Corrosion can lead to thread leakage of downhole casing , pipe perforation, thinning of wall thickness and possible collapse or burst, and lead to catastrophic accidents. The formation water is poured into the well from the corrosion perforated part of the casing, which results in the shutdown of oil and gas wells or the sharp increase of the water content of the produced products. If the formation in this area is high pressure, it may cause underground blowout. If the casing is corroded and perforated in a low pressure zone, it may cause production fluid to flow into the annulus outside the pipe and blowout, and the upper freshwater layer may also be polluted. If the corroded perforated part of casing can not be repaired, only abandoned wells are needed. For high pressure and high production gas wells, abandoning wells is a very complex operation. Therefore, it is of great significance to study the corrosion mechanism of downhole casing and take appropriate anti-corrosion measures.
External corrosion sources and corrosion mechanism of casing
The corrosion outside the casing mainly occurs in the free casing section without cement injection, and the cement sheath in the cement injection section can better protect the casing from corrosion. In the section with poor quality of cement injection, or the section with cement sheath damaged by downhole operation, casing may also suffer from external corrosion. With the occurrence of uniform corrosion and local corrosion, corrosion perforation occurs.
The main corrosion mechanisms of casing corrosion are multi-component induced corrosion, regional stray current corrosion, annular drilling fluid corrosion, metal defects of casing steel, operation damage and local stress concentration induced corrosion.
Measures for Reducing Corrosion of Casing Outer Wall
1. Avoiding too long Open hole section
If the open-hole section is too long, sand-mudstone profiles with high salinity or corrosive formation water may appear. At present, exploration and development wells only pay attention to the evaluation of oil and gas reservoirs, but pay less attention to the evaluation of formation water drilled through formation profile. Therefore, in the long open-hole section, it is not clear which formation section contains water or water content, which lays a hidden danger for the subsequent corrosion outside casing. In view of the strong corrosiveness of formation water in some sections of wells, it is necessary to inject cement to protect casing.
2. Using outer winding protective film of casing
The outer winding protective film of casing has very high corrosion resistance, which requires the ability to resist abrasion when running down, resist bite of casing pliers, or repair the bite in time on the drilling platform. The use temperature of winding protective film outside casing is different, and the price varies greatly. In order to reduce the cost, different outer winding protective films can be selected according to the downhole temperature. The winding protective film outside casing is the main technical direction to solve the corrosion of long open hole in deep well and cement injection section outside casing.
3. Choosing Corrosion Resistant Carbon Steel or Low Alloy Steel
API and ISO do not include the corrosion resistance of carbon steel or low alloy steel in the technical indicators, but the casing designer and purchaser can choose the casing with better corrosion resistance among many kinds of casing. Some stainless steels have strong resistance to Weightlessness Corrosion, but poor resistance to pitting corrosion. In the evaluation of corrosion performance, the anti-pitting ability is emphasized. It is forbidden to use casing with good Weightlessness Corrosion resistance but poor pitting corrosion resistance.
4. Improving Cement Quality and Using Sulfur Resistant Cement
Cement injection can effectively prevent casing corrosion on the premise that the quality of cement injection is good and the cement is not corroded by sulfate in formation. The corrosion of formation water to cement can be divided into two stages: cementing stage and the end of cement pumping. If the cement setting time is too long or the cement is weightless, formation water will invade. Water-invaded cement does not form cement stone or has poor cementing strength.
Improving the quality of cement injection and adopting appropriate sulfur-resistant cement are important technologies to prevent or slow down corrosion outside casing.
In summary, the main recommended measures for casing outer wall anticorrosion are: avoiding too long Open hole section, adopting outer winding protective film of casing, choosing corrosion-resistant carbon steel or low alloy steel, improving the quality of cement injection and adopting appropriate sulfur-resistant cement. Among them, the outer winding protective film of casing is one of the main technologies to solve the outer corrosion of casing. It has high technical and economic benefits and should be gradually popularized and applied.