Resistivindex
Resistivindex, commonly known as resistivity index, is a dimensionless parameter used in petrophysics and reservoir geology to describe how a rock’s electrical resistivity changes with fluid saturation and salinity. It is defined as the ratio of the rock’s resistivity at a given state (R_t) to its resistivity when the rock is fully water-saturated (R_o): I = R_t / R_o. The value is sometimes abbreviated as RI.
R_t is the resistivity measured under the conditions of interest, typically on core samples saturated with
In formation evaluation, resistivity index is used in conjunction with Archie’s equations to estimate water saturation
Limitations include its reliance on Archie's assumptions, which are best met by clean sandstones. Clay minerals,