Neutron Logging

a geophysical prospecting method based on the interaction of neutrons with rock formations. A thick-walled steel sleeve housing a neutron source and detector is lowered into a borehole. The detector is sensitive to the secondary radiation that results from the interaction of the neutrons with the atomic nuclei of the rocks (see). A filter made of paraffin, lead, or bismuth is placed between the source and the detector, thus preventing direct bombardment of the detector by the neutrons from the source. The detector signals, which are formed and amplified by electronic devices, are transmitted by cable to the surface for recording and analysis. A log—the variation of the signal counting rate with depth—is recorded as the unit traverses the borehole. Neutron logging was

Figure 6. Distribution of spin density in unit cell of iron, (a) Spin density of 3d electrons obtained by Fourier synthesis; the iron atom is in the upper left-hand corner; the numbers on the curves indicate density of magnetic moment in Bohr magnetons per A³; a is the period of the unit cell of iron, (b) The same as in (a) after subtracting out the spherically symmetric part of the spin density, (c) Distribution of the magnetization (in kilogauss) throughout the volume of the unit cell of iron; the magnetization results from polarization of the 4s electrons.

first performed in the USA by B. M. Pontecorvo in 1941; in the USSR, the development of neutron logging is associated with B. B. Lapuk and G. N. Flerov.

There are approximately ten kinds of neutron logging, which differ in the type of neutron source, the form of the secondary radiation, and the nature of the data obtained. In neutron-neutron logging, recordings are made of the thermal neutrons that result from the moderation of the fast neutrons of the source in the rock. In neutron-gamma logging, recordings are made of the gamma quanta that result from the capture of slow neutrons by nuclei. In these kinds of neutron logging, the relative amount of hydrogen in formations is determined with a continuous source. Since hydrogen is the most effective neutron moderator, the neutrons are moderated even at short distances from the source in rocks having pores filled with oil or water. For example, in sandstone with 20-percent porosity, the distance over which approximately 60 percent of the source neutrons (which have an energy of five megaelectron volts) become thermal neutrons is of the order of several centimeters. The number of thermal neutrons (or gamma quanta emitted during radiative capture) that reach the detector in this case is not large, since the distance to the detector is substantially greater (30–50 cm). With decreased hydrogen content in a formation, the slowing-down length increases, the neutrons become thermal in a region closer to the detector, and the number of detector counts increases. Thus, minima on the log correspond to beds with increased hydrogen content.

In addition to distinguishing porous beds, such as sandstone or limestone, containing water or oil, neutron logs make it possible to distinguish the denser beds, the boundaries of the beds, clay intercalations, and the boundaries between liquid and gas. The last capability makes neutron logging useful in prospecting for gas fields.

Neutron logging with a continuous source does not, however, make it possible to reliably distinguish water-saturated beds and oil-saturated beds, since water and oil are indistinguishable as neutron moderators. For this purpose, neutron logging with a pulsed source (pulsed neutron logging) is more effective. Formation water usually contains mineral salts, for example, NaCl, whereas there are no mineral salts in petroleum. Owing to the absorption of neutrons in chlorine, the lifetime τ of the thermal neutrons in a bed containing water is less than that in an oil bed. In pulsed neutron logging, the neutrons are emitted during short intervals of 1 to 10 microseconds, and only those signals from the detector that arrive at a time t > τ after the generation of the neutron pulse are recorded. In this case, the number of recorded signals will depend on τ. In a bed containing water, for which τ is not large, few neutrons remain by time t; thus, the intensity of the recording is small. In an oil-saturated bed, however, τ is larger and more neutrons remain. In areas with heavy mineralization of formation waters (200 grams of NaCl per liter), the instrument readings between water- and oil-saturated portions of the bed differ by a factor of ten. Pulsed neutron logging came into widespread use after the development of compact neutron pulse generators.

A scintillation counter and semiconductor detectors having high resolving power are used in neutron-gamma logging. Measurement of the spectrum of the gamma quanta generated by radiative capture permits elemental analysis of the rock. By using pulsed neutron logging, we can also determine the spectrum of the gamma rays that arise during the inelastic scattering of neutrons by nuclei. With this method it may be possible to distinguish oil-bearing beds according to carbon content—that is, regardless of the presence of salts in the formation waters.

In the USSR, neutron logging is one of the mandatory geophysical operations in all oil-bearing strata. Neutron logging is also used for seeking oil beds (horizons) that were missed in the drilling of old wells.

After the irradiation of rock by neutrons, the rock develops radioactivity, the measurement of which yields data on the composition of the rock (neutron-activation logging). Neutron-logging methods based on this are used in mineral prospecting and other geological studies.