Corrosion Fatigue Behavior Of High Strength Sucker Rod In High Water Conditions

The increase of the difficulty of crude oil extraction puts forward the requirement of the bearing capacity of the sucker rod.The application of the high-strength sucker rod is becoming more and more popular.Field applications have found that the service life of high-strength sucker rods is generally lower than the predicted life and is the main limiting factor to prevent its promotion.Produced fluid reinjection leads to the increase of the mineralization degree and the working conditions of the sucker rods are worse.Corrosion fatigue is becoming more and more serious causing huge economic losses to the oil field.It is of great significance to standardize the design of the sucker rod and to prevent and reduce the accident caused by the corrosion fatigue damage of the sucker rod.In this paper,by studying the problems in the process of using high-strength sucker rods,the stress corrosion and corrosion fatigue behavior of high-strength sucker rods in NaHCO₃ and NaCl solution were studied on the basis of the preliminary experiment of the project.Corrosion and stress tolerance corrosion sensitivity of high-strength H-grade rods 4330,30CrMoA and D-grade rods 20CrMoA,4142 in different concentrations of Cl^-and HCO₃^-solution were subjected to stress corrosion experiment.Based on the experimental results,in this paper the equivalent allowable stress of 4330 is calculated and the modified Goodman check formula of high strength sucker rod under the condition of corrosion is discussed.The main research results are as follows:?1?The existing fatigue life model and the corrosion fatigue damage mechanism of sucker rod string were analyzed.There is no suitable model and theory to predict the corrosion fatigue life of the sucker rod.Through single factor stress corrosion experiments the stress corrosion resistance of H-grade ultra-high strength rods is weaker than that of D grade rods.The 4330sucker rod is not suitable for high Cl^-concentration wells and is more suitable for the alkaline work conditions co-existence of HCO₃^-and Cl^-.In the highly corrosive high salinity conditions,it is recommended to use 30CrMoA preferred.In the use of D-grade sucker rod,the choice of20CrMoA rod is better than 4142.?2?The fatigue life of 4330 sucker rod material in corrosion environment with Cl^-is significantly lower than that in non-corrosive environment with the cliff and its life affected by the Cl^-concentration is not significant.The temperature promoted the effect of Cl^-on the fatigue life of the material.After the temperature was higher than 70?,the corrosion life gradually stabilized.The presence of HCO3^-plays a certain role in promoting the fatigue life of Cl^-ions but its effect is not significant.?3?The low-frequency corrosion fatigue S-N curve lgN=11.64-3.77lgS in the corrosive environment was fitted according to the finite data by performing the corrosion fatigue test of4330 at different stress levels.?4?According to the results of corrosion fatigue experiment,4330 sucker rod in the higher chloride ion concentration solution in the service,the proposed 4330 rod design correction Goodman check formula in the safety factor of 0.7 or less.Based on the experimental results of corrosion fatigue,the design formula of 4330 under the corrosion condition is explored again.The formula is given:[?_max]=0.846?_min+90.7.