The relation of formation factor to resistivity of formation
water and intergranular permeability has often been investigated,
and the general consensus is that this relation is closest when
established in a clean-sand aquifer in which water quality does
not vary substantially. When these restrictions are applied, the
following standard equation is a useful tool in estimating the
resistance of the formation water: F = Ro/Rw, where F is the
formation factor, which is a function of the effective porosity;
Ro is the resistivity of a formation that is 100 percent saturated
with interstitial water; and Rw is the resistivity of the water in
the saturated zone. However, arenaceous aquifers can have
electrical resistivities that are not directly related to
resistivity of water or porosity. Surface conductivity and ion
exchange are significant factors when the sediments are clay
bearing. The solid constituents are a major component of the
parameters needed to solve the equation for formation-water
resistivity and estimates of aquifer permeability. A correction
process needs to be applied to adjust the variables, Ro and F, to
the equivalent of clean sand. This report presents an empirical
method of using the neutron log and the electrical-resistivity
values from long- and short-normal resistivity logs to correct
for fine-grained material and the subsequent effects of low
impedance to electrical flow that are not related to the
resistance of formation water.
