Study On Corrosion Mechanism And Prevention Measures Of CO₂ Oil Wells

Based on the investigation of casing breakage of CO₂-containing oil wells in an oil field,the mechanism of casing breakage of CO₂ wells is investigated by analyzing the cementing quality,the composition of the produced water medium,the corrosive components,the corrosion products and the corrosion morphology.And the corrosion rate of the wells is also predicted.At the same time,the applicability and feasibility of different control measures are studied through weight loss method,electrochemical method,molecular dynamics simulation and corrosion detection method.The results show that the cementing quality has less effect on casing breakage of CO₂-containing oil wells.Meanwhile,the mineralization of produced water is high,and it is rich in CO₂ and corrosive ions.The casing failure is mainly caused by the corrosion of the inner wall of the casing.Therefore,the corrosion inside the casing has obvious CO₂ platform corrosion morphology.The corrosion products are FeCO₃ and a small amount of FeS and Fe₃O₄.The corrosion mechanism is mainly CO₂ corrosion,supplemented by SRB and a small amount of dissolved O₂ corrosion.Corrosion rate prediction results are consistent with that of the research on the broken and unbroken wells.Within a certain range,increase of temperature,flow rate and CO₂ partial pressure will lead to increase of casing corrosion rate.The Current corrosion inhibitors in oilfields present poor corrosion inhibition effect,their corrosion inhibition efficiency is below 65%,and the corrosion rate of the casing under simulated conditions is much greater than 0.076 mm/a.On the contrary,the new rosin tung oil imidazoline corrosion inhibitor is an anode type corrosion inhibitor.As the concentration of corrosion inhibitor increases,the polarization resistance and the corrosion inhibition increase.The best concentration of corrosion inhibitor is 100mg/L,and the inhibition efficiency can reach over 90%.The rosin tung oil imidazoline inhibitor displays higher adsorption energy than both the rosin and tung oil imidazoline,as well as the obvious synergistic mechanism of the corrosion inhibitors.Moreover,the active sites on the Fe surface can be effectively adsorbed and covered,and the adsorption strength increases with the increase of the corrosion inhibitor concentration.3Cr steel can be used as an economical corrosion-resistant casing material for CO₂-containing oil wells due to reducing the CO₂ corrosion rate significantly.The test results of tubing short connecting ring method are in good agreement with the prediction results of corrosion rate,which can accurately detect casing corrosion in different sections of wells.Finally,Based on the research results of different anti-corrosion technologies,a set of prevention and control measures system was proposed for the problem of corrosion cracking of CO₂ wells in an oilfield in China.