IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 5, OCTOBER 2005 1997 FOG-Based Navigation in Downhole Environment During Horizontal Drilling Utilizing a Complete Inertial Measurement Unit: Directional Measurement-While-Drilling Surveying Adrián G. Ledroz, Efraim Pecht, David Cramer, and Martin P. Mintchev, Senior Member, IEEE Abstract—Presently used surveying sensors in directional drilling processes include accelerometers and magnetometers arranged in three orthogonal directions. The magnetometers in these setups are negatively affected by external magnetic interferences induced by various sources. Therefore, expensive, heavy and lengthy protective nonmagnetic collars need to be installed. Fiber-optic gyroscopes (FOGs) in an inertial navigation setup have been proposed as an alternative to magnetometer-based downhole surveying. The present study explored the feasibility of utilizing a FOG-based tactical-grade inertial measurement unit (IMU) as a complete sur- veying sensor for measurement-while-drilling (MWD) processes downhole. Alignment and real-time navigation under laboratory conditions were demonstrated. Analysis of vibrations and temper- ature as possible factors limiting the accuracy of the navigation process was performed. Severe vibration effects were reduced using software techniques, and a shock-absorbing housing was suggested. The temperature range of the IMU is limited by the optical com- ponents of the device, but dynamic temperature changes within this range did not present a major problem. A downhole sub design demonstrated that the actual integration of the IMU requires only minor changes in the presently used drilling tools. The utilization of a tactical-grade IMU eliminates the necessity of nonmagnetic collars, which results in lower costs and improved accuracy. Index Terms—Horizontal drilling processes, inertial measure- ment unit, inertial navigation systems, measurement-while-drilling (MWD) processes, oilfield instrumentation. I. INTRODUCTION A. Horizontal Measurement-While-Drilling (MWD) T HE advantages of horizontal drilling in oil exploration and production can be appreciated from two different perspec- tives. First, it facilitates multiple well exploration from an off- shore platform and the accessibility of complex locations when the reservoir target is located under riverbeds, mountains or even cities [1]. Second, horizontal wells report higher productivity and longevity when compared to vertical ones [2]. Manuscript received August 8, 2004; revised December 14, 2004. This study was supported in part by the Natural Sciences and Engineering Research Council of Canada, in part by the Alberta Energy Research Institute, and in part by Phoenix Technology Services, LP. A. G. Ledroz was with the Department of Electrical and Computer Engi- neering, University of Calgary, Calgary, AB T2N 1N4, Canada. He is now with GyroData Inc., Houston, TX 77043 USA. E. Pecht and M. P. Mintchev are with the Department of Electrical and Computer Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada. (e-mail: mmintche@ucalgary.ca). D. Cramer is with Phoenix Technology Services LP, Calgary, AB T2P 3A8, Canada. Digital Object Identifier 10.1109/TIM.2005.853562 Fig. 1. Angles that determine the orientation of an object in the local (reference) frame: the pitch ( ), the roll ( ), and the azimuth ( ). A directional well is drilled vertically from the surface to the kick-off point at a given depth, and then is intentionally deviated from the vertical direction to a curve drilled at a specific build-up rate. In order to implement this complex drilling pattern, MWD equipment and a steerable setup need to be installed next to the drilling assembly [3]. The drilling assembly consists of diamond bit, high-speed motor with a bent housing, nonmagnetic drill collars which include magnetic surveying sensors, and a slick drill pipe. When compared to conventional rotary drilling tech- niques, drilling using downhole motors results in both cost and operational benefits [1]. Present MWD technology utilizes magnetic sensors and accelerometers. The orientation of the bottom-hole assembly (BHA) is determined using three-axis accelerometers and three- axis magnetometers arranged in three mutually orthogonal directions. The drilling process is interrupted at certain predeter- mined surveying stations, where the BHA becomes completely stationary. The magnetometers and the accelerometers then measure the three orthogonal components of the Earth magnetic field and the Earth gravity vector, respectively. The measures from the accelerometers are used to determine the pitch and the roll, while the measures from the magnetometers along with the pitch and the roll are used to calculate the azimuth (Fig. 1). 0018-9456/$20.00 © 2005 IEEE