3 IntroductionElectrical resistivity; a geophysical method= R. (A/L)
4 Factors affecting resistivity Soil typeFrequency of the currentDegree of saturation & ionic concentrationHydrocarbon saturationtemperature
5 Uses of resistivity measurements Resistivity of soil related toCompressive strengthElastic modulusDensityDegree of SaturationMax past pressureHydraulic conductivity
6 Archie’s Law Law relating Formation factor FF and porosity FF = Ro/Rw= a.-mFF describes pore geometry;removes the effect of pore fluid resistivityValues of the reservoir constant ‘a’ and the cementation exponent ‘m’
7 Factors affecting ‘m’ Shape of grain; angularity increases m Type of grain; clay high ‘m’, sand low ‘m’Tortuosity; higher tortuosity higher ‘m’AnisotropyOverburden pressure; higher pressure flattens the grains and angularity increases
8 Limitations of Archie’s eqn F= a/mArchie assumed a=1 and ‘m’ empirically found as 2.For = 0, F=QThe equation does not consider water saturation effect or the shale effect
12 ConclusionsThough the values of ‘a’ and ‘m’ influence the values of the estimated porosity, it has been now agreed that the water saturation Sw exerts more significant influence on the values of porosity and also the formation factor.Research should focus on methods for accurate estimation of Sw.
13 ConclusionsThe clay presence reduces resistivity causing overestimation of Sw. This may result in oil companies missing oil wells.Accuracy of Resistivity measurements is questionable. Inclined penetration of the probe combined with strong porosity gradients result in spurious high resistivity.