Analytical modelling in low-frequency electromagnetic measurements of steel casing properties

Published studies on low-frequency (10 Hz–100 kHz) electromagnetic measurements of oil-well steel casing properties are mainly experimental and focus on certain regions of the space and frequency distributions of an exciter magnetic field inside a casing. We present an analytical model, based on the work of Dodd and Deeds, and employ it to analyse the whole space–frequency characteristic. We show that the model predictions and sensitivity analysis are consistent with the known effects of coil separation and excitation frequency on the measurements. The modelled responses to pulsed excitation are in good quantitative agreement with the experiments. The model can be applied in optimal design of the measurement systems and model-based measurement of the casing properties.