Study On The Corrosion And Fatigue Behavior Of QT-900 Coiled Tubing

The corrosion behaviors of QT-900 coiled tubing and Ni-W alloy coating were studied in CO2 corrosion environment or H2 S and CO2 corrosion environment with high temperature and high pressure corrosion tests combined with the actual work process “into the well- place the air- into the well again”, using microscope, SEM and EDS testing methods, and the SSC resistance in H2 S environment of QT-900 coiled tubing and Ni-W alloy coating was also investigated. What’s more, the influence of Ni-W alloy coating on the physical and chemical properties of coiled tubing was studied.The results showed that the microstructures of coiled tubing were pearlite and polygonal ferrite. Ni-W alloy coating mainly consisted of Ni and W elements, while the content of Fe was low compared with the matrix coiled tubing. The average thickness of outer coating was 54.0μm, and the average thickness of inner coating was 98.0μm. The combining quality between the coating and the matrix was good. The influence of Ni-W alloy coating on the tensile properties of coiled tubing was small, while the hardness and the fatigue life of coiled tubing had been significantly improved. All of cracks in the fatigue failure of Ni-W alloy coating came from the surface coating and penetrated from coating to matrix. Cracks were crowed closing the leak location, but the cracks at 350 mm far from the leak location were more rarefaction.The corrosion of coiled tubing underground was mainly combined influenced by the stress and the environmental medias in the simulated depth at wellhead(that depth of 0m), 1500 m, 3300 m and 4400 m. Moreover, the H2 S partial pressure corresponded to 0.020 MPa, 0.030 MPa, 0.037 MPa, 0.040 MPa, and the CO2 partial pressure corresponded to 0.97 MPa, 1.10 MPa, 1.24 MPa, 1.33 MPa. The most serious part of corrosion was occurred in 1500 m underground, and the localized corrosion rate up to 4.4627mm/a after once into the well. The main corrosion products were mixture of Fe S and Fe CO3. Corrosion was dramatically exacerbated after twice into the well that the average corrosion rate was up to 1.5549mm/a which was about 3 times than once into the well. The content of corrosion products of Fe S significantly reduced after twice into the well, wtat’s more, the localized corrosion product film fell seriously.The corrosion resistance of coiled tubing was significantly improved by Ni-W alloy coating in the simulated at 60℃, the CO2 partial pressure was 1.07 MPa and the tensile stress was 439 MPa. The average corrosion rate of Ni-W alloy coating was 0.0160mm/a which belong to the slight corrosion, and it was about 1/40 compared with the ordinary coiled tubing.The surface of QT-900 coiled tubing and Ni-W alloy coating all significantly appeared cracks in H2 S stress corrosion standard tests. The properties of SSC resistance of QT-900 coiled tubing and Ni-W alloy coating were very bad.