TDT LOG BASICS
The thermal decay time log is a record of the rate of capture of thermal neutrons in a 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 log is also called pulsed neutron logs or neutron lifetime logs, and are often called by various service company abbreviation, such as TDT, PNL, NLL, PDK, etc. Thermal decay time, TAU, is the time for the neutron population to fall to 1/e (37%) of its original value. Neutron half-life is the time required for the neutron cloud to decay to one half its original concentration.

On older logs, the primary derived value from the pulsed neutron device is the neutron decay time (TAU), for Schlumberger logs and the Neutron Half Life (LIFE) for Dresser logs. These are related to the formation capture cross section (SIGMA), by the following equation:
      1: SIGMA = 4550 / TAU for the Schlumberger tool
      2: SIGMA = 3150 / LIFE for the Dresser tool

On modern logs, and many older ones, the SIGMA curve is displayed and the above calculation is not needed.

Where:
  SIGMA = capture cross section (capture units)
  TAU = neutron decay time (usec)
  LIFE = neutron half life (usec)

References:
 1.  Thermal Neutron Decay Time Logging Using Dual Detection
       J.T. Dewan, C.W. Johnstone, L. A. Jacobso, W. B. Wall, R.P. Alger,  SPWLA, 1973

The capture cross section SIGMA is defined as the relative ability of a material to "capture" or absorb free thermal neutrons. Chlorine has a high capture cross section and hydrogen has a low capture cross section.

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 logging tool 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, TAU, which is related to the macroscopic capture cross section of the formation, SIGMA, 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 Schlumberger 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 SIGMA (or more precisely, related to the formation decay rate, TAU).

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 SIGMA 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 SIGMA (or TAU) of the formation.

Other service companies offer similar tool designs.

The ratio of counts in the near to far spaced detector is recorded and used as an estimate of formation porosity, in a fashion similar to the CNL neutron log. Earlier TDT logs had only one detector, so no ratio porosity was available.

CAUTION: From personal experience, I have found that the dual detector TDT logs, especially older logs, do not give a good value for porosity in dolomite reservoirs. Always compare PHItdt to core or open hole log analysis whenever possible.

The water saturation is based on the sum of the capture cross sections, in a mathematical treatment similar to the sonic, density and neutron logs.

The response equation for the thermal decay time log follows the classical form:

      3: SIGMA = PHIe * Sw * SIGw (water term)
                     + PHIe * (1 - Sw) * SIGh (hydrocarbon term)
                     + Vsh * SIGsh (shale term)
                     + (1 - Vsh - PHIe) * Sum (Vi * SIGi) (matrix term)

This equation is solved for Sw by assuming all other variables are known or previously calculated.
      4: SWtdt = ((SIGMA - SIGMAM) - PHIe * (SIGHY - SIGMAM) - Vsh * (SIGSH - SIGMAM))
                     / (PHIe * (SIGW - SIGHY))
 

Where:
  PHIe = effective porosity (fractional)
  SIGMA = TDT capture cross section log reading (capture units)
  SIGMAM = capture cross section matrix value (capture units)
  SIGW = capture cross section for water (capture units)
  SIGHY = capture cross section for hydrocarbons (capture units)
  SIGSH = capture cross section for shale (capture units)
  SWtdt = water saturation from TDT (fractional)
  Vsh = shale volume (fractional)

RESERVOIR SATURATION LOG (RST)
The reservoir saturation tool (RST) is a combination of a modern carbon oxygen log and a standard pulsed neutron log. The dual-detector spectrometry system of the through-tubing reservoir saturation tool enables the recording of carbon and oxygen and dual burst thermal decay time measurements during the same trip in the well.

The carbon/oxygen (C/O) ratio is used to determine the formation oil saturation independent of the formation water salinity. This calculation is particularly helpful if the water salinity is low or unknown. If the salinity of the formation water is high, the dual burst thermal decay time measurement is used. A combination of both measurements can be used to detect and quantify the presence of injection water of a different salinity from that of the connate water.

Applications
  ■ Formation evaluation behind casing
  ■ Sigma, porosity, and carbon/oxygen measurement in one trip in the wellbore
  ■ Water saturation evaluation in old wells where modern open hole logs have not been run
  ■ Measurement of water velocity inside casing, irrespective of wellbore angle (production logging)
  ■ Measurement of near-wellbore water velocity outside the casing (remedial applications)
  ■ Formation oil volume from C/O ratio, independent of formation water salinity
  ■ Flowing wells (in combination with an external borehole holdup sensor)
  ■ Capture yields (H, Cl, Ca, Si, Fe, S, Gd, and Mg)
  ■ Inelastic yields (C, O, Si, Ca,and Fe)
  ■ Three-phase borehole holdup
  ■ PVL* Phase Velocity Log
  ■ Borehole salinity
  ■ SpectroLith lithology indicators Nuclear

TDT LOG CURVE NAMES

Modern TDT Logs - Dual Burst

Older TDT Logs - Single Burst

Curves	Units	Abbreviations
thermal decay time	usec	TAU
capture cross section	c.u.	SIGMA
casing collar	mv	CCL
* gamma ray	api	GR

EXAMPLES OF TDT LOGS

TDT-K type log with gamma ray GR, count rate ratio, sigma, near  and far count rate overlay N1 and F1. On many logs, TDT porosity TPHI replaced the ratio curve. Curve name abbreviations vary widely depending on era and service supplier. 
 

Gas detection example: density
neutron crossover from open
 hole logs in Track 1, near and far count rate crossover in Track 3.

Pulsed neutron (PDK-100) reservoir monitoring example. Original openhole analysis (Tracks 5 and 6), PDK saturation 10 years later (Track 4), and 13 years later (Track 3 )