Page 249 - Drilling Technology in Nontechnical Language

240 Drilling Technology in Nontechnical Language Second Edition

can identify various lithologies, pore fluids, hydrocarbon presence, and

other characteristics.
Electric logs are normally run on a special wireline that contains

electrical conductors within it. In difficult wellbore conditions (such as
high inclination, rough wellbore, or potential sticking conditions), the
logging tools can be attached to the bottom of drillpipe or coiled tubing
with the electrical cables run inside the pipe. Logging tools are also
available that either transmit the information to the surface using pressure
pulses in the mud while drilling or can record information within the tool
for downloading to a computer at the surface. These will be described in
more detail later in this chapter. The operating principles of the various
tools still apply, but the tools are run in the hole.
Below are described the major classes of electrical logging tools and
the basic operating principles of each.

Resistivity and induction tools

The electrical resistivity of a formation is related to the amount of
water contained within the formation (due to the porosity) and the electrical
resistivity of the formation water. Most sedimentary rocks do not conduct
electricity when no water is present within the rock.
Resistivity logs. Measure resistivity directly by passing a current
between electrodes touching the formation. This requires a conducting
mud (i.e., water based, not oil or gas based) to work.
Induction logs. Measure formation resistivity indirectly by inducing

an electrical flow in the formation using a coil and measuring the induced
current with another coil. Induction logs work well in oil-based mud, which
resistivity logs do not.
Microresistivity tools. Measure electrical resistance with a very
fine resolution at several different places around the circumference of the

wellbore. These can produce a color-coded picture of the wellbore with the
color varying by resistivity. It is possible to measure the dip of a formation
with a microresistivity tool, and as these tools also incorporate a north-
sensing tool, the direction that the formation dips can also be measured.
Sometimes, fractures can be recognized from the resulting images.

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