Geology Reference
In-Depth Information
insulating pad pressed firmly against the wallrock by
a power-driven expansion device (Fig. 11.8).The depth
of penetration is typically about 100 mm. Different
electrode arrangements allow the measurement of
micronormal, microlateral and microlaterolog apparent
resistivities that are equivalent to normal, lateral and
laterolog measurements with much smaller electrode
spacings. The log has to be moved very slowly and it is
normally used only in short borehole sections which are
of particular interest.
As the electrode spacing is so small, the effects of the
borehole diameter, drilling fluid and adjacent beds are
negligible.Very thin beds register sharply, but the main
use of the microlog is to measure the resistivities of the
mudcake and zone of invasion, which are needed to
convert log measurements into true resistivities.
Rubber pad
P
2
P
C
1
11.4.5 Porosity estimation
Porosity is defined as the fractional volume of pore spaces
in a rock.The method of
porosity estimation
is based on the
relationship between
formation factor F
and porosity
f
dis-
covered by Archie (1942).
F
is a function of rock texture
and defined as
Spring
R
R
f
w
F
=
(11.4)
where
R
f
and
R
w
are the resistivities of the saturated
formation and pore fluid, respectively (Section 8.2.2).
Porosity and formation factor are related by
f
=
aF
-
m
(11.5)
Fig. 11.8
The microlog.
where
a
is an empirical constant specific to the rocks
of the area of interest, and
m
a constant known as the
cementation factor
which depends on the grain size
and complexity of the paths between pores (
tortuosity
).
Normal limits on
a
and
m
, derived experimentally,
are given by
the apparent resistivity so that the output can be cali-
brated in ohm m.
The focusing of the log makes it sensitive to thin beds
down to the same thickness as the length of the central
electrode. The zone of invasion has a pronounced
effect which can be estimated from the results of nor-
mal and lateral logging and corrected using standard
charts.
062
.
<<
a
10
. ,
and
20
.
< <
m
30
.
11.4.4 Microlog
The
microlog
(or
wall-resistivity log
) makes measurements
at very small electrode spacings by using small, button-
shaped electrodes 25-50 mm apart mounted on an
11.4.6 Water and hydrocarbon
saturation estimation
Natural pore water is generally a good conductor of
electricity because of the presence of dissolved salts.
