Page 147 - Petroleum Geology

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(Fig. 6-9). This is applied to the wall of the borehole by a spring-loaded arm
that acts also as a Caliper (the hole diameter is thus also logged and record-
ed). When lowered down the hole with the caliper closed, the true mud
resistivity in the hole is measured and logged. Into the relatively impermeable
beds there is virtually no invasion by mud filtrate, and the two devices
measure essentially the same resistivity. Opposite permeable beds, where in-
vasion has led to the formation of a filter cake on the wall of the borehole,
the one-inch Inverse measures the resistivity of the mud cake (mostly) while
the two-inch Normal measures the resistivity of the mud cake and part of
the invaded zone. These two resistivities normally differ, so the two curves
on the log are separated. The caliper log opposite permeable beds may show
a hole diameter slightly less than the diameter opposite the relatively imper-
meable beds because of the mud cake, and it may even be less than the
nominal diameter of the hole.
The Microlog can therefore be used qualitatively to identify and measure
the thickness of the permeable parts of the sequence penetrated by the bore-
hole. Quantitative analysis leads to estimation of the Formation Factor,
through eq. 6.9, with all the reservations of that discussion and the discus-
sion of eq. 6.8 from which it was derived.

Fig. 6-9. Electrode arrangement of the Microlog.