Wellbore Positioning in Directional Drilling by Geopotential Field Anomalies

Abstract

When navigating the wellbore trajectory, the geomagnetic north can be used to obtain orientation, but interference with other nearby magnetic sources can drastically affect the value of the magnetic azimuth. A potential solution can be found by modelling the local magnetic field. Modelling the magnetic field can help estimating the magnetic gradient at nearby locations. Changes in orientation for a given location, could be detected by calculating the angle between the estimated and the measured gradient. In principle this should work provided the measurements are taken within the accuracy limits of the modelled field and the amount of time passed after the gradient estimation is short. To evaluate the possibility of using the anomalous geomagnetic field for orientation, this research required to estimate various sensor errors and the implementation of various algorithms that are required for inertial navigation. Experiments were performed with the sensor in various static positions, and the sensor moving on a track. Magnetometer measurements were used to model the magnetic field, with the intention to provide uncorrelated heading and velocity information. For the case of the moving sensor this information was fused with IMU measurements into an extended Kalman filter The results for the sensor orientation in various static positions showed that raw field measurements had a heading error of 17°, by modelling the field, the heading error reduced to 10°. For this case the magnetic field was modelled from the measurements from two points separated 1.2 m. A second test was performed by using 2 points separated by 0.60 m. The modelled field for this test produced a maximum orientation error of 7°. The results for the sensor moving on a track, showed that the heading error was under 8° for the first 6 seconds of input, but the error increased rapidly after that. One of the reasons for this is that the traveled distance obtained from the mechanization deteriorated very rapidly, and this affected the magnetic field modelling. An odometer / drilling rate might help mitigating this problem and the expected heading error is estimated to be under 8°.