Archie's assumption of saturation exponent equal 2 is valid only when the reservoir and the core are strongly water wet, the saturation exponent has been proven by many researchers, its strongly related to the reservoir wettability, pore-size distribution and displacement history and it can vary between 2 to 10 in value.

Determination of fluid saturations from electric well logs adopts a calculation procedure, which is highly sensitive to several parameters one in a particular saturation exponent (n). An accurate determination of initial oil in place in the early life of reservoirs or an evaluation of a developed reservoir is requires high accuracy water saturations (Sw) values.

This paper presents innovative petrophysical workflow using water saturation, true resistivity and reservoir total porosity values as inputs data; the geological zonation is critical; each selected zone will be cross-plotted to evaluate cementation exponent through Pickett plot and the irreducible water saturation factors using Sw-Phi crossplot. The mathematical derivation was done to develop a relation between the true formation resistivity (Rt) and the reservoir porosity in the irreducible water saturation intervals to predict saturation exponent utilizing the crossplot.

Case studies from three different giant oil fields located in the south of IRAQ have been selected to represent various applications scenarios of carbonate and shaly-sand oil-bearing formations to test the applicability of the proposed model.

Each field has a different set of data and requires a special treatment, varied from special and convectional core analysis, pressure test and NMR to only fullset of wireline data.

A quality verification for Archie saturation exponent "n" values has been done quantitatively with core analysis and qualitatively with in-situ and/or core wettability.

The proposed model success to evaluate saturation exponent for the studied reservoirs and show convergence with the verification methods.

It is concluded that the developed petrophysical workflow of this study provides a significant contribution to the determination of water saturation exponent (n) in simplified-robust way and consequently leads to better water saturation (Sw) estimation values, and it can be applied in any carbonate or shaly-sand reservoirs worldwide.

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