Chapter 15. pdf

15.7 Vertical Resolution
As with most tools the vertical resolution is defined by the source detector spacing for single detector
tools and the spacing between the two detectors for dual detector tools. The vertical resolution of
neutron tools is a little greater than these spacings. For the GNT tool the vertical resolution is 16
inches or 20 inches depending upon which of the two source-detector spacings possible for this tool
are used. The vertical resolution of the SNP tool is 16 inches, and for the CNL tool is 10 inches.
As usual, the true bed resolution (the thinnest bed for which the true neutron porosity reading in that
bed is measured) is approximately three times the vertical resolution (i.e., 36 to 48 inches). Thinner
beds will be noticed by the tool, but the apparent porosity will not be the same as the true porosity of
the bed.
The measuring point for these tools is half way between the source and the detector for single detector
tools, and half way between the two detectors for the dual detector tool.
15.8 Borehole Quality
The GNT tool is run centered in the borehole, and is therefore sensitive to the effect of caving and
wash-out due to the attenuation of gamma rays and neutrons by the larger annulus of drilling mud
around the tool. As the diameter of the borehole increases, more of the signal that the tool measures
represents the borehole, and less comes from the formation. Clearly, there is a degree of caving where
the signal measured by the tool is no longer representative of the formation, and then the tool cannot
be used. This problem is accentuated by the presence of chloride ions in the drilling mud (i.e., a salt
water-based mud), where attenuation by the drilling mud can lead to grossly erroneous porosity
readings.
The SNP and CNL tools are run pressed up against the borehole wall. In this case, the roughness of the
borehole wall due to caving or wash-out can result in the detectors or source not being pressed directly
up against the borehole wall. This will cause erroneous porosity readings.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 162
15.9 Mud Type
The tools can be used in most types of mud.
The GNT tool is sensitive to chloride-rich muds, and its results must then be corrected for the drilling
mud, mudcake and mud filtrate.
The SNP tool is not sensitive to the chloride effect and is run pressed against the borehole wall. The
mudcake tends to increase the apparent porosity because the mudcake has a high hydrogen index. This
effect can be corrected for.
The detectors of the CNL tool are sensitive to the chloride effect. However, the tool is run pressed
against the borehole wall so the effect of the drilling mud is not included in the measurement.
Furthermore, the use of two detectors automatically compensates for the effect of chloride-rich
mudcake and mud filtrate.
The density of the mud also affects the readings, because high density muds attenuate the radiation to
a greater extent. This is usually only a problem for the GNT tool, where the effect can be compensated
for using correction charts.