STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 979
The system is similar to the laterolog 3 used in wireline logging. A constant
1-k Hz AC voltage is maintained for all electrodes. The current flowing through
the center electrode is measured.
The resistivity range is 0.1 to 1000 C2.m. Beds as thin as 6 in. (15 cm) can
be adequately delineated.
Electromagnetic Resistivity. The measurement in electromagnetic resistivity
systems is similar to the wireline induction sonde resistivity. The frequency used
is 2 MHz instead of 20 kHz. This is due to the drill collars steel that would
completely destroy a 20-kHz signal. Early systems had one transmitter coil and
two receiver coils. Systems presently in use have two to four transmitters allowing
the recording of many curves with different depths of investigation. Figure 4-275a
shows the CDR, compensated dual resistivity tool of Anadrill.
Figure 4-275b is a schematic of the operating principle. Two signals are
measured: the wave amplitude reduction and the wave phase shift.
Two values of the resistivity can be calculated. The wave amplitude resistivity
(Rat,) appears to have a deep investigation radius: 35 to 65 in. according to
the formation resistivity. The phase shift resistivity (Rp,) appears to have a
shallow investigation radius: 20 to 45 in. An example of tool response is given
in Figure 4-276.
The deep penetration curve reads a value close to the noninvaded zone
resistivity and the shallow penetration curve reads a value much lower than the
invaded zone resistivity. The resistivity ranges for an acceptable accuracy are
0.15 to 50 a* m for the deep investigation radius (R,) and 0.15 to 200 R*m
for the shallow investigation radius (Rp,). The vertical resolution is 6 in. (15 cm).
Toroidal System Resistivity (after Gearhart-Halliburton). The system uses one
toroidal transmitter operating at 1 kHz and a pair of toroidal receiver coils
mounted on the drill collars. Figure 4-277 shows a sketch of a toroid.
The winding of the toroid acts as a transformer primary and the drill collar
as the secondary. The current lines induced by the drill collar are shown in
Figure 4-278.
The drill collar acts as a series of elongated electrodes in a way similar to
the laterolog 3 wireline sonde. The lower electrode, which is the drill bit, is
used to get the “forward” resistivity curve. A lateral resistivity measurement is
made between the two toroid receivers. An example of toroid logs is shown in
Figure 4-279.
The readings of both toroid curves seem to follow closely the ILd and ILm curves.
Example 15: Gamma Ray and Resistivity Interpretation
A typical set of logs recorded while drilling is shown in Figure 4-280. The
wireline caliper is shown in the gamma ray track. Displayed on this attachment
are gamma ray, RN,a curve, Pe curve, neutron and density curve. The delta-rho
curve is the quality curve check for the density log.
1. Draw a lithology description in the depth column.
2. Is the clean formation permeable? Why?
3. Does the porous zone contain hydrocarbons? What type? Give the boundaries.
4. Determine R,.
5. Compute the hydrocarbon saturation at 8400 ft assuming a = 1 and m = 2.
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