Abstract

This paper presents a new deep-reading logging-while-drilling electromagnetic (EM) logging method to detect bed boundaries ahead of bit. Unlike all existing EM logging approaches, the new method is based on the scattered electric field radiated by a magnetic dipole antenna. By analyzing the characteristics of electric tensor responses in layered formations, optimal look-ahead electric component is selected. The selected scattered field contributes to a large portion of the total field and is strongly sensitive to the boundary position. The measured voltage from the scattered electric component can be tens of times larger than that from the scattered magnetic fields and it attenuates slower. Thus, the detection capability improves significantly. A coaxial open-loop half-circle antenna is then proposed to measure the electric field in logging while drilling environment. A practical tool implementation equipped with two tilted close-loop antennas and two open-loop antennas is further developed for look-ahead application. Numerical results demonstrate that the detection depth of the new look-ahead tool can be up to 40 m under favorable conditions. Compared with current look-ahead logging tools, the new method not only significantly shortens the tool size, but also can recognize the boundary position and azimuth.