Page 16 - Well Logging and Formation Evaluation

6 Well Logging and Formation Evaluation

three energy windows to determine the relative contributions arising
from (1) uranium, (2) potassium, and (3) thorium in the formation. As
described later in the book, these data may be used to determine the
relative proportions of certain minerals in the formation.
• Spontaneous potential (SP): This tool measures the potential difference
naturally occurring when mud filtrate of a certain salinity invades the
formation containing water of a different salinity. It may be used to
estimate the extent of invasion and in some cases the formation water
salinity.
• Caliper: This tool measures the geometry of the hole using either two
or four arms. It returns the diameter seen by the tool over either the
major or both the major and minor axes.
• Density: The wireline version of this tool will typically have a much
stronger source than its LWD counterpart and also include a Pe curve,
useful in complex lithology evaluation.
• Neutron porosity: The “standard” neutron most commonly run is a
thermal neutron device. However, newer-generation devices often use
epithermal neutrons (having the advantage of less salinity dependence)
and rely on minitron-type neutron generators rather than chemical sources.
• Full-waveform sonic: In addition to the basic compressional velocity
(V p) of the formation, advanced tools may measure the shear velocity,
Stonely velocity, and various other sound modes in the borehole,
borehole/formation interface, and formation.
• Resistivity: These tools fall into two main categories: laterolog and
induction type. Laterolog tools use low-frequency currents (hence
requiring water-based mud [WBM]) to measure the potential caused by
a current source over an array of detectors. Induction-type tools use
primary coils to induce eddy currents in the formation and then a sec-
ondary array of coils to measure the magnetic fields caused by these
currents. Since they operate at high frequencies, they can be used in
oil-based mud (OBM) systems. Tools are designed to see a range of
depths of investigation into the formation. The shallower readings have
a better vertical resolution than the deep readings.
• Microresistivity: These tools are designed to measure the formation
resistivity in the invaded zone close to the borehole wall. They operate
using low-frequency current, so are not suitable for OBM. They are
used to estimate the invaded-zone saturation and to pick up bedding
features too small to be resolved by the deeper reading tools.
• Imaging tools: These work either on an acoustic or a resistivity princi-
ple and are designed to provide an image of the borehole wall that may