Chapter 15. pdf

15.3 Tool Operation
There are three main types of neutron tool, which are:
•
The Gamma Ray/Neutron Tool (GNT)
•
The Sidewall Neutron Porosity Tool (SNP)
•
The Compensated Neutron Log (CNL)
15.3.1
Gamma Ray/Neutron Tool (GNT)
This tool has a neutron source and a single detector that is sensitive to high energy capture gamma
rays and thermal neutrons and is non-directional. The tool can be run in either open or cased holes, and
in both cases is run centered. A 3-3/8 inches diameter tool is used in open holes, and a 1-11/16 or 2
inch diameter tool is used in cased holes. The source to detector spacing varies between tool
manufacturers, but is in the range 15.5 to 19.5 inches. Because the tool is centered, the detected
neutrons and gamma rays have to travel through both mudcake and drilling mud. Hence, this tool is
highly sensitive to changes in borehole quality, temperature, type of drilling mud, and mudcake
thickness. Correction curves are available from the tool manufacturers to correct the log data for
temperature, hole diameter, and the effect of drilling mud and mudcake.
Because the tool measures the thermal neutrons and the capture gamma rays, it is effected by the
process of neutron capture by chlorine. Since drilling mud, mud filtrate and formation waters may
contain significant amounts of dissolved chloride ions, the measurement can give erroneous values.
Erroneous values take the form of overestimated porosities in formations either drilled with muds
containing dissolved chloride ions, or salty formation fluids.
15.3.2
Sidewall Neutron Porosity Tool (SNP)
This tool is designed for use in open holes only. The tool has a source and a single detector with a 16
inch spacing, which are mounted on a skid that is pressed against the borehole wall. Often this will be
the same carrier that holds the formation density source and detector. Because the tool is pressed
against the borehole wall, the drilling mud does not affect the measurement, and the attenuation due to
the mudcake is reduced. However rough holes can cause misalignment of the either the source of the
detector with the borehole wall, and hence give erroneous readings.
The detector is sensitive to epithermal neutrons. These neutrons are not yet slow enough to take part in
absorption reactions with hydrogen and chlorine. Hence, the SNP tool readings are unaffected by the
presence of chlorine in high salinity muds and formation fluids.