1.
n. [Formation Evaluation]
The exponent,
n, in the relation of water
saturation, S
w, to
resistivity index, I (I = S
w-n) for a sample of
rock. It expresses the effect on the
resistivity of desaturating the sample, or replacing water with a non-conductive fluid. In petrophysically simple,
water-wet rocks (Archie rocks),
n is constant for different values of S
w, and a single average
n can be found for a particular
reservoir or
formation. A typical value is 2. In more complex rocks,
n changes with S
w, although often being about 2 near S
w = 1. In rocks with conductive minerals, such as
shaly sands,
n becomes increasingly lower as S
w is reduced. This change is negligible for high-salinity waters, but increases as the salinity is reduced. In shaly-
sand saturation equations, such as Waxman-Smits,
dual water, SGS and CRMM,
n is the intrinsic
n, determined with high-salinity water or with the
clay effects removed. The variation of I with S
w is then predicted, with varying success, by the different equations. In carbonates with multiple
pore types, such as fractures, vugs, interparticle
porosity and
microporosity,
n may change as each pore type is desaturated. A different
n may be used for a different range of S
w. In all cases,
n increases if any pores are
oil-wet. Values up to 8 have been reported in very oil-wet rocks.