Page 257 - Petrophysics 2E

230 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

100
so

~___~
1.0 0.05 0.1 0.5 1.0
Water Saturation (Sw): Fraction

Figure 4.17. Effect of clay conductance on resistivity index (courtesy of Core
Laboratories).

salinity and the amount of formation water. IR is also a function of the
amount, distribution, and type of clays present in the formation being
evaluated. For instance, the presence of cation exchangeable clays, such
as smectites, typically cause low IR values to be observed, whether
or not the rock contains hydrocarbons as illustrated in Figure 4.17.
The saturation exponent n is determined experimentally by saturating
a core sample with brine and measuring the rock resistivity &.

Then the brine is displaced with air, naphtha or live crude oil, and
the true resistivity R,, is measured after each increment of displacement.
The water saturation S, is determined by measuring the volume of water
produced and applying the material balance equations.
A plot of the ratio Rt& versus the water saturation on logarithmic
scale gives a straight line of slope -n, as illustrated in Figure 4.18 and the
following equation:

(4.55)

and

logIR = -nlogSw (4.56)

The slope of the straight line passing through S, = 1 when IR = 1 is
(-n), which can be calculated as follows:

(4.57)