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
T.J. Calvert, R.N. Rau, L.E. Wells, AIME, 1977
In conventional oil and gas:
In very heavy oil, tar, or bitumen, where invasion is
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 ATTEN 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.
Dielectric Phase Shift Parameters
Gas 18 20 18
Oil 20 30 25
Water 60 450 250 See Graph ==>
Quartz 42 46 46
Dolomite 48 50 48
Limestone 50 52 52
Shale 45 65 55
Solving the response equation for water filled porosity, we get:
4. PHIphz = (PHZ PHZma) / (PHZw PHZma)
5. SXOphz = PHIphz / PHIt
CAUTION: The porosity (PHIphz) derived from these logs is the water filled porosity. This is the flushed or invaded zone water content which is not total or effective porosity, except in water zones.
PHIphz = porosity from dielectric phase shift
Rept = resistivity from deep dielectric log
SXOphz = invaded zone water saturation
PHIt = total porosity from conventional log analysis
NOTE: The service company will display a porosity derived from phase shift on the log. However, this will be based on default parameters. You may need to recompute with the fluid, matrix, and shale properties based on actual lithology.
TEXTURAL PARAMETER "W" From Dielectric Phase Shift
Traditionally, the Archie parameters M and N are assumed or determined in the laboratory. The former method is prone to error and the latter may be impossible due to no core samples or results arrive too late to be useful. An alternate textural parameter, dubbed W by Baker Hughes and MN by Schlumberger, can be derived from the dielectric porosity and resistivity.
6. W = log (Rmf / Rxo) / log (PHIphz)
Using the Archie equation:
7. SWphz = ((RW / RESD) / PHIt^W)) ^ (1/W)
SWphz = water saturation using M = N = W
The above assumes M = N = W. Since W cam be calculated continuously over an interval, it can be helpful in refining SW in carbonates with varying pore geometry. In unconventional reservoirs where M and N are considerably less than 2.0, it may help to set N = W and M = 0.8 * W.
RESD may be from conventional array induction or from deepest dielectric resistivity if invasion is shallow enough. This log has excellent bed resolution, so it is worth trying to use porosity and resistivity from it if possible.
Porosity CROSSPLOTS FoR Dielectric LOG
These crossplots, courtesy of Baker Hughes, show dielectric phase porosity on a limestone scale versus other porosity tools.
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