Chapter 15. pdf

The Hydrocarbon Effect.
Note in Table 15.1 that the hydrogen indices of water and oil are
approximately the same because the effect of their different compositions on the partial concentration
of hydrogens per unit volume is approximately balanced by the difference in their density.
As with the formation density tool, the zone of investigation of the neutron tool is constrained to the
flushed zone. Hence, the tool will measure the hydrogen index associated mainly with the mud filtrate,

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 155
together with whatever remains of the hydrocarbons and formation water in the formation. Imagine,
for example that we are in an oil zone in a limestone, the mud filtrate almost completely replaces the
formation fluids such that there is a saturation of mud filtrate 
S
XO
, and a residual saturation of
hydrocarbons (1- 
S
XO
). The porosity read by the neutron tool is related to the actual porosity in the
formation by
(15.5)
If the hydrocarbon is oil, this equation reduces to 
φ
N 
≈
φ
, because, as we have seen in Table 15.1 the
hydrogen index of water and oils is similar; 
HI
mf 
≈
HI
hc
. This is true whether the mud filtrate is oil-
based or water-based.
(15.6)
If the hydrocarbon is methane gas with a density of 0.1 g/cm
3
, the equation reduces to
(15.7)
If the saturation in the flushed zone 
S
XO
= 0.7, Eq. (15.7) reduces to 
φ
N
= 0.77 
φ
. This is known as the
hydrocarbon effect in the neutron log
.