A true three-dimensional wellbore positioning method based on the earth ellipsoid

Abstract

In view of the problems existing in the current wellbore positioning method, a true three-dimensional wellbore positioning method was presented, and an example for analysis was given. The current positioning method uses the grid north as the reference datum to the north, positions in the horizontal plane based on the map projection coordinates, and positions in the vertical direction based on the elevation coordinates. It has inherent defects and errors, as the two positioning methods above are independent of each other, and only use the constant meridian convergence and constant magnetic declination at the wellhead to calculate the borehole trajectory for the whole well. Based on the earth ellipsoid and its calculating principle, the transformation relationship between the wellhead coordinate system, geodetic coordinate system and elevation coordinate system was revealed, and the true three-dimensional wellbore positioning method using the true north as the reference datum to the north was presented. Analysis results of an example show that the current positioning method yields a smaller vertical depth and a larger horizontal displacement for the target, and produces larger errors compared with the true three-dimensional positioning method. The true three-dimensional positioning method has fundamentally solved the problems existing in the current positioning method, accurately positioning the relative location between the target and the wellhead, and significantly improves the accuracy and reliability of borehole trajectory design. Key words: drilling theory, directional drilling, wellbore trajectory, wellbore positioning