Page 15 - Well Logging and Formation Evaluation

Basics 5

limitations placed by drilling, whose ability to steer the well and achieve
a high ROP is influenced by the placement of the LWD toolstring. LWD
data that may typically be acquired include the following:

• GR: natural gamma ray emission from the formation
• Density: formation density as measured by gamma ray Compton scat-
tering via a radioactive source and gamma ray detectors. This may also
include a photoelectric effect (Pe) measurement.
• Neutron porosity: formation porosity derived from the hydrogen index
(HI) as measured by the gamma rays emitted when injected thermal
or epithermal neutrons from a source in the string are captured in the
formation
• Sonic: the transit time of compressional sound waves in the formation
• Resistivity: the formation resistivity for multiple depths of investiga-
tion as measured by an induction-type wave resistivity tool

Some contractors offer LWD-GR, -density, and -neutron as separate
up/down or left/right curves, separating the contributions from different
quadrants in the borehole. These data may be extremely useful in steer-
ing horizontal wells, where it is important to determine the proximity of
neighboring formation boundaries before they are actually penetrated.
Resistivity data may also be processed to produce a borehole resistivity
image, useful for establishing the stratigraphic or sedimentary dip and/or
presence of fractures/vugs.
Other types of tool that are currently in development for LWD mode
include nuclear magnetic resonance (NMR), formation pressure, and shear
sonic.

1.2.2 Wireline Openhole Logging

Once a section of hole has been completed, the bit is pulled out of the
hole and there is an opportunity to acquire further openhole logs either
via wireline or on the drillstring before the hole is either cased or aban-
doned. Wireline versions of the LWD tools described above are available,
and the following additional tools may be run:

• Gamma ray: This tool measures the strength of the natural radioactiv-
ity present in the formation. It is particularly useful in distinguishing
sands from shales in siliciclastic environments.
• Natural gamma ray spectroscopy: This tool works on the same princi-
pal as the gamma ray, although it separates the gamma ray counts into