Principles of Neutron Logging
Neutron Source
Neutrons are emitted from the source at a high energy, usually between 1 and 10 MeV. However, they are not detected until their energy has been reduced to a fraction of an eV through collisions with atomic nuclei in the formation.
The rate at which energy is lost by the neutrons is a function of the elements in formation. Hydrogen is the most efficient element in slowing down, or moderating, neutrons. Porosity is inferred from the detection of these low energy, or thermal, neutrons because hydrogen is usually present only in the pore space of most water, oil, or gas bearing formations.
Neutron tools typically are calibrated based on a limestone matrix. Numerous corrections are made to the porosity determined from the neutron signal - corrections for borehole size, for formation matrices different from limestone, the presence of elements which have unusually high affinities for neutron absorption, etc. The measurement is most useful when it is plotted in conjunction with a density measurement, from which porosity can be computed. In this case differences in the neutron and density porosities can be used to infer fluid density parameters and whether the pore space contains oil/water or gas. Neutron and density tools respond the same in oil as they do in water.
Neutrons can be produced either from a so-called chemical source or by an electrically powered neutron generator. In either case, the neutrons produced are endowed with several MeV of energy.
The industry uses a chemical source which is quite common in porosity logging. The source consists of a mixture of beryllium and radioactive americium oxide powders. Emission of a neutron is the result of a 2-step process. First, an americium nucleus decays radioactively, emitting an alpha particle. The alpha particle then interacts with a beryllium nucleus and causes a neutron to be released. This process will now be discussed in more detail.
Alpha particles, which are emitted from the americium, consist of two protons and two neutrons. They are also the nuclei of helium-4 atoms, and are one of the products of the radioactive decay of heavy elements such as radium and uranium. Alpha particles carry two units of positive charge, and interact very strongly with other charged particles such as electrons. As a result they experience a tremendous amount of "drag" from electrons in nearby atoms, and are slowed very rapidly as they pass through solid matter. For example, the 5.5-MeV alpha particles emitted by americium will not penetrate an ordinary sheet of paper or the dead outer layer of human skin. It follows that none of the alpha radiation escapes from the source.
Most alpha particles simply slow down, gaining two electrons and thereby become helium gas inside the source (the volume of gas that is produced is never a problem for the structural integrity of the source). Occasionally, however, an alpha particle will interact with a beryllium nucleus in the AmBe source. This is a very rare event, which occurs only once for about every 17,000 alpha particles. When such an interaction does occur a neutron is emitted with energy of up to approximately 10 MeV 1. Neutrons are emitted from the source with equal probability in all directions.
The nominal activity of the neutron source refers to the americium material within the source. The stated activity of 8 Curies corresponds to the emission of about 8 * 3.7 * 1010 alpha particles per second. Since only one in 17,000 alpha particles causes emission of a neutron, about 17 million neutrons are emitted each second from such sources.
Slowing Down of Neutrons
Following emission from the source, high energy neutrons are moderated, or slowed down, through successive collisions with atomic nuclei in the formation, borehole, and logging tool. The collisions between the neutrons and nuclei are either elastic or inelastic. In elastic collisions, the nucleus and neutron are like hard spheres where energy is given up by the neutron to the nucleus. Both particles emerge from the collision with velocities based on the angle of the collision, the initial velocity of the neutron, and the relative masses of the neutron and the nucleus. The outcome of an inelastic collision is similar to the elastic, except that the neutron gives up additional energy to internal excitation of the nucleus. The nucleus will subsequently release this energy through some form of de-excitation, usually in the form of gamma ray emission.
Neutrons will continue to slow in this manner until they have the same energy on the average as the nuclei in the borehole environment. This is termed thermal energy, since its magnitude is proportional to the formation temperature. Typically this is a fraction of an electron-volt. Thus in the process of moderation from a value as high as 10 MeV to less than one eV, the neutron energy is decreased by a factor of between 107 and 108. The process of slowing down to thermal energies is termed the slowing down, or "moderation" phase, and at its end the neutrons are referred to as thermal neutrons.
The neutrons continue to move through and collide with nuclei in the formation after they are thermalized; however, their energy merely fluctuates with successive collisions about the thermal value. This second phase is called the thermal phase, or "diffusion" phase. The moderation phase ends when the neutrons are reduced to thermal energies. For neutrons with thermal energies, the likelihood of being captured by nuclei increases dramatically. The diffusion phase ends when the neutrons are captured by a nucleus in the environment, the borehole, the tool, or in the detectors in the tool.
1. "MeV" stands for 106 electron volts. The electron volt is the unit of energy typically used to refer to phenomena on the atomic and nuclear level. It is the energy gained by an electron when it falls through an electric potential of one volt, and is equivalent to 1.6x10-19 joules, or 3.8 x 10-20 calories.
Effects of Porosity on Neutron Moderation
A simple way to view the neutron measurement is first to think of the neutrons as forming a spherical cloud around the source. We realize immediately that at large enough distances the density of neutrons in this cloud must be vanishingly small, simply because of geometrical spreading. However, since it is thermal neutrons that are detected by the logging tool, let us imagine what the thermal neutron cloud is like.
As the neutrons exit the source, they are uniformly moving away from the source with their maximum energy. The only thermal neutrons at the location of the source will be those that have been slowed and drifted back in the direction from whence they came. The probability of a thermal neutron returning back to the source is relatively low, because neutron's initial direction is away from the source, and the fact that a neutron travels the farthest during its initial collision. Thus, while at the source we can expect that the density of high energy neutrons will be highest, the density of thermal neutrons will be low.
As we move away from the source the density of thermal neutrons increases. This occurs because as the distance increases the high energy neutrons both disperse and slow down through collisions. Thus, beyond some distance, most of the neutrons are thermalized. Due to geometrical spreading and the loss of neutrons to absorption by the environment, we can expect that the thermal neutron density will begin to decrease.
The radial extent of this cloud depends on the moderating properties of the formation. The more efficient the formation is at moderating the neutrons, the smaller will be the extent of the cloud; and vice versa for a less efficient formation. In thermal neutron tools, one or more detectors of thermal neutrons are positioned in the region of the cloud where the density of thermal neutrons decreases with increasing distance. The detection rate of the detector(s) is then established by the strength of the source, the efficiency of the detector, and the moderating properties of the formation. As the moderating properties of the formation increase, the neutron cloud will shrink in size and the count rate at the detector will decrease. Conversely, if the moderating properties decrease, the thermal neutron cloud will grow in extent, and the count rate at the detector will increase.
The reason the formation porosity is inferred from the neutron measurement is twofold: first, the pore space of formations is typically occupied by hydrogen rich compounds, either water or oil; and second, that hydrogen is the most efficient element at moderating neutrons because its nucleus is a single proton.
The moderating properties of elements follow the basic physics of collisions between particles of equal or different masses. In such collisions, the most energy is transferred during a "head-on" event; that is, when the motion between the particles is confined to the line between them defined by their relative positions at some time before the collision. Such a situation is not necessary, of course; and, indeed, it is rare. However, it is significant since it represents the most energy that can be lost by the neutron in any single encounter with another nucleus.
When the laws of conservation of energy and momentum are applied to such an event, it can be shown that the most energy that can be lost by the neutron is given by the expression:
where Eo = the energy of the neutron before the collision, Ef = the energy after the collision, m = the mass of the neutron, and M = the mass of the nucleus with which it collides.
The term in parenthesis is then the fraction of initial energy that can be lost in a head-on collision, and it can be evaluated for the most common elements in formation rocks: hydrogen, carbon, oxygen, silicon, and calcium. We do this in the table below:
It is clear from the entries in the third column of the table that while the presence of various elements will affect the moderation of neutrons (limestone, consisting of calcium, oxygen, and carbon, moderates somewhat differently from sandstone, consisting of silicon and oxygen), hydrogen in any appreciable amounts will dominate. Typical reservoir rocks, such as sandstone, dolomite, and limestone, have no hydrogen in their basic matrix material. Thus hydrogen will be present only in the pore space in either water, oil or gas. In such rocks, then, the moderation of neutrons is governed primarily by hydrogen in the pore space and only secondarily by the type of rock itself.
A thermal neutron detector (exactly how thermal neutrons are detected is discussed at length below) placed at an appropriate distance from the source will detect neutrons at a rate determined by their density in the thermal neutron cloud. If the moderating properties of the formation are changed, say by increasing the hydrogen-filled porosity of the formation, the radius of the neutron cloud will shrink, and the counting rate of the detector will decrease as well. This simple picture is useful for typical neutron tool design - higher counts are associated with low porosity, lower counts with high porosity.
Detection of Neutrons
Thermal neutrons, that is neutrons whose energies are less than one electron-volt, are detected by the Neutron sensor. How this occurs within the detector, and how other radiation such as gamma rays is discriminated against, is discussed in this subsection.
Radiation is detected through the ionization that occurs when it interacts with matter. One often hears the phrase "ionizing radiation" as a consequence. The detection technology, whether it is a Geiger-Mueller tube, a scintillator crystal, or something more exotic, works because the ionization caused by the radiation sets off a chain of events which can be detected in some fashion. The first detection of radiation by humans occurred when gamma rays from a radioactive ore, pitchblende, ionized atoms in an unexposed photographic plate. The effect of ionization on the photographic emulsion was similar to that of exposing the plate to visible light, and was then observable when the plate was subsequently developed. This 100-year old method of detecting radiation is still used in modern medical x-ray machines.
Unlike gamma rays, neutrons do not interact with atomic electrons to ionize matter directly. Instead they interact with the atomic nucleus itself. Broadly speaking, these neutron interactions can be divided into two areas: collisions with nuclei and absorption by the nuclei. As discussed above, fast neutrons, those with energies above a few electron-volts (eV), interact primarily through collisions. These collisions may be "elastic" or "inelastic" in nature. High energy neutrons will lose energy with each collision during the "moderation phase" of their life; once they have been slowed to thermal energies they continue to collide with nuclei during the "diffusion phase", but no longer lose energy, on the average. Absorption of neutrons may occur during either phase, but is much more likely to occur during the diffusion phase when the neutron is moving relatively slowly.
When a neutron is absorbed by a nucleus, a new isotope results. The new isotope always possesses excess energy and disposes of that energy through emission of one or more particles of radiation - usually gamma or beta radiation. However, the de-excitation mechanisms of certain nuclei are fairly unusual. The unique properties of the light isotope of helium, He3, make it very useful in neutron detection.
The He3 isotope itself has a high affinity for absorbing thermal neutrons. The absorption efficiency, for example, of a He3 tube is approximately 80% when filled with He3 gas at 10 atmosphere's pressure. Most thermal neutrons entering a He3 detector will therefore be absorbed by nuclei of the gas.
The second useful property of He3 is that, following absorption of a neutron, the He3 splits into a proton and tritium ion, rather than emitting other forms of radiation. Each time this absorption and subsequent split occurs, 765 KeV of energy is released, appearing as kinetic energy shared by the proton and tritium ions. Because they interact very strongly with matter (much more so, say, than gamma rays), the energy is given up in a very short distance from the absorption site.
He3 proportional counters exploit this reaction between He3 nuclei and thermal neutrons. Proportional counters are similar in construction to GM tubes. Like GM tubes, they consist of a cylindrical cathode with a central anode wire. As in GM tubes, electrons from ionization events are attracted to the anode wire and a cascade results. However, GM tubes and proportional counters are operated at different voltages and have as a result different operational properties. In GM tubes, the cascade results in a complete discharge of the tube. In proportional tubes the lower voltage means that a full discharge of the tube does not occur; rather, the electron cascade to the anode wire merely results in an amplification of the ionization event. The central wire in the tube is smaller than in GM tubes, producing a more concentrated electric field in the central region of the tube than is the case in a GM tube. The result is that most of the amplification of charge caused by successive ionizations in the cascade occurs near the central wire. Consequently, ionization events occurring almost anywhere in the tube will be amplified by about the same factor. Hence tubes operated in this fashion are called proportional counters.
The pulses from proportional counters are smaller than those from GM tubes, but the benefit is the additional information that is related to the amount of energy causing the ionization in the tube. This property renders proportional counters energy sensitive as are scintillation crystals.
Proportional counters can be rendered sensitive to neutrons by inclusion of appropriate materials. He3 gas is one such material. Each time absorption occurs a tritium (H3) nucleus and a proton (H1) are released with combined energies of 765 KeV. The strong ionization ability of these particles means that this amount of energy will be dissipated in the gas of the tube through ionization. Because only a few eV is required to produce an electron-ion pair, many thousands of electron ion-pairs will result. Since the same amount of energy is released with each absorption, the amplitude of each anode pulse resulting from neutron absorption will be essentially the same.
Other radiations in the environment - primarily gamma rays - can also produce pulses in the He3 proportional counters. However, the efficiency of He3 proportional counters for counting neutrons can approach 100%; whereas for gamma rays it is a fraction of one per cent. In addition under normal conditions there are simply very few gamma rays downhole that are in the energy range which could interfere with neutrons in the proportional counter. The consequence of this is that under normal conditions, the neutron count rate can be very high relative to the gamma rays that are detected by a He3 proportional counter, and most gamma ray counts that will occur will be of much lower amplitude than those from neutrons which will produce pulses of relatively high amplitude. The relatively low pulse amplitude produced by gamma rays allows them to be discriminated against relative to neutrons through setting suitable electronic thresholds. With appropriate proportional counter and electronic design, the only pulses exceeding the threshold will be those produced by neutrons.
Basic Neutron Tool Design
The heart of the Neutron tool is an 8-20 Ci source and near and far spaced banks of He3 proportional counters. The centers of these banks are approximately 30 and 60 cm from the source. Their positions relative to the source are chosen such that, over the expected range of measurement conditions, both banks of detectors will be in the region where the density of thermal neutrons decreases with increasing distance from the source over the expected range of porosity measurements.
Although one can infer porosity from count rates of a single detector, neutron logging tools typically use at least two sets of counters located at different spacings from the neutron source. As with many other logging sensors which use two or more differently spaced detectors, the combination of the signals from these two detectors can result in a measurement that is less sensitive to environmental effects.
The near and far spaced detectors of the tool actually consist of banks of several detectors each. There are three He3 tubes in the near bank and five in the far bank. The He3 detector banks are located on the insert some distance axially from the source. The detectors in each bank are configured on an arc. Because of mechanical considerations - the overall length of the source determined by the active volume and the securing mechanisms - the arcs of detectors are not exactly symmetric with the plane defined by the center of source activity and the axis of the tool. For this reason the response of each individual He3 detector is slightly different, and unique from all other detectors.
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