Fluid Identification - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 93

Fig. 7.9 Fluid density inertial tool
Capacitance (Dielectric) Tools
Another group of tools widely used to distinguish water from hydrocarbons depend
for their operation on the difference between the dielectric constant of water (~80)
and that of oil or gas (~6). A simple way to find the dielectric constant of a fluid is
to use the fluid itself as the dielectric between the plates of a capacitor. The capaci-
tance of the resulting capacitor may be found by classical methods such as including
it in an RC network and finding the resonant frequency.
A conventional design is shown in Fig. 7.10 . Two cylindrical metal tubes are
arranged so that well bore fluids flow through the annular space between them. The
raw readings of such a device are in terms of a frequency. Each tool will have a cali-
bration graph to convert a measured frequency to a watercut value. These tools
behave well provided that the continuous phase is oil. In practice, the measurement
may become unreliable if the watercut in the flowing mixture exceeds 30 %.
Array Capacitance Tools
In circumstances where the flow stream is not homogenized the simple cylindrical
instrument will not truly reflect the distribution of different phases across the actual flow
stream. Water, oil, and gas may become segregated when flow is in an inclined pipe or
a horizontally completed well. In such cases an array of capacitance measuring sensors
may be advantageously employed. Such an embodiment is illustrated in Fig. 7.11 .