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146 Reservoir geomechanics

Wellbore imaging

Wellbore imaging devices make it possible to obtain direct information on the distribu-
tion and orientation of fractures and faults encountered at depth. One family of wellbore
image tools is collectively known as the ultrasonic borehole televiewer (BHTV). Such
tools scan the wellbore wall with a centralized rotating ultrasonic (several hundred
kilohertz to ∼1megahertz) transducer that is oriented with respect to magnetic north
(Figure 5.3a from Zemanek, Glenn et al. 1970). The amplitude of the reflected pulse
is diminished when the wellbore wall is rough (such as where a fracture or bedding
plane intersects the well) and the travel time increases when the wellbore radius is
enlarged by features such as wellbore breakouts (Chapter 6). These devices provide
an image of both the acoustic reflectance and the radius of the wellbore such that it is
possible to construct three-dimensional images of a wellbore (Figure 5.3b after Barton,
Tessler et al. 1991). The reflectance depends on both the stiffness of the formation
and the smoothness of the wellbore wall. Figure 5.3d illustrates an unwrapped image
of the wellbore wall in which position around the well (with respect to north in this
case) is shown on the abscissa and depth is shown on the ordinate. The amplitude of the
reflected pulse is displayed as brightness. In such a display, planar fractures (or bedding
planes) cutting the wellbore (Figure 5.3c) have a sinusoidal appearance (Figure 5.3d)
resulting from the low amplitude of the reflected acoustic pulse along the intersection
of the fault plane and wellbore wall. The dip direction is obvious in the unwrapped
image (the direction of the lowest point where the fracture leaves the wellbore) and the
amount of dip is determined from

−1
Dip = tan (h/d) (5.5)

where h is the height of the fracture as measured at the top and bottom of its intersection
with the wellbore and d is the diameter of the well. In Chapter 6,we will demonstrate
another important application of borehole televiewer data in the context of analysis
of stress-induced compressional wellbore failures (or breakouts) as the time it takes
the pulse to travel to/from the wellbore wall (and knowledge of the acoustic velocity
of the wellbore fluid) enables one to reconstruct the detailed cross-sectional shape of
the wellbore wall. Ultrasonic wellbore imaging is now available from a number of
geophysical logging companies. While the details of operation of these types of instru-
ments are slightly different (such as the number of pulses per rotation, the frequency of
the ultrasonic transducer and the way in which the transducer beam is focused on the
wellbore wall), the fundamental operation of all such tools is quite similar.
Figure5.4bisanunwrappedimageofwellborewallmadewiththeothertypeofimage
data used widely in the oil and gas industry, an electrical imaging device that uses arrays
of electrodes on pads mounted on arms that press against the wellbore wall. The imaging
device (Figure 5.4a from Ekstrom, Dahan et al. 1987) monitors the contact resistance