“The theory of 2 MHz resistivity tool and its application to measurement-while-drilling,” The Log Analyst, Vol. XXV, No. 3, pp. 35-46, May-June 1984.
A medium frequency (2 MHz) resistivity tool built on a steel drill collar is analyzed. This tool, called EWR (Electromagnetic Wave Resistivity), is used for measurement-while-drilling (MWD) applications. consists of a transmitting coil producing an electromagnetic field in the surrounding formation. A pair of receiving coils senses the electromagnetic field and the receiver electronics detects the difference in the phases of the received signals, which is directly related to the formation resistivity. MWD data are stored in the tool for recovery after a bit run, or telemetered to the surface while drilling. Vertical resolution of the tool is approximately six inches (6") and depth of investigation (diameter) is about 50 inches. Often the virgin formation resistivity, Rt, is measured, since the depth of invasion is frequently shallow for MWD. This article first covers the mathematical analysis, computation technique, and conversion of phase difference to formation resistivity (Rt). Then borehole correction charts, effects of invasion, and example logs obtained by this tool are discussed. They are presented such that understanding the EWR''s response characteristics and application does not depend on reading of the more theoretical sections.