Study On The Factors And Mechanism Of Corrosion Of Casing String Of Combustion Drive Oil

Autoclave tests, Salt spray tests, SEM, EDS and XRD were used to study the corrosion behavior of oil well pipe of N80, BG90 H, BG90H-3Cr and BG90H-9Cr used for combustion drive oil well. Moreover, the corrosion mechanisms of the four kinds of pipes were investigated by the electrochemical analysis.The results from simulating air injection environments showed that the oxidation corrosion rates of N80, BG90 H, BG90H-3Cr and BG90H-9Cr decrease in turn. In the wet air environment, pitting occurred on all of materials in different degrees, and the maximum corrosion rate of N80 is up to 5.3655mm/a.The results from simulating producing environments showed that the average corrosion rates of N80, BG90 H, BG90H-3Cr, BG90H-9Cr decrease in turn at all of environments. The corrosion rates of the four kinds of pipes appeared a peak at 120℃ in CO₂+H₂S environment, and local corrosion of the four kinds of pipe were serious at about 120℃. In CO₂+H₂S+O₂ environment, with the increase of temperature,the average corrosion rates of all the materials account for increase,and local corrosion is most serious at about 150℃. The dissolved oxygen increase the average corrosion rates of the oil well pipes, and raise the temperature where the corrosion rate peak occurred..The corrosion behaviors of the four kinds of materials in CO₂+H₂S+sand environment are similar to that in CO₂+H₂S environment. The sands in the fluid swash the product film and passivating film, which results in the corrosion rates of the pipe materials increase entirely and aggravates the local corrosion, which is more obvious at high temperature.Polarization curves of four materials showed that with the increase of temperature, N80, BG90 H and BG90H-3Cr of cathode and anode area of Tafel is obvious, all the reaction mechanisms is activation control, and the cathodic Tafel slopes of the three materials was significantly higher than that of anode, cathode control the corrosion. While the anodic polarization curve of BG90H-9Cr appeared obvious passivation phenomenon, which indicated that corrosion was controlled by anode. Four kinds of materials have two time constants at all temperatures, i.e. the impedance spectroscopy consists of high-frequency capacitive arc and low-frequency capacitance arc, as the alloy element BG90H-3Cr containing high Cr, the polarization resistance is relatively high, low corrosion rate, The content of Cr BG90H-9Cr up to 9%wt, forming Cr rich corrosion film on the surface of a material is more compact, the polarization resistance is far greater than the other three materials, and with the increase of temperature, the polarization resistance decreased, the corrosion rate increased. EIS and polarization curve test analysis results are consistent, namely N80, BG90 H, BG90H-3Cr at 60 ℃, the corrosion current density was maximum, and the corrosion current density of BG90H-9Cr is much lower than that of N80, BG90 H, BG90H-3Cr, and with the increase of temperature, the corrosion current density increases.