Method and apparatus with high resolution electrode configuration for imaging in oil-based muds

Abstract

Various disclosed resistivity imaging tools and methods provide a high-resolution electrode configuration for imaging in oil-based imaging in oil-based muds. Some tool embodiments have a sensing surface that comprises: a measurement electrode, a focus electrode surrounding the measurement electrode, and a return electrode surrounding the focus electrode. The sensing surface can be provided on an extendable sensor pad or on the wall-contacting portion of a stabilizer. Some method embodiments include measuring the measurement electrode current while driving a voltage signal between the measurement electrode and the return electrode. The voltage signal may simultaneously or sequentially provide energy at different frequencies. The resistivity measurements are combined with tool position and orientation measurements to form a borehole wall image. Robust and reliable performance is expected in the hostile conditions often experienced by logging while drilling (LWD) tools, coupled with the ability to make micro-resistivity measurements with a resolution approaching that of electrode-grid tool designs.