Rama Rau, Ray Davies, Michael Finke and Michael Manning
Halliburton Logging Services, Houston, Texas
ABSTRACT
Dielectric logging, introduced in the 1970’s, added a new dimension to electromagnetic logging and has proven to be a valuable addition to formation evaluation. A new 1 GHz, high-frequency dielectric logging tool has been developed with distinctive features relative to prior tools. The sensors are deployed using a pad on a flexible arm body instead of an in-line configuration, thereby providing better sensor contact with the formation. Multiple antennas provide four independent measurements of formation dielectric properties with four different depths of investigation and four different vertical resolutions. Continuous operation of the transmitter and receiver channels, in addition to overcoming switching transients, also improves the ability to measure phase and amplitude of weak signals in highly attenuating (conductive) formations. A backup arm optionally deploys a microlog pad to provide additional information on mudcake thickness, and to provide independent verification of thin-bedding structure observed with the dielectric pad.
One of the dielectric measurements is derived using incident and reflected signals at the transmitter, independent of the antenna characteristics. In effect, this provides one set of dielectric properties at a zero source-to-detector spacing and hence maximizes vertical resolution. Internal calibration over the dynamic range of the tool is made before and after logging, and is recorded in the calibration summary. Transformation of the calibrated phases and amplitudes to dielectric constant and 1 GHz resistivity is performed in real-time and is based on algorithms developed from mathematical modeling results.
Log examples from several wells illustrate the improvements described above. Comparisons of conventional porosity logs with water-filled porosity computed from the HFD log are also made.