Geology Reference
In-Depth Information
porosity from laboratory measurements). A practical
problem with Gassmann
from an analysis of the sonic slownesses in different
directions (e.g. Horne et al.,
2012
). In the absence of
such data the VTI constants may be derived by
analysing data from a number of wells, each with
different hole deviations through the same geology
(
Fig. 8.56
).
The analysis of log data in shale sections neces-
sarily focusses on averaged sonic data and the detec-
tion of a broad trend with increasing hole angle.
Figure 8.57
shows an example from Brevik et al.
(
2007
). It is evident that in these data there is little
effect of hole deviation out to an angle of 30° and
also that the general level of compressional and shear
wave anisotropy (as determined from the fractional
increase in velocity at 90° compared to 0°) is of the
order of 15%
s model is that the various
parameters are not independent; vary one and it will
affect the others. Thus, without independent shear
and porosity information the invasion problem is
under-constrained. For example, if there is no meas-
ured V
s
, then it would need to be predicted, but to do
this knowledge of the fluid modulus is required.
'
8.4.5 Sonic correction for anisotropy in
deviated wells
Deviated wells can present a range of problems in
seismic-to-well ties. One particular problem is that
sonic log velocities in shales are generally higher than
vertical velocities (
Chapter 5
) owing to the presence
of transverse isotropy (VTI). Misties can result (e.g.
Gratwick and Finn,
1995
). It is therefore recom-
mended that interpreters consider the potential for
an anisotropic influence on sonic velocities in deviated
wells and make an appropriate correction to the sonic
logs. It should be remembered that it is not really
hole angle that is important but relative bed dip (e.g.
Vernik,
2008
). The reader is recommended the work
of Hornby et al., (
2003
), Rowbotham et al.(
2003a
),
Brevik et al.(
2007
), Vernik (
2008
) and Wild (
2011
).
If cross dipole data are available in a deviated hole
then it is possible to obtain the anisotropic constants
20%.
The simplest approach to this issue is to determine
the anisotropic coefficients in Thomsen
-
s(
1986
,
2002
)
anisotropic equations (see
Chapter 5
,
Eq. (5.2)
) that
make a match to the change of velocity with hole
angle (e.g. Rowbotham et al.,
2003a
; Vernik,
2008
).
The problem can also be addressed as part of an
inversion scheme (e.g. Hornby et al.,
2003
)orinthe
application of an anisotropic rock physics model (e.g.
Xu et al.,
2005
).
There is usually significant scatter in the data
from a multi-well analysis as shown in
Fig. 8.57
. This
is probably related to lateral variability in shale
'
a)
Figure 8.56
Schematic illustration of
borehole deviation and sonic slowness in
shales; (a) several wells with different hole
angles through the same shale formation
(orange), (b) decreasing shale slowness
(i.e. increasing velocity) with increasing
deviation, (c) horizontal vs vertical
slowness (after Horne et al.,
2012
).
1
2
3
4
b)
c)
Horizontal
Slowness
Deviation
191
