This article is based on
Chapter 3 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell
Books 1986 Republished as "Crain's Logging Tool
Theory" in 2004 and updated annually through 2016. Last
update May 2017.
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
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.
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
SIGMA = 4550 / TAU for the Schlumberger tool
SIGMA = 3150 / LIFE for the Dresser tool
modern logs, and many older ones, the SIGMA curve is displayed
and the above calculation is not needed.
SIGMA = capture cross section (capture units)
TAU = neutron decay time (usec)
LIFE = neutron half life (usec)
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
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
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.
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,
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
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.
water saturation is based on the sum of the capture cross sections,
in a mathematical treatment similar to the sonic, density and
response equation for the thermal decay time log follows the classical
= PHIe * Sw * SIGw (water term)
+ PHIe * (1 - Sw) * SIGh (hydrocarbon term)
+ Vsh * SIGsh (shale term)
+ (1 - Vsh - PHIe) * Sum (Vi * SIGi) (matrix term)
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))
PHIe = effective porosity (fractional)
SIGMA = TDT capture cross section log reading (capture
SIGMAM = capture cross section matrix value (capture
SIGW = capture cross section for water (capture units)
SIGHY = capture cross section for hydrocarbons (capture
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
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.
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.
■ Formation evaluation behind casing
■ Sigma, porosity, and carbon/oxygen measurement in one trip in the
■ 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
■ Formation oil volume from C/O ratio, independent of formation water
■ 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
Sample pf an RST log used in time-lapse
resercoir monitoring mode.
TDT LOG CURVE NAMES
Modern TDT Logs - Dual Burst
|near count rate
|far count rate
|* TDT Porosity
Older TDT Logs - Single Burst
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 )