Casing Inspection - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 228

Figure 13.8 illustrates the response of an electromagnetic thickness log when
encountering a hole in the tubing string. Note the presentation is calibrated in actual
tubing wall thickness. This rescaling requires that the operator make some calibra-
tion readings in a casing of the type present in the well. It is quite common to see
quite large differences in thickness between adjacent stands due to a number of
variables such as the drift diameter of the pipe, the weight/ft, the relative magnetic
permeability of the steel used, etc.

Fig. 13.8 Tubing thickness measurement . Courtesy TGT ®

The electromagnetic-type tools are good at fi nding vertical splits in pipe since
the magnetic-fl ux lines pass perpendicular to the casing wall. A horizontal circum-
ferential anomaly is less well defi ned.

Pipe Analysis Log (PAL)

Another closely related measurement uses a slightly different technique and forms
the basis of the Pipe Analysis Log (PAL), also known as the Vertilog. Two electro-
magnetic measurements are of interest in the context of the pipe analysis tool;
magnetic- fl ux leakage and eddy-current distortion.