The thermal decay time (TDT) log is based on a measurement of the rate of decay (absorption) of thermal neutrons in a formation. The principle of such logs has been described in detail elsewhere. In Ref. 4 the special features (capture gamma ray detection, Sliding Gate detection system) of the thermal neutron decay time log are described.

Chlorine is by far the strongest neutron absorber of the common earth elements, and the thermal neutron decay time, T, is determined primarily by the sodium chloride present in the formation water. Like the resistivity log, therefore, the thermal neutron decay time measurement is sensitive to the salinity and amount of formation water present in the pore volume. Unlike the resistivity log, this log can be run in cased hole.

Also, the thermal decay time log is relatively unaffected by drilling and completion conditions for the usual borehole and casing sizes encountered over pay zones. Consequently, when formation water salinity permits, this log can detect the presence of hydrocarbons in formations that have been cased, as well as changes in water saturation during the production life of the well. The log is thus useful for evaluating oil wells, for diagnosing production problems, and for following reservoir performance. With a version of the tool that is 1 11/16 in. in diameter, it is possible to enter a producing well through the tubing under pressure without having to kill the well.

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