Abstract
Summary Horizontal drilling increases the productivity of the drilling processes, but inevitably requires a measurement-while-drilling surveying system employing three-axis accelerometers to determine the inclination and the tool face angles, and three-axis magnetometers to provide the azimuth. The magnetometer measurements are affected by drillstring magnetic interference and downhole ore deposits, which necessitates the installation of the magnetometers inside costly nonmagnetic drill collars. The aim of this article was to examine the usefulness of the fiber-optic gyroscope (FOG) to replace magnetometers for real-time monitoring of the azimuth. System analysis of the effect of the severe downhole environment on the FOG's overall performance is presented. This analysis includes modeling the effects of shock forces, vibrations due to mud pump noise, bending vibrations due to drill collar whirling and linear coupling between the axial and transverse modes of vibration. The results obtained show that the FOG has excellent immunity to shock and vibration. Moreover, since magnetic fields have only a minor effect on the FOG, it can be installed directly behind the drill bit, thus eliminating nonmagnetic drill collars. This improves the measurement accuracy and eliminates the cost associated with nonmagnetic drill collars.
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