Quantitative Study of the Applicability of Fiber-Optic Gyroscopes for MWD Borehole Surveying
- Noureldin Aboelmagd (U. of Calgary) | Herb Tabler (Intl. Downhole Equipment Ltd.) | Dave Irvine-Halliday (U. of Calgary) | M.P. Mintchev (U. of Calgary)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2000
- Document Type
- Journal Paper
- 363 - 370
- 2000. Society of Petroleum Engineers
- 1.6.1 Drilling Operation Management, 1.4.4 Drill string dynamics, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 1.10 Drilling Equipment, 1.6 Drilling Operations, 1.5 Drill Bits, 1.12.1 Measurement While Drilling, 1.11 Drilling Fluids and Materials, 1.6.6 Directional Drilling
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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.
During the last decade oil companies and drilling contractors began to show a great deal of interest in horizontal drilling.1 It was suggested that by "kicking off" above an oil bearing formation, drilling into it at an angle and then following along within the formation horizontally, the productivity and longevity of a producing well could be increased many-fold.2 However, this necessitates the installation of directional measuring equipment behind the drill bit. Horizontal well technology involves drilling a vertical hole (usually using conventional rotary drilling) to an appropriate depth. The horizontal drilling equipment is then installed with bent housing adjusted to an appropriate offset angle (usually less than 3°).2 The assembly is installed inside the hole and rotated so that the offset points toward the desired azimuth direction. Subsequently, a window is cut through the casing using a special bit and the kick-off continues from that point with the ongoing azimuth angle being monitored using three-axis magnetometers. The inclination (the deviation from the vertical direction) and the tool face angle (the angle between the tool's main body and the horizontal plane) are determined using three-axis accelerometers.3
The major weaknesses of the present directional sensing instruments stem from the use of magnetometers to monitor the azimuth and from the hostile environment in which these devices must work. The problem encountered with the use of magnetometers is the presence of massive amounts of steel around the drilling rig.4 The abundance of ferromagnetic material necessitates the separation of the magnetometers by nonmagnetic drill collars (Montel metal).4 The cost of nonmagnetic drill collars can run higher than $30,000 for a single installation. Aside from the cost, the use of nonmagnetic drill collars introduces a second problem: the magnetometers must be separated from the bearing assembly and the drill bit, which means that they must be installed about 30 ft behind the drill bit. Elimination of the nonmagnetic drill collars could reduce the distance between the instrument package and the drill bit by approximately 50%. The third problem associated with the use of magnetometers is their lack of reliability when used underground due to the deviation of the Earth's magnetic field from ore deposits.
In the search for a replacement for the magnetometers it seems that some of the technological devices used for satellite guidance systems can be adapted for the azimuth measurement in horizontal drilling applications.5 Such devices must have a long-term accuracy, a long mean time between failure (MTBF), and should be able to withstand high temperatures as well as substantial vibrations and shocks which are considered the most hostile phenomena in the downhole environment. The present commercially available navigation devices such as mechanical gyroscopes and ring laser gyroscopes cannot be adapted for downhole drilling application. Mechanical gyroscopes contain moving parts which cannot perform properly in the harsh environment existing downhole.6 In addition, they have a small MTBF (9,000 hours) with high drift rate and need frequent maintenance. Although ring laser gyroscopes are of high accuracy, their size is larger than the minimum instrument size for downhole applications.5
In the present study we investigate whether the fiber-optic gyroscope (FOG) could be suitable for downhole drilling applications. This type of gyroscope is superior to its mechanical counterpart since it contains no moving parts, thus allowing high reliability (high MTBF) with no need for frequent calibration or maintenance.6 In addition, the FOG's exhibit low environmental sensitivity since they can withstand relatively high temperatures, shocks and vibrations.6 Moreover, currently available FOG's are of small size (1.6 in. in diameter, 0.5 in. thick) with a drift rate of less than 0.5 deg/hr, long MTBF (60,000 hours), no gravitational effects and excellent immunity to vibration and shock forces.7,8 With its small dimensions and high environmental insensitivity, the FOG can be mounted inside the bearing assembly of the drill pipe between the mud motor and the drill bit. Thus, significant improvement in the surveying accuracy can be provided since the FOG can monitor the azimuth angle at a position closer to the drill bit.
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