Quantitative study of the applicability of fiber-optic gyroscopes for MWD borehole surveying

Horizontal drilling increases the productivity of the drilling processes, b ut inevitably requires a measurement-while-drilling surveying system employ ing three-axis accelerometers to determine the inclination and the tool fac e angles, and three-axis magnetometers to provide the azimuth. The magnetom eter measurements are affected by drillstring magnetic interference and dow nhole ore deposits, which necessitates the installation of the magnetometer s inside costly nonmagnetic drill collars. The aim of this article was to e xamine the usefulness of the fiber-optic gyroscope (FOG) to replace magneto meters for real-time monitoring of the azimuth. System analysis of the effe ct of the severe downhole environment on the FOG's overall performance is p resented. This analysis includes modeling the effects of shock forces, vibr ations due to mud pump noise, bending vibrations due to drill collar whirli ng 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 n onmagnetic drill collars. This improves the measurement accuracy and elimin ates the cost associated with nonmagnetic drill collars.