Publication History: 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 Mar 2022 by Sandra Bleue. This webpage version is the copyrighted intellectual property of the authors.

Do not copy or distribute in any form without explicit permission.

Electromagnetic PROPAGATION LOG BASICS
Three tool types measure formation dielectric permittivity.  Two are legacy tools, the oldest being the DLT, a low frequency tool consisting of coils on a mandrel, operating in the 50 to 200 megahertz range.  The second legacy tool, the high frequency EPT or Electromagnetic Propagation Tool, transmitted 1.1 GHz from a pad contact device with two transmitter receiver pairs.  The newest dielectric tool is the Array Dielectric Tool (ADT Schlumberger mnemonics). The ADT scans 4 frequencies between 20 MHz and 1 GHz.   The Baker DLT tool restricts the frequency range to 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.

The newer array (scanner) tools can be useful in carbonate reservoirs, where electrical properties are unknown. 


 

The electromagnetic principles involved in dielectric logging tools are described by Maxwell's Laws.

 

Reference:
 1. Electromagnetic Propagation - A New Dimension In Logging

    T.J. Calvert, R.N. Rau, L.E. Wells, AIME, 1977


LOW FREQUENCY DIELECTRIC LOGS (DLT)
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.

 

HIGH FREQUENCY Electromagnetic PROPAGATION LOGS
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. PHIept = PHIe only in water zones.
 

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.



MULTI FREQUENCY array DIELECTRIC LOGS (ADT)
 

The ADT measures formation dielectric at four frequencies, 20, 100, 500 MHz and 1 GHz. The ADT consists of four receiver antennas symmetrically arranged around two transmitter antennas, plus 2 shallow mud-cake probes on an articulated pad.  A total of 9 measurements are made per depth level, outputting permittivity e and conductivity s for each frequency.  Processing using a geometrical model gives a radial resistivity profile of the reservoir.  The magnitude of epsilon quantifies the fluids and rock types near the wellbore.  The signal phase shift and attenuation is transformed to water-filled porosity and compared on the log to total porosity. The textural arrangement of fluids and matrix is calculated and displayed as the Archie m=n curve. The depth of investigation is 1-4”, with a vertical resolution of 1”.   

Multi frequency dielectric tool skid and transmitter - receiver layout ==>
 

 


DIELECTRIC
LOG CURVE NAMES
 

DLT  Log    
Curves Units Abbreviations
permittivity unitless EPSILON
resistivity ohm-m Rept
attenuation db/m ATTEN
* gamma ray api GR
caliper in or mm CAL
porosity from DLT % of frac PHIEPT
     
     
EPT Log    
Curves Units Abbreviations
EPT travel time nsec/m TPL
signal level (near) db LNEAR
signal level (far) db LFAR
attenuation db/m ATTEN
* gamma ray api GR
caliper in or mm CAL
porosity from EPT % of frac PHIEPT
     
     
ADT LOG    
Curves Units Abbreviations
permittivity unitless EPSILON
invaded zone resistivity ohm-m RXOadt
* gamma ray api GR
caliper in or mm CAL
water filled porosity % of frac PHIadt
dielectric water saturation % of frac SWadt
archie m=n unitless m=n

EXAMPLES OF Electromagnetic PROPAGATION LOGS


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 hydrocarbon. 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 porosity from  dielectric scanner (ADT) and EPT with total porosity (track 5) shows bypassed pay in rough boreholes (x690-x710 and x740-x755), (track 4). The skid mounted multiple detectors on the ADT reduces borehole effects.


Comparison of array induction, carbon oxygen, and dielectric phase logs.
The multi-frequency dielectric dispersion measurement clearly enables interpretation of the hydrocarbon zones from X260-X280 feet and X410-X440 feet in (Track 5). The standard triple combo log was overly optimistic, showing freshwater zones as potential oil zones.
 

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