Comparative analysis of drill pipe corrosion performance

Drill pipe corrosion is a common problem in drilling projects, and it is becoming more and more serious as drilling develops towards deep/ultra-deep wells, deep-sea wells, and highly corrosive wells. In drilling operations, to meet the needs of various drilling processes, drilling fluid systems such as brine and potassium-based polymers are used, and contain a variety of additives, which are highly corrosive under high temperature and high pressure underground. At present, the main materials of drill pipes are oil-quenched (representative steel type is 36CrNiMo4) and water-quenched materials (representative steel type is 4130H). Through appropriate heat treatment processes, the conventional mechanical properties of the above two types of materials can meet the various requirements of API Spec 5DP-2009 "Drill Pipe Product Specification" for drill pipes. However, there is no systematic comparative analysis test on the corrosion resistance of the above two types of materials. Since the corrosive media encountered by drill pipes are mainly dissolved oxygen, CO2, and H2S;

1. Material analysis
The chemical composition of drill pipes is made of 36CrNiMo4 and 4130H. Compared with the two materials, the 36CrNiMo4 drill pipe is Cr-Ni-Mo alloyed, the 4130H drill pipe is Cr-Mo alloyed, and the carbon content of the 4130H drill pipe is significantly lower. Mechanical properties and metallographic analysis results of 36CrNiMo4 and 4130H drill pipes. Both 36CrNiMo4 and 4130H are S steel grades, and their strength levels are the same.

2. Comparative analysis of corrosion performance
2.1 Oxygen corrosion test
Electrochemical corrosion tests were conducted on 36CrNiMo4 and 4130H drill pipes and the dynamic potential polarization curves of the two materials in 3.5% NaCl solution were measured. The cathode polarization curve is steeper than the anode polarization curve, and the corrosion potential is close to the anode equilibrium potential. From the electrochemical kinetics, it can be seen that the corrosion of the two materials in a 3.5% NaCl solution is mainly controlled by the cathode. The cathodic polarization curve drops sharply with the increase of corrosion current density. This is because the polarization caused by the retardation of oxygen diffusion continues to increase so the entire cathodic process is mainly controlled by the diffusion process of oxygen. Whether at room temperature or 60°C, the oxygen corrosion rate of the 4130H drill pipe and 36CrNiMo4 drill pipe is the same. To further verify the brine drilling fluid taken from the oil field, an oxygen corrosion test was carried out in an autoclave. Under the condition of saturated oxygen, the corrosion test results of 36CrNiMo4 and 4130H drill pipes in brine drilling fluid also show that the oxygen corrosion rates of the two materials are the same. Through the above comparative test, it can be seen that the dissolved oxygen corrosion of the drill pipe is oxygen depolarization corrosion. Since the corrosion of the drill pipe is controlled by the diffusion of dissolved oxygen, the material of the drill pipe has little effect on the corrosion rate.
2.2CO2 corrosion test
A CO2 corrosion test was carried out on the drill pipes of 36CrNiMo4 and 4130H materials to determine the CO2 corrosion rates of the two materials in different environmental media. CO2 corrosion test solution ratio. Under the condition of saturated CO2, the CO2 corrosion test conditions and test results of 36CrNiMo4 and 4130H drill pipes, and the corrosion morphology of 36CrNiMo4 and 4130H drill pipes in solution 1 and solution 2. Table 6 CO2 corrosion test conditions and test results of 36CrNiMo4 and 4130H drill pipes Table 5 CO2 corrosion test solution ratio mg/L: Whether solution 1 or solution 2 is used for the CO2 corrosion test, the CO2 corrosion rate of 36CrNiMo4 drill pipe is lower than that of 4130H. Among them, in the corrosion environment of solution 1, the corrosion rate of 36CrNiMo4 drill pipe is 27.4% lower than that of 4130H; in the corrosion environment of solution 2, the corrosion rate of 36CrNiMo4 is 12.1% lower than that of 4130H. In addition, in different corrosion environments, the corrosion rate of the 36CrNiMo4 drill pipe in solution 2 is 49.1% higher than that in solution 1, and the corrosion rate of the 4130H drill pipe in solution 2 is 23.3% higher than that in solution 1. It can be seen that although the content of corrosion-resistant alloying elements such as Ni and Mo in 36CrNiMo4 is significantly higher than that in 4130H, resulting in differences in the CO2 corrosion resistance of different materials, it is still not as great as the impact of the environmental medium.
2.3 Sulfide stress corrosion test
Using the constant load method, the 36CrNiMo4 and 4130H drill pipes were tested for sulfide stress corrosion cracking (SSC) resistance in solution A of NACETM0177-2005 "Laboratory test method for metal resistance to sulfide stress cracking and stress corrosion cracking in H2S environment". The results of the SSC resistance test of 36CrNiMo4 and 4130H drill pipes. The drill pipes made of both materials broke in a very short time due to their high strength, and are not suitable for use in sulfur-containing environments; however, in comparison, the SSC resistance of the 36CrNiMo4 drill pipe is significantly lower than that of the 4130H material, and as the loading stress level decreases, the superiority of the SSC resistance of the 4130H drill pipe becomes more and more obvious. When the loading stress is 80% SMYS, the fracture time of the 4130H drill pipe is about 1.9 times that of the 36CrNiMo4 material; when the loading stress is 60% SMYS, the fracture time of the 4130H drill pipe is about 5.0 times that of the 36CrNiMo4 material. Studies have shown that the corrosion of steel in aqueous solutions containing H2S belongs to hydrogen-induced cracking stress corrosion, which is caused by the hydrogen released by the cathode reaction entering the steel and enriching certain key parts. When w(Ni)≦2% and hardness 22＞HRC, the reason why the SSC resistance of nickel-containing steel is lower than that of nickel-free steel is that the hydrogen evolution overpotential of nickel-containing steel is low, which promotes cathode hydrogen evolution, causing more hydrogen to enter the steel, increasing the concentration of diffusible hydrogen in the steel, and thus causing the SSC resistance of the steel to decrease.

3. Conclusion
(1) The oxygen corrosion rates of 36CrNiMo4 and 4130H are the same.
(2) The CO2 corrosion resistance of 36CrNiMo4 drill pipe is better than that of 4130H.
(3) The sulfide stress corrosion resistance of the 36CrNiMo4 drill pipe is significantly lower than that of 4130H.