Schlumberger

Corrosion Evaluation and Monitoring

Why Evaluate Corrosion?

Wireline corrosion evaluation saves money throughout the life of a well, particularly for deep or remote wells, wells expected to have a long lifetime, and wells producing CO2 or H2S. Monitoring for weak points and predicting problem areas makes it possible to target—and budget for—prevention. Corrosion or damage prevention is less costly than repair, but repair expense can be minimized by precisely identifying any point of failure.

What causes corrosion?

Electrochemical corrosion can occur when a metal is immersed in a conductive medium. Galvanic or bimetallic corrosion occurs when two metals in contact are immersed in the same fluid (the most obvious example is a battery in which the two metals are very different). Slight differences between casing joints, defects and impurities can also trigger galvanic action. Concentration cell corrosion occurs when the same metal is immersed in a fluid that varies in composition from one point to another. The change in composition may be due to differences in pH or the concentrations of dissolved gas (e.g., oxygen) or dissolved salts. Galvanic and concentration cells exist on scales ranging from kilometers to millimeters or smaller. Other corrosion processes that can cause problems downhole are stress corrosion, hydrogen embrittlement, and chemical corrosion.

In addition, noncorrosive metal loss can be caused by abrasion from produced fluids and solids or by mechanical wear. The nature of corrosive attack can be relatively uniform, as in general corrosion, or highly localized, as in pitting, where penetration can be very rapid.

Hydrogen sulfide, carbon dioxide, and oxygen and chloride ions are known to promote corrosion, though the mechanisms by which they act are very different. The effects of corrosion can be reduced (and in some cases eliminated) by using resistant metals or inert protective coatings, or by introducing chemical inhibitors into the fluid. Corrosion can be controlled with electrochemical means by applying cathodic protection through the placement of sacrificial anodes, but more commonly a DC current is impressed on the casing from a rectifier and groundbed.

Corrosion monitoring

Because downhole corrosion problems are many and varied, we provide a variety of measurements—acoustic, electrical, and mechanical—to identify and quantify corrosion, determine the cause, and monitor:

  • inside diameter
  • casing thickness
  • scale detection
  • hole detection
  • metal-loss indicators
  • mechanical wear
  • corrosion rate
  • condition of multiple strings
  • string composition
  • effectiveness of anticorrosion systems.

Keep a Closer Eye on Your Casing

EM Pipe Scanner
Quantitatively evaluate casing damage and corrosion without having to pull completion tubing by using the
EM Pipe Scanner electromagnetic casing inspection tool.

Visit EM Pipe Scanner page