INFLUENCE OF STEEL CASINGS ON ELECTROMAGNETIC SIGNALS

Most wells in oil fields are cased with metal. The feasibility of meas uring low-frequency magnetic fields through such steel-cased wells is of great interest for monitoring fluid movement during production. We report on experimental and modeling results regarding the effect of st eel casing on electromagnetic signals transmitted through them. This e ffect includes the attenuation and phase shift of a signal diffusing t hrough the casing wall. Sample casings are studied under low field str ength in the linear regime where the casing's electrical properties ar e independent of the applied signals. Frequency spectra are measured f or transverse-electric and transverse-magnetic polarizations from 0.1 Hz to 1 kHz. Numerical models, developed to simulate the casing respon se, match the experimental data very well. Finally, studies of the eff ects of the earth's magnetic field and casing remanent magnetization s how that these steady biases do not affect the ac signal detection.