Chapter 15. pdf

Formation
Wire or 
other tool
Sprung-loaded
arm
End of tool
or other tool
Source
Near Detector
Far Detector

---

Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 158
Figure 15.5
Log presentation for the GNT tool.
Figure 15.6
Log presentation for the SNP tool.
6
16
CAL (in)
16
6
Bit Size (in)
D
e
p
th
 (
m
)
1050
1055
3360
160
GNT (NAPI)
100
0
SGR (GAPI)
6
16
CAL (in)
16
6
Bit Size (in)
D
e
p
th
 (
m
)
1050
1055
-15
45
SNP (%)
150
0
GR (GAPI)

---

Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 159
Figure 15.7
Log presentation for the CNL tool.
15.5 Calibration
15.5.1
The GNT Tool
The primary standard for this tool is the API Neutron Calibration Pit at the University of Houston.
This pit contains three blocks of carbonate with accurately known porosities that are filled with fresh
water that are drilled to accept a tool. The tool is calibrated in 19% porosity Indiana limestone, for
which an API value of 1000 API neutron units is defined. The response of the tool is then checked in
1.9% Carthage Marble, 26 % Austin limestone, and 100% fresh water. A tool tested at Houston is used
as a secondary reference for the calibration of other tools at the wireline company’s workshops around
the globe. A wellsite check on the calibration of a tool is also carried out before and after running the
tool using a neutron source of accurately known radioactivity.