Crain's Petrophysical Handbook - ELECTROMAGNETIC PROPAGATION and DIELECTRIC LOGS

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ELECTROMAGNETIC PROPAGATION and DIELECTRIC LOGS

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Electromagnetic PROPAGATION LOG BASICS
Dielectric (DPT) logs and electromagnetic propagation (EPT) logs are logs of the high-frequency dielectric properties of reservoir rocks. Dielectric propagation logs work between 20 and 200 MHz. Logs made above 200 MHz and into the GHz range are known as electromagnetic propagation logs.

The dielectric log usually includes two curves: the relative dielectric permittivity, epsilon, which is unitless, and the resistivity in ohm-m. At the frequency used, water molecules have a strong effect on the dielectric properties, so that both relative dielectric permittivity and conductivity increase with the volume of water present. Relative dielectric permittivity can be used to distinguish hydrocarbons from water of any salinity. However, the effect of salinity is more important than the salinity effect with the higher frequency electromagnetic propagation log, and the interpretation is more complex. The advantage of the dielectric propagation log is that the lower frequency permits a larger depth of investigation and therefore an analysis of the undisturbed zone.

Electromagnetic propagation logs record propagation time and attenuation in a shallow region Propagation time can be transformed into water filled porosity and attenuation is used as a shale indicator. Water filled porosity, along with effective porosity, can give Sxo in conventional oil and gas, or Sw in heavy oil or bitumen. Both EPT and DPT measure about 6 inches vertically.

Logging while drilling propagation resistivity logs work between about 100 kHz and 10 MHz. Induction and laterolog tools work in the 10 to 100 kHz range. Below about 100 kHz, the measurements are based on the properties of standing waves, not of propagation.

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.

Simplified log analysis of EPT is based on a time average equation similar to the sonic log Wyllie equations:
    1: TPo = (TPL^2 - ATTN^2 / 3604)^0.5
    2. PHIept = (TPo - (1 - Vsh) * TPma - Vsh * TPsh)
                    / (TPw - TPma)

where:
ATTN = EPT attenuation (db/m)
TPL = EPT log reading (nsec/m)
TPma = EPT matrix value (nsec/m)
TPw = EPT water value (nsec/m)
TPsh = EPT shale value (nsec/m)

In conventional oil and gas:
3: Sxo = PHIept / PHIe
If shale corrections were ignored:
4: Sxo = PHIept / PHIt

In very heavy oil, tar, or bitumen, where invasion is minimal:
5: Sw = PHIept / PHIe
OR 6: Sw = PHIept / PHIt if shale corrections were ignored in finding PHIept.

Depth plots of both PHIept and PHIe are commonly shaded to show the difference, which is the hydrocarbon volume. This difference is helpful in locating hydrocarbons and oil water contacts in conventional oil, and may be close to actual hydrocarbon volume in heavier oils.

Charts are available to correct ATTN and TPL for spreading, salinity, and temperature. Some computer software ignores the shale correct so you get water filled porosity plus clay bound water instead of just water filled porosity.

The EPT method is somewhat insensitive to water salinity and matrix properties as both terms have relatively narrow ranges (TPma ~~ 8 and TPw ~~ 70). It is especially useful in fresh water oil or gas reservoirs, where resistivity methods lack sufficient resolution to detect hydrocarbons easily.

More elaborate methods to solve for water filled porosity are encoded in commercial software, using the real and imaginary (phase and amplitude) information buried in the electromagnetic signal. The CRIM and CTA methods are documented in Schlumberger's "Log Interpretation Principals and Applications" manual. The dielectric constant of materials varies with the electromagnetic frequency of the logging tool, so these more exotic methods are required with dielectric (low frequency) logs, since propagation time is not recorded on these logs.

Electromagnetic PROPAGATION LOG CURVE NAMES

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


DPT Log
Curves	Units	Abbreviations
DPT permitivity	unitless	EPSILON
resistivity	ohm-m	Rept
attenuation	db/m	ATTEN
* gamma ray	api	GR
caliper	in or mm	CAL
porosity from EPT	% of frac	PHIEPT

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).

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.