It has been estimated that 1 percent of the total operating costs of the petroleum industry could be saved by correct application of existing corrosion protection technology. Wireline corrosion evaluation services may be used to save money throughout the life of a well. Corrosion control is particularly cost-effective for deep or remote wells, those expected to have a long lifetime or for wells producing CO2 or H2S.
By predicting problem areas, prevention budgets may be spent wisely. It is worthwhile to monitor for weak points since corrosion or damage prevention is cheaper than repair. Finally, precise identification of failure can be used to minimize repair expense.
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 being 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, the composition of which varies from one point to another. The change in composition may be due to differences in pH or in 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, 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. This can be achieved by placement of sacrificial anodes, but more commonly a DC current is impressed on the casing from a rectifier and groundbed.
Corrosion Monitoring
Downhole corrosion problems are many and varied, requiring many techniques for their detection and monitoring. Schlumberger gives you a combination of measurements; acoustic, electrical and mechanical, so that the right choice can be made to get the answers you need:
