stress. Electrical images are similar to those obtained
by dipmeter logs. Modern image logs provide a good
overview of the lamination and also fractures. This
may be compared with sedimentological logging of a
core (Fig. 16.37).
16.5.9.3 Uncertainties of Image Logs
The principal drawback of optical imaging is that it
requires a transparent fluid in the borehole. Unless
transparent fluid can be injected ahead of the lens,
the method fails. This requirement has limited the
application of downhole cameras. The electrical imaging tool does not provide an absolute measurement of
formation resistivity but rather a record of changes in
resistivity. The resolution of electrical microimaging
tools is governed by the size of the buttons, usually a
fraction of an inch. In theory, any feature that is as
large as the buttons will be resolved. If it is smaller, it
might still be detected.
Acoustic imaging amplitudes are governed by several factors. The first is the shape of the borehole wall
itself: irregularities cause the reflected energy to scatter so that a weaker reflected signal is received by the
transducer. Examples of these irregularities are
fractures, vugs and breakouts. Borehole televiewers
work best where the borehole walls are smooth and
the contrast in acoustic impedance is high. The scattering or absorption of acoustic energy by particles in
the drilling mud can affect image resolution. This
problem is more serious in heavily weighted muds,
which are the most opaque acoustically and cause loss
of image resolution.
Fig. 16.36 (a) Measurement principle of electrical imaging
(microresistivity imaging) devices illustrated by Schlumberger’s
Formation MicroImager (FMI) and (b) Example of breakout
detection using an acoustic imaging borehole televiewer.
Breakouts are indicated by the low acoustic amplitude of the
reflected signal, shown here as darker areas. The breakouts are
rotated because of a drilling-induced slippage of localised faults.
(Courtesy of Schlumberger)
16 Well Logging: Principles, Applications and Uncertainties
423
by dipmeter logs. Modern image logs provide a good
overview of the lamination and also fractures. This
may be compared with sedimentological logging of a
core (Fig. 16.37).
16.5.9.3 Uncertainties of Image Logs
The principal drawback of optical imaging is that it
requires a transparent fluid in the borehole. Unless
transparent fluid can be injected ahead of the lens,
the method fails. This requirement has limited the
application of downhole cameras. The electrical imaging tool does not provide an absolute measurement of
formation resistivity but rather a record of changes in
resistivity. The resolution of electrical microimaging
tools is governed by the size of the buttons, usually a
fraction of an inch. In theory, any feature that is as
large as the buttons will be resolved. If it is smaller, it
might still be detected.
Acoustic imaging amplitudes are governed by several factors. The first is the shape of the borehole wall
itself: irregularities cause the reflected energy to scatter so that a weaker reflected signal is received by the
transducer. Examples of these irregularities are
fractures, vugs and breakouts. Borehole televiewers
work best where the borehole walls are smooth and
the contrast in acoustic impedance is high. The scattering or absorption of acoustic energy by particles in
the drilling mud can affect image resolution. This
problem is more serious in heavily weighted muds,
which are the most opaque acoustically and cause loss
of image resolution.
Fig. 16.36 (a) Measurement principle of electrical imaging
(microresistivity imaging) devices illustrated by Schlumberger’s
Formation MicroImager (FMI) and (b) Example of breakout
detection using an acoustic imaging borehole televiewer.
Breakouts are indicated by the low acoustic amplitude of the
reflected signal, shown here as darker areas. The breakouts are
rotated because of a drilling-induced slippage of localised faults.
(Courtesy of Schlumberger)
16 Well Logging: Principles, Applications and Uncertainties
423