Cathodic protection of steel casing pipe

With the rapid development of the petroleum industry, severe corrosion of casings, buried pipelines, tank bottoms, especially casings has had a serious impact on the production, development, management and environment of oil fields.

Oil well casing is an additional layer of protection in order to protect the internal main oil pipeline from being squeezed by the stress of the external stone and soil layers and enhance its transportation stability. Since oil wells are thousands of meters deep underground, the deep soil casing is under tremendous pressure. With the long-term use of pipelines, oil well casings will suffer varying degrees of corrosion, holes, fractures, deformation, ruptures, and serious or even damage after a minimum of 16 months and a maximum of 5 years. This can cause significant losses to the well's performance.

1. Casing corrosion problem

Corrosion exists in both the inner and outer casing walls of oil and gas wells, among which internal corrosion is more serious. In addition, there are serious corrosion defects in the welds of the casing, such as weld corrosion of the casing. Well sections with serious corrosion are generally above 1000m. In order to solve the corrosion problem of the inner wall of oil well casing, technologies such as biocides, corrosion inhibitors, pH value adjustment, and closed oxygen isolation are generally added to the annular space of the oil well, and low-strength casing is selected in sulfur-containing oil and gas fields.

2. Cathodic protection prevents casing corrosion

In order to solve the corrosion problem of the outer wall of steel casing, cathodic protection technology should be used. At present, it has been recognized by people in the field of anti-corrosion at home and abroad. Cathodic protection is the most economical, reasonable and effective technical measure to prevent soil corrosion of underground steel structures. Cathodic protection is a safe and mature anti-corrosion technology. It has the characteristics of strong protection ability, simple construction and low investment. This technology is increasingly used in oil and gas, petrochemical, municipal construction and other sectors.

3.Cathodic protection of oil well casing has several characteristics.

Due to the depth of the oil well and the complex environment it faces, the impressed current protection method in cathodic protection must be used to effectively ensure the cathodic protection efficiency of the oil well casing. Secondly, in order to cope with the pressure of deep soil layers and possible corrosive environments, deep well-type anodes must be used as auxiliary anodes in the impressed current method. Deep well anodes have the advantages of being less affected by climate, stable grounding resistance, small stray current interference, small footprint, long service life, and easy installation. They are suitable for situations with dense pipe networks and harsh working conditions.

In practical applications, in areas with many or dense oil wells, one deep anode well is required for every two to four wells to serve as the anode end of the cathodic protection system. Deep anode wells are usually located near several oil wells to ensure that the cathodic protection current can cover all oil wells within the range.

Finally, based on the above two points, the impressed current cathodic protection of oil wells must also use a customized potentiostat to provide sufficient, appropriate cathodic protection current. The cathodic protection current will be emitted by the potentiostat, flow to the anode deep well as the anode section, and then be transported through the soil to several nearby oil ell asings, where it will become the cathode and thus protected, and finally return to the constant potential from the wellhead confluence point on the surface instrument, forming a complete closed loop.

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