Environmental Engineering Reference
In-Depth Information
Figure
13.5
shows an interpretation of casing-potential profi le logs run before
(left panel) and after cathodic protection was installed. Note that the metal loss has
been reduced to practically zero (right panel) by the application of appropriate
cathodic protection.
Fig. 13.5
Casing-potential profi le before (
left
) and after (
right
) cathodic protection. Courtesy
Baker Atlas
Electromagnetic Devices
The most common inspection tools used to assess casing corrosion are of the elec-
tromagnetic type. They come in two versions, those that attempt to measure the
remaining metal thickness in a casing string and those that try to detect defects in
the inner or outer wall of the casing. Although frequently run together, these tools
will be discussed separately.
Electromagnetic Thickness Tool (ETT)
The electromagnetic thickness tools are available under a variety of trade names,
such as ETT (Schlumberger), Magnelog (Baker Atlas), Electronic Casing Caliper
Log (McCullough), and Defectoscope (TGT). They operate in a manner similar to
open-hole induction tools. Each consists of a transmitter coil and a receiver coil. An
alternating current is sent through the transmitter coil. This sets up an alternating
magnetic fi eld that interacts with both the casing and the receiver coil (see Fig.
13.6
).
The coils are spaced about three casing-diameters apart to ensure that the fl ux lines
sensed by the receiver coil are those that have passed through the casing. The signal
Search WWH ::
Custom Search