Geology Reference
In-Depth Information
In sandstone and limestone all hydrogen ions are
present in pore fluids or hydrocarbons, so the hydro-
gen ion concentration is entirely dependent upon the
porosity. In shales, however, hydrogen can also derive
from micas and clay minerals. Consequently, the litholo-
gy must be determined by other logs (e.g. gamma log)
before porosity estimates can be made in this way. Simi-
lar count times and logging speeds to other radiometric
methods are used.The method is suitable for use in both
cased and uncased boreholes. An example is given in
Fig. 11.12.
refracted in the wallrock and part of its energy returns to
the sonde as a head wave (Section 3.6.3). Each sonic
pulse activates a timer so that the differential travel time
between the receivers can be measured. If the sonde is
tilted in the well, or if the well diameter varies, different
path lengths result. This problem is overcome, in a
borehole-compensated log, by using a second source on
the other side of the receivers (Fig. 11.13(b)) so that the
tilt effect is self-cancelling when all four travel paths are
considered.
Porosity
f
may be estimated from the sonic measure-
ments (see Section 3.4). For a rock whose matrix velo-
city (the velocity of its solid components) is
V
m
and pore
fluid velocity is
V
w
, the formation velocity
V
f
is given by
11.8 Sonic logging
The
sonic log
, also known as the
continuous velocity
or
acoustic log
, determines the seismic velocities of the for-
mations traversed.The sonde normally contains two re-
ceivers about 300 mm apart and an acoustic source some
900-1500 mm from the nearest receiver (Fig. 11.13(a)).
The source generates ultrasonic pulses at a frequency of
20-40 kHz.
Since the wallrock invariably has a greater velocity
than the drilling fluid, part of the sonic pulse is critically
1 1
VV V
f
=+
-
f
f
(11.12)
w
m
The velocity of the matrix can be determined from cut-
tings and that of the fluid from standard values.
Sondes of the dimensions described above have trans-
mission path lengths that lead to penetrations of only a
few centimetres into the wallrock and allow the discrim-
ination of beds only a few decimetres in thickness. How-
ever, they are greatly affected by drilling damage to the
wallrock and, to overcome this, longer sondes with
source-geophone spacings of 2.1-3.7 m may be used. In
addition to providing porosity estimates, sonic logs may
be used for correlation between boreholes and are also
used in the interpretation of seismic reflection data by
providing velocities for the conversion of reflection
times into depths. An example is given in Fig. 11.14.
Sonic logs can also provide useful attenuation infor-
mation, usually from the first P-wave arrival. Attenua-
tion (Section 3.5) is a function of many variables
including wavelength, wave type, rock texture, type and
nature of pore fluid and the presence of fractures and fis-
sures. However, in a cased well, the attenuation is at a
minimum when the casing is held in a thick annulus of
cement and at a maximum when the casing is free. This
forms the basis of the cement bond log (or cement
evaluation probe) which is used to investigate the effec-
tiveness of the casing. Other techniques make use of
both P- and S-wave travel times to estimate the
in situ
elastic moduli (Section 5.11). See also the description of
vertical seismic profiling in Section 4.13.
(a)
(b)
S
1
S
R
1
R
2
R
3
R
4
R
1
R
2
S
2
Fig. 11.13
(a) A simple sonic log. (b) A borehole-compensated
sonic log.
