Thermal Decay Time Log

Definition - What does Thermal Decay Time Log mean?

Thermal Decay Time Log, also known as Thermal Decay Time, is a log record that captures the rate of thermal neutrons in a formation after it is bombarded with a burst of 14 Mev neutrons. In a tool, an electronically operated neutron generator is present that produces the neutron pulses which spread in the borehole and the formation. Thermal decay time logs are used to measure the decay or dying time of the short lived neutron pulse that is produced from a pulsed neutron decay logging tool.

Petropedia explains Thermal Decay Time Log

Thermal decay time log is known by various names in the O&G service provider industries such as Thermal Decay Time or Neutron Life Time Logs or Pulsed Neutron Logs and these names are abbreviated as TDT, NLL, PNL, etc. The neutron generator produces high speed thermal neutron pulses in the borehole as well as formation regions. The electronic circuit on the pulsed neutron decay logging tool then records the detection times relative to a time reference related to the neutron production bursts. The downhole and uphole circuit controls the operation of the tool and with the surface system, processes the spectrum of die-away or decay detection times. From these data the system estimates earth formation properties which are then used in the evaluation of the reservoirs. The logging tool can be logged through open or cased hole. The detector is designed to detect gamma rays resulting from capture of thermal neutrons which is produced as a result of slowing down and the high-energy source neutrons getting thermalized or the thermal neutrons.

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