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