Corrosion Mechanism and Anti-corrosion Process of Oil Well Casing

The damage to the wellbore structure of oil fields and water wells not only affects the normal production of damaged wells, but also affects the normal development of adjacent wells and even blocks, and poses a great threat to the stable production of the entire oil field. The main factors that cause damage to the structure of oil and water wells are: chemical action, physical action and quality problems of oil and water wells. Among them, the chemical corrosion of the casing accounts for a very large proportion. The main cause of corrosion damage is the formation water containing sulfide which corrodes the casing, and the electrochemical action of the surface causes the casing to corrode. With the extension of the development time of the oil field, the corrosion of the oil well casing is becoming more and more serious, which directly affects the further exploitation of the oil field.

1. Research on the corrosion mechanism of oil well casing
1.1 Causes and mechanisms of soil corrosion
Soil is an extremely complex multi-phase system composed of three phases of solid, liquid and gas. In the soil, there are a large number of capillary micropores between the soil particles, and the gaps are filled with air and water. Water exists in various forms in the soil. Soil is an ionic conductor due to the presence of water, so the soil can be regarded as a corrosive electrolyte. Due to the above characteristics of soil, soil corrosion and other electrochemical corrosion processes have different characteristics, namely the transfer of oxygen. Oxygen is transported through the solution body in the solution, and through the pores of the soil when the soil is corroded. In soils with different structures and humidity, oxygen permeability will vary greatly. Oxygen concentration batteries will play a big role when the soil is corroded. The soil pH value in the study area is greater than 8, which indicates that the soil in this area is alkaline, and the content and content of the soil are high.

1.2 Causes and mechanisms of chemical corrosion
The electrochemical corrosion of the metal is due to the corrosion of the galvanic cell when the metal is in contact with the electrolyte solution. For example, if two electrodes of zinc and copper are placed in a dilute solution, when they are connected with a wire, a current flows through the wire. This device is called a primary battery. The lower potential is the anode, and the higher potential is the cathode. In the copper-zinc battery, the zinc potential is lower as the anode, which will continuously lose electrons and become Zn2 + into the solution, that is, zinc is continuously oxidized and dissolved; the copper potential is high as the cathode, which only serves to transfer electrons, so that H + in the acid gets electrons It is reduced to hydrogen gas escaping from the surface of copper, and copper itself has no change. If two metals of copper and zinc are brought together and immersed in an electrolyte solution (such as dilute), the same change will occur. In this case, zinc is the anode, and the electrons it loses flow through the copper in direct contact with it and are accepted in the solution on its surface, so zinc continuously becomes electrons into the solution, and zinc is corroded. It can be seen that the corrosion of the metal in the electrolyte solution is due to the formation of the primary battery. Such batteries are called corrosive primary batteries.

The annulus corrosion of the oil jacket of the water injection well is mainly caused by the combined action of SRB bacteria, CO2, chemical compounds and a small amount of dissolved oxygen in the sewage. Because the liquid in the annulus is relatively stationary, the water injection in the annulus space is corrosive during the long-term water injection process. As the depth increases and the temperature increases, it creates favorable conditions for the growth and reproduction of bacteria. Common bacteria include sulfate reducing bacteria (SRB), iron bacteria and saprophytic bacteria (slime forming bacteria). Among them, SRB bacteria cause the most serious corrosion, accounting for about 50% of the total corrosion. Sulfate-reducing bacteria are anaerobic bacteria with strong adaptability. Their suitable survival temperature is 25 ~ 37 ° C, and their suitable pH value is 6.7 ~ 7.3. The corrosion principle is to reduce S-in water to S and cause corrosion. The overall corrosion reaction is as follows: SRB4Fe + S0-24 + 4H20- + FeS + 3Fe (0H) 2 + 20H-injection water of the injection well usually contains a certain amount of catch compounds and sulfates, these salts are different for the ring sleeve The degree of corrosion, and the products after these substances corrode the metal are mainly solubles, so there is no shielding effect for subsequent corrosion.

2. Research on cathodic protection and anti-corrosion technology of sacrificial anode
2.1 Cathodic protection technology of oil well casing
There are two types of cathodic protection technology: sacrificial anode cathodic protection and forced current (impressed current) cathodic protection.
Sacrificial anode cathodic protection technology. Sacrificial anode cathodic protection technology is to use a metal or alloy with a potential that is more negative than the metal to be protected to electrically connect with the protected metal, relying on the current generated by the metal with a more negative potential to continuously corrode and dissolve to protect other metal.

As a sacrificial anode material, it must meet the following requirements: ① must have a sufficiently negative stable potential; ② self-corrosion rate is small and the corrosion is uniform, and a high and stable current efficiency; ③ high electrochemical equivalent, that is, the electricity generated per unit weight Large flow rate, ④ The anode polarization should be small during work, and the product is easy to dissolve; ⑤ Corrosion products do not pollute the environment and have no pollution; ⑥ Wide source of materials, easy processing, and low price. Forced current cathodic protection technology. The forced current method is to pass a cathode current directly from the external DC power supply to the metal to be protected, so that the metal to be protected becomes a cathode to achieve the purpose of cathodic protection.

According to the solubility of anodes, auxiliary anodes can be divided into three categories: soluble anodes (such as steel, aluminum), slightly soluble anodes (such as high-silicon cast iron, graphite), and insoluble anodes (such as platinum, platinum plating, and metal oxides). DC power supply is a power source for forced current. Its basic requirements are stable and reliable, can run continuously for a long time, and adapt to various environmental conditions.

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