Casing Inspection - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 225

Electrical-Potential Logs
An electrical-potential log determines the fl ow of galvanic current entering, or leaving,
the casing. This will indicate not only where corrosion is taking place, and the
amount of iron being lost, but also where cathodic protection will be effective. The
magnitude and direction of the current within and external to the casing is derived
mathematically from electrical-potential measurements made at fi xed intervals
throughout the casing string. In order to achieve reliable results from this kind of
survey, the borehole fl uid must be an electrical insulator; that is, the hole must be
either empty or fi lled with oil or gas. Mud, or other aqueous solutions, will provide
a “short” that invalidates the measurements. The log itself is a recording vs. depth
of the small galvanic voltages detected. Figure 13.4 illustrates the tool used to make
the measurements of potentials and current fl ow.
Fig. 13.4 Casing-corrosion
tool. Courtesy Schlumberger