Page 123 - The Geological Interpretation of Well Logs

- SONIC OR ACOUSTIC LOGS -

Table 8.8 Acoustic information used to discriminate reservoir A. HORIZONTAL FRACTURE
characteristics.
TELEVIEWER
LOG TUBE WAVE

Characteristic | Compressional Shear Stoneley
Slowness Slowness Slowness

Fractures - + ee

Permeability + + ae
(m)
Lithology KK RK a
depth

ee
-
+
Porosity

3K
Fluids
-
AK

275 -
*** required, + often useful, — not neeeded
after Paillet et af, 1992

T
T
9 50 400
permeability in both porous sediments and fractured wave made energy in % of energy
crystalline rocks is well established in the literature in unfractured rock
(Paillet et af, 1991), Full waveform measurements, in
STONELEY ATTENUATION
ent
:
-
fact, give one of the few opportunities to estimate in situ
permeability in boreholes. The Stoneley (or tube) wave
can be looked on as a pressure pulse moving along the t
borehole. When this pulse encounters an open fracture,
—=+— >
pressure is released into it and the pulse is attenuated fracture
(Figure 8.355). It is especially important to note that it is N
only open fractures that will be detected: surface studies
do not allow differentiation between fractures which are
closed and fractures which are open in the subsurface. eT,
With open fractures a reflected, secondary Stoneley
wave is produced with a strength related to the amount
of pressure released. Thus, under the right conditions, not B. SCHEMATIC DIAGRAM
only can open fractures be detected but also their Figure 8.35 The effect of an open fracture on Stoneley (tube)
permeability (Homby ef ai., 1992). Unfortunately not waves. A) energy loss shown on a tog compared to open,
horizontal fractures interpreted from a borehole televiewer.
only open fractures create these effects but also hole
B) schematic illustration of Stoneley waves attenuated across
washouts and significant formation boundaries.
an open fracture (modified from Paillet, 1991).
Although research has tended to concentrate on
fractures in crystalline rocks and hard formations (i.e.
Paillet 1991; Hornby e7 ai., 1992), some work suggests -lithology (and porosity)
that the experience gained in these formations can also Neither the detail nor the resolution of lithological infor-
be used in sedimentary successions (e.g. Mari et ai., mation from the full waveform sonic are as good for
1994). It is clear that information from array sonic borehole work as the information supplied by the
analysis must be combined with information from bore- standard open hole tools (Chapter 11). However, there is
hole imaging tools (Figure 8.35), core analysis and, considerable need to try to define the lithological content
where available, test flow rate information (Paillet, of seismic sections and the information for this can be
1991). Borehole imaging tools supply information on extracted from the full waveform sonic. The determina-
the immediate borehole environment which includes tion depends on the variations of elastic parameters
drilling induced fractures. The array sonic data provides between rock types. A simple way of characterising
information on fractures penetrating up to metres away lithologies is to plot compressional slowness against shear
from the borehole, and more tikely to be providing flow, slowness (Pickett 1963; Leslie and Mons 1982). With
although as indicated, there may be confusion with these plots, sandstones generally show ratios of 1.6 to
effects not caused by fracturing. 1.75 while limestones show a ratio of approximately 1.9
In the same way as Stoneley wave attenuation occurs (Figure 8.36). Unfortunately, in detail such relationships
with open fractures, attenuation may also be caused by are generally only valid for one set of data (see also
any permeability. Typically, such attenuation will be Section 8.5) and cannot be used in a general application
more marked in hard formations, such as limestones, (Paillet et ai., 1992}. None the less, full waveform sonic
where the acoustic contrast between formation and mud- data are invaluable as input to seismic interpretation and
filled aperture is greatest. processing.

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