Electric and magnetic dipole antennas of a directional resistivity logging-while-drilling tool for “look-around” boundary detection

Abstract

Directional resistivity logging-while-drilling tools play an important role in geosteering. In contrast to the widely used tool with magnetic dipole transmitters and receivers, tools are investigated with electric and magnetic dipoles as transmitters and/or receivers for “look-around” boundary detection. Measurement schemes are selected based on their look-around boundary detection abilities. Those measurement schemes are studied regarding the sensitivity and depths of boundary detection, the optimal transmitter-receiver distance and frequency, and the effects of a drill collar using the coefficient propagator method and the finite-difference method. Higher sensitivity to a boundary often implies a shallower depth of investigation. More specifically, the measurement schemes using an electric dipole transmitter and receiver are the least sensitive to the boundary; however, such schemes have the maximum depth of detection and the strongest signal strength. The schemes using electric and magnetic dipole transmitters and receivers have a moderate depth of detection and signal strength. The measurement schemes with a magnetic dipole transmitter and receiver have the highest sensitivity to the boundary and the weakest signal strength. The drill collar affects the response of all of the measurement schemes. However, this effect cannot be corrected with a simple coefficient for the schemes using an axial electric dipole transmitter, but it can be corrected for others. The optimal frequencies and optimal transmitter-receiver distances of those schemes have two types: the changing one that changes with the distance to boundaries and the fixed one that usually has the lowest frequency or the shortest transmitter-receiver distance. The characteristics of each measurement scheme will guide the tool designs and determine its role in the application and method of interpretation.