Study on the Oxygen Content of Oxygen-Reducing Air Flooding Based on Anticorrosion Considerations

Oxygen-reduced air flooding (ORAF) is an enhanced oil recovery process developed on the basis of air flooding and nitrogen flooding. In the process of oil displacement, oxygen-reduced air is injected into low-pressure and low-temperature reservoirs. It is expected that oxygen injected into oxygen-reduced air would undergo a low-temperature oxidation reaction with some crude oil under formation conditions to generate carbon dioxide [1, 2]. Compared with the state of high-temperature and high-pressure reservoirs, the degree of the low-temperature oxidation reaction is lower, and the generated carbon dioxide is very small [3]. The resulting flue gas mixture is mainly nitrogen, oxygen, and a small amount of carbon dioxide. In the oil displacement process, the gas that plays the main role in oil displacement is nitrogen, and carbon dioxide provides less power. Because the reaction degree of crude oil low-temperature oxidation reaction under the conditions of low-temperature and low-pressure reservoirs is not high and the oxygen consumption is low, if the air is injected into such reservoirs for oil displacement, the oxygen content in production wells may be too high, and there is a risk of explosion. For safety reasons, Chinese operators began to consider applying ORAF to such reservoirs, such as Changqing, Yumen, Qinghai, and Huabei Oilfields in China [4]. In addition, it can be predicted that ORAF technology would attract more and more attention in the future, especially in low permeability reservoirs with low temperature and low pressure.

Oxygen corrosion in the process of ORAF can be divided into oxygen corrosion and carbon dioxide corrosion. Higher oxygen partial pressure in gas-injection wells shafts would greatly accelerate oxygen corrosion. Therefore, serious corrosion during gas injection may occur on the inner wall of the pipeline or casing and on the surface gas injection line. At the same time, with the increase in oxygen concentration, the carbon dioxide generated by the reaction would also increase, which may lead to serious carbon dioxide corrosion in production wells [5]. The experiment [6] shows that under the condition of low-temperature and low-pressure reservoir, the oxidation reaction degree of crude oil is low, the carbon dioxide produced is low, and the degree of carbon dioxide corrosion in production wells would not be too great. In the process of ORAF, special attention should be paid to oxygen corrosion, especially in pipeline and casing pipe of gas-injection wells shaft with high temperature and pressure.

Oxygen corrosion is obviously different from other corrosion types such as carbon dioxide, hydrogen sulfide corrosion [7-11], microbial corrosion [12, 13], or other corrosion [9, 14-17]. There are two main reasons: on the one hand, the oxidation products formed by oxygen corrosion are relatively loose [18] and have poor protection against the matrix; on the other hand, oxygen, as a strong oxidant, directly participates in the cathodic reduction reaction in the corrosion process, and the corrosion rate increases with the increase of oxygen concentration in a certain range. This means that the oxygen concentration is not only related to the oxygen corrosion rate but also directly related to the oxygen-reduced cost of oxygen-reduced air.

Aiming at the law and mechanism of oxygen corrosion, many scholars have made in-depth research on it. Cox and Roetheli studied the effect of oxygen concentration on corrosion rate and corrosion product of steel in oxygen-containing water. It was pointed out that at low oxygen concentration, the corrosion rate is proportional to oxygen content, and the corrosion product is iron oxide, while at high oxygen concentration, the formation of protective gel-like iron hydroxide on metal surface slows down the corrosion rate of oxygen [19]. Yepez et al. and Yu et al. showed that the content of dissolved oxygen is one of the main corrosion factors of steel. Even a small amount of oxygen can cause serious pitting to steel. At the same time, oxygen would cooperate with carbon dioxide to corrode, aggravating the corrosion rate of steel [20, 21]. Li studied the effects of temperature, pressure, and oxygen content on oxygen corrosion rate and pointed out that the most important way to control high-temperature corrosion is to control oxygen content and water content [22]. Pournazari and Asselin analyzed the effects of dissolved oxygen and temperature on aluminum alloy steel. It was found that the effect of dissolved oxygen on corrosion rate was significantly greater than that of temperature on corrosion rate [23]. Bai et al. pointed out that when the concentration of chloride ion in the corrosive medium is low, the oxygen corrosion rate increases with the increase of chloride ion concentration. However, when the concentration of chloride ion increases to a certain extent, the expulsion effect of chloride ion on oxygen begins to dominate, and chloride ion blocks the contact between oxygen ion and metal surface, which makes the corrosion rate begin to decrease [24]. Yu et al. found that in the dynamic corrosion experiment, due to the shear effect of fluid, the looser corrosion products on the metal surface peel off, and a smoother and denser corrosion product film is formed on the metal surface, which slows down the progress of oxygen electrochemical reaction. However, on the contrary, in a static environment, porous and loosely piled corrosion products form oxygen concentration cells on the metal surface, aggravating the degree of pitting [25]. Zhong et al. found that under high-temperature and high-pressure conditions, the oxygen corrosion rate is very high, and two layers of corrosion products are formed on the metal surface. The outer layer is porous and loose, and the inner layer is flat and compact. In addition, the composition and structure of the corrosion products are not affected by oxygen partial pressure and temperature [26]. Qi et al. studied the oxygen corrosion mechanism of gas injection wellbore in the process of nitrogen injection flooding and pointed out that the corrosion rate increased with the increase in temperature and pressure [27]. Wang et al. studied the oxygen corrosion behavior of S135 drill pipe steel in drilling fluid. The research results show that the corrosion of drill pipe steel is mainly caused by oxygen reduction, and the corrosion rate mainly depends on the limiting diffusion current density of oxygen. The increase of the oxygen content of the drilling fluid and the flow rate of the drilling fluid will accelerate the oxygen corrosion rate of drill pipe steel, while the temperature and salt content of the drilling fluid have dual effects on the oxygen corrosion rate of drill pipe steel [28]. Yang and Xu divided the oxygen corrosion into overall corrosion and local corrosion, and their research showed that the higher the dissolved oxygen content, the faster the corrosion rate; the lower the pH value, the more serious the metal corrosion, and the free carbon dioxide in the water will accelerate the corrosion; high-temperature fluid cooling is most prone to oxygen corrosion [29]. Rosli et al. found that a high concentration of oxygen will oxidize ferrous ions into iron ions, which will lead to the formation of iron oxides and thus accelerate corrosion [30]. The oxygen corrosion law of steel in low oxygen environment and liquid environment has been widely studied, and the indoor oxygen corrosion experiments under high pressure and high temperature have also been studied. However, in the high-pressure oxygen-reducing air environment, the appropriate oxygen content of oxygen-reducing air is still not clear to meet the requirements of low corrosion rate and low gas production cost at the same time. In addition, the research on the oxygen corrosion law of different working conditions under this oxygen content condition is also lacking.

N80, J55, and 3Cr steels are usually used as wellbore steels in oilfields, so these three steels were selected as research objects. In this paper, the control variable method and weight-loss method combined with Ultra-Depth Three-Dimensional Microscope were used to study the corrosion behavior of the above three steels with oxygen content. By analyzing the production cost of deoxidized air with different oxygen content, the appropriate oxygen content of deoxidized air was obtained. Under this oxygen content of the oxygen-reduced air, the corrosion behavior of the three steel materials under different total pressure, temperature, and coexistence of air and water was further studied. This study can provide certain data support for the selection of oxygen content in oxygen-reduced air and the corrosion protection of gas-injection wells shaft in the process of high-pressure gas injection by ORAF and provide technical support for effective and safe development of low permeability reservoirs.

The main experimental materials were shown in Table 1, and Table 2 shows the main chemical components of the experimental hang-parcel. Figure 1 shows the specifications of the coupon, the test coupon was suspended on the bracket through the left hole in Figure 1, the bracket was made of nonmetallic material, and the suspension mode was shown in Figure 2.

The main experimental instruments were shown in Table 3, and Figure 3 was a schematic diagram of a high-temperature and high-pressure corrosion experimental instrument. Figure 2 was a structure diagram of high-temperature and high-pressure corrosion device. In the experiment, the partial pressure of oxygen is realized by the oxygen valve, the nitrogen is pressurized by the oil-free air compressor and the gas pressurization system, and the reaction pressure is controlled by the nitrogen inlet valve.

1. The hang-parcel was polished step by step with 200 # sandpaper until 1200 # a mirror surface. The size of the polished hang-parcel was measured by Vernier caliper (accuracy 0.02 mm); then the measured hang-parcel was placed in a container containing acetone solution and soaked for 5 minutes to remove the grease on the surface of the hang-parcel. The degreased hang-parcel was immersed in anhydrous ethanol to remove the acetone solution on the surface of the hang-parcel; after 5 minutes, the coupon was taken out, and the absolute ethanol on the surface of the coupon was wiped off by absorbent cotton, then the hang-parcel was placed on filter paper, and finally, it was placed in a desktop electric heating constant temperature drying oven for 60 minutes to dry; after 1 hour, the dried hang-parcel was taken out and weighed with an electronic balance (accuracy 0.1 mg).

2. After recording the number, the hang-parcel was successively hung on the bracket in the kettle; the kettle cover was installed, and the nitrogen inlet pipeline and the oxygen inlet pipeline were connected in turn. The valve of the nitrogen inlet pipeline was opened, and the reactor was filled with dry nitrogen of high purity for 30 minutes to remove the impurity gas in the reactor; the nitrogen inlet valve was closed, and the reactor was heated to the required temperature; after heating to the experimental temperature, the oxygen inlet pipeline valve was opened, the reactor was slowly filled with dry oxygen to make it reach the oxygen partial pressure required by the experiment, and then the oxygen inlet valve was closed; the gas pressurization system was started to pressurize the nitrogen, the nitrogen inlet valve was opened, and the pressurized nitrogen was slowly injected into the reactor. When the reactor reaches the total pressure required for the experiment, the nitrogen inlet valve was closed; the beginning time of the experiment at this moment was recorded, and the corrosion time was 168 hours.

3. Experimental parameters of each condition:

   • O₂ content was 1% (oxygen partial pressure 0.3 MPa), 3% (0.9 MPa), 5% (1.5 MPa), 8% (2.4 MPa), 10% (3.0 MPa), and 12% (3.6 MPa), total pressure was 30 MPa, and temperature was 60℃ in a dry gas environm

   • The temperature was 25℃, 45℃, 60℃, and 70℃, respectively, the total pressure was 30 MPa, the oxygen content was 5% (1.5MPa), and the air environment was dry.

   • The total pressure was 20, 25, 30, and 35 MPa, respectively, the oxygen content was 5% (the oxygen partial pressure was 1.0, 1.25, 1.5, and 1.75 MPa respectively), and the temperature was 60℃ in a dry gas environment.

   • The total pressure was 20, 25, 30, and 35 MPa, respectively, the oxygen partial pressure was 1.5 MPa, the temperature was 60℃ in a dry gas environment.

   • Fill oxygen-reduced purified water into the kettle, so that the hang-parcel was semi-immersed in liquid, the total pressure was 30 MPa, the oxygen content was 5% (1.5MPa), and the temperature was 60℃.

4. After the experiment was completed, the hang-parcel was taken out, and the blank hang-parcel was also taken out. The two groups of hang-parcel were put into an ultrasonic cleaning machine containing hydrochloric acid pickling solution to remove corrosion products. The proportion of pickling solution was as follows: 35 mL hydrochloric acid (concentration 37%), 315 mL purified water, and 3.15 g hexamethylenetetramine. The cleaning temperature is 30℃, and the cleaning time was 15 minutes. After pickling was completed, the sample was put into anhydrous alcohol for dehydration, then the dehydrated hang-parcels were dried in the drying oven for 1 hour, and then taken out and weighed.

5. Calculate the corrosion rate: Calculate the average corrosion rate of the hang-parcel according to the following weight loss method corrosion calculation equation:

where V is the corrosion rate, mm/a; m₁ is the mass before the corrosion hang-parcel test, g; m₂ is the quality of the corroded hang-parcel after cleaning, g; m₃ is the mass of the blank hang-parcel before cleaning, g; m₄ is the quality of the blank hang-parcel after cleaning, g; S is that surface area of the hang piece, cm²; t is the corrosion time of the hang-parcel, h; ρ is the density of hang-parcel, g/cm³.

The depth of the pitting pit was measured by the software matched with Ultra-Depth Three-Dimensional Microscope, and the local corrosion can be calculated by the following formula according to the measured depth of the pitting pit:

where: V’ is corrosion rate, mm/a; Δh is depth of corrosion pit, mm; t is corrosion time of hang-parcel, a.

According to Dalton’s law, the ratio of the number of particles in a single gas component to the total number of particles in the mixed gas is equal to the ratio of the partial pressure of a single gas to the total pressure of the mixed gas, so the ratio of the partial pressure of oxygen to the total pressure in the experiment indicates the oxygen content in the mixed gas. Figure 4 shows the relationship curve between current and potential measured under different oxygen content conditions. It can be seen that the corrosion potential moves negatively with the increase of oxygen content in the mixed gas, and the corrosion current of the cathode and anode increases significantly. The corrosion rate of steel increases exponentially with the increase of oxygen content. Figure 5 shows the diagram of corrosion laws of three kinds of gas injection shaft materials under different oxygen contents. The results show that the corrosion rates of the three steels increase with the increase of oxygen content under the conditions of total pressure 30 MPa, temperature 60℃, and dry environment. When the oxygen content was more than 3%, the corrosion rate of N80, J55, and 3Cr steels gradually slows down. When the oxygen content was more than 5% (oxygen partial pressure >1.5 MPa), the corrosion rates of the three steels tend to be stable. When the oxygen content was 5% (oxygen partial pressure was 1.5 MPa), the corrosion rates of N80, J55, and 3Cr steels were 0.01970 mm/a, 0.01695 mm/a, and 0.01500 mm/a, respectively. According to the National Association of Corrosion Engineer (NACE) standard RP-0775-2005 “Practice Preparation, Installation, Analysis, and Interpretation of Corrosion Coupons in Oilfield Operations,” it belongs to slight corrosion (<0.025 mm/a). Among the three kinds of steels, 3Cr steel has the best oxygen corrosion resistance, and N80 has the worst.

As can be seen from Figure 6 [4], when the air was deoxidized by the air separation unit, with the decrease of oxygen content in the air, the cost of oxygen reduction was gradually increasing. Especially when the oxygen content was reduced to less than 5%, the cost of oxygen reduction increases sharply. On the other hand, the cost of producing 5%–10% oxygen concentration oxygen-reduced air has little difference. Therefore, the oxygen content of oxygen-reducing air during the high-pressure injection process of oxygen-reducing air was recommended to be 5%. There were two reasons for this: On the one hand, under the condition of 5% oxygen content dry gas, the corrosion rates of the three steels were only slight corrosion (<0.025 mm/a). On the other hand, the manufacturing price of oxygen-reduced air with 5% oxygen concentration was similar to that of oxygen-reduced air with 10% oxygen concentration; 5% oxygen concentration oxygen-reduced air can still ensure that the manufacturing cost of oxygen-reduced air was maintained at a relatively low price.

The cost of producing oxygen-reducing air with 5% oxygen content was low. Under this condition, the corrosion rate is relatively slight, which indicates that 5% oxygen content in oxygen-reduced air has a good economy and low corrosion. However, the oxygen corrosion behavior was affected not only by the oxygen content but also by the temperature, total pressure, and corrosive medium environment. Therefore, it is very important to understand the corrosion behavior of the oxygen-reducing condition under different temperature, total pressure, and gas-water coexistence environment.

Figure 7 shows the corrosion rates of three steels at different temperatures. As can be seen from Figure 7, under a dry environment with a total pressure of 30 MPa and an oxygen content of 5% (1.5 MPa), the corrosion rates of the three steels increase with the increase in temperature. Surprisingly, under the condition of the oxygen content of 5% (1.5 MPa) and 70℃, the highest corrosion rate of the three steels was 0.0246 mm/a, and the lowest was 0.0176 mm/a, which means that in this case, no matter which kind of steel was used as the gas injection pipe, it can be used normally for decades, even if no corrosion inhibition measures were taken. Even when the temperature was 70℃, the three kinds of steel in the NACE RP0775 standard basically belong to slight corrosion (<0.025mm/a), which was far lower than the corrosion inhibition standard of oil and gas fields (usually no more than 0.1 mm/a). The corrosion resistance of 3Cr steel was better than that of J55 steel in the process of corrosion.

Figure 8 shows the corrosion rates of three steels under the same oxygen content and different total pressures. It can be seen from Figure 8 that when the temperature was 60℃ and the oxygen content was constant, the corrosion rates of the three steels gradually increase with the increase of the total pressure. However, due to the high oxygen partial pressure (>1.0 MPa) in the experimental range, the growth trend of the corrosion rates of the three steels was relatively slow.

Figure 9 shows the oxygen corrosion rate under the same oxygen partial pressure and different total pressure. It can be seen from Figure 8 that when the oxygen partial pressure was constant, the effect of the change of the experimental total pressure on the oxygen corrosion rate was not very obvious. Compared with Figure 8, it can be preliminarily judged that the partial pressure of oxygen was one of the important factors affecting the corrosion rate.

When the total pressure was 35 MPa and the oxygen content was 5% (oxygen partial pressure was 1.75 MPa), the corrosion rate of three kinds of steel was less than 0.025 mm/a, which belongs to slight corrosion (NACE RP0775 standard), which was far lower than the corrosion inhibition standard of oil and gas fields. Even if no corrosion inhibition measures are taken, they can be used for decades.

Under the conditions of 60℃, total pressure 30 MPa, 5% oxygen content (1.5 MPa), and coexistence of gas and water (experimental water is pure water), three steel materials were subjected to high-temperature and high-pressure corrosion experiments. Figure 10 shows the diagram of corrosion laws of three kinds of gas injection shaft materials under different oxygen contents. The results show that the corrosion rates of the three steels increase with the increase of oxygen content under the conditions of total pressure 30 MPa, temperature 60℃, and air-water coexistence environment. When the oxygen content was more than 5%, the corrosion rate of N80, J55, and 3Cr steels soared.

Table 4 shows the average corrosion rates of three kinds of steel under different conditions. In the coexistence of gas and water environment, even if pure water was used as the liquid phase, the corrosion rate of the three materials was still much higher than that in the gas environment, which was about forty-three, forty, and six times of that in the dry gas corrosive medium. This shows that in the liquid environment, the wellbore will face high corrosion risk. If the appropriate corrosion inhibition measures are not taken, the wellbore will appear the failure problem of depression and perforation in a short time. Under the condition of the coexistence of gas and water, N80 and J55 steels were extremely serious corrosion (>0.254 mm/a) in NACE RP0775 standard, 3Cr steel was moderate corrosion (0.025–0.125 mm/a), but considering that the liquid corrosion medium used in this experiment was pure water with low ion concentration, it is equivalent to adopting corrosion inhibition measures. Therefore, comprehensive analysis shows that in the downhole liquid environment, when the oxygen content was reduced to 5%, it is possible to meet the corrosion inhibition standard of oil and gas fields by using appropriate corrosion inhibition measures, and it is difficult and uneconomic to achieve the corrosion protection standard by reducing the oxygen content [25].

In the dry gas environment, there was little difference in the corrosion rate under different pressure, temperature, and oxygen concentration in the experimental range, which makes the macrocorrosion morphology of each hanging piece have no great difference, and the surfaces of the samples were smooth. Figure 11 shows the macromorphology of the surface before and after cleaning under the condition of 30 MPa, 60℃, and 5% oxygen content dry air. It can be seen from Figure 11 that under the condition of dry oxygen-reducing gas, after the completion of the corrosion test, the corrosion of the three kinds of steel surfaces was very slight, and the metallic luster can be seen. Only reddish-brown corrosion products can be seen in some small areas. After cleaning, only very small pitting traces can be seen in some parts, which can be ignored.

When the dry air condition was changed into the coexistence environment of gas and water and the other conditions were 30 MPa, 60℃, gas phase air oxygen content was 5%, the corrosion rate and corrosion morphology have qualitative changes. Figure 12 shows the macromorphology of the surface before and after cleaning under the condition of gas-water coexistence. It can be seen from a1–c1 in Figure 10 that there were obvious corrosion interfaces on the surface of the three samples, and the corrosion of most parts of the upper part of the sample was slight, showing metallic luster. The corrosion trace of the lower part of the sample was obvious, and a thick layer of reddish-brown corrosion products can be observed on the surface of N80 and J55 steel. The structure of the corrosion products was loose and easy to peel off. The corrosion products on the surface of 3Cr sample were thin and had a faint metallic luster. In Figure 10 a2–c2, it can be observed that the pitting traces on the surface of 3Cr steel are slight, and only a few shallow pitting traces appear in some areas. There were dense pitting corrosion pits at the gas-water interface between N80 steel and J55 steel. From the interface to the bottom of the coupon, the corrosion morphology gradually changes from dense pitting corrosion morphology to long groove corrosion morphology.

Figure 13 shows the three-dimensional surface corrosion morphology of the coupon after removing the corrosion products. It can be seen from Figure 13 that pitting corrosion of N80, J55, and 3Cr steels occurs in different degrees, among which the pitting corrosion of N80 steel was the most serious with more pitting corrosion areas and higher corrosion degree. The corrosion degree of some pits in J55 was almost the same as that in N80. The pitting corrosion depth of 3Cr steel varies, but the overall corrosion degree was generally low.

The local corrosion pit depth and local corrosion rate of the three steels were shown in Table 5. The local corrosion rates of N80, J55, and 3Cr steels in the presence of gas and water are 26.0704 mm/a, 20.7767 mm/a, and 2.7397 mm/a, respectively, which means that pitting corrosion and perforation of N80 and J55 steel may occur in a few months in the presence of oxygen in the presence of gas and water if proper corrosion inhibition measures were not taken. Although the local corrosion rate of 3Cr was relatively low compared with the other two, it was still in very serious corrosion in NACE standard, and appropriate anticorrosion measures were also needed. Among the three steels, 3Cr steel has the strongest pitting corrosion resistance, followed by J55 steel and N80 steel.

Since the metal in the air will occur oxygen depolarization corrosion. Among them, the oxygen depolarization corrosion process is as follows: first of all, the oxygen in the air enters the solution, and then the dissolved oxygen is uniformly diffused in the solution through the convection, and then due to the diffusion, action oxygen passes through the diffusion layer to the metal surface to form adsorbed oxygen, and finally, the oxygen molecules are reduced on the cathode. The electrochemical corrosion mechanism of oxygen in air is shown in Figure 14. The electrochemical reaction of oxygen in air or neutral solution is as follows:

Anodic reaction:

Cathode reaction:

In the solution, Fe²⁺ will react with OH⁻ to form Fe (OH)₂. Fe(OH)₂ can be further oxidized to form Fe(OH)₃ in the oxidizing environment. The final corrosion products are Fe₃O₄, Fe₂O₃, or FeOOH [31], as shown in formula 5–9:

Under the condition of dry oxygen-reducing air, the water content in the air is very low, so it is impossible to form a continuous water film on the metal surface, and only a small water film is formed in some areas of the metal surface. Due to the small thickness of the water film, it is more beneficial for oxygen in the air to transport to the electrode surface. As a result, the metal will not corrode in the area not covered by water film, while the area covered by water film will undergo electrochemical corrosion. When the corrosion develops to a certain extent, pits (local corrosion) will be formed on the material surface.

The content of dissolved oxygen in the solution is positively correlated with the oxygen partial pressure, which is controlled by the oxygen partial pressure [32]. When the oxygen content or oxygen partial pressure is small, the electrochemical corrosion rate of metal surface in solution is mainly determined by the diffusion rate of dissolved oxygen in water to the metal surface. Within this range, the oxygen corrosion rate increases rapidly with the increase of oxygen partial pressure (oxygen content). When the oxygen content or oxygen partial pressure is large, the oxygen corrosion rate is mainly controlled by the ionization reaction rate on the metal surface. Therefore, it can be observed from Figure 4 that when the oxygen partial pressure (oxygen content) increases to a certain extent, the oxygen corrosion rate begins to stabilize.

The influence of temperature on the corrosion rate has a dual role: on the one hand, the increase in temperature will accelerate the diffusion rate of oxygen and the reaction speed of the electrode, resulting in an increase in the corrosion rate; on the other hand, the increase in temperature will reduce the solubility of oxygen in solution [32], thereby slowing down the corrosion process. According to Figure 7, in the temperature range of 25°C–70°C, the heating effect of temperature dominates, so in this temperature range, the oxygen corrosion rate increases with the increase in temperature.

In a closed system, on the one hand, the solubility of oxygen in solution increases with the increase of the total pressure of the system, accelerating corrosion; on the other hand, an increase in the total pressure of the system will make the metal surface corrosion higher density, thereby reducing the oxygen corrosion rate [33]. However, in a gaseous environment, the oxygen corrosion rate itself is low, and the corrosion product is less formed, so the corrosion rate increases slowly with the total pressure of the system (as shown in Figure 8). The partial pressure increase of oxygen increases the solubility of oxygen in solution, improves the diffusion rate of dissolved oxygen to the metal surface in solution, and leads to an increase in oxygen corrosion rate.

Under the condition of the coexistence of gas and water, the oxygen concentration cell is formed due to the uneven distribution of oxygen content above and below the liquid surface. The air part on the liquid surface has a sufficient oxygen supply and can be regarded as the cathode, which is protected. The metal part below the liquid level, due to the relatively low dissolved content in water, makes this part of the metal become anode, so the corrosion degree is obvious. The corrosion rate of 3Cr steel under this condition is lower than that of the other two steels. The reason is that 3Cr is enriched in the corrosion product film and forms stable amorphous Cr (OH)₃ or Cr₂O₃, which makes the film more stable. At the same time, the corrosion product film dominated by Cr (OH)₃ has a certain cation selective permeability, which can effectively prevent anions from penetrating the corrosion product film to reach the metal surface and reduce the corrosion rate of the material [34].

Table 6 is the summary table of corrosion rate under various experimental conditions.

1. The oxygen content of oxygen-reducing air during the high-pressure injection process of oxygen-reducing air was recommended to be 5%. At this time, the corrosion rate of the wellbore pipe string can meet the oilfield anticorrosion standards, and the oxygen reduction cost can be maintained in a low range.

2. Under the condition of oxygen reduction, no matter how the temperature (25℃ and 70℃) and total pressure (20 and 35 MPa) change, the corrosion rate of N80, J55, and 3Cr steels is still lower than the corrosion inhibition standard of the oilfield.

3. The corrosion rate of N80, J55, and 3Cr steels in the water-gas coexistence oxygen-reduced environment was much higher than that in the dry condition, which indicates that the existence of water in the corrosive medium was the key factor to promote the qualitative change of the corrosion rate. Once there was water in the shaft fluid environment, the corrosion rate of steel would greatly increase.

4. Under the condition of pure gas injection, N80 steel with lower cost can be used because the corrosion rate under this condition is still slight corrosion (<0.025 mm/a) in the NACE standard, but the actual working condition of the oilfield may be more complex. However, due to the fact that the actual working conditions of the oilfield may be more complex, for the production method of the liquid-containing oxygen-decreasing air drive, it is necessary to take single or multiple anticorrosion measures such as anticorrosion steel, composite material continuous pipe, corrosion inhibitor, anticorrosion coating, and so on to meet the on-site gas injection and anticorrosion requirements and ensure the production safety of the on-site oxygen-reduced air flooding.

The authors gratefully expressed their thanks for the financial supported by the Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University (Ministry of Education & Hubei Province) (NO. UOG2022-31) and the Open Fund of Hubei Key Laboratory of Drilling and Production Engineering for Oil and Gas (Yangtze University) and the Foundation of the National Natural Science Foundation of China (No. 51306022).

The authors declare that they have no conflicts of interest.

The data for this study are available in this manuscript and supplementary material.