induction log, used to measure formation conductivity, is generally applicable to boreholes
drilled with moderate to non conductive drilling fluids and to empty or air drilled boreholes.
Downhole electrical resistivity/induction surveys are run to provide data for the
evaluation of drilling mud invasion, to determine true formation resistivity/conductivity and
flushed zone resistivity, to determine pore water resistivity/conductivity, and to potentially
provide a means for correlation across wells. Resistivity data also are used in conjunction
with other log measurements to estimate permeability and in situ mineralogy. Applicability
of a particular resistivity or induction tool to a specific borehole environment is a function of
the formation electrical properties, the properties of the borehole fluid, and the desired
resolution of the survey. In general, a wide array of electrode and coil configurations are
available for borehole applications. The development of focused, multiple electrode resistivity
and multi coil induction sondes have improved the vertical resolution and depth of penetration
of electrical tools.
The spontaneous potential (SP) tool records the electrical potential produced by the
interaction between formation pore water, conductive drilling fluid, and ion selective
formation components. The SP curve can potentially differentiate between porous and
permeable zones and non porous, impermeable zones, and can define layer boundaries and
estimate ground water resistivity. However, the SP curve cannot be recorded in boreholes
filled with non conductive drilling fluids or cased boreholes because the fluid does not
provide electrical continuity between the SP electrode and the formation. Similarly, if the
borehole fluid filtrate in the formation and the natural formation water have approximately
equal resistivities, the SP curve deflections will be small and the curve will appear featureless.
Electrical noise and anomalous potentials are common problems on SP logs, a result of
insufficient electrical insulation of the steel cables used to lower the SP electrodes into a
borehole. Surface or subsurface electrical sources, and weather effects also are possible
sources of anomalous potentials.
Nuclear Methods
Nuclear radiation tools are used to measure passive or induced radiations from the
nuclei of the atoms comprising a formation. Commonly applied nuclear tools are the natural
gamma ray, gamma gamma, and neutron devices that can be applied in either cased or open
boreholes filled with any type of fluid.
Conventional gamma ray logging is a passive process that uses a sonde containing a
scintillation counter to measure the total natural radioactivity emitted by the formation. The
measured total radioactivity is a linear combination of source radiation from potassium,
thorium, and uranium bearing formation elements. Natural Gamma Ray Spectrometry (NGS)
logging is a refinement of conventional gamma ray logging that uses five window (energy
levels) spectroscopy to resolve the total natural gamma ray spectra into the potassium,
thorium, and uranium components. The tool has a sodium iodide scintillation detector to
November 1992
4 31






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