STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

972 Drilling and Well Completions
The steel housing rarely exceeds 0.5 in. (12 mm) and a calibration is done in
terms of API units, arbitrary units defined in a standard calibration pit located
at the University of Houston.
The MWD total gamma ray tools cannot be calibrated in the standard pit, since
they are too large. Their calibration in API units is difficult because it varies with
the spectral content of the radiation. By spectral matching the MWD logs can be
made to closely resemble the wireline logs. The logs which were recorded by the
MWD companies in counts per second (cps) are now recorded in API units.
Another difference between the wireline logs and the MWD logs is the logging
speed. With a wireline, the sonde is pulled out at a speed of 500 to 2,000 ft/min
(150 to 600 m/min). The time constant used to optimize the effect of the statistical
variations of the radioactivity emission, varied from 2 to 6 s. Consequently,
the log values are somewhat distorted and inaccurate.
In MWD, the recording speed is the rate of penetration which rarely exceeds
120 to 150 ft/hr or 2 to 2.5 ft/min, two orders of magnitude less than the
logging speed. Counters can be made shorter and time constant longer (up to
30 s or more). This results in a better accuracy and a better bed definition.
Figure 4-269 shows an example of comparison between an MWD gamma ray
log and the wireline log ran later.
To summarize, the total gamma ray measurements are used for real-time
correlation, lithology identification, depth marker and kick-off point selection.
Direction-Focused Gamma Rays. It is important to keep the trajectory of
horizontal or nearly horizontal wells in the pay zone. By focusing the provenance
of the gamma rays it is possible to determine if a shale boundary is approached
from above or from below.
The tool shown in Figure 4-270 has its scintillation detector inserted in a
beryllium-copper housing, fairly transparent to gamma rays. A tungsten sleeve
surrounds the beryllium-copper housing, with a 90" slot or window running from
top to bottom. Figure 4-270 is a sketch of the tool cross-section. The center of
the window is keyed to the reference axis of the directional sensor. Consequently
the directional sensor indicates if the window is pointing up or down.
By rotating the tool, one can differentiate between the level of gamma rays
entering from the top and the lower part of the borehole. A sinusoidal response
is recorded which depends on the following:
distance from the bed boundary.
gamma ray intensity of the bed in which the tool is in
the contrast of radioactivity at the boundary.
the shielding efficiency of the tungsten sleeve.
An example of the log ran is a horizontal borehole as shown in Figure 4271.
The depths on the log are along the hole depths. Vertical depths are shown in
the higher part of the log with a representation of the true radioactivity of each
bed. The following observations can be made:
Approaching formation bed boundaries are detected by concurrent separation
and displacement of the high and low gamma counts. These are shown in
Figure 4271 at measured depth intervals (7970-7980 ft) and (8010-8020 ft).
Radioactive events occur in the measured depth interval (8,100-8,200 ft)
with no displacement of the low/high side gamma ray logs. The radioactive
events must be perpendicular to the gamma detector and could be indications
of vertical natural fractures in the formation.