| With the increase of the depth in oil and gas wells, the rigorous service condition and mechanical characteristics put forward new requirements and challenges to the material resistance to stress corrosion performance. High strength steel is a kind of casing and drill pipe material which is promoted to be used in deep wells and ultra-deep wells. Field practice shows that with the improvement of strength, the material's ability to resist environment sensitive fracture presents downward trend. Therefore, the study of high-strength steel's ability to resist environment sensitive fracture is one of the ways to effectively guarantee the casing and drill pipe's security and the integrity of the structure in the process of service.Based on fracture mechanics and stress corrosion cracking theory, fracture toughness K\c and KISCC of high strength casing has been determined. The stress corrosion cracking behavior of high strength steels were evaluated in organic salts drilling fluid by slow strain rate tests (SSRT), constant load test and four-point bending test, etc. Meanwhile, the stress corrosion crack mechanism of the steel in the organic salts drilling fluid medium has been researched. Crack propagation character of high strength steel under different temperatures was studied by K-R resistance curve. The behavior of stress corrosion crack of two high strength steels in organic salt drilling fluid are investigated by using round bar tensile samples with notch The results can be used to determine whether the high strength was safe and reliable, and to provide experimental date for the production of high-strength steel with high resistance to stress-corrosion cracking.The specific research achievements are as follows:(1) Fracture toughness (Kic) of high strength casing steel (140-2) has been determined by using three-point bend specimen, four-point bend specimen, compact-tension (CT) specimen, the double cantilever beam (DCB) specimen, round bar tensile specimen, single edge notch bend specimen and double edges notch bend specimen And, the test results are givea Based on this study, a process for testing fracture toughness of high-strength steel was provided. Meanwhile, the problem that the thickness of high strength steel does not meet the requirement of the standard test methods is sloved. The plane-strain fracture toughness (Kic) is 133.94 MPa·m1/2.(2) Aiming at fracture toughness.Kiscc of a high strength steel, the organic salts drilling fluid were simulated under the temperature of 150? and the pressure of 30 MPa by three-point bend specimen with notch and the double cantilever beam (DCB) specimen with pre-existing crack. The fracture morphology analysis shows that the surface of the three-point bend specimens with notch has not the crack. DCB specimens have no obvious damage. And, there is not a significant change in mechanical parameters of DCB specimens. The plane-strain fracture toughness (KISCC) is 131.19 MParm1/2.(3) The LF-100-100-V-304 slow strain rate test machine made by CORTEST is used in this experiment. Aiming at the stress corrosion cracking behavior of a high strength steel (140-2), the air working conditions, the annulus protection fluid and the organic salts drilling fluid were simulated under the temperature of 60?,100?,150? by slow strain rate tests (SSRT). The results showed that the high strength steel stress corrosion cracking tendency increase as the temperature grows, and so do the stress corrosion cracking susceptibility index F(a) and F(8). Besides, the tendency of increase of F(a) is more significant than that of the F(8). The maximum sensitivity index of stress corrosion of the sample in organic salt drilling fluid media is 9.66%, which is much lower than 25%, indicating that there is not obvious stress corrosion cracking tendency for high strength casing in simulated operating condition.(4) The stress corrosion cracking (SCC) behavior of high strength drill pipe steel (165-1) was investigated in organic salts drilling fluid by constant load test. The experiment equipment is 70MPa?300? multiphase flow dynamic cycle high temperature and high pressure autoclave. The temperature was 100? and 150?, the pressure was 30MPa, and the loading proportion is 80% of nominal yield strength (165ksi). The results showed that the high strength drill pipe steel with high resistance to stress corrosion cracking was safe and reliable. The fracture morphology of the specimens was investigated with scanning electron microscope (SEM). Fracture mode is ductile as lots of dimples are visible. There is no brittle crack source in the organic salt slurry medium, and there is no old stress corrosion crack. Compared to tensile experimental results at 25? in the atmospheric air medium, with the temperature's rising, the tensile strength of samples are all in downward trend in the stimulated working condition, the elongation of two kinds of samples are all in up ward trend in the stimulated working condition. The specimens have no obvious damage. And, there is not a significant change in mechanical parameters of the specimens.(5) The stress corrosion cracking (SCC) behavior of ultra-high strength drill pipe steel (165-2) was investigated in organic salts drilling fluid by four-point bending test, SEM/EDS analysis technique. The temperature was 150?, the pressure was 30MPa, and the loading proportion is 80% and 100% of nominal yield strength (165ksi). The results showed that it was chlorine ion that resulted in transgranular stress corrosion cracking. The stress corrosion crack mechanism of the steel in the organic salts drilling fluid medium was anodic dissolution. There exists corrosion pits on the surface of samples. The chlorine and sulfur content was high in the corrosion products based on EDS analysis. The chlorine ion originates the stress corrosion crack, and the sulfur accelerated the crack.(6) The force-displacement curve of fracture process for drill materials under different low temperatures was obtained by using an instrumented impact tester (mode ZBC2302-D). The relationship between the displacement (S) and crack extension (?a) is ? second function. Crack extension Aa and fracture toughness KR were calculated by K-R resistance curve. Charpy impact testing is largely employed for obtaining a material's CVN (Charpy impact energy) in petroleum engineering. However, it can't directly be used for quantitative safety assessment by the means of fracture mechanics. The technique given here can be used to research the high strength casing steel's resistance to the propagation of an (already existing) crack.(7) The behavior of stress corrosion crack of two kinds of high strength steel casing (140-1 and 140-2) in organic salt drilling fluid are investigated with the constant strain test. The temperature was 100? and 150?, the pressure was 30MPa, the sample was the round bar with notch and the loading proportion was 80% and 100% of nominal yield strength (140ksi). The results showed that the high strength steel of 140-2 exhibits high mechanical properties and superior resistance to stress corrosion cracking, compared to 140-1. As can be seen, the optimization of chemical elements is beneficial to improve comprehensive performance and reduce the environmental fracture sensitivity.Through these investigations, the sensitivity of stress corrosion cracking of high strength steel in organic salt drilling fluid are mastered, and the stress corrosion crack mechanism of the steel in the organic salts drilling fluid medium was discussed. Based on the experimental results and discussions, conclusions were drawn and proposed about different fracture toughness tests and stress corrosion cracking evaluation methods. The research results have important theoretical and practical significance. |
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