APPLICATION OF CHARGED PARTICLE ACCELERATORS TO SOLVING PROBLEMS IN WELL LOGGING

V. M. ZAPOROZHETS, V. V. SUDIN

(Presented at the Second Interuniversity Conference on Electron Accelerators)

The effectiveness of radiometric methods for studying borehole sections can be significantly increased by creating small-sized charged-particle accelerators suitable for operation in drilled wells—borehole generators of neutrons and gamma quanta.

The advantages of borehole accelerators over natural sources of radioactive radiation are associated with the possibility of obtaining, with their help, radiation of greater intensity and higher energy, and also—most importantly—with the possibility of controlling the energy and intensity of the radiation.

For safety reasons, the power of radiation sources made from radioactive substances must, in borehole investigations, be limited to a comparatively small value, amounting for polonium–beryllium neutron sources to no more than 10 curies of polonium, and for gamma-quantum sources to no more than 0.05 curies. A borehole accelerator can be switched on at the depth of the well and is therefore safe during transport and during preparatory work at the well. Increasing the power of the accelerator is limited only by its design.

Increasing the power of radiation sources will make it possible to increase the rate of measurements in radioactive logging by reducing the statistical scatter of their results. This increase is proportional to the square root of the ratio of the source powers, with unchanged measurement accuracy. We shall not be mistaken in saying that the efforts expended on developing borehole accelerators will be repaid by an increase in the rate of measurements in neutron logging and in investigations of wells by the method of induced radioactivity.

It is known that the energy of gamma quanta of natural and artificially produced radioactive substances does not exceed 3 MeV. The energy of most neutrons emitted by radium–beryllium and polonium–beryllium sources does not exceed 5 MeV, and only for very few neutrons does it exceed 10 MeV. With accelerators it is possible to obtain gamma quanta and neutrons of considerably higher energy.

An invaluable advantage of accelerators is the possibility of obtaining monoenergetic radiation, as well as the possibility of controlling the radiation process, in particular obtaining radiation pulses of specified duration, shape, complexity, and repetition frequency.

The noted features of borehole accelerators make it possible to undertake the development of new methods of radioactive logging that are not feasible when using stationary radiation sources made from radioactive substances.

Let us consider some of these methods.

The use of a gamma-quantum generator with sufficiently high radiation energy and intensity makes it possible to raise the question of developing gamma-neutron logging, in which rocks are irradiated with gamma quanta and the intensity of the photoneutron radiation of the rocks produced thereby is recorded.

Table 1, compiled from data in [1, 2, 3, 4], gives the photoneutron characteristics of deuterium, beryllium, and the principal elements composing sedimentary rocks of oil and gas fields and present in formation waters that usually saturate these rocks.

As can be seen from the table, there is a considerable difference among rock-forming elements in the magnitude of the cross section of the reaction \((\gamma, n)\), which apparently may serve as a basis for their selective differentiation in rocks by the gamma-neutron logging method. Of particular interest are the low threshold energies of the reactions \(D(\gamma, n)\) and \(Be(\gamma, n)\).

The use of the reaction \(D(\gamma, n)\) appears promising for identifying oil-bearing beds in a well section by the increased deuterium content in oil (especially in its light fractions), which exceeds its content in formation waters by a factor of 2–3 [3, 5, 6, 7].

To assess the possibility of differentiating rocks from gamma-neutron logging data, let us consider the photoneutron properties of sedimentary rocks. The photoneutron properties of a rock can be characterized by the macroscopic effective cross section \(\sigma_m\) of the \(\gamma,n\) reaction, related to the cross sections \(\sigma_i\) of the nuclei of the atoms making up the rock by the following relation:

\[ \sigma_m = \sum_i n_i \sigma_i , \]

where \(n_i\) is the number of nuclei of the \(i\)-th kind in \(1\ \mathrm{cm}^3\) of rock.

The values calculated by this formula for the ratio of \(\sigma_m\) of rock to \(\sigma_m\) of water for various rocks and for various gamma-quantum energies are shown in Fig. 1. The values of \(\sigma_i\) used in the calculation were taken from [2, 3, 4, 8, 9, 10]. The same figure also shows the dependence of the ratio of neutron migration lengths in rock and in water, constructed from data in [11, 12]. This dependence characterizes the differentiation of rocks by neutron logging methods.

As can be seen from Fig. 1, there is significant differentiation of rocks according to their photoneutron properties, exceeding, at some values of gamma-quantum energy, their differentiation according to neutron properties. Of particular interest is the dependence of the differentiation on gamma-quantum energy, the possibility of varying which during investigations will therefore provide a substantial increase in the resolving power of the method.

Thus, for example, at gamma-radiation energies below 4–6 MeV, only photoneutrons from deuterium and beryllium will be recorded.

Let us now consider other methods of radioactive logging that are feasible with the use of borehole accelerators.

The use of a gamma-quantum generator in gamma—gamma logging makes it possible to increase the depth of investigation by increasing the ener-

gamma rays emitted by the generator. In addition, increasing the source power makes it possible to carry out investigations with probes of larger size, whose use ensures greater sensitivity of the method to changes in the density of rocks.

Fig. 1. Photoneutron characteristics of the principal sedimentary rocks

\(\delta_m\) — macroscopic cross section of the \(\gamma,n\) reaction.

\(E_\gamma\) — energy of the gamma rays,

\(M=\sqrt{2L_f^2+L^2}\) — migration length

\(L_f\) — slowing-down length of fast neutrons,

\(L\) — diffusion length of slow neutrons.

With the aid of accelerators it is possible to obtain electrons of considerably higher energy than those emitted by natural and artificial \(\beta\)-emitters. This provides the basis for developing a new method of “electron logging,” in which the wall of the borehole is irradiated by a stream of electrons and the resulting bremsstrahlung is recorded; its intensity will apparently depend on the lithological characteristics of the rocks.

The use of a stationary high-power neutron generator will substantially facilitate the study of rocks intersected by a borehole by means of the methods of induced radioactivity of long-lived elements that are currently employed (for example, the method of sodium activation). At a high energy of the generated neutrons (\(14\ \text{MeV}\)), it may prove feasible to record the induced activity of aluminum, during whose activation also-

a radioactive isotope \(Na^{24}\) is also formed. Along with this, the development of new methods of induced radioactivity of short-lived elements, feasible with a pulsed neutron generator, is of great interest, since it can provide additional data on the composition of rocks and the fluids saturating them. Thus, when neutrons of energy \(14\ \text{MeV}\) interact with the nuclei of atoms of rocks, radioactive isotopes of nitrogen and aluminum may be formed; these arise as a result of activation by fast neutrons of oxygen and silicon and have half-lives, respectively, of 7.5 sec and 2.3 min. This makes it possible to count on the use of a borehole neutron generator for identifying, in the cross section of a well, rocks containing an increased amount of oxygen and silicon, in particular sandstones. The results of laboratory work [9] show that the activity induced by neutrons with an energy of \(14\ \text{MeV}\) is considerably higher in sandstone samples than in limestone and clay samples.

The use of a neutron generator operating in a pulsed mode provides the basis for developing a new method of neutron gamma-ray logging, in which rocks are irradiated by pulses of neutron radiation and the gamma radiation arising during the first inelastic collisions of neutrons with the nuclei of atoms of the rocks is recorded.

One may also raise the question of developing new methods of neutron–neutron logging with a pulsed neutron generator.

In one of them, the density of slow neutrons is recorded in the intervals between periodically following short pulses of neutron radiation, the time interval between the end of pulse generation and the start of recording of neutron density being chosen so large that only those neutrons are recorded which have migrated for a long time in the rock. In this way, it may be possible to increase the depth of investigation in neutron logging and to reduce the influence of the well on its readings.

Another new method of neutron–neutron logging is based on recording the changes with time in the density of thermal neutrons at some distance from an accelerator generating short pulses of neutron radiation. In this way, information may be obtained on important neutron parameters of rocks associated with their lithological properties.

The use of ion accelerators as neutron generators is considered in [14]. It is of interest to assess the possibility of using, for this purpose, a betatron with a tungsten–beryllium or tungsten–deuterium target. This can be done on the basis of the following data.

In Fig. 2, compiled from experimental data published in [1, 2, 8, 9, 16], the dependence on gamma-quantum energy of the yield of photoneutrons from thick targets made of beryllium and deuterium is shown. The bremsstrahlung gamma radiation of a betatron is characterized by a continuous spectrum [16]. Figure 3 shows what part of the gamma radiation of a betatron will have an energy exceeding the threshold of the \(\gamma, n\) reaction for beryllium and deuterium at different maximum betatron energies. The calculated data obtained for a tungsten target from the data of [19] are compared, as a check, in this figure with experimental data obtained with a gold target [17].

Using the data shown in Figs. 2 and 3, one can calculate

Fig. 2. Dependence of the yield \(N\) of photoneutrons in neutrons/sec per 1 gamma quantum of energy \(E_\gamma\), per 1 g of \(Be\) and \(D\), for a thick target located at a distance of 1 cm from the source of gamma quanta.
\(1 — D,\quad 2 — Be\)

Fig. 3. Dependence, on the maximum energy \(E_{\max}\) of bremsstrahlung, of the content \((k)\) in the bremsstrahlung gamma-radiation spectrum of betatron gamma quanta with energy \(E\) greater than the threshold energy \((E_{\text{thr}})\) for the beryllium and deuterium reactions.
\(1 — E > E_{\text{thr}}\) for \(Be\);
\(2 — E > E_{\text{thr}}\) for \(D\);
— calculation;
— — experimental results according to data of [23]

Fig. 4. Dependence of the yield \(N\) of photoneutrons, in neutrons/\(\mu\text{A}\), from \(1\) g of \(D\) and \(Be\) on the maximum energy of the betatron bremsstrahlung gamma radiation, for a thick target located at a distance of \(1\) cm from the betatron target.
\(1\)—\(D\), \(2\)—\(Be\).

Fig. 5. Spectrum of photoneutrons from beryllium and deuterium when a betatron is used as the source of gamma radiation, with maximum bremsstrahlung energy \(E_{\max}\).
\(1, 2\)—\(Be\); \(3–4\)—\(D\); \(1–3\)—\(E_{\max}=5\) MeV; \(2–4\)—\(E_{\max}=10\) MeV.

on the dependence of the photoneutron yield from thick targets of deuterium and beryllium under the action of betatron radiation having different maximum gamma-quantum energies (Fig. 4). The energy spectrum of photoneutrons, calculated from data [1, 2] for several values of the maximum gamma-quantum energy, is shown in Fig. 5.

Table 1

Photoneutron properties of deuterium, beryllium, and the principal elements composing sedimentary rocks of oil and gas fields and included in formation waters.

The data of the calculations carried out show that even with a comparatively small maximum energy of betatron gamma radiation it can be used as a neutron source. The results of studies [18, 19] show that it is reasonable to use targets of deuterium or beryllium up to 200 g for the betatron. Limiting ourselves to a value half as large, we obtain that, at a maximum energy of the betatron gamma radiation of 5 MeV, the photoneutron yield per \(1\,\mu\mathrm{A}\) of current on a tungsten target will be, for deuterium, \(8\cdot 10^{10}\)

and for beryllium, \(1.6 \cdot 10^{10}\). Assuming that the current to the betatron target lies within the range \(0.001\)–\(0.01\ \mu\text{A}\) (19), we find that the yield of photoneutrons from a deuterium target will be within the range \(10^8 \div 10^9\) neutrons per second, and from a beryllium target within the range \(10^7 \div 10^9\) neutrons per second.

The betatron can be used as a neutron generator in laboratory work on neutron–neutron logging. When used for neutron gamma logging, its major drawback will be the considerable intensity of the gamma radiation it emits.

REFERENCES

1. Segre, E. (editor). Experimental Nuclear Physics. Vols. I and II, IL, 1955.

2. Vlasov, N. A. Neutrons. Gostekhizdat, 1955.

3. Montalbetti, R., Katz, L., Goldenberg, J. Photoneutron cross sections. Phys. Rev., 91, p. 659, 1953.

4. Hartly, W. H., Stephens, W. E., Winhold, E. J. Cross sections for photoneutron emission induced by the lithium gamma-rays. Phys. Rev., 104, p. 178, 1956.

5. Filippova, N. S. Hydrogen isotopes in petroleum. DAN SSSR, 3, 29, 1, 1935.

6. Rankama, K. Isotopes in Geology. IL, 1955.

7. Isotopes in Geology. Collection of articles, IL, 1954.

8. Zeviger, J. S. Sum rules for photodisintegration of the deutron. Phys. Rev., 97, p. 970, 1954.

9. Allen, Z. Photodisintegration of deutron. Phys. Rev., 98, p. 705, 1955.

10. McDaniel, B. D., Walner, R. L., Stearus, M. B. Neutron yields from photodisintegration by gamma-rays from Lithium. Phys. Rev., 80, p. 801, 1956.

11. Kantor, S. A. Fundamentals of the theory of neutron logging. Applied Geophysics, issue 13, Gostoptekhizdat, 1955.

12. Bulashevich, Yu. P. Theory of neutron logging as applied to exploration of petroleum and coal deposits. Izv. AN SSSR, ser. geography and geophysics, 12, 2, 1948.

13. Caldwell, R. L. Nuclear physics in petroleum exploration research. World Petroleum, April, 1956.

14. Zaporozhets, V. M., Filippov, E. M. The use of charged-particle accelerators for the investigation of wells by radioactive logging methods. Applied Geophysics No. 19, Gostoptekhizdat, 1958.

15. Gusev, N. G. Handbook on Radioactive Radiations and Protection. Medgiz, 1956.

16. Schiff, L. J. Energy-angle distribution of thin target Bremsstrahlung. Phys. Rev., 83, No. 2, 1951.

17. Starfeld, N., Koch, H. W. Differentiale cross section measurements of thin target Bremsstrahlung, produced by 2.7-to 9.7 Mev lestraus. Phys. Rev., 102, No. 6, 1956.

18. Cockroft, J. D., Duckworth, J. C., Merrison, A. W. High-energy electron accelerators as pulsed neutron sources. Nature, 163, No. 4153, p. 869, 1949.

19. Hine, G. I., Senftle, F. E. Neutron yield from Be \((\gamma, n)\) Be\(^8\) reaction. Phys. Rev., 80, p. 904, 1950.

20. Akimov, Yu. M., Kononov, B. A., Sokolov, L. S. On the extraction of an electron beam from a betatron chamber. Report at the Conference on Electron Accelerators, Tomsk, February 15–20, 1958.

21. Zaporozhets, V. M., Sulin, V. V. Gamma-neutron logging. Report at the Conference on Electron Accelerators, Tomsk, February 15–20, 1958.

22. Zaporozhets, V. M., Kantor, S. A. Pulsed neutron logging. Report at the Conference on Electron Accelerators, Tomsk, February 15–20, 1958.

23. Filippov, E. M. Gamma–gamma logging with a gamma-quantum generator. Report at the Conference on Electron Accelerators. Tomsk, February 15–20, 1958.