Page 258 - Petrophysics 2E

FORMATION RESISTIVITY FACTOR 23 1

0.001 0.01 0.1 1.0
Water Saturation (Sw). Fraction
Figure 4.18. Resistivity index us. water saturation for a range of measured saturation
exponents (courtesy of Core Laboratories).

The value of n is affected by wettability, overburden pressure, nature and
microscopic distribution of the reservoir fluids, and types and amounts
of conductive clays (or measuring the slope directly from the graph).
Anderson examined the effects of wettability on the saturation exponent
and found that [ 131 :

n is essentially independent of wettability when the brine saturation
S, is sufficiently high to form a continuous film on the grain surfaces
of the porous medium and, consequently, to provide a continuous
path for a current flow. This continuity is common in clean and
uniformly water-wet systems. The value of the saturation exponent
n in these systems is approximately 2 and remains essentially constant
as the water saturation is lowered to its irreducible value, hi.
In uniformly oil-wet systems with low brine saturations, large values
of the saturation exponent, 10 or higher, should be expected.

Table 4.5 shows what typically occurs in an oil-wet core as the water
saturation drops [13]. An examination of this table shows that below
a certain brine saturation, the exponent n begins to increase rapidly.
For instance, in the case where the non-wetting brine saturation is
reduced by oil injection, the value of n increases from 4 to 7.15 as S,
drops from 34.3 to 33.9. This rapid rise of n in oil-wet systems, as the
brine saturation decreases, is due to an increase in the resistivity of the
system. The resistivity increase is due to disconnection and eventual
isolation and trapping of a portion of the brine by oil. This portion