Chapter 15. pdf

15.5.2
The SNP and CNL Tools
These tools are calibrated in blocks of limestone, sandstone and dolomite of high purity and accurately
known porosity in test pits. The tools are calibrated, not to give readings in API neutron porosity units,
but to give the porosity directly in percent. Limestone is chosen as the calibrating lithology, so the
readings are given in limestone porosity units.
IMPORTANT
The common use of limestone porosity units as the calibrating unit for these logs
means that the reading of porosity in a limestone formation will be exact, but the reading of porosity in
these units in any other formation lithology will need correction (Fig. 15.8).
6
16
CAL (in)
16
6
Bit Size (in)
D
e
p
th
 (
m
)
1050
1055
-15
45
CNL(%)
100
0
SGR (GAPI)

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 160
Figure 15.8
Correction chart for obtaining porosity values for lithologies other than limestone.
The SNP tool is calibrated at the wellsite uses a U-shaped polyethylene baffle (artificial formation)
with a two position baffle, where the geometry of the block and baffle arrangements are known to
result in readings equivalent to 11% porosity and 22% porosity.
The calibration of the CNL tool is checked at the wellsite before and after each logging run by the use
of a neutron source of accurately known activity placed a standard distance from each detector.
15.6 Depth of Investigation
The depth of penetration of the tool readings depends directly upon the amount of hydrogens in the
formation. Thus, for porous rocks that contain water or hydrocarbons (i.e., many hydrogens), there is
much neutron scattering and absorption, and hence the depth of investigation is small. For low
porosity rocks containing water or hydrocarbons, there are fewer hydrogens in the formation and the
depth of investigation is larger.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 161
The presence of gas in a formation increases the depth of investigation because, although the gas may
contain hydrocarbons, the gas has a vary low density compared to liquid water and oil, and hence a
very low hydrogen index.
The presence of shales decreases the depth of investigation because even low porosity shales contain
many waters (i.e., hydrogen) that are bound to the surface of the clay crystals.
The depth of investigation of the CNL tool in a water saturated formation of 35% porosity is about 12
inches, and that of the SNP tool in the same formation is about 8 inches.