Paramagnetism shielding in drilling fluid

Abstract

In drilling operations, drilling fluid containing magnetic materials is used when drilling a well. The materials can significantly shield the Earth’s magnetic field as measured by magnetic sensors inside the drilling strings. The magnetic property of the drilling fluid is one of the substantial error sources for the determination of magnetic azimuth for wellbores. Both the weight material, cuttings, clay and other formation material plus metal filings from the tubular wear may distort the magnetometer readings. This effect is obviously linked to the amount and kind of magnetic material in the drilling fluid, and the development of corrective means has therefore highlighted the drilling fluid. The influence on directional Measurement While Drilling (MWD) from drilling fluids has been studied using finite element modeling techniques. The simulations have been performed for several cases with realistic representations of MWD tool geometries and varying location of Bottom Hole Assembly (BHA) versus the wellbore. The wellbore, tools and magnetic fields were modeled by finite element methods. The wellbore is modelled as a perfect circular cylinder. With unsymmetrical geometry the placement of the magnetic MWD sensors may become more sensitive to magnetic shielding effects, and small position changes may result in significant errors in the measured magnetic field components, both attenuation and amplification [1]. One important result is that for situations with perfect axial symmetry, the magnetometer readings are attenuated proportionally to the square of the magnetic susceptibility. Since the magnetic susceptibility is a small number, this means that the effect on magnetometer readings is generally negligible. However, if the symmetry is broken, the distortion on the magnetometer readings can be increased significantly. This means that segregation of cuttings, metal filings or weight material can strongly influence the strength of the measured magnetic fields [1]. It has been shown sometimes to cause significant errors in the accuracy of drilling hole positioning using magnetic surveying. Here we present a general physical approach for correcting the measured magnetic fields from paramagnetism by the paramagnetic material for such in drilling fluid. Based on information of the paramagnetic properties of the drilling fluid and the well geometry, applied to a sufficiently long straight section of a well [2] this paper will show how the magnetic field in a cylindrical wellbore can be calculated analytically by using conformal mapping .