| CO2 EOR technology can improve the recovery rate of low osmotic pressure oil fields while permanently storing CO2 underground,which is an important industrial-level means to ensure energy supply and achieve CO2 emission reduction.CO2 EOR technology needs to inject CO2 into the reservoir after pressurizing it,which will cause serious CO2 corrosion of the gathering&transportation pipelines.The existing low-alloy steel pipes can no longer meet the requirements of the CO2 flooding service environment.At present,the corrosion problem of the pipeline only can be improved by brushing organic coating on the inner wall of low-alloy steel pipe,adding corrosion inhibitors and using organic materials such as glass fiber reinforced plastics.But in terms of long-term,sustainable and applicability,the result is not satisfactory.However,the cost of useing corrosion-resistant materials such as high-Cr stainless steel and Ni-based alloys will be unbearable.Therefore,the development of special steel pipes suitable for CO2 flooding environment has important economic value and strategic significance.In this paper,the Thermo-Calc software was used to calculate the composition optimization design scheme of low C and Cr steel.Through the accelerated corrosion test of high temperature and autoclave,combined with electrochemical,EBSD,AES and SKPFM analysis methods,the corrosion rate and corrosion product film structure of different Cr content test steels in the simulated CO2 EOR environment were compared,and the corrosion behavior and corrosion resistance mechanism of Cr steel with low C were revealed.Meanwhile the difference of weight loss rate and weight gain rate of the samples with and without oxide scale was compared and analyzed.Combined with the morphology characterization,composition and phase analysis of the corrosion products,the corrosion behavior of low C medium Cr steel with oxide scale was clarified.On this basis,the industrial trial production of low C medium Cr gathering&transportation pipe was carried out,and its mechanical and corrosion resistance performance was evaluated.The following main conclusions were obtained:Thermo-calc calculation shown that when the content of C,Si and Mn are controlled in the range of 0.03-0.07%,0.2-0.3%and 0.1-0.3%,respectively,the solution of Cr can be effectively improved and precipitations can be reduced,which is conducive to improving the corrosion resistance of steel.On this basis,three typical lowC medium Cr steels,5Cr,7Cr and 9Cr,were prepared in the laboratory.With the increase of Cr content,the bainite content in the steel increased,and the strength of the steel increased accordingly.Its corrosion resistance is significantly affected by the content of Cr element and the medium temperature.In a typical CO2 EOR environment with CO2 partial pressure of 0.6 MPa,HCO3-concentration of 3000 mg/L,Cl-concentration of 10000 mg/L,Ca2+ concentration of 125 mg/L,and Mg2+ concentration of 150 mg/L,in the range of 30-110℃,the corrosion rates of the two test steels 5Cr and 7Cr showed a trend of first increasing and then decreasing with the increase of temperature,and the peak value appeared at 90℃.The corrosion rate of 5Cr steel was significantly affected by temperature.When the temperature increased from 30℃ to 90℃,the corrosion rate increased from 0.1201 mm/a to 1.8698 mm/a,by 14.6 times,and when the temperature increased to 110℃,the corrosion rate decreased to 0.3965 mm/a.7Cr steel was less affected by temperature.When the temperature increased from 30℃ to 90℃,the corrosion rate increased from 0.1494 mm/a to 0.3642 mm/a,and the maximum corrosion rate was only 2.4 times that at 30℃.9Cr steel was not significantly affected by temperature,the corrosion rate was always maintained below 0.06 mm/a in this temperature range.The exposed low C medium Cr steel substrate started with slight pitting corrosion.Pitting was preferentially formed around grain boundaries or secondary phases.The Cr(OH)3 generated inside the pitting pit will inhibit the vertical expansion of the pitting corrosion,the corrosion started to develop along the grain boundary,and the corrosion type changes from local corrosion to uniform corrosion.When the exposed grain boundaries were fully corroded,new pitting was formed in the ferrite grains,and the corrosion type exhibits uniform corrosion+local corrosion characteristics.After a short period of vertical expansion,the new pitting develops laterally,and the corrosion type becomes uniform corrosion again.After such a cycle,a structural corrosion profile was formed on the matrix surface due to pitting corrosion selectively between ferrite and bainite.The corrosion driving force of the three low C medium Cr steels comes from the potential difference between different phases of ferrite and bainite,and between grain boundary and intragranular.The bainite potential is high and the ferrite potential is low.Increasing the Content of Cr leads to the increase of bainite ratio,which is the key to improve the corrosion resistance of low C medium Cr steel.With the increase of Cr content,the high potential bainite content increases,and because Cr tends to segregate at the grain boundary,the higher Cr content,the smaller the potential difference between grain boundary and intragranular,and the more uniform the surface potential distribution,which can reduce the corrosion of high angle grain boundaries..In addition,the segregation of Cr element will also occur at the low angle grain boundaries between the bainite laths,and the low angle grain boundaries can provide a channel for the diffusion of Cr element,which is easy to form a Cr(OH)3-rich product film.The three low C medium Cr steels can form a double-layer corrosion product film in the CO2 corrosion environment,the inner layer is amorphous Cr(OH)3,FeCO3,and the outer layer is granular FeCO3,CaCO3.In the initial stage of corrosion,a Cr(OH)3-rich inner product film can be formed on the surface of low C medium Cr steel.Fe passes through the inner film in the form of Fe2+ and dissolves into the solution,and then CO32-in the solution combines with Fe2+ to form FeCO3 and begins to deposit on the surface of the inner film.With the increase of Fe2+ concentration in the solution,the first nucleated FeCO3 particles grow and stack,and the later nucleated FeCO3 particles fill the stacking gap to form a complete outer layer.With the increase of Cr content,the content of Cr(OH)3 in the inner product film increases and becomes denser,which is beneficial to hinder the further occurrence of corrosion.In the actual service process,the scale existing on the inner wall of the steel pipe is first in contact with the conveying medium.The results of the research on the CO2 corrosion resistance of the low C and medium Cr steel samples covered with scale show that the oxide scale of such samples is a three-layer structure,the inner layer is Fe3O4,the middle layer is Fe2O3,and the outer layer is mainly Fe3O4 mixed with Fe2O3.Cr2O3 is distributed between the inner layer and the middle layer,and inside the Fe2O3 package of the outer layer.The impedance mainly comes from the oxide skin itself.During the corrosion process,the outer and middle layers of the oxide scale with low Cr content are prone to peel off,and the inner layer with high Cr content has a certain protective effect on the substrate and delays the occurrence of corrosion.When the corrosion of the inner oxide scale is completed,the corrosion law of the sample conforms to that of the sample without scale.Based on the above research,the industrial trial production of 5Cr and 7Cr steel pipes was carried out on the existing production line of Tianjin Pipe Corporation.The yield strength of the 5Cr steel pipe after quenching and tempering at 930℃+600℃ is 560 MPa,the tensile strength is 630 MPa,and the impact energy at room temperature is≥ 250 J,after water quenching at 930℃ and tempering at 680℃,the yield strength and tensile strength of 7Cr steel pipe are 600 MPa and 665 MPa,respectively,and the impact energy at room temperature is 235 J.Under the simulated CO2 EOR gathering&transportation conditions,the average corrosion rate of 5Cr steel pipe is 0.0965 mm/a,and that of 7Cr steel pipe is 0.0331 mm/a,which are 25.6%and 8.8%of the active Q345D(corrosion rate is 0.3770 mm/a)pipe,respectively,which greatly improves the corrosion resistance of steel pipe.In summary,in view of the serious corrosion problem of metal pipelines caused by CO2 EOR technology,this paper proposes a design scheme for the composition of low C medium Cr steel,and reveals its corrosion behavior and corrosion resistance mechanism in CO2 EOR environment.On this basis,the industrial trial production and evaluation of low C medium Cr gathering&conveying pipes represented by 5Cr and 7Cr has been realized,which provides a theoretical basis for the design and application of special corrosion-resistant pipes for CO2 EOR environment,and provides technical support for China to achieve the goal of "emission peak and carbon neutrality" successfully. |
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