Chapter 15. pdf

Table 15.1
Hydrogen index calculations for some reservoir minerals and fluids.
Compound
Formula
A
i
n
i
n
H
ρρ
b
HI
Pure water
H
2
O
16
1
2
1.000
1.000
Oil
(CH
2
)
x
12
1
2
0.780
1.003
Methane
CH
4
12
1
4
ρ
m
2.25
×
ρ
m
Gas
C
1.1
H
4.2
12
1.1
4.2
ρ
g
2.17
×
ρ
g
Quartz
SiO
2
28, 16
1, 2
0
2.654
0.000
Calcite
CaCO
3
40, 12, 16
1, 1, 3
0
2.710
0.000
Gypsum
CaSO
4
.2H
2
O
40, 32, 16
1, 1, 6
4
2.320
0.4855
Note the following:
•
If the tool is in 100% water (a large tank of the stuff, say), the 
HI 
= 1.000. This is equivalent to a
rock of 100% porosity saturated with water. So we have a fixed point that 
HI 
= 1 represents 
φ
 
= 1.
•
If the tool is in a pure limestone rock with zero porosity, the 
HI 
= 0.000, because there are no
hydrogen atoms in the solid matrix of calcite. So we have a second fixed point where 
HI 
= 0
represents 
φ
 
= 0.
•
If the tool is in a pure limestone with a given porosity 
φ
, the hydrogen index will be directly
proportional to the amount of water in the formation. Hence 
HI 
= 
φ
for completely water saturated
limestones.
Thus the hydrogen index is a proxy measure of porosity if the solid minerals of the formation contain
no hydrogen and if the pores are completely saturated with water. But, it is the hydrogen index that
controls the count rate observed by the neutron tool. Hence we have a tool that can measure porosity in
water saturated formations where the matrix minerals contain no hydrogen.
In practice, it is not only hydrogen that affects the passage of neutrons in a formation. As we can see
from Fig. 15.2, other rock forming elements have an effect on the slowing down of neutrons in the
formation, but their effect is small compared with hydrogen, and when it comes to neutron absorption,
both chlorine and hydrogen have a part to play. The small errors that are introduced by assuming that
it is just hydrogen that affects neutrons are overcome by calibrating the tool. The tool is calibrated in
pure limestone, and hence does not give true porosities. Instead it gives equivalent porosities that
would occur if the rock was measuring limestone. This will be discussed in further detail in Section 4.