Drilling Fluid affects MWD Magnetic Azimuth and Wellbore Position

摘要

The drilling fluid may degrade the accuracy of magneticazimuth and borehole position significantly so that itcomplicates the drilling operation and reduces the possibilityto hit the planned targets. As an example: Azimuth is distortedup to 5° because of the magnetic properties (susceptibility) ofthe drilling fluid; and the wellbore is displaced 50 meter in oneof the analyzed survey sections.Survey data from 30 well sections in the North Sea and theNorwegian Ocean are analyzed in order to see whether theweight material in the drilling fluid may affect the magneticcharacteristics for the drilling fluid. The cross-axial magneticfield intensity is attenuated more when applying ilmeniteinstead of barite as weight material. Another significantfinding is that the attenuation is stronger in 12 ?" sectionsthan in 17 ?" and 8 ?" sections.Laboratory experiments also indicate that drilling fluidswith barite are less susceptible than those with ilmenite, andthat fresh fluids are more favourable than used fluids.Attenuation factors for cross-axial magnetic components,which are derived from laboratory measurements and asimplified model of the downhole conditions, become muchlower than those derived directly from the survey data. Thiscontradiction is explained by some physical andoperational conditions.Guidelines have been established for how to control,reduce and eliminate the negative effect on the wellborepositions. The precautions and actions, which are presented,are dependent on the composition of the drilling fluid, thedrilling operation, the wellbore directions, the magneticinterference sources, the accuracy of the geomagneticreference field and the requirements for wellbore positionaccuracy and reliability.The ultimate method for getting around this problem is tomeasure azimuth with a gyroscopic tool. However, multistationanalysis and corrections of magnetic survey data is alsoa satisfactory solution when certain operational andgeometrical requirements are met.