Page 187 - Introduction to Petroleum Engineering

174 WELL LOGGING
There have been many variations in the design of resistivity logging tools. The
variations can be placed into two categories: electrode tools and induction tools.
Current from an electrode in an electrode tool travels through the formation and back
to another electrode. “laterolog” is the name for the modern variation of electrode
tools. By contrast, induction tools have two sets of wire coils: coils for generating
magnetic fields that induce current in the formation and coils for sensing the magnetic
field produced by the induced current in the formation. Historically, electrode
tools were used for water‐based drilling fluids, while induction tools were used for
oil‐based drilling fluids. Those restrictions have faded with modern designs. Some
laterologs can measure formation resistivity of cased holes.
The depth of investigation varies with the design of the electrodes in a laterolog
or design of the coils in an induction tool. Most tools can simultaneously measure
shallow, medium, and deep resistances as shown in Figure 9.6. Some tools, “array”
tools, report measurements at five depths from about 1 up to 7 ft.

9.5 OTHER TYPES OF LOGS

In the previous sections, we have highlighted the most commonly used logs. A few
other logs are introduced here. These logs illustrate the variety of logs that are being
developed and used to meet industry challenges.

9.5.1 borehole Imaging
Borehole imaging techniques are designed to produce centimeter‐scale images of the
rocks comprising the borehole wall. They include optical, acoustic, and electrical
imaging techniques. Consequently, borehole imaging is not used in a cased hole.
Borehole images can be used to identify fractures and fracture orientation, observe
structural and stratigraphic dip, detect breakouts in the borehole wall, analyze small‐
scale sedimentary features, and assess net pay in thin beds.

9.5.2 Spectral Gamma‐Ray Logs
Uranium, thorium, and potassium are the main sources of natural gamma radiation.
In the standard gamma‐ray logging tool, gamma rays from all three elements and any
other radioactive elements are combined. The energy, or wavelength, of gamma rays
from these elements differ and can be separately sensed. Spectral gamma‐ray sensors
can distinguish radiation from uranium, thorium, and potassium. This information
can be useful for analysis of some formations.

9.5.3 Dipmeter Logs
A dipmeter is designed to determine the direction and angle of dip of rock strata
adjacent to the borehole. The spatial orientation of a plane requires measuring
the elevation and geographical position of at least three points on the plane.