One of the most meaningful ways to obtain reservoir fluid saturations is by means of down-hole, wire-hole, and electrical resistivity measurements. In this technique the electrical resistivity of the formation is directly measured by means of a down-hole logging tool. These measurements are then related to down-hole saturations by means of Archie's equations. In order to apply Archie's equations, appropriate values for the a, m and n constants must be determined.

At AGAT, both the formation resistivity factor and hydrocarbon resistivity index are determined for reservoir rock.

A number of core plugs are chosen such that they represent the range of porosities typical for the reservoir. Each plug is cleaned, and its porosity, gas permeability and grain density are determined. The plugs are saturated with deaerated reservoir brine, sealed in plastic sleeves and raised to the temperature of the reservoir. Net overburden pressure is applied to the plastic sleeves and the resistivities of the core plugs are measured. The resistivity of the reservoir brine is measured at reservoir temperature and the formation resistivity factors are calculated.