Observability Analysis for INS Alignment in Horizontal Drilling

Contemporary surveying in measurement-while-drilling (MWD) processes incorporates measurements from three-axes accelerometers and magnetometers. Unfortunately, magnetometer-related problems limit the navigation performance of this technique. The introduction of fiber-optic-gyroscope (FOG)-based inertial navigation system (INS) in MWD aims at overcoming these limitations. However, drifts in the measurements provided by the INS might be prohibitive for the long-term utilization of this modern navigation-method downhole. One of the main obstacles precluding the elimination of these measurement drifts is the limited observability of the azimuth angle state provided by the INS. This paper explores the feasibility of utilizing a FOG-based tactical-grade inertial measurement unit (IMU) as a complete surveying sensor for a MWD processes downhole by implementing an innovative in-drilling alignment (IDA) procedure. During IDA, the IMU is exposed to controlled dynamics that excites azimuth-related states. This allows better and faster alignment that can reduce long-term navigation drifts, thus improving the overall accuracy in INS-based MWD processes. It is suggested that one take advantage of the longitudinal space available in the drilling-pipe system and impose controlled motion on the IMU to excite its states and increase its observability. Theoretical simulations and analytical approximations exploring the IDA idea have shown reduction in the steady-state azimuth-error variance and in the time required to achieve convergence with the increase of the acceleration-controlled motion. Several practical aspects of implementing this approach are evaluated and compared.