Resistivity Logs

By Richard Yeomans

Resistivity logs are an important - if not imperative - part of drilling and reservoir engineering. They are a method of characterising reservoir rocks by measuring their electrical resistivity.

Rocks by nature are basically insulators so electrical current finds it hard to flow through them. The formation fluid however is a conductor and electrical current is easily able to flow through it. It should be noted that when the formation fluid has high concentrations of hydrocarbons these act essentially as an 'infinite' resistor. This key property difference between the formation fluids allows us to identify potential hydrocarbon baring formations.

Fig 1: Resistivity of Formation (Archie's Equation)

Where n, m and a are empirically determined parameters, Rw is the fluid resistivity, Sw is the water saturation. Rt is the total resistivity observed of the formation .

It is important to also realise that the resistivity log will also be effected by the relative porosity of the formation. If the formation is tight and highly compacted the resistivity log will also show largely high and can be mistaken for a potential HC zone if not calibrated properly. We should always compare and relate to the density log (from which we can determine porosity from) before drawing any immediate conclusions about the hydrocarbon potential.