Introduction of OCTG corrosion

1. The harmfulness of corrosion in petroleum industry

According to the statistics of relevant departments, the average annual corrosion scrap rate of metal equipment and machines is about 4%. If we reduce the loss caused by corrosion to the national economy by one percentage point, that is, from 4% to 3%, we can recover the corrosion loss of 60-80 billion yuan. The petroleum industry has realized that corrosion is a very serious problem, and the economic loss caused by the corrosion of drill pipe and wellbore string is particularly prominent. Once the failure occurs, it will cause: casualties, environmental pollution, abandoned well, drilling stop, well drop and other consequences. Thousands of corrosion failure accidents occur every year in oilfields all over the country. In addition to wasting a lot of raw materials, some of them also cause serious environmental pollution and personal casualties. According to incomplete statistics, the direct economic loss caused by corrosion in China's oil industry every year is more than one billion yuan, and the indirect loss is even more amazing, which can not be ignored by all departments.

2. Corrosion status of petroleum special pipe

The corrosion of oil casing, gathering pipeline and storage tank is mainly caused by the products of oil and gas well, shallow water and injection liquid, and the main media are H2S, CO2, dissolved oxygen and SRB. Drilling tools are often affected by many media including drilling fluid, such as pitting corrosion, corrosion fatigue, gravitational tension and rotation stress corrosion damage and fracture.

2.1 corrosion characteristics of oil pipe / gathering pipeline

There are sulfate reducing bacteria, carbon dioxide, bicarbonate ion and chloride in the produced water, which cause under scale corrosion and thread corrosion. When hydrogen sulfide occurs in wells (especially in gas wells), tubing damage is dangerous. The main causes of corrosion damage are hydrogen embrittlement HIC and SSCC. The tubing is in a complex stress state under the action of self weight, external pressure and other factors. The tensile stress is an important factor to promote the damage of metal protective film and matrix corrosion. When Sr2 + and Ba2 + scales appear, they can not be cleaned by conventional pickling method, which seriously affects the normal production. Wax deposition still exists in oil pipes and gathering pipelines, and high frequency of wax removal seriously affects normal production. The erosion corrosion of oil pipe and gathering pipeline is also a common form of corrosion failure, including abrasion, cavitation, etc. The corrosion pattern (CO2 corrosion) of the inner wall of tubing in an oil field is typical uniform internal corrosion and typical corrosion perforation.

2.2 corrosion characteristics of drill pipe

The main failure modes are excessive deformation, puncture or fracture and surface damage. Excessive deformation is caused by the working stress exceeding the yield limit of the material; piercing or fracture accounts for a large proportion of drill string failure accidents, mainly due to stress corrosion, fatigue and corrosion fatigue. Surface damage mainly includes corrosion, wear and mechanical damage. 70% of the accidents are caused by perforation or fracture in the transition zone of drill pipe thickening. The prerequisite for piercing is the existence of cracks. The corrosion of the outer wall of the drill pipe is light and uniform, but the corrosion of the inner wall of the drill pipe is very uneven. The corrosion at the junction of the inner thickening transition zone and the pipe body is serious. There are many pitting pits, and the cracks are originated at the bottom of these pitting pits. Internal coating is the main solution to solve the corrosion fatigue failure of drill pipe.

2.3 corrosion characteristics of casing

Casing corrosion is mainly caused by corrosion of sulfate reducing bacteria, CO2, bicarbonate and chloride in produced water (formation water). The serious corrosion of screw thread is mainly due to the combined action of bacterial corrosion, crevice corrosion and stress corrosion. Periodic friction accelerates the corrosion perforation of casing, which is characterized by directivity. The low cementing quality results in the loss of the protective effect of the casing cement layer.

2.4 external corrosion of oil well pipe during storage and transportation

During the storage and transportation of oil well pipes, the corrosion forms: atmospheric corrosion (O2 corrosion), corrosion under scale (failure of protective layer, formation of local oxygen concentration cell of hot-rolled oxide scale and other surface attachments), salt spray (during sea transportation), acid fog corrosion (typical in Chongqing); should cause the manufacturer (long-term anti-corrosion coating, export sea transportation, appearance quality) Attention of users (protective measures should be taken for storage and transportation). Requirements for external protective coating: good water resistance (7D); good salt spray resistance (grade 1-2); good weather resistance (no discoloration in various environments); good adhesion (grade 1); good impact resistance (3j).

3. Corrosion factors in OCTG service environment

3.1 dissolved oxygen: the dissolved oxygen in drilling fluid is the main cause of drill pipe corrosion, resulting in ferric hydroxide precipitation. It has two accelerated corrosion mechanisms, one is the depolarization of oxygen, the other is the direct attack of oxygen on metal at anode.

3.2 hydrogen sulfide: in the presence of sulfide, hydrogen atoms enter the steel and cause hydrogen embrittlement. Maintaining the pH value of drilling fluid above 8 is an effective way to control hydrogen sulfide corrosion. In addition, sulfide stress cracking of steel is closely related to temperature, external stress or residual stress, microstructure and chemical composition of steel.

3.3 carbon dioxide: forms carbonic acid after dissolving in water, and the corrosion characteristic is pitting corrosion. The corrosion strength depends on the partial pressure and pH value of carbon dioxide.

3.4 SRB: bacterial corrosion, especially sulfate reducing bacteria (SRB), is one of the main corrosive bacteria in casing corrosion.

4. Main influencing factors of OCTG corrosion

4.1 pH value: hydrogen ion is a cathodic depolarizer. Generally, the smaller the pH value is, the more serious the metal corrosion is. When the pH value is higher than 6.6, the corrosion rate begins to decrease.

4.2 Cl -, total salinity: Cl - is the main factor to destroy the passive film and the accelerator to produce pitting pits. In many environments, such as CO2 corrosion, it is the main factor to destroy the CO2 corrosion film and accelerate the pitting. With the increase of salinity, the conductivity of the liquid is improved, and the increase of harmful ions accelerates the corrosion.

4.3 temperature: generally, the corrosion rate increases with the increase of temperature. The main reason is that the reaction speed increases with the increase of particle diffusion speed.

4.4 velocity: This is an extremely important and complex factor. It includes physical, mechanical and electrochemical contents, mainly including the following three forms of damage, namely cavitation, abrasion and corrosion.

4.5 dissolved oxygen: the concentration of dissolved oxygen is the main influencing factor of corrosion, and the deoxidation of drilling mud and sewage reinjection is very important.

4.6 others: such as water content, protective film, impurities, oxygen content, bacteria and other influencing factors.

5. Corrosion mechanism of OCTG

5.1 the main corrosion mechanism of electrochemical corrosion: the ferrite of steel is used as the anode, and the cathode is carbon body with higher potential, forming corrosion couple, and the anode dissolves gradually, that is, electrochemical weight-loss corrosion.

5.2 CO2 corrosion characteristics: local corrosion, pitting corrosion, ringworm corrosion and mesa pit corrosion, high perforation rate.

5.3 H2S corrosion characteristics: formation of stress corrosion cracking (SSCC). Hydrogen sulphuric acid is easy to ionize, resulting in hydrogen corrosion of low strength steel and hydrogen embrittlement of high strength steel.

5.4 function of water in medium:

① Water is an essential medium for electrochemical reaction.

② Both corrosion and CO2 corrosion are caused by dissolving with CO2 in water to form acidic substances (carbonic acid and hydrogen sulphuric acid).

③ Acidic conditions aggravate ionization and electrochemical corrosion.

5.5 corrosion characteristics and mechanism of sulfate reducing bacteria (SRB): the casing is in the state of stagnant water and anoxia for a long time, and SRB propagates in large numbers, reducing sulfate to H2S, resulting in serious corrosion. Hydrogen corrosion of low strength steel and hydrogen embrittlement of high strength steel are the main forms.

5.6 stress corrosion: due to the great weight of the drilling tool itself and the rotation of the drilling tool, especially the drilling of directional wells, the friction between the drilling tool and the formation or casing is very large, that is to say, there is great stress, and the interaction in the corrosive medium causes corrosion cracking.

5.7 mechanical wear and man-made impact: especially in the presence of stress, local wear or scratch will become the source of corrosion failure.

6. Basic anti-corrosion measures for oil well pipes

6.1 according to the standard design and material selection, different materials and steel grades should be selected for different service environments. High alloy tubes such as stainless steel tubes are also used in corrosive environment (the cost is more than 4 times that of carbon steel), but stainless steel tubes also have the problem of Cl - pitting corrosion.

6.2 take appropriate and effective protective coating. Such as anti-corrosion coating and metal coating protection technology;

6.3 other auxiliary means shall be adopted, such as adding corrosion inhibitor and closed water injection system to reduce the dissolved amount of O2, CO2 and H2S; using sulfate reducing bacteria bactericide.

6.4 adopt cathodic protection, establish corrosion monitoring system, grasp corrosion trends, and take preventive measures.

7. Performance requirements of coating for oil well pipe

The essence of oil well pipe anti-corrosion is shielding. It is to prevent the contact of H2S, Cl - with CO2 and the acidic compounds formed by dissolving in water with the pipe body. The specific requirements are: the coating surface is smooth, defect free and good adhesion; it has a certain temperature resistance; it has a good barrier to corrosive media in the environment; the chemical properties of the coating material are stable; it has reliable mechanical properties.

8. Function of internal anti-corrosion coating

Due to the electrical insulation of the coating, it can resist corrosion; due to the smooth surface, it can improve fluid efficiency; due to the smooth surface and chemical properties, it can reduce wax deposition; due to the chemical properties of the coating, it can reduce scaling; it can improve the reuse rate of pipes; it can avoid fishing and workover operations; it is conducive to improving the water quality of water injection.

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