Laboratory sulfide stress cracking (SSC) tests were performed on specimens taken from a QT-900 coiled-tubing test string in an attempt to define zones of acceptable sour service. SSC tests were performed at room temperature in brine fluid environments with HS partial pressures (PHS) ranging from 0.01 to 10 bars and pH levels from 2.8 to 4.5. In addition, the effectiveness of a new inhibitor for crack prevention was tested. SSC testing, which included NACE Method A tensile, four-point bent beam and slow strain rate test specimens, were performed on as-milled as well as fatigue-cycled tubing. Fatigue cycling of the QT-900 test string was performed on a pressure bending fatigue machine with tubing samples cycled from 33% to 75% of average tested fatigue life. Test specimens were taken from parent metal, bias weld, seam weld (ERW), and butt weld locations. As a result of this test program, acceptable zones of service in sour environments have been proposed although further testing may alter boundary locations.
After the failure of two QT-900 strings in high HS content wells in Canada in 2002, it was obvious that more understanding of the inherent SSC susceptibility of QT-900 coiled tubing (CT) was needed. Previous to these failures, the use of QT-900 CT in sour wells had been very successful. For example, Canadian operations used QT-900 tubing in 112 wells containing 0.03% to 35% HS without incident prior to these two failures. Investigation of the QT-900 string failures (1.500- and 1.750-in. OD) showed two common characteristics of the failures besides being exposed to sour fluids:
all SSC cracking occurred at OD mechanical damage, and
HCl acid was used in both wells.
It is well known1 that corrosion activity and hydrogen permeation rates increase in sour environments with increasing acidity of the environment. Interestingly, 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 1.500-in. OD string failed at areas damaged by semi-circular gripper blocks (Fig. 1) and the 1.750-in. OD string failed at areas damaged by vee gripper blocks (Fig. 2).
Besides the presence of high amounts of HS, both of the common factors (mechanical damage and low pH from the introduction of acid) were important, and it is conjectured that the failures might not have occurred if either of these factors had been mitigated. However, after examining these and other failures involving HS, it was believed that the guiding principle should be:
"It's the environment, stupid."
This take-off of a political statement from President Bill Clinton's 1992 presidential campaign seems appropriate for CT in sour service operations. Although other factors, such as the presence of mechanical damage, strength and hardness, ductility, chemistry, fatigue cycles, etc. 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. Therefore, in this paper, we have concentrated on determining how various sour environments affect the SSC resistance of QT-900 CT.