Research on high-power and high-efficiency emission of cross-well electromagnetic logging

Cross-well electromagnetic (EM) logging has useful implications in oil, gas, and water monitoring, as well as carbon sequestration estimation. Difficulties in the efficient propagation of EM waves through a steel casing inhibit cross-well EM logging at distances greater than 300 m. This study investigates whether high-power transmission and high-efficiency transmitting antennas can resolve the challenges of long-distance cross-well EM logging through steel casings. Theoretical analysis shows that the emission moment should be at least 12 000 A·m2. By calculating the equivalent relative permeability and selecting appropriate coil materials and magnetic cores, the maximum emission magnetic moment can realize a high-power transmission. Based on series and parallel resonant circuits and the AC/DC cable core multiplexing power supply method, the effective transmission bandwidth is broadened and transmission efficiency is increased three-fold. The emission unit is fabricated and tested with a single, fiberglass, and double casing. We observed that the cross-well EM technology performs better for nonmetal casing wells than for metal casing wells. The amplitude and phase curves between two fiberglass casing wells in the range of 425 m are quite smooth. However, the receiving signals in the double-layer steel casing drop to only a few tenths of the value in the fiberglass casing wells, highlighting the difficulty of long-distance EM-wave transmission when using steel casings. Thus, studies on EM transmission through four or more layers of steel casings should be conducted in the future.