Water Saturation From Pulsed Neutron Logs
Logs that come under this designation are the Thermal Decay Time
Log (TDT) or the Neutron Lifetime Log (NLL). They are also called
Pulsed Neutron Logs (PNL). They are
primarily affected by the presence of chlorine and hence can differentiate
between salt water and hydrocarbons. They do this by generating
a burst of neutrons and monitoring the decay time of the neutrons.
The tool emits a burst of neutrons and the decay time of the
neutrons are recorded by measuring the neutron count rate zt
the detectors versus time. This process is repeated as the
tool is moved up the borehole.
Pulsed neutron devices usually have a series of `gates' for
measuring count rates at different times after the neutron burst
has taken place. This method is used to measure borehole and background
effect, and correct for them.
Since
the neutrons are generated, rather than emitted by a radioactive
chemical source, the tool is very attractive to those who fear
the consequences of losing a radioactive source in a producing
well.
The
new generation of dual spacing detector devices minimize the effects
of casing and tubing, so that no corrections are necessary.
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)
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.
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)
WHERE:
SIGh = log reading in 100% hydrocarbon
SIGi = log reading in 100% of the ith component of matrix rock
SIGMA = log reading
SIGsh = log reading in 100% shale
SIGw = log reading in 100% water
PHIe = effective porosity (fractional)
Sw = water saturation in uninvaded zone (fractional)
Vi = volume of ith component of matrix rock
Vsh = volume of shale (fractional)
This
equation is solved for Sw by assuming all other variables are
known or previously calculated.
SWtdt  Water Saturation from TDT log.
4:
SIGW = 22.0 + 0.000404 * WS
5:
IF PHIe > 0
6: THEN SWtdt = ((SIGMA  SIGMAM)  PHIe * (SIGHY  SIGMAM)
 Vsh * (SIGSH  SIGMAM))
/ (PHIe * (SIGW  SIGHY))
7: OTHERWISE SWtdt = 1.0
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)
WS = water salinity (ppm NaCl)
NUMERICAL
EXAMPLE:
1. Assume data as follows:
PHIe = 0.28
SIGW = 84 cu
SIGMAM = 10 cu
SIGHY = 22 cu
SIGMA = 25.5 cu
Vsh = 0.20
SIGSH = 37 cu
SWtdt = ((25.5  10)  0.28 * (10  22)  0.20 * (37  10)) /
(0.28 * (84  22)) = 0.39
2.
If zone contained gas:
SIGHY = 9 cu
SWtdt = (25.5  10)  0.28 * (10  9)  0.20 * (37  10)) / (0.28
* (84  9)) = 0.49
Nomograph for water saturation from TDT log
Porosity
from TDT LOGS
In
the case of the dual spacing devices, the ratio of the corrected,
or net, count rate from each detector is calculated. This is the
same approach that is used for the CNL and, like the CNL, porosity
can be derived from the pulsed neutron ratio. Similarly, gas effects
must be taken into account.
The
illustration below shows how porosity can be derived from the TDT ratio curve.
Equations to represent this chart are available but are complex
and seldom used. Most modern TDT logs present a porosity curve
equivalent to a CNL style neutron log. There was a short period
when TDT porosity in dolomite was badly in error. Always compare
TDT porosity to other sources.
Porosity from TDT log
Limits to use of tdt for saturation calculations
The
capture cross section is relatively inaccurate in low salinity,
low porosity situations. The chart shown below is used to
determine under what conditions the log can be used. The C/O
curve on modern tools often helps locate hydrocarbon zones in
fresher water situations.
Find useful range of TDT log here
To
overcome this inaccuracy problem, older logs were run in multiple
passes and the SIGMA curves summed to reduce statistics. Typically,
five runs were summed. More modern tools have better signal to
noise ratio and do not need multiple passes. However, saturation
may still be inaccurate when salinity is less than 50,000 ppm.
Check with the service company for useful salinity / porosity
ranges on current tools as specifications are constantly changing
The
current Schlumberger tool is called the Reservoir Saturation Tool
(RST) and the term TDT may disappear as newer tools replace older
ones.
Choosing
Analysis Parameters
SIGMAwater (SIGW)
is best derived from water salinity, which in turn can be
derived from water resistivity:
8: WS = 400000 / FT1 / ((RW@ET) ^ 1.14)
9:
SIGW = 22.0 + 0.000404 * WS
WHERE:
BHT = bottom hole temperature (degrees Fahrenheit or Celcius)
BHTDEP = depth at which BHT was measured (feet or meters)
DEPTH = midpoint depth of reservoir (feet or meters)
FT = formation temperature (degrees Fahrenheit or
Celcius)
FT1 = formation temperature (degrees Fahrenheit)
RW@FT = water resistivity at formation temperatures (ohmm)
SUFT = surface temperature (degrees Fahrenheit or Celcius)
WS = water salinity (ppm NcCl)
SIGMAhydrocarbon
(SIGHY)
ranges between 0 and 23, with a default of 22 cu for typical
oil and 9 cu for gas. See graphs below.
SIGMA values for oil, gas, and water
SIGMAshale
(SIGSH) ranges between 20 and 45. You can look at a depth plot of
your log, find the nearest, fairly thick, shale as observed on
the gamma ray log and read the average of the SIGMA curve over
the same interval. If GR is not a good shale indicator, try density
neutron separation or shallow resistivity
A
crossplot of GR vs SIGMA will do the same thing (as long as radioactivity
is a function of shale minerals and not uranium). Find the cluster
of high GR values representing shale and pick the corresponding
SIGMA shale.
SIGMAmatrix (SIGMAM)
can be taken from chartbook tables or can be calculated from the
SIGMA log chrve if porosity is known from conventional log
analysis. The values in the chartbook
tables do not work well because real rocks are not pure minerals.
A method for finding SIGMAM from the log data itself uses the
following equation:
9. SIGMAM = (SIGMA  PHIe * SIGW) / (1  PHIe)
This
eliminates the salt in the water in the porosity (SIGMA salt =
770) and accounts for any other minerals in the sandstone (for
example an iron rich cement where SIGMA iron = 220). Most real
rocks have SIGMA larger than the values in the tables in chartbooks.
You can vary SIGMA matrix point by point or take an average of
several calculated values.
WHERE:
SIGMAM = capture cross section of matrix (capture units)
SIGMA = capture cross section log reading (capture units)
SIGW = capture cross section of water (capture units)
PHIe = effective porosity (fractional)
This
should be done in a clean porous interval containing water.
MATRIX PARAMETERS FOR PURE MINERALS
Caution: these values are for pure minerals and values for real
rocks are often higher.
MINERAL 
SIGMAM 
Quartz
SiO2 
4.3 
Calcite
CaCO3 
7.3 
Dolomite
CaCO3.MgCO3 
4.8 
Feldspars 

Albite
NaALSi3O8 
7.6 
Anorthite
CaALSi2O8 
7.4 
Orthoclase
KAlSi3O8 
15.0 
Evaporites 

Anhydrite
CaSO4 
13.0 
Gypsum
CaSO4.2H2O 
19.0 
Halite
NaCl 
770 
Sylvite
KCl 
580 
Carnallite
KCl.MgCl2.6H2O 
370 
Borax
Na2B4O7.10H2O 
9000 
Kermite
Na2B4O7.4H2O 
10500 
Coal 

Lignite 
30
+/5 
Bituminous
coal 
35
+/ 
Anthracite 
22
+/5 
IronBearing
Minerals 

Iron
Fe 
220 
Geothite
FeO(OH) 
89.0 
Hematite
Fe2O3 
104 
Magnetite
Fe3O4 
107 
Limonite
FeO(OH).3H2O 
80.0 
Pyrite
FeS2 
90.0 
Siderite
FeCO3 
52.0 
IronPotassium
Bearing Minerals 

Glauconite
(green sands) 
25
+/5 
Chlorite 
25
+/15 
Mica
(Biotite) 
35
+/1 
Illite
Shale 
37
+/5 
Others 

Pyrolusite
MnO2 
440 
Manganite
MnO(OH) 
400 
Cinnabar
HgS 
7800 

