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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.
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 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 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 enery 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 is necessary for the following factors:
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.
Apparent porosity increases below 22 percent and decreases above 22 percent because of mudcake on the borehole wall.
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.
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 (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)
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:
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:
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 mirror 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: