# Archie equation

There are two equations that Archie (1942) proposed that may be combined into a single equation, which is generally know as "the Archie equation" for determining water saturation.

The formation volume factor *F*, is defined as

The resisitivity index *I*, is defined as

the ratio of the actual resistivity of the formation, *R*_{t}, and the value that is should have if its pore space is completely filled with formation water, *R*_{0}.

In this form, the desired, but unknown water saturation *S*_{w} may be solved from log measurements of porosity , and formation resitivity *R*_{t}, when values are known for the cementation exponent *m*, a value for *a* if other than unity, the saturation exponent, *n*, and the formation water resistivity *R*_{w} at formation temperature.

In general, *n*, the saturation exponent is taken to be 2, and *a* to be 1. Cementation exponent varies from 1.5 - 2.5.

## External links

- Archie's law — Archie's equation on Wikipedia

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