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 formation water or oil, while R_o represents the resistivity of the same rock when saturated with water at 100 percent saturation. The resistivity index increases as factors that hinder electrical conduction rise, such as the replacement of conductive brine by hydrocarbons or the presence of low-salinity formation water, which reduces conductivity. In practice, I is influenced by pore geometry, porosity, brine salinity, temperature, and fluid saturation.

In formation evaluation, resistivity index is used in conjunction with Archie’s equations to estimate water saturation and to interpret log responses. For example, the Archie relation Sw = (a * R_o / (phi^m * R_t))^(1/n) connects resistivity measurements to water saturation, where a, m, and n are empirical constants. A higher RI typically indicates reduced conductive pathways (e.g., oil-bearing zones or higher salinity contrasts), aiding differentiation between hydrocarbon-bearing and water-filled intervals.

Limitations include its reliance on Archie's assumptions, which are best met by clean sandstones. Clay minerals, cation exchange, surface conduction, temperature effects, and unconventional lithologies can distort RI measurements. Consequently, resistivity index should be used with other petrophysical data for robust reservoir characterization.