PART
II, PAGE 3
WHAT
ARE NEUTRONS?
A
neutron is a neutral particle approximately the same mass as a proton. Neutrons interact almost exclusively with atomic
nuclei. Neutron energies are measured in electron volts (ev), thousands of ev
(Kev) and millions of ev (Mev).
Neutrons are classified by energy as follows:
Fast
Neutrons: Above 500 KeV
Intermediate: 1 KeV to 500 KeV
Slow: Below 1KeV
Slow
Neutrons are classified further:
Epithermal: .1 KeV to 1 KeV approx.
Thermal: Below .1 KeV approx.
Thermal
neutrons are referred to as “thermal” because they are in thermal equilibrium.
Neutron
sources:
Spontaneous
fission – Cf-252 produces neutrons through a process of spontaneous
fission. The neutrons produced from this
process have energies between 250 KeV and 2 MeV. The energies are too low for some
applications but are acceptable for others. This source has the advantage of
being compact however, it is expensive.
Alpha –
neutron sources – This type of source is
most commonly used. Alpha particle
sources such as Pu, Po, Am, are combined with Be (beryllium). As the alpha particles bombard the beryllium,
neutrons are produced. The energies of these neutrons are between 1 MeV and 12
MeV. The higher energy levels permit deeper penetration and greater depths of
investigation.
Neutron
generators – Portable neutron sources can be made using a Deuteron beam
impinging on a tritium target and in the process generating neutrons. These neutrons have an energy of 14.1 MeV. A high voltage source is used to create the
energy necessary to drive the beam of deuterons into the target. Because the
reaction is caused by an electronic circuit – this source can be turned on or
off. Neutron generators are expensive
and operate for only 100 to 200 hours.
Because of the high energy it is not suitable for water content logging.
NEUTRON
TOOLS:
Neutron
tools are designed to accommodate one of the sources described above and at a
fixed spacing, a detector is used to detect the resulting captured neutron
reaction products.
Four
detector types are used:
BF3
counters – This is a common detector similar to a Geiger-Mueller tube. The
efficiency is fairly low.
He3
counters – This is a more modern detector similar to the BF3 detector but 4 to
20 times more efficient.
Epithermal
neutron detectors – This is a cadmium shielded gas type detector.
Gamma
ray detectors – This detector is used to detect gamma rays of capture rather
than neutrons.
NEUTRON
FLUX:
Neutron
flux is the product of the number of neutrons per cubic centimeter and their
velocity. The units are neutrons per
square cm per second.
Neutrons
emitted from a neutron source are subjected to interaction thru collision with
atoms in the formations surrounding the borehole. Most of the collisions are elastic –
resulting in elastic scattering of neutrons. Each collision results in energy
loss. Collisions with silicon atoms
require 297 collisions to reduce the energy of a 14 MeV neutron to .01 eV
(thermal). Smaller atoms, for example,
hydrogen require only 19 collisions to thermalize the same neutrons. The result
of the reduced energy level to thermal is that the neutron is “captured” by an
atom and a gamma-ray of capture is generated.
Hydrogen is therefore the most efficient in slowing down and reducing
the energy of neutrons. In formations
containing 100 percent water saturation, neutrons are thermalized within 8
centimeters. The same formations having 1 percent water saturation will
thermalize neutrons within 18 to 26 centimeters. (Kreft 1974) The depth of
investigation is dependent on porosity and as porosity deceases, the depth of
investigation increases. It has been estimated that 90 percent of the response
in a dry sand will come from a depth of investigation of less than 58 cm. (Ferronski et al. 1968)
Neutron
flux can be visualized as spheres having a common center and at increasing
diameters, neutrons having less energy – the outermost sphere having the
highest concentration of thermal neutrons and gamma rays of capture.
BOREHOLE
COMPENSATION:
Borehole
compensation is necessary for the following factors:
Gap:
Gap
corrections are used to correct for the gap between the tool and the borehole
wall. In tools that are designed for eccentric
logging – for example sidewall neutron tools, a caliper presses the tool
against the side of the borehole. Sidewall neutron tools reduce the effect of
the borehole. Dual spacing Neutron tools are also designed to reduce the effect
of the borehole on neutron response.
Logging companies publish correction charts for gap for both one and two
detector tools.
Apparent
porosity increases as standoff (gap) increases.
Mudcake:
Apparent
porosity increases below 22 percent and decreases above 22 percent because of
mud cake on the borehole wall.
Hole
size:
The
distance neutrons travel is largely determined by the volume of hydrogen. It is important to measure and have knowledge
of borehole size and rugosity –
particularly washouts. Charts that
correct for hole size are published. Increased borehole size increases porosity
1 percent for each inch directly.
Salt:
Because
sodium chloride is not very effective in slowing neutrons, yet is contained in
the pore space in solution with water, corrections are applied to porosity for
sodium chloride content in the pore space.
Casing:
Casing
(cement) affects neutron logging. Each
inch of cement causes apparent porosity to increase 2.66 percent.
POROSITY
FROM NEUTRON TOOLS:
A
Neutron tool gives an “indication” of porosity.
Neutron
tools are calibrated in API Neutron Units. The API test facility located at the
Tool
suppliers develop a transform to convert API Neutron Units to Porosity for the
tools they produce. Since the tools are calibrated in a Limestone matrix –
corrections must be made for logging in other matrix materials.
CORRECTION FOR OTHER MATRIX:
Neutron
tools are calibrated for a limestone matrix.
When neutron logging is done in other rock matrices, the value of
neutron porosity must be corrected.
Correction
charts are used for
this purpose.
CORRECTION
FOR HYDROGEN INDEX:
Neutron tools are calibrated for a
specific fluid, usually fresh water. The hydrogen index (IH) of some
oils is the same as that of water. The hydrogen index of others is not the
same. Hydrogen index also varies with temperature and pressure. See chart.
Since not all borehole water is fresh water,
the response for the corrected neutron log to water is FNW.
For a neutron log ran in water
saturated, shale free matrix other than the matrix for which it was calibrated:
FN = FNW * F + FNma * (1-F)
CORRECTION
FOR SHALE:
Corrections
must be made for out of matrix log data.
If a neutron log has been calibrated for limestone matrix, the logged
porosity will be in error if the matrix differs from limestone.
Sandstone
– clay/shale sequences are common in logging.
In a
sandstone matrix, apparent porosity decreases 4 percent.
In a
Dolomite matrix, apparent porosity increases 6 percent.
Shale
contains water in the form of “bound water”.
The water content is measured as porosity. If a measure of water content is desired – no
correction is necessary. If the
objective is to accurately measure porosity, then a correction must be applied.
To
determine Shale Volume (Vsh) from a gamma log:
Vsh
= (L – C)/ (S – C)
Where:
L =
gamma reading in the zone of interest
C =
gamma reading in Clean sand formation
S =
gamma reading in shale
When
density and neutron logs are available; Shale volume can be calculated as
follows:
Where:
F(n) = Neutron Porosity
F(d) = Density Porosity
F(nsh) = Neutron Porosity in Shale
F(dsh) = Density Porosity in Shale
CORRECTION
FOR GAS:
Gas is
less dense than water and contains less hydrogen in the same pore space. When
it is known that gas is present in a formation, a correction is necessary. View
a chart that shows the effect
of gas and shale on the neutron porosity log compared to a density porosity log
overlay on the same scale.
Correction
charts are published for this purpose.
In summary,
we have porosities from acoustic (sonic), density, and neutron logs. Each
measured porosity involves uncertainties caused by assumptions about matrix and
fluid properties. Each is affected differently by shale and hydrocarbons whose
amount and properties must be determined somehow. Because of these effects and
uncertainties, it is necessary to combine equations to obtain more accurate
values of porosity.
POROSITY FROM TWO LOGS:
OVERLAYS:
One
common log interpretation procedure is to superimpose density
and neutron logs, both in terms of porosity and both on the same scale.
If the
true porosity is constant and gas is present, FD will be greater than F and FN will be less than F. The logs are mirror images.
IF
shale is present and rma is assumed to be other than shale
then FD will be slightly less than F and FN will be greater than F. The logs will be mirroring images and have different scales. The
size of the effect will depend on F.
The
effect will be greater for a compensated neutron log than for an epithermal
neutron log (Truman et al., 1972).
Learn
more about CROSS PLOTS:
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