Resistivity
Calibration Technique for Micro-
Resistivity
Image Logs: Applications for Thin-Bedded, Shaly Sand Reservoirs, Gulf of Mexico
GLOBE, MICHAEL J., WAYNE K. CAMP and ROGER D. HAIRR
A new technique is presented for calibrating resistivity
measurements
recorded by micro-
resistivity
image devices. This technique involves converting
the recorded electrical power measurements to a scaled true
resistivity
. The
scaled micro-
resistivity
curve represents an average of multiple sensors on a
given pad or pad and flap combination. This scaled average curve agrees well
with standard electric log shallow
resistivity
measurements over equivalent
intervals. The scaled image log allows for accurate and quantitative
resistivity
comparisons that is not possible with the standard processed image logs.
Pseudo-color borehole images were generated from the scaled micro-resistivity
curves and compared with core data from two offshore Gulf of Mexico fields to
evaluate their reliability. Dynamic normalization image enhancement techniques
resulted in greater resolution of
apparent
stratigraphic details in the
uncalibrated images. Dynamic normalized uncalibrated images may reflect real
resistivity
variations in the well bore, resolving subtle textural variations in
grain size and clay morphology, and thus may be preferred for certain log
interpretation applications. The calibrated images exhibited better correlation
to lithologic features observed from the cores. The scaled images allow for
quantitative
resistivity
variations which may be useful in characterizing
hydrocarbon saturations within the flushed zone of the wellbore. The scaled
images are preferred for net pay and net sand counts within thin-bedded shaly
sand reservoirs.