Chapter%2014 - Petrophysics MSc Course Notes 14 The...

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Petrophysics MSc Course Notes The Litho-Density Log Dr. Paul Glover Page 139 14. THE LITHO-DENSITY LOG 14.1 Introduction The litho-density log is a new form of the formation density log with added features. It is typified by Schlumberger’s Litho-Density Tool ( LDT ). These tools have a caesium-137 source emitting gamma rays at 0.662 MeV, a short-spaced and a long-spaced detector in the same way as the basic formation density tool. However, the detectors are more efficient, and have the ability to recognize and to count separately gamma rays which have high energies ( hard gamma rays : 0.25 to 0.662 MeV) and gamma rays which have low energies ( soft gamma rays : 0.04 to 0.0 MeV). The hard gamma rays are those that are undergoing compton scattering. The count rates of these gamma rays (in the energy window 0.25 to 0.662 MeV) are used in the conventional way to measure the formation density (Chapter 13). The final density value obtained is more accurate than the basic formation density tool because the harder gamma rays are less prone to attenuation by borehole effects, and there is a smaller spacing between the two detectors that has reduced statistical fluctuations in the count rates. The soft gamma rays are those that are undergoing photo-electric absorption. This effect can be used to provide a parameter which is dependent upon the atomic number of the formation, and therefore immensely useful in lithological recognition. 14.2 Theory 14.2.1 Compton Scattering and Photo-Electric Absorption Figure 14.1 shows the energy spectra of gamma rays as they are emitted from the source, and after travelling through various distances of the rock. At the radiation source, all gamma rays have a well defined energy of 0.662 MeV, represented by the sharp peak in Fig. 14.1. After travelling through the rock the gamma rays undergo compton scattering and loose energy, so the initially sharp peak moves to lower energies. Each of the gamma rays undergoes a different number of collisions dependent upon chance, and hence looses a different amount of energy. Thus the peak is not only moved to lower energies, but is also dispersed (becomes wider). In Fig. 14.1, Curve A represents the initial energy spectrum, Curve B represents the energy spectrum after the gamma rays have traveled a small distance through the rock, and Curve C represents the energy spectrum of the gamma rays after they have traveled an additional small distance through the rock. However, some of the most scattered gamma rays now have energies close to 0.2 MeV. Once below the 0.2 MeV threshold, the gamma ray can be completely absorbed by the atoms in the rock, and there is a given probability that this will occur depending upon whether the soft gamma ray encounters an electron under the correct conditions. This is called photo-electric absorption, and is a completely different process from compton scattering. The result is that gamma rays attaining energies less than 0.2 MeV are eaten away from the energy distribution. Curve D in Fig. 14.1 shows the energies of the
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Chapter%2014 - Petrophysics MSc Course Notes 14 The...

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