1887
Volume 4, Issue 3
  • ISSN: 0263-5046
  • E-ISSN: 1365-2397

Abstract

In exploration, seismie methods are used to locate interesting formations. Well logging measurements are often relied upon to make the quantitative determination of formation petrophysical properties in these formations. One important formation property is porosity, which can be estimated using acoustic or nuclear measurements. The nuclear radiation scattering techniques used for its estimation employ gamma ravs or neutrons. Thc motivation for the use of gamma rays for the determination of formation porosity is straightforward. The electron density of the formation affects the scattering and transmission of gamma rays in a well known manner: the logarithm of the gamma ray flux at the detector depends linearly on the reciprocal of the electron density. A measurement of the gamma ray flux yields the density. To convert the density measurement of the formation into porosity, the appropriate electron density of the rock matrix must be known as well as that of the formation fluid, although this second factor is of somewhat less importance than the former. Therefore it is important to know the rock lithology in order to use the appropriate matrix density for extracting the correct porosity value from the density measurement. The application of neutron scattering to determine porosity is somewhat more complicated due to the nature of neutron interactions with matter. In porosity logging applications, the most important phenomenon is elastic scattering of the neutrons with the formation nuclei. In this process the kinetic energy of the neutron is reduced at each scattering depending on the scattering angle and, more importantly, the mass of struck particle: the closer its mass is to the neutron mass, the more efficient the collision can be in reducing the neutron energy. For this reason, hydrogen is the most efficient moderator of neutrons in earth formations. A determination of the moderating efficiency of a formation should be related to its hydrogen content. A measurement of the spatial distribution of multiply scattered low energy neutrons is sufficient to characterise the moderating efficiency of the formation. As in the case of density logging, the accurate interpretation of the neutron measurement depends on a knowledge of the lithology of the formation. However, unlike the density measurement, the nature of the fluid filling the pores is quite important. In this artiele the measurement principles of the neutron porosity device will be reviewed and related to some useful formation parameters. The interpretation of the measured parameters in terms of porosity will be discussed and shown to be dependent on the rock lithology, the pore fluid composition and the presence of shales or c1ay minerals.

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/content/journals/10.3997/1365-2397.1986005
1986-03-01
2024-03-28
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  • Article Type: Research Article
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