This paper presents some of the fmdings from a joint industry research program supported by major oil and gas producers, coiled tubing (CT) manufacturers and material producers. Phase I of this program determined the cracking resistance of CT material in wet H2S environments which included a database of field failure scenarios. In Phase II of this program, cyclic fatigue tests were conducted on specimens pre-exposed to aerated brine, sour brine, sweet brine, and stimulated acid service environments. Tests showed that the duration of exposure in aerated brine and sour brine environments did not affect the average number of cycles to failure. There was no significant difference among the different grades of coiled robing in each separate environment. However, exposure to the evaluated environmental conditions severely decreased the fatigue life of the coiled tubing in comparison to the as- received (control) set of samples.
INTRODUCTION AND BACKGROUND
Coiled tubing (CT) is a cost-effective tool utilized by the well logging, oil and gas industries. Before 1985, coiled tubing was made of relatively low strength steel (<60 ksi specified minimum yield strength - SMYS grades) and was considered a consumable product 1. Today, the technology has expanded considerably and CT service life has been extended. In addition, CT is now used in service conditions that could not have been previously performed with conventional techniques. The potential for this CT tool along with recent developments had sparked an interest in fi~her extending service life. To perform this task, stricter manufacturing process controls were implemented to improve materials performance. The API RP 5C7 material, for instance, was developed through strict quality control measures 2. Yet although many measures have been taken to improve quality, significant concerns remain about the effect of increasingly demanding environments.
A survey was conducted by Maldonado et al. 3 to report the most common failure mechanisms and environments. Regular service conditions included several different types of environments such as brine or water, inhibited acid (with HC1 and HF), hydrogen sulfide, nitrogen, crude oil, cement, atmospheric and geothermal. The survey findings show that pitting, tensile overload and fatigue were the most common failure mechanisms. In addition 42 percent of the failures were corrosion related. The survey also determined that most failures occurred at well depths between 5,000- 10,000 ft (1,524- 3,048 m) and 64 percent of the failures occurred in a range of 11-50 strain cycles. Most of the materials reported were 70, 80, or 100 grades suggesting the absence of a unified material standard in terms of strength. Generally speaking, the higher SMYS steels are better suited to withstand the high stresses placed on CT during use. However, there was a lack of data concerning the effect of service conditions on CT materials.