Low-Frequency Corrosion Fatigue Test Study of Sucker Rods under High-Salinity Well Fluids in Deep CBM Wells

Corrosion fatigue test is the most direct and effective method to study the corrosion fatigue characteristics of sucker rod. At present, the commonly used test method is the high frequency fatigue test, but the working state of sucker rod is typical low-frequency and high-cycle corrosion fatigue, and the test with high frequency will reduce the impact of corrosion. Alloy steel 4330 is widely used in coalbed gas well high strength sucker rod, but the research on its low frequency corrosion fatigue life is relatively few. Therefore, in this paper, the corrosion fatigue test method of axial low-frequency and high-cycle was adopted to study the corrosion fatigue characteristics of 4330 steel sucker rod through the corrosion fatigue test under different typical corrosion media, temperature, and stress levels. The results show that the fatigue life of 4330 sucker rod drops sharply when the Cl− concentration in high salinity well fluid exceeds the threshold value of 155 mg/L. When this threshold is exceeded, the downward trend slows down. It can be seen that the significant factor affecting the corrosion fatigue life of 4330 material is not the concentration of Cl−, but the existence of Cl−. The presence of HCO3− promotes a further decrease in the corrosion fatigue life of the 4330 sucker rod by Cl−. The corrosion fatigue life of 4330 sucker rod decreases with the increase of temperature. When the well fluid temperature is less than 50 °C, the impact is relatively significant. When the well fluid temperature is more than 70 °C, the decline trend of corrosion fatigue life slows down. Based on the fitted S-N curve (stress-fatigue life curve), it is calculated that the fatigue limit of 4330 sucker rod at the stress ratio of 0.6 is 196 MPa in the solution of 10,000 mg/L Cl− at room temperature. These could provide valuable theoretical and technical guidance for design and selection of high-strength sucker rod in high-salinity corrosion well fluid environment.