A new dielectric dispersion tool delivers a highly accurate shallow resistivity measurement in a variety of borehole environments. This new-generation dielectric tool differs from previous tools by incorporating a new antenna array on a fully-articulated pad, thus avoiding many of the environmental effects that plagued dielectric logging in the past. A further enhancement is that the new tool makes measurements at multiple frequencies from approximately 20 MHz to 1 GHz with collocated transverse and longitudinal transmitter and receiver arrays.

The new tool was recently used for the first time in Australia in several wells drilled with oil-based muds. Results from the dielectric tool were compared with results from traditional methods in two of these wells. These wells tested Triassic-aged fluvial and deltaic sands and shales on the Northwest Shelf, offshore Australia. In high-resistivity pay zones with resistivity greater than 100 ohm.m, the shallow resistivity measurement from the dielectric tool was superior to standard shallow array induction measurements and, in some places, seemed more representative than even the deep array induction measurements.

Dispersion processing uses the data acquired at multiple frequencies to calculate water-filled porosity. In pay sands, by combining the water-filled porosity from dielectric dispersion measurements with total porosity from density-neutron, water saturation can be calculated that is independent of formation salinity and does not require special core analysis measurements of electrical properties. The salinity of the water in the formation can be determined from conductivity and permittivity dispersion when conditions are favorable. Pre-job planning is essential because not all borehole conditions allow for accurate evaluation of all applications.