Casing Inspection - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 232

Interpretation

Measurements made by the pipe analysis tools are generally only suitable for quali-
tative interpretation. This is because any voltages induced in the sensor coils are
dependent not only upon the size of any fl aws in the casing, but also upon the mag-
netic permeability of the casing, the logging speed, and the abruptness of a defect.
The measurement is therefore primarily used to locate the presence of small defects
in the casing, such as pits and holes; defects such as a gradual decrease of the wall
thickness cannot be detected. These devices will give zero signal in the two extremes
of no casing and perfect casing (except at the collars), so the electromagnetic thick-
ness tool should also be used to measure the casing-wall thickness, in order to obtain
a complete picture of the state of the casing.
Since two sets of data are recorded with one set infl uenced by defects occurring
anywhere in the casing and the other by faults on the inner surface, it can be inferred
by examination whether the casing is damaged on the inner or outer wall, assuming
that there is no defect within the casing. Although the magnetic-fl ux bulges away
from both sides of the casing at the location of a defect, the effect is greater on the
side of the fl aw, hence for the fl ux-leakage measurement, smaller defects can be
detected on the inner surface than on the outer surface.
Eddy-current measurements are not able to detect fl aws with a diameter smaller
than about 0.39

″ , while the fl ux-leakage limit is somewhat lower (0.25 ″ ). This

means that it cannot be determined whether an anomaly less than 0.39 in size is on

the inner or outer surface. If a defl ection is noted on the eddy-current measurement
but not on the fl ux measurement, it is assumed that the defect on the inner wall is
less than 1 mm deep, and can usually be ignored. In addition, the fl ux-leakage read-
ings show events that are not due to casing damage, but rather to the presence of
localized magnetization in the casing. If a reference survey is run in new casing then
a time-lapse technique can be used to determine casing damage at a later time.
Typical logs for a variety of defects and anomalies are represented in Figs. 13.12 ,
13.13 , 13.14 , 13.15 , 13.16 .

″

Fig. 13.12 Casing collars. Courtesy Schlumberger