Abstract
Safe and economical determination of wellpath in directional drilling is traditionally achieved by the measurement-while-drilling (MWD) method which implements geopotential sensors, i.e., magnetometers and accelerometers. However, inaccuracies in determination of the wellpath arise because of random and systematic errors in measurements. In general, inclination is under good control with gravity measured by accelerometer, while azimuth requires a number of corrections as it also requires magnetic measurement which involves multiple sources of errors such as sensor errors, poorly-modelled crustal magnetic variation, drillstring magnetization, etc. These errors must be completely reduced or minimized to obtain an accurate wellbore position. A magnetic-free system and method to determine a borehole azimuth in the directional drilling is investigated in this study. We show that a borehole azimuth can be properly determined by using a system of coupled accelerometers mounted on measurement-while-drilling (MWD) sensors using Gravity in-Field Referencing (GiFR). In order to reduce errors due to relative oritation of the coupled drillstrings, we developed the Quaternion-based GiRF which considers the relative diaplacements and rotations between the two sets of accelerometer resulting in determining an improved azimuth.
