Page 213 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

182 Reservoir Engineering

where N = number of gamma rays observed at time t
No = number of gamma rays observed at t = 0
t = elapsed time (microseconds)
2 = time constant of the decay process, microseconds

Of most interest is z since it is strictly a function of the decay rate of the
neutron cloud (or rather the slope of the exponential function). From z the
capture cross section, X, can be calculated
= 4,550/~ (5-117)
The tools that are available to measure 2 include:

1. TDT-K (with 3 moveable gates or detectors)
2. DNLL (dual neutron lifetime log) (which uses 2 gates),
3. TDT-M (with 16 fixed gates), and
4. TMD (thermal multigate decay) (which uses 6 gates).
In general, these tools all perform the same function: they measure the decay
rate of the neutron cloud in the formation. This is accomplished by using a
series of windows to measure near and far-spacing counting rates, as well as
background gamma ray rates. The first gates are not triggered until all neutrons
in the cloud in the formation are thermalized. At this point neutron capture
has started. By using certain gating times and gate combinations, the slope of
the straight portion of the decay curve is measured and related to X. In addition,
the ratio between the short-spacedetector and long-spacedetector counting rates
is also calculated and is related to porosity. (It is similar in principle to the CNL
porosity device used in openhole logging.)
Log Presentations. Figure 5-107 is an example of a DNLL log presentation.
Most other TDT logs are presented in a similar way, except that the number of
curves varies from company to company. The log shown in this figure consists of
5 curves:
1. Gamma ray curve (Track 1).
2. Gate 1 counting rate (CPM).
3. Gate 2 counting rate (CPM).
4. Ratio curve (= CNI. ratio).
5. Sigma curve (x).

In addition, the pips located on the left side of track 2 are the corrected
casing collar locations.
Gate 1 and Gate 2 show the raw data from the detectors, the ratio curve shows
relative hydrogen concentration (= water-filled porosity), and the G curve shows
the capture cross-section. Some logs also show a T curve, but it is normally
omitted [217].
Interpretation. Interpretation of pulsed neutron logs is very straightforward.
It relies on knowledge of three parameters (four in hydrocarbon-bearing zones):

1. &og (capture units).
2. Luix
3. L,,
4. ~hvdrncardon