1.
n. [Formation Evaluation]
The value
a in the relation of
formation factor (F) to
porosity (
phi): F =
a / phi^{m}. The value
a is derived empirically from best fits of measured values of F and
phi on a group of
rock samples. It has no clear physical significance, although it has been related to grain shape and tortuosity. In the
saturation equation, it always occurs associated with the water resistivity as (
a * R
_{w}). It is sometimes claimed that
a must be 1 since at
phi = 1, F must be 1. However, a material with
phi = 1 is not a rock:
a is essentially an
empirical factor for rocks and as such can take any value. A wide range of values has been found, from 0.5 to 5.