Optimum Design And Corrosion Protection Of Aluminum Alloy Drill Pipe In Scientific Ultra-deep Drilling

In order to adapt to the more difficult drilling environment,some special and unconventional drilling techniques should be adopted in scientific ultra-deep drilling.Compared with the steel drill pipe,the aluminum alloy drill pipe with its unique superiority(lighter weight,higher specific strength,deeper drilling depth and less energy consumption)is the essential scheme of the drill string design used in scientific ultra-deep well.However,the weak thermal stability,the low anti-corrosion performance and structural failure in the difficult drilling environment is the main weakness of aluminum alloy drill pipe,which has greatly affected the application of aluminum alloy drill pipe under the scientific ultra-deep well.Based on the engineering application of aluminum alloy drill pipe in scientific ultra-deep wells,this paper establishes the selection method and comprehensive evaluation technology of drill string material,carries out optimum design and simulation analysis of drill pipe,and completes the theoretical prediction and experimental control of corrosion protection of aluminum alloy drill pipe,so as to complete the optimization of aluminum alloy drill pipe technology,and design and manufacture long-life and high reliability aluminum alloy drill pipe.The results are as follows.(1)Based on the high temperature and other working conditions and the requirements of drill string design,2219 and 2618 aluminum alloys are selected as the evaluation objects.The solution aging of 2219 aluminum alloy was 535?/1h,165?/20h,2618 was 530?/1h,200?/20h.The mechanical properties of 2219 aluminum alloy were better than 2618 aluminum alloy after long time heat exposure at ambient temperature and worse than 2618 aluminum alloy at transient high temperature.The element Zr in 2219 aluminum alloy can slow down the solute diffusion rate in the process of heat exposure,and it is beneficial to improve the residual strength after heat exposure.According to the fuzzy comprehensive evaluation,2618 aluminum alloy is more suitable for drilling string in high temperature section.(2)The difference between the results of tensile strength theoretical calculation and that of finite element analysis is 4.49%,and that of bench test is 2.49%;the difference between the results of torsional yield strength calculation and that of finite element analysis is 6.67%,and that of bench test is 6.23%.Under the condition of the same contact chamfer,the fatigue life of the contact site decreases with the increase of the drill pipe rotary speed.In the case of the invariant rotary speed of the drill pipe,the contact fatigue life will increase with the addition of contact chamfer.While the drill pipe bending moment is less than a certain critical value and the rotary speed is relatively low,the fatigue failure will not occur in the contact site.(3)The corrosion of aluminum alloy drill pipe used in SK-2 ultra-deep well at the connection point of aluminum rod and steel joint is the most serious.A large amount of Fe element is detected,and the main components of the corrosion products are Al₂O₃ and Al(OH)₃phase.The samples with MAO coating had larger corrosion potential,smaller corrosion current density and higher impedance.MAO samples had smaller weight loss compared with untreated samples,processing stable corrosion potential and corrosion current,and no large-area pitting corrosion occurred.The results of corrosion simulation analysis show that the electrolyte potential on one side of the aluminum alloy electrode is higher than that on the other side of the steel joint electrode;the maximum corrosion location is aluminum/steel galvanic corrosion;the corrosion depth increases with the increase of drilling depth;compared with the influence of temperature,the load has relatively small influence on corrosion.