Experimental study on spatial magnetic field distribution characteristics around casing strings

Abstract:

To address the insufficient understanding of casing magnetic field distribution characteristics in passive magnetic ranging （PMR） technology， this study investigates the spatial magnetic field distribution laws around casings. A testing system consisting of an FVM-400 three-axis fluxgate magnetometer and non-magnetic supports was established. Based on magnetization theory analysis， axial and radial magnetic field tests were conducted on typical 133/8 and 95/8 inches casings under single and combined conditions using the control variable method. The results indicate that the axial magnetic field of the casing exhibits sinusoidal-like fluctuation characteristics， with the peak and valley values of the magnetic field gradient concentrated in the collar and end regions. The radial magnetic field intensity decays exponentially with distance. Under the experimental conditions， the magnetic field approaches the geomagnetic field reference value at distances greater than 2 m axially from the end face and 8 m radially from the axis. The magnetic field morphology of combined casings is significantly influenced by the magnetic pole combination at the collar： same-direction combinations form a single strong peak at the collar， while reverse combinations exhibit "double-peak and double-valley" characteristics， with the former showing greater magnetic field intensity and influence range. Additionally， casings of the same specification exhibit individual differences in remanent magnetism. This study reveals the influence mechanisms of casing size， remanent magnetism， and magnetic pole combination on the spatial distribution of the magnetic field， providing an experimental basis and fundamental data support for refining PMR theoretical models and enhancing magnetic field signal analysis capabilities under strong interference backgrounds.