Measurement-while-drilling surveying of highly inclined and horizontal well sections utilizing single-axis gyro sensing system

Abstract

In the oil industry, when developing a plan for drilling horizontal wells, measurement-while-drilling (MWD) surveying becomes one of the important prerequisites for the successful completion of the drilling process. MWD surveying of horizontal wells determines the position and the orientation of the bottom hole assembly (BHA) in real-time during the drilling operation. The BHA orientation is determined by its inclination from the vertical direction as well as its azimuth. The present MWD surveying system incorporates three-axis accelerometers and three-axis magnetometers mounted in three mutually orthogonal directions. This magnetic surveying system suffers from the deviation of the magnetic field measurements due to the massive amount of steel around the drilling rig. A new method utilizing the fibre optic gyroscopes (FOG) technology was suggested to replace the magnetic surveying system. It was reported that a single FOG mounted inside the bearing assembly with its sensitive axis along the tool spin axis could be incorporated with three-axis accelerometers to continuously survey the near-vertical well section. This study aims at suggesting a surveying methodology for highly inclined and horizontal well sections utilizing FOG sensors. At each surveying station, the intrinsic uncertainties of the surveying sensors and the other vibration-induced noise are reduced using a transversal finite impulse response filter. The inclination is then computed by processing the accelerometer measurements, while the gyro measurement is utilized to determine the azimuth. In addition, optimal estimation techniques based on Kalman filtering are used to improve the azimuth accuracy and to limit the effect of the drift of the surveying sensors over time. This study indicates that gyroscopic surveying utilizing the FOG technology can be a reliable solution for MWD surveying of horizontal wells.