What are the Anti-corrosion Methods for Offshore Tubing and Casing?

The corrosion of oil casing is related to the partial pressure of corrosive gas, ambient temperature, water content, salinity, flow rate, pH value and other factors. The corrosion characteristics of oil and gas fields in different sea areas are different. The temperature of the developed oil layers of most oilfields in the Bohai Sea is between 60 and 150, and the partial pressure of CO2 is between 0 and 1.5MPa. The pH value of the formation water is basically above 7.5, and the Cl- of the formation water is generally low, basically below 5000 ppm. The partial pressure of CO2 in gas wells in the East China Sea is between 0.3 and 1.5 MPa, and the temperature is basically between 100  and 150 . The pH value of the formation water is basically above 7.5, and the Cl- of the formation water is generally low, basically below 5000ppm. The temperature of the development zone in the eastern South China Sea oilfield is mostly between 65°C and 132°C, and the partial pressure of CO2 is mostly lower than 0.4MPa, with the highest being 1.68MPa. Some oil fields contain high levels of hydrogen sulfide. The pH of the formation water is basically below 7, and the Cl- of the formation water is relatively high, between 15,000 and 25,000 ppm. The development horizon temperature and CO2 partial pressure of the western South China Sea oilfield are mostly distributed in the area of 59<160, 0.09MPa<3.67MPa. The chloride ion content is mainly distributed around 15000ppm.

Common anti-corrosion methods for offshore tubing and casing:

1. Cement sealing anti-corrosion technology

In the earliest stage, the main reason for casing damage is that the cement is not high enough to return to the outer water layer, which causes the corrosive gas in the water layer to erode the casing surface and cause corrosion. Therefore, in the 1980s, a strategy to increase the cement back height was proposed to increase the corrosion resistance of the casing. Through this technology, the degree of casing corrosion is reduced, but it does not completely prevent casing corrosion, so the corrosion resistance of the casing is not improved from the root cause.

1. Use corrosion-resistant alloy pipe

Since 2000, the anti-corrosion program of offshore tubing and casing began to seek solutions from the material to improve the anti-corrosion performance of the tubing and casing, thereby completely solving the problem of tubing and casing pipe corrosion. In recent years, through indoor experiments, under certain temperature and pressure conditions and corrosive gas conditions, Cr-containing pipes such as 1Cr, 3Cr, 9Cr, 13Cr have been evaluated, and a set of anti-corrosion patterns suitable for offshore oil and gas field tubing and casing has been developed.

1. Inject corrosion inhibitor

From the beginning of the completion process, if the formation contains oily corrosive gas, chemical anti-corrosive agents will be added to the well fluid to prevent scale, wax and corrosion. Therefore, the use of chemical methods for anti-corrosion of tubing and casing is also a conventional method at home and abroad. Depending on the oil and gas field itself, intermittent or continuous injection of corrosion inhibitors can be used for anti-corrosion, and the injection volume also varies according to the corrosive environment of the oil and gas field. Since the addition of corrosion inhibitors needs to be continuous, on the one hand, the cost is high, and it will also have a certain impact on on-site production.

1. Use anti-corrosion coating tube

For tubing and casing pipes, anti-corrosion coatings are often used at home and abroad to prevent them from being infiltrated in corrosive gases, which is also an effective anti-corrosion method. Some oil fields use the nitriding process for the internal and external anti-corrosion of the tubing, and its wear resistance is good, but this method of anti-corrosion on the inner and outer walls will accelerate the corrosion of the inner wall of the casing to a certain extent. At the same time, the nitriding tube is relatively rough, which will make the tubing Yuzaka is more serious than other types of tubes. In addition, some oil and gas fields use epoxy powder coating, titanium nano coating, epoxy ester phenolic coating and other methods to prevent corrosion.

1. Strengthen corrosion monitoring

With the advancement of science and technology, more and more methods of monitoring corrosion have been applied: such as anode excitation technology, radiation activation technology, thin layer activation technology (TLA), optical fiber sensing technology, resonance frequency measurement method, laser method to determine oxide film Thickness method, penetrant flaw detection method, potential gradient method, holographic interferometry, Raman spectroscopy, etc. Through these different monitoring methods, it is possible to monitor the corrosion of the tubing and casing in an all-round way, and provide better means and methods for the control of the corrosion of the tubing and casing.