Fundamentals of Fluid Flow in Porous Media
Formation Resistivity Factor:
Resistivity Index (RI)
Another fundamental electrical property of porous rocks is the resistivity index, RI. It is defined as:
Where Ro is the resistivity of the sample saturated with electrolyte (salted water) and Rt (true resistivity) is the resistivity of the sample which is saturated with electrolyte and hydrocarbon. As previously mentioned hydrocarbons are non-conductive materials. So always Rt ≥ Ro → RI ≥ 1. SI value decreases by increasing the water saturation till unity at fully water saturation. Various empirical correlations have been found that relate the RI to water saturation in the medium. They are of the form
Where n is the resistivity index exponent (also known as saturation exponent) and c is some function of tortuosity. Archie’s correlation gives n = 2.0 for consolidated sands, while William’s correlation gives n = 2.7.
Combination of equations (2‑50), (2‑51) and (2‑55) yields:
According to equation (2‑56) initial water saturation of the formation could be predicted by knowing the value of Rt, Φ and Rw. Usually Rt and porosity evaluated by the log data and Rw calculated according to the formation water salinity.