Corrosion phenomena in oil fields are not only very common, but also one of the three major public hazards in oil fields, which has a very negative impact on the comprehensive benefits of oil fields. Among the various facilities in the oil field, the presence of water injection wells is of great significance for the stable and efficient production of oil fields. The components of these injection wells, especially the downhole pipelines, are not only damaged by various corrosive media including soil, crude oil, atmosphere, etc., but also are in contact with water, especially oilfield sewage rich in various impurities. Face more corrosion threats. The following factors are particularly damaging to the injection well pipe.
First, the injection water in the injection well usually contains a certain amount of chloride and sulfate, which have different degrees of corrosion to the pipeline. Moreover, the products after the corrosion of the metal pipes by these substances are mainly soluble, so there is no shielding effect for the subsequent corrosion. Not only that, active ions such as chloride ions can also destroy existing passivation films or other protective films on the surface of the pipe. Thus, the damaged portion becomes the anode of the primary battery reaction, and the undestroyed portion becomes the cathode. The result of this battery reaction is that the corrosion is more severe in the already corroded areas, resulting in so-called pitting, which eventually leads to local perforation. Because this type of corrosion has forced a large number of water injection pipes to be scrapped before reaching the design life, it has received much attention.

The chemical corrosion caused by dissolved gases in the water is also quite impressive. In oil fields, it is very common to send oily sewage into a water injection system. However, the oily sewage contains corrosive gases such as dissolved oxygen, hydrogen sulfide or carbon dioxide. When the conditions are suitable, it is easy to corrode the steel pipe and facilities of the water injection system. For example, the presence of carbon dioxide causes a significant drop in the pH of the sewage, making the acid corrosion of the system more pronounced. There is a large amount of carbon dioxide in the water, which causes a large amount of hydrogen ions to be generated, and the iron in the steel is largely eroded in this environment. As an anode of electrochemical corrosion, iron atoms lose some of their own electrons, become soluble iron ions, enter the solution, and hydrogen ions get electrons, which are combined into hydrogen, so that as long as the water contains sufficient carbon dioxide gas, the steel is easily sourced. The ground is corroded, and the water injection pipe gradually loses its role.

In many cases, oxygen is dissolved in water, which can increase the rate of corrosion of steel at room temperature by more than 10 times, although over time The corrosion rate will gradually drop to a stable value, but this value is still more than twice the corrosion rate in pure water. Therefore, the dissolved oxygen in the process of injecting the water injection into the injection well will destroy the base of the pipeline to a large extent and shorten the life of the water injection pipeline.

When both oxygen and carbon dioxide are present in the oil field wastewater. On the one hand, dissolved oxygen promotes the occurrence of electrochemical corrosion and erodes the surface of the pipeline. On the other hand, the acidic environment caused by the presence of carbon dioxide will destroy the protective layer that may appear on the surface of the pipeline. The result of the joint action of the two sides is that the water injection pipe is quickly corroded but it is difficult to find the product of corrosion.

It is worth noting that sewage in sulfur-containing fields contains large amounts of hydrogen sulfide, which creates new corrosion under these conditions. On the one hand, similar to carbon dioxide, the presence of hydrogen sulfide facilitates the depolarization and corrosion reaction of hydrogen ions, causing a large amount of metal components such as iron on the surface of the water injection pipe to be corroded as a reaction anode, which is produced in a normal salt water environment. The iron ions will form sulfides such as Fe9S8 with sulfur ions. This poorly soluble compound deposits on the surface of the pipe, but its loose structure does not provide good protection for the pipe. More deadly, the penetration of hydrogen from electrochemical corrosion into the interior of the pipe can make the pipe more brittle, resulting in many micro-cracks that are difficult to see by the naked eye. In addition, the formation of sulfides can easily cause crack propagation in the pipeline. The stress eventually leads to the rupture of the pipe. Therefore, in the sulfur-containing injection wells, it is very important that the water injection pipeline resists the corrosion of hydrogen sulfide.

In many injection wells, there is no trace of hydrogen sulfide in the initial injected water, but because of sulfate-reducing bacteria (usually represented by SRB) There is still a very typical phenomenon of hydrogen sulfide corrosion. There are many kinds of such bacteria, and the range of survival is also wide. The common ones are active in an environment with a temperature of 30 ° C - 35 ° C and a pH of about 5.5 - 9.0. The actual observation found that in the injection wells, it is usually more than 500 meters underground, and the SRB is very active in the near-dead environment between the oil casings. This kind of bacteria can not only reduce the production of hydrogen sulfide by sulphate, but also increase the content of hydrogen sulphide in the water, and make the water quality deteriorate into "black water". It can also cause substances such as FeS to block and destroy the pipeline in the water, thus making the water injection pipeline comprehensive. The destruction. Therefore, inhibiting the reproduction of SRB has become an important part of the corrosion prevention of water injection wells.

In summary, there are many corrosion factors in oilfield injection wells. The protection of water injection pipelines requires detailed analysis of the characteristics and comprehensive effects of various factors. And take corresponding measures to achieve better results. At present, many scientific research institutions and oilfield service providers in China are committed to introducing new technologies and new products for anti-corrosion of oilfield water injection pipes, such as the newly introduced nano-coating oil pipes of Beijing Anton Donger Engineering Technology Co., Ltd. The anti-corrosion and anti-scaling of the tubing have obvious effects. It is believed that in the near future, with the promotion of these products and technologies, the overall anti-corrosion level of China's oilfields will be greatly improved.