Chapter 15. pdf

15.10
 
Uses of the Neutron Log
15.10.1
Determination of Porosity
The main use of the neutron log is to provide porosity information.
The tool is sensitive to the amount of hydrogens in the formation and to a less extent upon other
elements. It is assumed that the contribution to the measurement by elements other than hydrogen is
negligible, and that the contribution to the measurement from hydrogen comes entirely from the fluids
fully occupying the pore space.
However, in real rocks elements other than hydrogen that exist in the rock matrix do contribute to the
measurement (e.g., chlorine in formation water, mud filtrates and in some evaporite formations), and
hydrogen is present in the matrix (e.g., bound water in shales).
The problem is partially overcome by calibrating the tool to give the porosity in limestone units. Pure
limestone saturated with fresh water is chosen because it contains no elements which contribute
significantly to the measured signal other than hydrogen.
The porosities that are read by the tool are accurate in limestone formations that contain fresh water.
The porosities that are read by the tool in other lithologies or with other fluids need to be corrected by
a chart such as that shown in Fig. 15.8.
There are three effects that are not corrected for in the log data, that need to be briefly mentioned.
The Hydrocarbon Effect.
The presence of hydrocarbon liquid (oil) does not effect the tool response
as it has approximately the same hydrogen index as fresh water. Hydrocarbon gas, however, has a
much lower hydrocarbon index resulting from its low density, and its presence will give rise to
underestimations
in porosity (Fig. 15.9).

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 163