| For a long time, CO2and H2S corrosion were the main corrosion types in the oil and gas production and transportation. However, in the process of oil and gas exploitation, such as nitrogen injection, CO2underground reinjection, injection of corrosion inhibitor, reinjection of produced water and equipment maintenance, it was inevitable to bring amounts of O2into the underground. In recent years, with the increasing discovery of the heavy oil reservoir, exploitation of the heavy oilfield faced severe challenges. In order to improve the heavy oil fields recovery ratio, the multiple thermal fluid injection stimulation technology was widely promoted. A certain amount of O2in the multiple thermal fluids, combined with the CO2in underground reservoir could cause serious corrosion on the down-hole string and other production facilities which would result in great risk of failure. At present, the corrosion of the metal in CO2and O2co-existence environment of oil and gas field was not received enough attention, and the corrosion characteristics and corrosion mechanism were unclear.This paper focused on the corrosion behavior of common carbon steel and low alloy steel used in oil and gas fields. The simulated high temperature and high pressure corrosion environment with CO2-O2was conducted by using the self-made high temperature and high pressure autoclave and in-situ electrochemical test equipment. And the composition, morphology and structure of the corrosion products of carbon steel and low alloy steel in both high temperature and high pressure CO2corrosion environment and CO2-O2co-existence corrosion environment were analyzed using modern surface analysis methods (SEM, EDS and XRD) and electrochemical detection methods. Also the corrosion mechanism of carbon steel and low alloy steel in CO2-O2environment was studied. The main research results obtained were as follows:(1) The simulation experiments of N80steel in high temperature and high pressure with different conditions of gases were investigated. The results demonstrated that dendritic characteristic of localized corrosion was formed in mixed oxygen and carbon dioxide condition and the corrosion films of N80steel had a double-layer structure. The outer corrosion product was loose, porous and barely protective. The inner corrosion product was composed of FeCO3and little amount of Fe2O3and FeOOH. The corrosive medium could pass through the corrosion product via the pore, and Ferric oxide formed as inner corrosion product destroyed the integrity of the inner layer, which caused localized corrosion. Cl" gathered only at the very bottom of the pits, which caused the extension of corrosion pits in depth and the formation of dendritic characteristic of localized corrosion ultimately. Besides, the difference of potential, O2concentration between inside and outside of corrosion pits and the decrease of pH value inside the pits aggravated the process of corrosion in corrosion pits.(2) The simulation experiments of3Cr steel in high temperature and high pressure with different conditions of gases were investigated. The results indicated that the corrosion product of3Cr in the condition of carbon dioxide and oxygen was loose and porous, and was composed of FeCO3, FeOOH and Fe2O3. There was no evident enrichment of Cr in the corrosion product, and3Cr steel showed localized corrosion with shallow pits. The electrochemical results indicated that inner film resistance Rf1, outer film resistance Rf2, and the charge transfer resistance R, were lower than those in pure CO2condition, and the double-layer capacitance Cd1, and film capacitance Cf1,Cf2were higher than those in pure CO2condition. The protectiveness of corrosion product films in the condition of carbon dioxide and oxygen was worse than that in pure CO2condition. In the condition of carbon dioxide with oxygen, corrosion product formed in the surface of3Cr steel was composed of various substances, which led to the loose and porous corrosion product and it was negative to the formation of Cr(OH)3layer which enhanced the protectiveness of corrosion films dramatically in pure CO2condition. Therefore, hydrogen corrosion and oxygen reduction of3Cr steel in acid medium were promoted.(3) The corrosion characteristics of3Cr steel changes with corrosion time in high temperature and high pressure aqueous environment containing95%CO2and5%O2were investigated. In the initial stage, the integrity and density of the inner layers were improved and the resistance of films Rf increased with the prolong of the corrosion time. With further increase of the corrosion time, the iron oxidation from Fe2+into Fe(OH)3precipitation reacted easily with sufficient dissolved oxygen. Fe(OH)3formed in local region of inner film occupied the position of Cr(OH)3and caused non-uniform distribution of Cr element in the inner film. It made Rf decrease and pitting corrosion occur. With the increasing corrosion time, corrosion product with higher content of Cr formed in the bottom of pits and R, and Rf increased, which restrained the corrosion development in depth. As a result, the localized corrosion with shallow pits appeared on the surface of3Cr.(4) The characteristics of the corrosion films in the high temperature and high pressure environment containing CO2and different content of dissolved oxygen indicated that with a trace increasing content of dissolved oxygen (less than2.19ppm), the density of corrosion films would increase and the corrosion rate would decrease. However, the content of the dissolved oxygen further increasing (greater than2.19ppm) would destroy the integrity of corrosion films, leading to the occurrence of localized corrosion. The pH value on the surface of the electrode could be reduced by both the Cr element in the steel and the trace dissolved oxygen. The critical degree of super-saturation of FeCO3was decided by pH value and affected the crystalline state of FeCO3. With the lower of pH value, the size of grain decreased and the corrosion films became more protective. The further increasing of the dissolved oxygen would make it easy to form Fe(OH)3and then localized corrosion occurred. |
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