The following factors are particularly damaging to the well for water injection wells.

First of all, injected water injection wells usually contain a certain amount of chloride and sulfate salts, these salts have different degrees of corrosion on the pipeline, and these substances corrode the metal pipe after the product is mainly soluble, so for subsequent Corrosion without any shielding effect. Not only that, chloride ions and other ions can also damage the pipeline surface of the existing passivation film or other protective film. In this way, the damaged portion becomes the anode of the primary cell reaction, and the unspoiled portion becomes the cathode. As a result of this cell reaction, corrosion has become more intense at places where corrosion has occurred, creating what is known as pitting corrosion, which eventually leads to localized perforation. Due to this type of corrosion, a large number of water injection pipelines are forced to be scrapped before reaching their designed life expectancy.

The chemical attack caused by the dissolved gases injected into the water is also quite noticeable. In oil fields, it is very common practice to send oily wastewater into a water injection system. However, the oil-contaminated water contains corrosive gases such as dissolved oxygen, hydrogen sulfide or carbon dioxide. When the conditions are right, it is easy to corrode the steel pipes and facilities of the water injection system. For example, the presence of carbon dioxide will produce a significant decline in sewage pH, the system of acid corrosion is even more pronounced. There is a lot of carbon dioxide in the water, which leads to a lot of hydrogen ions, and the iron in the steel is greatly eroded in this environment. As the anode of electrochemical corrosion, the iron atom loses some of its own electrons and becomes soluble iron ions, which enter the solution, and the hydrogen ions get electrons and combine to become hydrogen gas, so as long as the water contains sufficient carbon dioxide gas, the steel can easily be continuously supplied Corrosion, water pipes and therefore gradually lose their role.

In many cases oxygen is dissolved in water to increase the rate of corrosion of the steel by more than 10 times at room temperature, although the rate of corrosion gradually decreases to a steady value over time, but this value is still pure Water corrosion rate more than twice. Therefore, the infiltration of oxygen into the oilfield wastewater during the injection into the injection well will largely destroy the pipeline matrix and shorten the life of the injection pipeline.

When oxygen and carbon dioxide coexist in oilfield wastewater. On the one hand dissolved oxygen to promote the electrochemical corrosion occurs, erosion of the pipe surface, on the other hand, the presence of carbon dioxide in the acidic environment will undermine the pipe surface protection layer may occur. The result of a combination of both is that the water pipe quickly corrodes but it is hard to detect any corrosion. It is noteworthy that the sewage in sulfur-containing oilfields contains a large amount of hydrogen sulfide, which creates new corrosion under these conditions. On the one hand, similar to carbon dioxide, the presence of hydrogen sulfide is conducive to hydrogen ion depolarization corrosion reaction, resulting in the surface of the water pipe and other metal components as a reaction anode is greatly corrosive, in normal saline environment, produce The iron ions and sulfide ions generated Fe9S and other sulfides. This sparingly soluble compound deposits on the pipe surface, but its loose structure does not

To the pipeline to bring good protection. Even more lethal is that the hydrogen generated by electrochemical corrosion can penetrate the inside of the pipe to make the pipe more brittle, resulting in a lot of micro-cracks that are not visible to the naked eye. In addition, the generation of sulfides is also easy to cause crack propagation in the pipe Of the stress, eventually leading to the rupture of the pipe. Therefore, in sulfur-filled water injection wells, it is very important that the water injection pipe be resistant to hydrogen sulfide corrosion.

In many injection wells, there is no trace of H2S in the initial injected water, but very typical hydrogen sulfide corrosion is still present due to the presence of sulfate-reducing bacteria (usually SRB). There are many kinds of such bacteria, the scope of survival is relatively wide, a few common in the temperature of 30 ℃ -35 ℃, PH value of 5.5-9.0 around the activity. The actual observation shows that SRB is very active in water injection wells, which are usually above 500 meters underground and near the dead water environment between oil casing. This bacterium can not only reduce the sulfate to produce hydrogen sulfide, so that the content of hydrogen sulfide in the water increases, making the deterioration of the water become "black water", and can produce substances such as FeS in the water blockage and destruction of the pipeline, resulting in a comprehensive water pipeline The destruction. Therefore, inhibiting the breeding of SRB has also become an important part of injection wells anti-corrosion.

In summary, there are many corrosion factors in oilfield water injection wells. The protection of water injection pipelines needs to analyze the characteristics and comprehensive effects of various factors in detail and take appropriate measures to achieve better results.