Geology Reference
In-Depth Information
1.
Lithology
2.
Bed thickness
3.
Porosity
4.
Fluid type and amount
5.
Fluid flow vector
6.
Permeability
7.
Trace element chemistry
8.
Fracture orientation
9.
Rock strength
Discussed below are logging techniques to infer these properties. Problems
in geophysical logging of oil wells include the presence of mud cake and of
formation disturbance by drilling fluids. The presence of these materials
disturbs the geophysical measurements.
Density Logs
The density log employs a cesium source of gamma rays shot into the
formation at 45° away from the hole axis. The receiver is a scintillation counter
collimated at 45° to the hole also but at a right angle to the source direction. The
method uses Compton scattering and assumes a direct relationship between
electron density and bulk density. This is actually a surprisingly accurate
assumption because the ratio is very close to 2:1 for mass compared to number
of electrons. The level of gamma radiation caused by the scattering is
proportional to the total number of electrons in the formation near the sonde.
Neutron Logs
Nonradioactive elements emit gamma radiation if they are sprayed by a
stream of neutrons. The neutron source for logging is americium or beryllium,
which produces a constant population of high-energy neutrons. Hydrogen
selectively absorb these neutrons, and as the neutrons are absorbed, gamma rays
are emitted. The more hydrogen in the rock, the faster neutrons are absorbed
and the more gamma rays are emitted. Water or oil absorbs neutrons; therefore,
porosity is generally proportional to hydrogen content as indicated by the
neutron log. The neutron log is a porosity determination tool.
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