Geology Reference
In-Depth Information
higher than in vertical wells). It should be noted that
in deviated wells rig source checkshots are not particu-
larly useful, as it can be difficult to accurately correct
the slant ray times to vertical. Walk-above VSP shots
are the best dataset for this purpose.
Commonly used drift functions are linear trends
with knee points (e.g.
Fig. 4.3
), polynomial fits and
spline fits. The knee points for linear trends should be
located at major changes in lithology or at unconfor-
mities where there are changes in velocity in order to
avoid generating spurious reflections if the calibrated
velocity log is used for synthetic generation. Spline fits
are reasonable if there are a large number of points
such as in a VSP, but care should be taken when
applying a spline fit to sparse checkshots. Polynomial
fits are useful in basin fill sequences where there is a
simple compaction trend and linear fits with knee
points are appropriate in basins with a number of
prominent unconformities and variations in litho-
logical style. If linear fits are made between each
checkshot (i.e. simply using the checkshots as the time
vs depth function) it is important to check the effect
on the velocities.
When using the time
V
p
(m/s)
Drift (ms)
TWT(ms)
2000 6000 0
607 -10
0
10
6000
7000
8000
depth relations in a well tie
the interpreter has a choice of whether to use the
calibrated velocity log or the original log in the well
tie match. Given that the differences should not be
large the choice is not usually important. Many
workers favour using the calibrated time
-
Figure 4.3
Log (depth-time) calibration: column 1, black - V
p
log,
red - calibrated velocity log, blue - velocities from checkshots;
column 2, blue - integrated depth-time curve from V
p
, red -
calibrated depth-time curve (i.e. with drift applied); column 3, blue
crosses - drift points, black - drift curve fitted to the data using linear
segments with knee points.
depth rela-
tionship with the original (upscaled) velocity log,
although benchmark tests appear to show slightly
better ties when using the calibrated velocity log
(Roy White, personal communication).
-
Drift, in general, is a real effect of the subsurface,
principally related to velocity dispersion between log
and seismic frequencies (e.g. Stewart et al.,
1984
). Each
formation usually has its own drift characteristics.
Usually, sonic log velocities are higher than seismic
velocities, giving rise to positive drift (i.e. shorter inte-
grated log times compared to the seismic) (
Fig. 4.3
). In
cases where the sonic velocity is less than the seismic
(i.e. negative drift), the log data should be checked for
hole problems such as washouts. It has been common
practice for positive drift corrections to be applied
linearly to the data whereas negative drift corrections
are preferentially applied to the lower velocities. This is
based on the assumption that the negative drift is likely
to be due to erroneously low sonic velocities in zones
where hole conditions are poor. In deviated wells, drift
may also be in part caused by velocity anisotropy (for
example shale velocities in deviated wells are often
4.3 The role of VSPs
Vertical seismic profiles (VSPs) are useful in the well
tie process because they provide a link between wells
and seismic at the correct scale. The essence of the
VSP method is to record a surface seismic source
using down-hole geophones. The simplest geometry
is for a vertical well (
Fig. 4.4
). A string of geophones is
deployed in the well and, by shifting them up between
shots, it is possible to record signals at a large number
of levels. For example, records might be obtained at
50 ft spacing over an interval of 5000 ft in the well, to
give 100 levels in all. At each level, the geophone will
record down-going waves (such as the direct arrival,
the leftmost raypath in
Fig. 4.4
) and up-going waves
(such as the two reflections in
Fig. 4.4
). Multiples are
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