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The following figures show the resuk of electromagnetic measiurements on 4 (four)
WelUngton shale samples. The electromagnetic parameters (conductivity, dielectric constant, and dissipation factor) are plotted on the z-axis, as a function of frequency and the
amount of water loss in percent.
Figure 5.27 shows the conductivity of four WeUington shale samples WNl, WN2,
WN4, and WN5 as a function of frequency and water loss. Sample WN3 was not included
in the research as that sample cracked during preparation time. AU plots show a trend of
declining conductivity as frequency decreases and also as the amount of water loss increases. Figures 5.27a and 5.27c show that the two measurement methods more or less create a
continuous trend. Figures 5.27b and 5.27d, however, show a distinct gap in the frequency.
One explanation for the gap is due to heterogeneity of the sample; as different measurement
techniques read different areas, slight changes in properties wUl be recogiuzed more on one
method than the other. The low frequency setup that uses the parallel-plate method will
read the whole sample surface and across the sample thickness, basically averaging the
property of the medium. The high frequency setup uses an open-ended probe that has a diameter of less than half of the sample diameter; thus this probe only reads whatever is right
on the center of the sample surface. Hence a high spot on the center of the sample will easily skew the high frequency measurements.
To better look at the changes of conductivky as water content decreases at logging
tool frequencies, one can do a cross-section cut on Figure 5.27 parallel to the water loss axis
at the specific tool frequency. Figures 5.28 through 5.33 shows conductivity of sample
WelUngton WNl at 35 Hz (Deep Laterolog tool), 280 Hz (Shallow Laterolog tool), 20 kHz
(induction type tool), 10 MHz, 250 MHz (MWD tool), and 1.1 GHz (dielectric tool). Ignoring the first data point tiiat has a lower conductivity than tiie others, the conductivity
drops rapidly in the first 5% water loss and then levels off, as seen at 35 Hz, 280 Hz, and
20 kHz in Figure 5.28, 5.29, and 5.30, respectively. This drop in conductivity is less defined
at 10 Mhz and vanishes at 250 MHz and 1.1 GHz.
Figure 5.34 shows the dielectric constant as a function of frequency and water loss
in percent, and Figures 5.35 up to 5.40 show the dielectric constant at various logging tool
frequencies as a function of water loss.
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