This article is based on
Chapter 3 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell
Books 1986 Republished as "Crain's Logging Tool
Theory" in 2004 and updated annually through 2016.
Updated Dec 2018.
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
Two classes of tools are available for measuring the
formation dielectric constant. The first one is
low-frequency dielectric constant logging tools using coils
on a mandrel, and more recently on a sidewall skid. They
operate in the 50 to 200 megahertz range. The second type is
high-frequency, 200 MHz to 1.1 GHz, using microwave antennae
on a pad contact device.
The first type was known as dielectric logs (DLT). The high
frequency tools became known as electromagnetic propagation (EPT)
logs. Modern versions of both types that operate at multiple
frequencies are called array dielectric tools. The
Schlumberger tool scans 4 frequencies between 20 MHz and 1
GHz. The Baler tool covers 47 to 200 MHz.
All these tools can be used to estimate invaded zone water
saturation Sxo = PHIept / PHIt. Under the right conditions
(shallow invasion, reasonably deep investigation) the Sxo
may approach the undisturbed zone water saturation SWept.
The measurement is relatively independent of water salinity
at salinities above 10,000 ppm NaCl so it is a helpful guide
to spotting hydrocarbons in fresh water environments.
Its major use is in heavy oil wells, such as those in
California and western Canada, and in EOR projects where
water, CO2, and chemical floods have confused the original
water resistivity regime.
electromagnetic principles involved in dielectric logging tools are
described by Maxwell's Laws, summarized below and given in more
1. Electromagnetic Propagation - A New Dimension In Logging
T.J. Calvert, R.N. Rau, L.E. Wells,
HIGH FREQUENCY Electromagnetic PROPAGATION
EPT logs measure propagation time (TPLP and signal
attenuation (ATTEN). Both are strongly affected by water so
water filled porosity can be calculated from these values.
As a 1 GHz tool, depth of investigation was very shallow. In heavy oil, where invasion is shallow, this water volume
is close to the irreducible water. PHUept = PHIe only in
Logging tools today use a skid-mounted transmitter-receiver
array. Older tools were mandrel style and
measurements were more seriously affected by rough or large
borehole conditions. Bed resolution is a function of the antenna
spacing and the firing mode. End-fire gives resolution of an
inch or two and broadside a foot or a little more. The
attenuation curve on the EPT was an excellent shale indicator in
laminated shaly sands, compared to the 3 foot resolution of a
gamma ray log.
EPT tool concept, with two transmitters and two receivers, in
end-fire (left) and broadside
(right) measuring modes. Region investigated (red) varies with
resistivity contrast between Rxo
and Rt, tool frequency, and firing mode.
LOW FREQUENCY DIELECTRIC
On low frequency DLT tools, curves presented varied
considerably but might include attenuation, phase shift, relative dielectric
permittivity, or resistivity. The advantage of
the DLT propagation log is that the lower frequency permits
a larger depth of investigation and therefore an analysis of
the undisturbed zone may be more likely.
MULTI FREQUENCY DIELECTRIC
Newer array dielectric logs measure at 4 different
frequencies, giving a resistivity profile at 4 depths of
investigation. They also measure signal phase shift, which
can be transformed into water filled porosity and presented
on the log. The Schlumberger array tool uses 20, 100, 500,
and 1000 MHz for its electromagnetic scans. It makes a total
of 9 measurements at each of the 4 frequencies at each depth
level. The Baker array tool restricts itself between
47 and 200MHz.
Multi frequency dielectric
tool skid and transmitter - receiver layout ==>
LOG CURVE NAMES
EPT log with deep induction, gamma ray, neutron and density.
Zone A is gas (density neutron crossover, PHIept is low), Zone B
is oil (no crossover, PHIept is low), Zones C, D, and E are wet
(PHIept = PHIxdn).
EPT porosity overlay on density neutron porosity shows residual
PHIept (red) and PHIxdn (blue) in an oil zone. Oil water contact
is located where red and blue curves meet near bottom of log at
about 6860 feet. Resistivity contrast (Track 2) does not define
contact clearly. Porosity curve separation is only a
hydrocarbon locator, as this is light oil, with invasion.
Comparison of array induction, carbon oxygen, and dielectric
The multi-frequency dielectric dispersion measurement clearly
enables interpretation of the hydrocarbon zones from X260-X280
feet and X340-X440 feet in (Track 5). The standard triple combo
log was overly optimistic, showing freshwater zones as potential
Comparison of dielectric scanner results (Track 4) and EPT
results (Track 5) shows high hydrocarbon volume in rough
borehole. The skid mounted multiple detectors on the DPT reduces