Fundamentals of Fluid Flow in Porous Media

 

Chapter 2

Formation Resistivity Factor:

Resistivity Index (RI)

Another fundamental electrical property of porous rocks is the resistivity index, RI.  It is defined as:

Where R_o is the resistivity of the sample saturated with electrolyte (salted water) and R_t (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 R_t ≥ R_o → 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 R_t, Φ and R_w. Usually R_t and porosity evaluated by the log data and R_w calculated according to the formation water salinity.

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<< EFFECT OF CONDUCTIVE SOLIDS

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MULTI-PHASE SATURATED ROCK PROPERTIES >>

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References

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Questions?

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