| Corrosion of casing and tubing is the main problem in H2S/CO2 sour oil and gas fileds. H2S, CO2 and other corrosive medium can cause not only general corrosion and localized corrosion, but also stress corrosion cracking (SSC), hydrogen induced cracking (HIC) and other unexpected brittle fracture accident, which can cause huge economic losses and serious casualties and environment problems. Corrosion behavior of P110, TP110-3Cr, TP110S, TP110SS in different corrosion medium under different temperature in sour gas environment were studied in this paper:corrosion rate of four casing at different CO2 and H2S partial pressure under different temperature were tested by weight loss method, the corrosion film morphology and phase composition were analyzed by SEM, XRD, EDS; according to the relevant standards, SSC performance was studied by constant load test and bending beam test method; resistance to electrochemical corrosion ability was also studied through the test material polarization curves and electrochemical impedance spectroscopy by simulating oilfield environment; and the annulus protection fluids were evaluated. Research on influence of casing corrosion factors and corrosion mechanism can reference to 110 casings materials in sour oil field on the safe use of experimental and theoretical basis.The results show that:average corrosion rate of four kinds of casing material in the simulation of CO2/H2S in corrosion environment is relatively low, localized corrosion probability is also smaller, so in CO2/H2S coexistence sour fields, average corrosion and local corrosion are not the main influencing factors, and resistance to stress corrosion cracking performance ability is the key factor.Average corrosion rates of TP110S and TP110SS are smaller, which don’t show obvious local corrosion occurs at room temperature and have passed SSC resistance testing and performance in a simulated environment of SSC performance testing, and can be used in sour gas field environment. Although the corrosion rates of P110 and TP110-3Cr are smaller, they are failed to pass the SSC resistance performance and cannot be used in sour gas field.Temperature, partial pressure of carbon dioxide and hydrogen suifide partial pressure have obvious influence on corrosion rate. At 90 degrees ℃, under the same partial pressure of H2S, the corrosion rate is significantly improved with the increasing of CO2 partial pressure; which under the same partial pressure of CO2, the corrosion rate decreased significantly with the increasing of H2S partial pressure; at 140 degrees ℃, under the 3.0MPa partial pressure of CO2, the corrosion rate equally obvious decline with the increasing of H2S partial pressure. In all experimental conditions, under 90 degrees ℃,0.1 MPa CO2 partial pressure and 1.4 MPa H2S partial pressure, and under 140 degrees℃,3.0MPa CO2 partial pressure and 1.4 MPa H2S partial pressure, the uniform corrosion rate is lowest, especially TP110S and TP110SS, their average corrosion rate is lower than in 0.1mm/A.Analysis of surface corrosion products by EDS, XRD shows that, in H2S/CO2 coexistence simulated environment, H2S corrosion is the leading one. Surface film is mainly composed of FeS, which is beneficial to reduce the uniform corrosion rate.Effect of temperature on electrochemical corrosion characteristics of change is more complex:to P110 and TP110-3Cr, with the increasing of temperature, material corrosion potential is reduced gradually, and anode corrosion current increases, the corrosion rate increases too. For TP110S the corrosion current was not monotonic changes with the increasing of temperature. Polarization curves of 50 degrees ℃ and 70 degrees ℃ are very close, and for polarization curve of 90 degrees ℃, the anodic polarization curves polarization rate is higher, so that the corrosion current in -0.6V and higher is obvious decreased than over the range of 50 degrees and 70 degrees ℃. The TP110SS polarization curves show that, at 90 degrees ℃, The corrosion current density in certain potential ranges (-0.6V) is maximum, but because its anodic polarization rate, in a certain potential ranges above -0.6V, anode corrosion current ratio at 90 degrees ℃ is much lower than those at 70 degrees℃. The polarization curves of the four casings show that:at 50 degrees ℃, corrosion current of TP110SS is minimum; 90 degrees C, corrosion current of TP110S is minimum. Material impedance spectroscopy indicates that:capacitance arc radius of PI 10, TP110-3Cr, TP110S are increased, their corrosion resistance, corrosion resistance are improved.Suggests on annulus protection fluid selection of various technical indicators:CaCO3 and CaSO4 inhibition rate≥90%, BaSO4 inhibition rate≥40%, inhibitor rate≥80%, sterilization rate≥90%, average corrosion rate< 0.2mm/A. Evaluation results shows:annulus protection fluid bactericidal rates of CT/TPK-1S and CT/TPK-1 are 70% and 76%, and inhibitor rates of CaCO3, CaSO4 and BaSO4 of two annular protecting fluid are much lower than the threshold value specified by standards. |
PDF Full Text Request