Drilling fluids contain magnetic contaminations that negatively affect Measurement While Drilling directional tools and causes damage to the equipment in contact with the fluid. The effect is relevant while drilling long deviated wells. Ditch magnets are routinely installed in the fluid system to remove magnetic particles while drilling, with the purpose of protecting the Shale Shaker screens from large metallic debris, serve as monitoring tool to detect troubles in the drilling operations and clean the fluid from magnetic particles. In this paper we describe field data for the operation and efficiency of the ditch magnets. An extensive set of samples of drilling fluid and of ditch magnets debris have been taken during offshore operations and have been brought onshore for detailed investigations. The material extracted has been assessed as constituted mainly by steel swarfs and steel fines. The magnetic materials still present in the used drilling fluid have been extracted and quantified by a novel method that provide higher extraction rates and better accuracy than the methods currently employed in the industry, allowing to assess the actual content of magnetic contaminant. The magnetic susceptibility of the fluid has been measured and compared with values predicted for a known concentration of magnetic contaminants, allowing for the evaluation of the bias induced by them on the magnetometers of the MWD tools employed. The capability of ditch magnets to remove the magnetic contaminants from fluid is quantified and compared with other available methods, like gravity separation, with special focus on the removal of magnetic particles in the range of few microns. Operational details and current issues in the deployment of ditch magnets are reviewed, and the most viable directions for improvement are briefly discussed.

Magnetic shielding of the Measurement While Drilling (MWD) directional tools and damages to mud pumps, down-hole tools and casing/drill-pipe and difficulties in understanding logging results are some of the main problems caused by steel and magnetic contaminated drilling fluids. In order to have these problems significantly reduced, all the magnetic contaminants should ideally be removed from the drilling fluid by the use of ditch magnets. Hence, the magnets and their ability to remove magnetic waste from the drilling fluid was evaluated by field evaluations. Data from a field test of a new ditch magnet system where all the drilling fluid is forced to flow into an area with a strong magnetic fields have been compared with data from an operation with a traditional ditch magnet system in order to see if there are any significant differences in the total amount of magnetic waste material collected from the two systems. It is shown how drilling length, inclination and casing size may affect the production of magnetic debris entering the drilling fluid, and hence, show the well construction dependence of the ditch magnet performance.