The Design of BHA and the Placement of the Magnetometer Sensors Influence How Magnetic Azimuth is Distorted by the Magnetic Properties of Drilling Fluids

Abstract

Abstract 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 problem has been studied by laboratory experiments and analyses of downhole survey data. However, there are several inconsistencies in the results, and the phenomenon is not fully understood. The geometric properties of the BHA and the magnetic directional tool are focused upon in this study. The influence on directional 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 BHA versus the wellbore. 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. With a collar based tool, that is non-symmetric by nature, the model shows a complex distortion picture. The influence varies with the tool face angle, and shows both attenuation and amplification. The distortion maxima and minima for the two cross-sectional magnetometers do not coincide. These findings underline how hard it will be to find a straight-forward corrective action. The results from the simulations give increased knowledge about the influence from the magnetic properties of the drilling fluid. This is essential for accuracy estimation and multistation analysis of magnetic survey data. Furthermore, the results demonstrate the importance of considering the geometry of the BHA and the tool, and of the sensor placement when performing the mechanical design.