Resistivity is the ability of a substance to impede the flow of an electrical current.

This is a very important rock property in formation evaluation as it helps to differentiate between formations filled with salty waters (good conductors of electricity) and those filled with hydrocarbons (poor conductors of electricity).

Hence, a difference in resistivity exists between rocks filled with hydrocarbons and those filled with formation water.

Resistivity and porosity measurements are used to obtain values of water saturation to help evaluate producibility of the formation.

Two categories of resistivity logs are: induction and laterolog.

The laterolog tools use electrodes to inject a current on the formation and to measures voltages at different points in the tool.

The induction tools use coils and magnetic fields to develop currents in the formation whose intensity is proportional to the conductivity of the formation.

The intensity of these currents is measured on receiver coils in the tool.

Applications

•Rt, Rxo and Di in invaded formations

•Rv and Rh in anisotropic formations

•High-resolution Rt in deviated wells and/or dipping beds

•Pre-invasion Rt measurement in deeply invading formations

•Pre-washout Rt measurement in unconsolidated formations

•LWD replacement for wireline array resistivity tools