(484c) Experimental Investigation into Shielding Effect of Magnetic Powder on Drilling Fluid

Drilling fluid contains various weight–powders such as magnetite and iron powders, and is prone to be contaminated magnetically. Such contamination in the drilling fluid shields the Earth's magnetic field measured by the magnetic sensors, and might contribute significantly to errors in directional surveying of wellbores. A series of laboratory measurements were initiated to understand such a magnetic shielding effect. In the measurement, a single axis fluxgate magnetometer was used, and the vertical component of the Earth's field was measured. It is found that there were very strong shield effects when magnetometer was buried in dry magnetite powders and dry iron powders. Further measurements showed, however, that such shield disappeared when the iron powders was mixed into a well defined drilling fluid, whereas the magnet field damped significantly in the same drilling fluid with a presence of magnetic powders. Neither of the two findings is so far known to the surveying of wellbores, and could be used to illustrate how directional magnetic positioning tools give erroneous readings.