Environmental Engineering Reference
In-Depth Information
A secondary use for PNC tools is as an indicator of the flow of water. Although
it was never intended as a flowmeter some quirks of nuclear physics allow a tool that
was designed for formation evaluation can also be used for flow metering. The
mechanism that makes this possible is called nuclear activation. It takes place when
fast neutrons emitted from the PNC's neutron generator collide with atoms of oxy-
gen and “activate” them. Oxygen atoms have nuclei that include eight neutrons and
eight protons. In the activation process an extra neutron is temporally added to an
oxygen nucleus producing an unstable isotope (Oxygen 17). This in turn decays
with the emission of a beta particle and gamma rays leaving behind an atom of
nitrogen. The process has a short half life of just over 7 s. The gamma rays that are
emitted in this activation process can be detected with the PNC tool itself.
Any oxygen atom is fair game for this process whether it be in the form of water
(H
2
O) or part of another compound (e.g. CaCO
3
). The application in flow metering
is by “tagging” water in a flow stream and detecting gamma ray increases over
background as the activated water flows past the tool.
Figure
5.19
gives a schematic of a typical PNC tool. Note that there are three
gamma ray detectors. The “upper” detector is a conventional gamma ray detector
run in continuous mode to serve as a measure of natural formation radioactivity
(as used for depth correlation, etc.). Two additional gamma ray detectors (referred
to as the “near” and the “far” detectors) are used when PNC tools are measuring Σ.
These are sampled at specific short time intervals to gauge the rate at which gamma
rays of capture, emanating from the formation, die away as a function of time. In the
flow metering mode, the gamma ray detectors are used to time, when the activated
oxygen atoms pass in front of the detectors. For this purpose the “far” and the
“upper” detectors are used.
Fig. 5.19
Typical pulsed
neutron tool
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