TDT. The Thermal Decay Time Log is a record of the rate of capture of thermal neutrons in a portion of formation after it is bombarded with a burst of 14-MeV neutrons. An electronic neutron generator in the tool produces pulses of neutrons which spread into the borehole and formation.
The neutrons are quickly slowed down to thermal energies by successive collisions with atomic nuclei of elements in the surrounding media. The thermalized neutrons are gradually captured by elements within the neutron cloud, and, with each capture, gamma rays are emitted. The rate at which these neutrons are captured depends on the nuclear capture cross sections which are characteristic of the elements making up the formation and occupying its pore volume. The gamma rays of capture which are emitted are counted at one or more detectors in the sonde during different time gates following the burst, and from these counts the rate of neutron decay is automatically computed. One of the results displayed is the thermal decay time, t, which is related to the macroscopic capture cross section of the formation, S, which is also displayed.
Because chlorine is by far the strongest neutron absorber of the common earth elements, the response of the tool is determined primarily by the chlorine present (as sodium chloride) in the formation water. Like the resistivity log, therefore, the measured response is sensitive to the salinity and amount of formation water present in the pore volume. The response is relatively unaffected by the usual borehole and casing sizes encountered over pay zones. Consequently, when formation water salinity permits, Thermal Decay Time logging provides a means to recognize the presence of hydrocarbons in formations which have been cased, and to detect changes in water saturation during the production life of the well. The TDT log is useful for the evaluation of oil wells, for diagnosing production problems, and for monitoring reservoir performance.
The TDT-K system utilizes two detectors and two variable time gates (plus a background gate) to sample the capture gamma radiation decay following the neutron burst. The width and positions of the time gates. as well as the neutron burst width and burst repetition rate, are varied in response to signals that are related to S (or more precisely, related to the formation decay rate, t, where t = 4550/S).
The TDT-M system utilizes sixteen time gates and one of four possible neutron burst widths and burst repetition rates. Counts from the sixteen gates are combined to form two “sum” gates (plus a background gate) from which S is computed. As in the TDT-K system, the combination of gates used to form the “sum” gates, as well as the burst width and repetition rate, are selected according to S (or t) of the formation.
The ratio of counts (R) in the near-spaced to far-spaced detector is recorded and used as an estimate of formation porosity. TDT is a mark of Schlumberger.
