Geology Reference
In-Depth Information
separation between transmitters and receivers,
together with a more sophisticated depth referenced
compensation technique, gave superior results. The
next development was to lengthen the source
to factors such as stress, mud weight, pore pressure,
temperature and speed of logging. The next step is to
identify bad hole effects and other errors followed by
replacing bad logs or missing sections.
In some situations there may be a need to scale log
responses to maintain consistency across a group of
wells. This is most likely to be true of gamma ray logs
but caution is advised when considering this process
for sonic and density logs. They are compensated tool
readings that in good hole have a high degree of
accuracy. Applying such scaling to density and sonic
logs may remove real differences. A useful summary
of the key steps in log quality assessment has been
given by Yan et al.( 2008 ).
Bad hole effects such as washouts are fairly
straightforward to identify from the caliper and dens-
ity correction logs whilst the effects of filtrate invasion
on the density and sonic signature are more subtle.
Often the process is iterative, involving comparisons
of logs from different wells and pseudo-logs generated
from transforms, rock physics models or rock prop-
erty templates. These comparisons are commonly
made in relation to a key reference, such as depth
below mudline or stratigraphic zone.
Sometimes it is difficult to identify where log QC
stops and log interpretation begins. Indeed these pro-
cesses are often intertwined. Once a petrophysical
evaluation has been generated rock physics models
can be fit to the log data, providing a means of
evaluating log quality and identifying potential errors.
This can be done by comparing predicted and meas-
ured curves in the depth domain or/and in the cross-
plot domain.
A useful tool for both QC and interpretation pur-
poses is the
receiver
separation (up to around 15 ft), introduce a large array
of receivers and record the wave train in a time
window, giving rise to
-
tools.
Slownesses are derived from the waveforms principally
but not exclusively by using semblance/coherency
techniques ( Fig. 8.42 ).
A limitation of these tools, however, is that if the
shear wave is slower than the mud arrival then no
shear arrival is generated. In order to measure shear
velocity in
'
monopole array sonic
'
formations, the dipole source was
developed, which generates a
'
slow
'
wave, essen-
tially an undulation of the borehole wall. This wave
shows significant dispersion, i.e. velocity varying with
frequency, and in waveform processing a correction is
applied to the flexural velocity based on a modelled
dispersion curve.
It is not easy to say what the depth of investigation
is for the sonic measurement. Although the arrival
time of the first signal is along the path with the
shortest travel time, the depth of penetration depends
on the velocity profile close to the borehole wall as well
as the source
'
flexural
'
receiver separation. For conventional
borehole sonic tools the penetration is limited, prob-
ably around 5 cm. It may typically be in the range
15
-
40 cm for longer spaced sonics and dipole sonics.
Modern sonic logging tools are becoming ever
more sophisticated and increasingly being used in
the understanding of fractures and formation geome-
chanics. A benefit of the dipole source is that it is
directional and can therefore be used to investigate
anisotropy. In cross dipole mode the sonic tool can
derive information on the anisotropy of the formation
based on comparisons of fast and slow shear waves
(discussed in Chapters 5 and 7 ; e.g Haldorsen et al.,
2006 ). The latest sonic scanner technology is effect-
ively a 3D acoustic imaging tool which can be used not
only to measure isotropic velocities but also to char-
acterise intrinsic and stress-induced anisotropy.
-
'
'
degaard and
Avseth, 2004 ). Rock physics templates are essentially
crossplots such as porosity vs velocity or acoustic
impedance vs V p /V s with the background rock physics
model annotated. Shale and sand end members are
indicated as well as hydrocarbon equivalents. An
example is shown in Fig. 8.43 . The template provides
a useful context for understanding the various effects of
changing porosity, fluid content, shale content, cement
volume and effective pressure described in Chapter 5.
rock physics template
(e.g.
8.4 Data QC and log edits
Making sure that the log data are of good quality is a
key part of using rock physics to create seismic
models. The petrophysicist will typically check the
depth registration of various logs and make standard
corrections for environmental effects on tools related
8.4.1 Bad hole effects
Adverse hole conditions (borehole washouts and
damaged formations) can lead to bad density and
sonic logs, which are a frequent cause of poor well
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