Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 152

pulsed neutron generator, and (2) gamma ray detectors at different distances from
the neutron generator. A generalized neutron tool is shown in Fig. 11.14 .
The cloud of neutrons produced by the initial neutron burst from the generator
results in a cloud of thermal neutrons in the vicinity of the tool. This cloud dies
away due to capture by chlorine atoms or other neutron absorbers in the formation.
If there is plenty of chlorine present (i.e., high water saturation), then the cloud of
thermal neutrons disappears quite quickly. If, however, hydrocarbons are present
(i.e., low water saturation), then the cloud of thermal neutrons decays much more
slowly. The rate of decay is measured by monitoring how many capture gamma rays
enter the gamma ray counter(s) as a function of time.
Fig. 11.14 Generalized pulsed neutron tool
Fig. 11.15 Thermal-neutron decay curves for oil, gas, and water-bearing formations