Effect of Downhole Electrode Length on Active Magnetic Ranging Technique for Relief Wells

Currently, low-frequency injection current-based active magnetic ranging is key to efficiently linking relief and accident wells. The ranging technology excitation process is complex, and the detected magnetic signals are generated by current excitation on the tubular column of the accident well, closely related to the downhole electrode length. Using the principles of magnetic ranging technology for relief wells, the effect of downhole electrode length on magnetic field distribution around accident well casings was analyzed through experimental and numerical methods. The results show that in isotropic formations, magnetic induction intensity around the accident well increases with downhole electrode length when casing conductivity far exceeds formation conductivity. The magnetic induction intensity curve decreases as the distance between the downhole electrode and the casing axis increases. The decrease in peak magnetic induction intensity speeds up with longer downhole electrode lengths. Considering the insertion capability of rigid downhole electrodes, an optimization of electrode length for a well showed that possibility within the borehole decreases when electrode length exceeds 9.6 m. This study provides guidance for rescue well magnetic ranging operations and tool development.