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. Results for water saturation are consistent with conventional calculations from mineral-based log analysis, magnetic resonance data, and Dean-Stark core-plug saturation measurements.
Several other applications were tested, even though prejob planning indicated only a low-moderate chance of success. Determination of shale porosity, assumed to be water-filled, was tested against two types of core analysis which showed the tight sand and shale porosities to be in the 2 to 5 p.u. range. Dielectric dispersion results in shale intervals were generally close to the core plug porosity in one well and the total magnetic resonance porosity in the other.
In a gas reservoir where significant oil-based filtrate invasion took place, the water-filled porosity was very similar to the bound-fluid volume measured by the magnetic resonance tool. This observation reveals a new application for the tool as an alternative for faster bound-fluid logging.
Another application was formation water salinity determination. Pre-job planning suggested a moderate chance of success only for reservoir-quality sandstones. In clean sands, the dielectric results showed a fair match compared to in-situ salinity measurements from core plugs. However, in shaly sand and shale, the results are possibly affected by the high conductivity of clay-bound water. Further work has been proposed to improve the salinity inversion in shale.
