Dead Reckoning Method for Tracking Wellbore Trajectories Constrained by the Drill Pipe Length

Two key factors for calculating the trajectory of a wellbore are the attitude and interval of the survey stations. A measurement while drilling (MWD) system that relies on magnetometers and accelerometers may fail to measure the wellbore attitude in an environment with magnetic anomalies. In addition, in the case of a wellbore trajectory with large variations in the attitude, the commonly used minimum curvature method (MCM) can result in large deviations from the planned wellbore trajectory. In this study, a novel dead reckoning (DR) method was developed that is constrained by drill pipe length subdivisions for improved tracking accuracy of wellbore trajectories with a large variety of attitudes. A gyro MWD system based on fiber optic gyroscopes (FOGs) and accelerometers is utilized to continuously calculate the wellbore attitude. The variations in the attitude angle within the wellbore can be tracked by using the proportional relationship between increments in the attitude and drill pipe length subdivisions, which can be synchronized with the gyro MWD update frequency. Simulations and experiments were performed to verify that the proposed method could accurately track wellbore trajectories with large variations in the attitude. In the simulation, the proposed method demonstrated a mean trajectory deviation of less than 0.5 m over a distance of 30 m, which was markedly lower than the mean deviation of 2.5 m by the MCM. In the slope experiment, the proposed method demonstrated substantially better tracking accuracy of the wellbore trajectory than the MCM. Measurements from an actual wellbore with large variations in the attitude confirmed that the proposed method reduced the tracking error by up to 3 m compared to the MCM.