Petroleum Engineers

49-16 PETROLEUM ENGINEERING HANDBOOKSPOHlANtOUIPOItNllAlmillivolts: ::::,:i lid : .-.- __-*.j-r.L!-8 _: * :.;..:.:.v; ‘1, +)>+.h i :..__::..~~ 3-N -!:: :t-.,:. ::‘: :::Ij y;*: ::+-:..::: 7’:I.. ::77,,.I:1..:,:: :+:.::~ :. ‘:Fig. 49.21~ISFkonicpresentation.Fig. 49.20-DualInduction Laterolog 8 presentation.directly from the logarithmic scale. The linear scale ismore easily correlated with earlier logs. This format hasgained acceptance as the standard for resistivity logs.The ISF log in combination with the sonic log requiresa modification of this grid usage because Track 3 isneeded for the sonic At curve. The grid selected is shownin Fig. 49.21.Skin EffectIn very conductive formations the induced secondarycurrents are large, and their magnetic fields are important.The magnetic fields of their ground loops induceadditional EMF’s in other ground loops. This interactionbetween the loops causes a reduction of the conductivitysignal recorded by the induction log. This signal reductionis known as “skin effect.”Induction logs usually are automatically corrected forskin effect during recording. The correction is based onthe magnitude of the uncorrected tool response treated asif it were from a homogeneous medium. A secondaryskin-effect correction may be required when the mediasurrounding the sonde are not of uniform conductivity.Such corrections are incorporated in various interpretationcharts.Geometrical FactorWhen conductivities are not high, skin effect may beneglected, and the response of induction logs can bedescribed in terms of conductivities and “geometricalfactors” of the volumes surrounding the tool. Thegeometrical factor, G, of a volume having a specificgeometrical orientation with the sonde is simply the fractionof the total signal that would originate with thatvolume in an infinite homogeneous medium. For computationof geometrical factor to be practical, it isnecessary to assume that the volumes conform to symmetryof revolution about the sonde.The magnitude of the signal in conductivity units is theproduct of the geometrical factor and the conductivity ofthe material, and the total signal sensed by the tool is the