Research On Corrosion Behavior And Mechanism Of Aluminum Alloy Drill Pipe Material In Drilling Fluid

As China's surface shallow resources have been basically ascertained,in order to protect China's energy security,it is urgent to explore deep oil and gas resources.At the same time,in order to promote the development of earth sciences,the depth of China's mainland scientific drilling is advancing toward the goal of a single hole of 10,000 meters.In deep oil and gas drilling and scientific drilling wells,traditional steel drill pipes have relatively large potential safety problems because of insufficient specific strength.Lightweight aluminum alloy drill pipes are ideal substitutes for steel drill pipes,but experience abroad and domestic shows that aluminum alloys Drill pipe in the higher pH drilling fluid will have serious corrosion problems,so it is necessary to study the corrosion behavior and corrosion mechanism of aluminum alloy drill pipe in drilling fluid.Fully immersed test method is used to affect the commonly used drilling fluid components(bentonite,polymer additives,sulphonation additives),temperature,immersing time,pH value,steel compounding,corrosion inhibitor addition,and in-situ drilling fluid,etc.The corrosion behavior of the material was studied.The single component solution of the drilling fluid can cause corrosion to the aluminum alloy drill pipe material.The corrosion methods are pitting corrosion,and the mechanical properties after corrosion have little change;the surface loss rate and mechanical property loss rate gradually increase(1.85%up to 6.73%)with increasing temperature(20?120?);The corrosion mode is gradually changed from pitting to uniform corrosion;With the increase of corrosion time(24-720h),mechanical property loss rate gradually increase(0 up to 3.7%).Within 720h,all the corrosion methods were pitting corrosion.As the pH value(10?12)increased,the surface damage and mechanical properties loss rate gradually increased(1.85%up to 5.22%).The corrosion mode changed gradually from pitting to uniform corrosion;compounding with steel will accelerate the corrosion of aluminum alloy material in the drilling fluid,but the acceleration process is slow,and the obvious acceleration effect occurs when the corrosion time is 336 hours.Under the coupling conditions(temperature,time,pH value),orthogonal experiments were carried out to determine that the significant factors influencing the corrosion of aluminum alloy drill pipe materials in drilling fluids are temperature and pH.The remaining strength decreases rapidly at pH=10?11,and changed little at pH 11?12.When the temperature in the 20?70 ? residual strength decreased faster,and at 70?120 ? the decline tended to be slow.It is suggested that the pH of the drilling fluid for aluminum alloy drill pipe should be controlled below 10,and drilling parameters should be calculated according to the residual strength of drill pipe of 90%.The SEM,EDS,FT-IR were used to analyze the surface morphology and corrosion products of the etched aluminum alloy specimens.The results showed that when the temperature and pH were lower,the corrosion mechanism was pitting.When the temperature and pH increase,corrosion mainly takes corrosion cracks,and the increase of pH will promote the development of cracks.The second phase particles of Cu,Mg,Si and other elements in the alloy are the sensitive areas of corrosion.After the second phase particles fall off,corrosion pits are formed.The corrosion products mainly consist of Al,C,O,S and other elements.The range of 2500?1500cm-1 and 600?400cm-1 is the main identification area of the corrosion products of aluminum alloy drill pipe material in the polysulfone drilling fluid.