Drilling Fluid affects MWD Magnetic Azimuth and Wellbore Position

Abstract

Abstract The drilling fluid may degrade the accuracy of magnetic azimuth and borehole position significantly so that it complicates the drilling operation and reduces the possibility to hit the planned targets. As an example: Azimuth is distorted up to 5° because of the magnetic properties (susceptibility) of the drilling fluid; and the wellbore is displaced 50 meter in one of the analyzed survey sections. Survey data from 30 well sections in the North Sea and the Norwegian Ocean are analyzed in order to see whether the weight material in the drilling fluid may affect the magnetic characteristics for the drilling fluid. The cross-axial magnetic field intensity is attenuated more when applying ilmenite instead of barite as weight material. Another significant finding is that the attenuation is stronger in 12 1/4" sections than in 17 1/2" and 8 1/2" sections. Laboratory experiments also indicate that drilling fluids with barite are less susceptible than those with ilmenite, and that fresh fluids are more favourable than used fluids. Attenuation factors for cross-axial magnetic components, which are derived from laboratory measurements and a simplified model of the downhole conditions, become much lower than those derived directly from the survey data. This contradiction is explained by some physical and operational conditions. Guidelines have been established for how to control, reduce and eliminate the negative effect on the wellbore positions. The precautions and actions, which are presented, are dependent on the composition of the drilling fluid, the drilling operation, the wellbore directions, the magnetic interference sources, the accuracy of the geomagnetic reference field and the requirements for wellbore position accuracy and reliability. The ultimate method for getting around this problem is to measure azimuth with a gyroscopic tool. However, multi-station analysis and corrections of magnetic survey data is also a satisfactory solution when certain operational and geometrical requirements are met. Introduction The drilling fluid is one of the factors which may contribute significantly to errors in directional surveying of wellbores for petroleum production, because the magnetic properties of the drilling fluid affect the sensor readings in MWD magnetic directional tools1. The phenomenon appears briefly as a damping of the measured cross-axial components of the Earth's magnetic field intensity, and often it dominates over most other relevant error sources. The magnetic susceptibility for a material describes how it becomes magnetized by an external magnetic field4. Drilling fluids have mostly small positive susceptibility values, and are said to be paramagnetic. The paramagnetic property is due to the base fluid, the weight material and steel particles from drillstring and casing wear. A magnetic drilling fluid will act like a shield when we are observing the Earth's magnetic field intensity with a tool which is surrounded by the fluid. Due to the geometry the shielding effect appears mainly on the cross-axial, transverse to the wellbore direction, components of the magnetic field intensity. Throughout this paper SI units are employed for magnetic susceptibility. The attenuation of the cross-axial components will cause azimuth errors. Fig. 1 illustrates the azimuth errors caused by a 1% scale factor error on the cross-axial magnetic readings. 1% or more is typical for some drilling fluids. The magnetic dip angle is 72° which reflects North Sea conditions. There are shown results for the two standard methods for calculating azimuth:All the three sensor readings are used, both the two cross-axial ones and the axial one.Only the two cross-axial sensor readings are used directly. The axial one just determines the magnetic interference in the axial direction.