S partial pressures ranging from 0.145 to 145.0 psi and pH levels from 2.8 to 4.5. The effectiveness of a new inhibitor for crack prevention was tested. SSC testing, which included Natl. Assn. of Corrosion Engineers (NACE) Method A tensile, four-point bent-beam (FPBB), and slow-strain-rate (SSR) test specimens, was performed on as-milled as well as fatigue-cycled tubing.
Introduction
After the failure of two QT-900 strings in high-H2S-content wells in Canada in 2002, it was obvious that a better understanding of the SSC susceptibility of QT-900 CT was needed. Before these failures, use of QT-900 CT in sour wells had been very successful. Canadian operations used QT-900 CT in 112 wells containing 0.03 to 35% H2S without incident before these two failures. Investigation of the QT-900 string failures [1½- and 1¾-in. outside diameter (OD)] showed two common characteristics of the failures besides the exposure to sour fluids: (1) all SSC cracking occurred at OD mechanical damage, and (2) hydrochloric acid was used in both wells.
All SSC cracks in both strings occurred at damage located on the seam weld, even though equivalent damage was present at other areas around the tubing. The 1½-in.-OD string failed at areas damaged by semicircular gripper blocks, and the 1¾-in.-OD string failed at areas damaged by V gripper blocks.
Besides the presence of high amounts of H2S, both of the common factors (mechanical damage and low pH from the introduction of acid) were important, and the failures might not have occurred if either of these factors had been mitigated. Although other factors such as strength and hardness, ductility, chemistry, and fatigue cycles are important, they are not as fundamentally important as the environment in which the tubing is being operated. If the tubing becomes hydrogenated because of the interaction of H2S on the metal surface, tubing properties will change significantly. The full-length paper concentrates on determining how various sour environments affect the SSC resistance of QT-900 CT.
Experimental Domains
The testing was performed on undamaged samples with the knowledge that mechanical damage is important and needs to be minimized and monitored by nondestructive inspection techniques. To offset this deficiency, full 30-day SSC tests were run even though workover strings are not exposed to sour fluid for this length of time during operations.
Defining domains of sour-service severity is a common approach for operations involving sour-service production using carbon- and low-alloy-steel components. The domain concept, which has been adopted by European Federation of Corrosion (EFC) Publication Number 16 and NACE MR0175/Intl. Organization for Standardization (ISO) 15156, covers room-temperature (i.e., 75°F) conditions. Testing at a temperature of 75°F is more severe for carbon and low-alloy steels than testing at elevated temperatures. CT can become hydrogenated downhole at higher temperatures, leading to failure of the tubing at the top when the string has cooled somewhat and is going over the gooseneck or onto the reel. Therefore, it is logical that these domains can and should be used as a guide in determining suitable environments where CT can be used successfully in sour fluids.