Two electrodes installed on a microresistivity measuring device measure resistivity values of a borehole at very small intervals. The microlog detects permeable zones based on differences in resistivity measurements from two electrodes with different depths of investigation. A positive separation between the measurements indicates an invaded, and thus permeable, formation where the mudcake is present. The microlog provides high resolution identification of permeable intervals for qualitative evaluation, though not quantitative permeability measurements.

1. TOPIC
MICRORESISTIVITY LOG

2. MICRO-RESISTIVITY LOG
Micro log is a resistivity log of a bore
hole which Is measured by two
electrodes that are installed on a pad of
a microresisitivity measuring device.

3. PRINCIPLE
• All micro-resistivity logs are based on the application of Ohm's Law.
• The flexible oil-filled microlog pad is pressed against the borehole wall by
arms and springs.The face of the pad has three small in-line electrodes
spaced 1” in.The electrodes record "microinverse" log and "micronormal" log
simultaneously.
• In an invaded permeable zone, Rmc is usually significantly lower than Rxo.The
micronormal device has a greater depth of investigation than the
microinverse. It is, therefore, less influenced by the mudcake and reads a
higher resistivity when mudcake is present.
• In impermeable formations, the two curves read approximately the same
resistivity or have a small negative separation, and the resistivities are
usually much greater than in permeable formations.

4. • Positive separation occurs in a permeable zone. Although the microlog curves
identify permeable formations, quantitative inferences of permeability are not
possible.
• Under favorable circumstances, Rxo values can be derived from microlog
measurements using charts provided by the service companies.
• Rmc values for this purpose can be measured directly or estimated from other charts,
and h mc is obtained from comparing the caliper curve to bit size.The limitations of
the method are as follows:
• The ratio Rxo/Rmc must be less than approximately 15 (porosity more than 15%).
• The value of hmc must be no greater than 0.5 in. [1.3 cm].
• Depth of invasion must be greater than 4” in.Otherwise, the microlog readings
are affected by Rt.

6. INSTRUMENTATION
Two electrodes are used in micro resisitivity log

7. UNIT
The unit of micro-resisitivity log is Ohm-m

8. USES
• The microlog is still used qualitatively for its ability to detect
permeable intervals with a fine vertical resolution, but not for the
evaluation of Rxo.
• The measurement comprises two short-spaced devices with different
depths of investigation, providing resistivity measurements of small
volumes of mudcake and formation adjacent to the borehole.
• The presence of a mudcake, identified by a "positive" separation of
the two curves, indicates an invaded and, therefore, permeable
formation.
• As a qualitative log, the microlog is usually presented on a linear
grid.