Induction logging vertical

At the end of the 1960 s serious attention was paid to other modifications to induction logging. One of them is based on the use of transient fields, when measurements are performed in the absence of the primary magnetic field (Kaufman and Sokolov, 1972). The study of the secondary fields, caused by induced currents in a medium with either cylindrical or horizontal interfaces allowed one to describe the radial and vertical responses of the two-coil probe and find the most optimal range of time for measurements. 

It is appropriate to notice here that in logging methods based on application of direct current the direction of current lines depends on the relation between resistivities of borehole, invasion zone, and bed. At the same time in induction logging, when the source of the field is a vertical magnetic dipole located on the borehole axis, current lines, regardless of the distribution of resistivity in a radial direction, present themselves as circles located in horizontal planes with their centers on the borehole axis, and they do not intersect the cylindrical surface between media with different conductivity. 

Usually in induction logging the vertical component of the magnetic field is measured on the borehole axis (r = 0). In accord with eqs. 5.23-5.25 we obtain equations for this component expressed in units of the primary field ... 

Besides, improvement of the vertical response of a multi-coil induction probe due to measurements at higher frequencies allows us to apply induction logging in a more resistive medium, inasmuch as the ratio between the quadrature component of the electromotive force and that of the primary field increases. 

Track 4 compares the saturation-calculation results of a conventional shaly sand analysis using standard induction log data (which is essentially measuring horizontal resistivity) to a 3DEX- (tensor) based laminated shaly sand analysis that incorporates vertical resistivity as well. Water saturation in the pay zone has been reduced to approximately 25%. It means that an additional 40-50 ft. of pay has been identified and quantified by having vertical resistivity from 3DEX and using it in the petrophysical analysis. 

In highly deviated wells the readings of conventional induction tools depend on resistivity anisotropy, which was undetectable in vertical wells. To perform the interpretation of logs in horizontal wells it is therefore important to have a modeling code making it possible to take anisotropy into account. 

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