Wireless electromagnetic telemetry for metal cased wells: A novel approach with comprehensive channel analysis

Abstract

Low-frequency electromagnetic telemetry has been widely adopted for downhole data acquisition in the oil and gas industry. However, its application in cased wells has been limited by the high magnetic permeability and low resistivity of the metal casing, which causes the casing to absorb most of the electromagnetic waves transmitted by the downhole transmitter, resulting in inaccurate downhole data acquisition. We propose a novel approach to address this issue by suspending a transmitter (tens of meters in length) in the casing and connecting it electrically to the metal casing through two connectors. By forming a circulation along the transmitter and the casing, low-frequency electromagnetic waves can penetrate through the casing and reach the formation for telemetry. To further enhance the signal received by the ground antenna, we conduct a comprehensive analysis of the complex downhole-to-surface communication channel. Notably, the channel, which consists of the formation, the casing, and the transmitter, is divided into a near field and a far field for separate analysis. Quantitative assessments of four crucial factors on the ground-received signal strength are also conducted, i.e., electromagnetic wave frequency and location of the excitation source in the transmitter, as well as the length and location of the transmitter in the well. An 80-meter telemetry device is designed and optimized for integration into the well test. Experimental results confirm the consistency between the theoretically analyzed and measured attenuation law of the ground-received signal, indicating the effectiveness of our wireless electromagnetic telemetry technique for cased wells.