Casing and Tubing

This section looks at the types of corrosion that can occur externally on casing, internally on casing and in the casing/tubing annulus. Many of the corrosion issues in these applications also occur in other areas in the oilfield.

External Casing Corrosion

External casing corrosion can occur for a number of reasons, such as exposure to water zones, differences in formations the casing passes through, or when the casing acts as an anode to other metals in the area (such as surface equipment and other wells). Cementing the casing can prevent some of this corrosion, and some wells are completely cemented for this reason. Cathodic protection (see the prevention page)is another common method to reduce corrosion.

Internal and Annulus Corrosion

Corrosion in the annulus between the casing and the tubing is dependant on the type of fluid in this area. When a well is completed with a packer, as in the illustration above, this fluid is usually a combination of drilling mud, brine and possibly produced oil or gas. The composition of this fluid determines the types and the amount of corrosion. Packer fluids can be designed to meet the corrosion mitigation needs of a particular well, for instance, using oil based drilling mud can help prevent corrosion in the annulus. If the well is completed without a packer, the annulus space is filled with wet gas above the fluid level and produced fluids below. The composition of the fluids and the gas can cause corrosion, especially if acid gasses are present. (see the section on H₂S for more information)