Page 231 - The Geological Interpretation of Well Logs

- IMAGE LOGS -

generally the one that is the most sensitive and contains Sedimentary structures, and image facies are more effec-
the most character (Figure 13.28). In comparisons, it is the tively analysed with the electrical images.
amplitude image which is compared with the electrical In summary, electrical images are favoured for sedi-
image. The amplitude log is an indication of both acoustic mentary analyses where water-based muds are used, the
impedance and borehole wall roughness. In this way it BHTV for fracture and tectonic studies or where oil-
provides lithological information (Figure 13.34). For based mud is used. However, the choice is often imposed
example, a dense carbonate presents a smooth borehole by drilling conditions.
wall and returns a high amplitude signal while coals have
low reflectance and retum a low amplitude signal. The
13.9 Some examples of acoustic
acoustic impedance variations of the borehole wal] may be
dampened somewhat by the acoustic impedance contrast imaging tool interpretation
between the mud and the borehole wall. Best signals come
from intervals where this contrast is considerable. — structural dip
Used in the simptest way, acoustic images provide a high
Time of flight response is clearly quite different from
the amplitude response. Time of flight records borehole quality dipmeter, the dip and azimuth taken from sine
geometry. It is therefore sensitive to hole ovality, a sensi- wave fitted to surfaces using the work station. This was
tivity which is used in the identification of breakouts indeed the early use to which the too] was put (Rambow,
1984). The data from such a process are generally much
(Section 13.9). However, it will also be affected by voids
on the borehole wall such as open fractures, which will less scattered than the comparable data from the electri-
return no signal (Figure 13.28). Lithological effects wil] cal images (Section 13.5) and can be effectively used as a
be minimal except where slight borehole size differences structural dip.
are caused by different lithologies.
The complementary nature of the two presentations — fractures
helps in their mutual interpretation. For example, com- By far the most common use of the acoustic log is in the
parison between amplitude and time of flight logs can examination of fractures (Paillet et a?., 1990). This is as
indicate whether a fracture is open or closed. An open much the case in the hydrocarbon as in the non-hydro-
fracture gives a response on the amplitude log through carbon industries (water, geothermal etc.). The advantage
ioss of signal, and also on the time of flight log as no for all is that the images allow the identification, mea-
signal is returned. A filled fracture will provide an image surement and recognition of fracture type in the
on the amplitude log (depending on acoustic impedances) subsurface.
but no image on the time of flight log (Taylor, 1991). In Jaboratory experiments, fractures have been
detected down to a width of 0.025 mm (0.001") (see
— electrical images vs. acoustic images Resolution). In practice, detection in the subsurface is at
There is a tendency to compare the acoustic images with around 0.5 mm (0.02") to 1 mm (0.04") while two frac-
the electric images, not just because they are both images tures must be separated by about 8 mm (0.3") to be
but also because of commercial competition. Of course recognised (Dudley, 1993), In core to image compar-
isons, the logs are seen to detect perhaps only 25% of all
the two should be compared, not in a competitive sense
fractures although possibly 50% of the larger, more
but to find out which tool to use for a particular set of
important ones with apertures above 0.5 mm (0.02")
circumstances.
There are two obvious differences between the tools. (Dudley, 1993). In highly controlted cases, all the impor-
The acoustic tools can be used in any fluid including oil- tant fractures are seen on the acoustic images. The
based muds, the electrical tools cannot: the acoustic tools example shown previously is of fractures in a carbonate
give a full 360° coverage, the electrical tools give partial (Figure 13.28).
coverage, from 20%-90% (Table 13.1). The claimed Although the detection of a fracture and its orientation
resolution and detection are similar for both tools measurement in the subsurface is a major step, ‘Just to
find a fracture is not enough’ (Nelson, 1985). There is a
although in practice, the electrical images can be inter-
preted at a much finer scale for many features than the need to identify the type of fracture and, as more image
acoustic images (see Resolution). logs become available, it is clear that dnlling induced
From experience it is found that the acoustic tools have fractures are far more common than was originally
a good sensitivity to fractures. The full 360° coverage thought (Lincecum e7 al., 1993).
is essential to study fractures which are often irregular, One difficulty with the study of fractures is the damage
branching and non-planar (Laubach et al., 1988). The that occurs to them during drilling, of spalling, chipping
acoustic logs are also good in the study of borehole and erosion by circulating mud. For example, highly
geometry and breakouts. The acoustic caliper, along dipping surfaces are generally eroded away or broken at
with ihe images are excellent for this (Figure 13,33). the high and tow borehole crossing points (Figure 13.30)
However, definition of sedimentary features and lith- (Paillet et ai, 1985). Such breaks can be seen on the
ological boundaries is generally poor to moderate. fractured carbonate example (Figure 13.28). In a similar
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