Page 68 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

56 Reservoir Engineering

As the water saturation is reduced toward zero, the resistivity approaches that
of the clays rather than approaching infinity as with clean sands. Relationships
between resistivities and fluid content in the presence of conductive solids have
been presented in the literature [78,79].
Early investigations, using data from the Woodbine sand of east Texas,
suggested that the saturation exponent, n, may range from 2.3 to 2.7 [80,81].
Wyllie and Spangler [82] presented data (see Figure 5-38) for several natural
and synthetic porous media that showed a variation of the saturation exponent
from 1.4 to 2.5. Other investigators found that the distribution of fluids within
the core sample, at the same water saturation, could affect the resistivity index
for both sandstones [83] and for limestones [84]. The exponent, n, was also
found to vary depending upon the manner in which the conducting wetting
phase saturation was varied [82,85].

I

H
X
W
0
E
>-
k
1
t-
E
v)
W
a

0.4 0.6 0.0
FRACTIONAL WATER SATURATION, Sw
Figure 5-38. Relationship between resistivity index and water saturation for
several media [82].