Corrosion of Casing Pipe

The corrosion of casing pipe is related to the metallurgical property of pipe material and service environment. Metallurgical properties of the pipe include: chemical composition, heat treatment and material structure. Service environmental effects include: medium pH, solution composition and concentration, temperature and pressure, medium flow rate and so on. Carbon steel and low alloy steel and H2S, CO2 and Cl- and other corrosive media contact, there will be two types of corrosion, one for the H2S caused by environmental sensitive cracking, including hydrogen induced cracking (HIC), ladder (SWC), stress-directed hydrogen induced cracking (SOHIC), soft zone cracking (SZC) and sulfide stress cracking (SSC); the other is weight loss corrosion (MLC), including comprehensive corrosion, pitting and crevice corrosion. According to the corrosive medium, generally have dissolved gas corrosion, dissolved salt corrosion and bacterial corrosion.

Dissolved gas corrosionOxygen dissolved in oil and gas fields (mainly from the ground into the mud and water injection) can cause corrosion of carbon steel. Oxygen containing less than 1 mg / L in the solution may cause severe corrosion. If CO2 or H2S is present at the same time, the corrosion rate will increase sharply. The main factors affecting the oxygen corrosion of oxygen concentration, pressure, temperature and so on. The corrosion rate of carbon steel in oil and gas fields depends on the oxygen concentration and the oxygen diffusion barrier. The corrosion rate is faster, and as the corrosion process progresses, The

CO2 is often present as an associated gas in oil and natural gas in oil and gas. In addition, the use of CO2 as a hydraulic agent to improve oil and gas recovery will bring CO2. CO2 can be dissolved in water to produce H2CO3, reducing the pH value of the solution, causing the anode iron to dissolve and cause corrosion. The United States LittleCreek oil field during the implementation of CO2 flooding field test, did not take any protective measures, tubing corrosion rate of up to 1217mm / a, less than 5 months time, the wall was corroded perforation. The CO2 oil associated gas content in the buried hill structure of North China oilfield is 42%, which makes the corrosion rate of low carbon steel reach 3 ~ 7mm / a. CO2 can lead to severe localized corrosion, perforation and stress corrosion (SCC). Factors that influence the CO2 corrosion rate of carbon steel are mainly related to CO2 partial pressure, temperature, pH value, Cl- and HCO3-, and the CO2 partial pressure plays a decisive role. When the partial pressure of CO2 is less than 0.021MPa, almost no corrosion occurs. When the partial pressure is between 0.021 and 0.21MPa, pitting occurs in different degrees. When the partial pressure is more than 0.21MPa, serious local corrosion occurs. Local corrosion of CO2-containing oil and gas wells is often selectively occurring at some point in the well due to temperature. Many domestic and foreign oil and gas wells contain H2S. H2S in oil and gas, H2S is released from the formation of sulphate-reducing bacteria (SRB) in the formation of neutralizing the sulfate in the chemical additive. The aqueous solution of H2S is acidic and increases the corrosion rate. The content of H2S in the South Texas gas field is as high as 98%, the highest in the world, and the H2S content in the Alberta gas field is 81%. The content of H2S in Zhaoanzhuang gas field in Hebei is 92%. Carbon steel in H2S aqueous solution will produce hydrogen depolarization corrosion, H2S concentration is low, can generate a dense iron sulfide film, effectively prevent the Fe ions through, thereby significantly reducing the corrosion rate of metal; H2S concentration is high, the generation Of the iron sulfide film was black loose layered or powder, not only can not prevent the passage of Fe ions, but with the formation of macro MAC batteries. Dissolved salts and CO2 in water also have a certain effect on H2S corrosion.

Corrosion of bacteriaAs the annular part of the liquid is relatively static, the ring spaceOf the injection of water with the depth of the increase in temperature, for the breeding and breeding of bacteria to create favorable conditions. Common bacteria are sulfate-reducing bacteria (SRB), iron bacteria and mucous bacteria. Which is the most serious corrosion caused by SRB bacteria, accounting for more than 50% of total corrosion. SRB is a kind of organic matter for the bacteria, the temperature rise of 10?, SRB growth rate increased by 1.5 to 2.5 times, beyond a certain temperature, SRB growth will be inhibited or even death. The optimum pH for SRB survival is 7.0 to 7.5, and the metabolic activity of SRB will decrease. SRB corrosion principle is to reduce the sulfate to divalent sulfur, divalent sulfur and iron reaction to produce black FeS, resulting in casing corrosion. In addition, SRB cell aggregates and corrosion products into the formation with the injection of water may also cause stratigraphic blockage, resulting in increased water injection pressure, water injection decreased, a direct impact on crude oil production.

Dissolved salt corrosionThe corrosion rate of dissolved salts of carbon steel in oilfields is significantThe impact of the. Carbon steel in the neutral and alkaline salt solution is the main occurrence of oxygen depolarization corrosion, will produce protective passivation film, therefore, than in acidic salt solution corrosion rate is smaller. The presence of Ca2 + and Mg2 + ions increases the salinity of the solution, which increases the ionic strength and aggravates the local corrosion. The presence of HCO3 inhibits the dissolution of FeCO3, which facilitates the formation of the corrosive product film and facilitates the passivation of the metal surface, thereby reducing the corrosion rate. Cl- is the main anion that causes corrosion of the oil casing. On the one hand Cl - because the radius is small, easy to penetrate the corrosion product film, and adsorbed on the metal surface of Fe2 + ions combined to form FeCl2, carbon steel and low alloy steel to promote corrosion.

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