Coiled tubing (CT) is routinely exposed to different concentrations of acid in stimulation and cleanout jobs. Routine maintenance measures are generally taken, such as the use of inhibitors and proper cleanup and storage practices. However, a large number of CT strings suffer from corrosion and corrosion-fatigue related damage. Though it is difficult to isolate string damage solely from corrosion fatigue, this paper addresses the effect of acid with and without inhibitors on corrosion-fatigue properties of the grade-90 pipe through experimental testing. The results of testing demonstrate that exposure to acid without an inhibitor and during fatigue had the most pronounced effect on fatigue life. The simultaneous action of corrosion and fatigue produced more pits and cracks than would have occurred by corrosion or fatigue separately.

CT exposure to acid before, during, and after bending can have different effects on fatigue life. This paper describes an experimental study that was conducted to observe these effects. A strip of metal, 1-in. wide and 6-in. long, was cut from a round 1.75-in. OD, 1.56-in. wall thickness of 90-ksi yield CT and underwent one-way, bent-beam fatigue testing. This paper will present S-N curves generated for samples in three environments: air, acid, and acid with inhibitor at deflection levels ranging from 0 to 40 mm. Finite element analysis (FEA) was used to correlate deflection to strain range levels of 0.5 to 4.0%.

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