Variable In-Field Geomagnetic Referencing for Improved Wellbore Positioning in Directional Drilling

Abstract

Abstract Safe and economical determination of wellbore trajectory in directional drilling is traditionally achieved by measurement-while-drilling (MWD) methods which implement magnetic north-seeking sensor packages. However, inaccuracies in determination of well path rise because of random and systematic errors in measurements of the sensors. Multi-station analysis (MSA) and magnetic in-field referencing (IFR) have already demonstrated the potential to decrease the effects of errors due to magnetization of drillstring components along with variable errors due to irregularities in magnetization of crustal rocks in the vicinity of wells. Advanced MSA methodologies divide a bore-hole into build and lateral sections and utilize average reference values of total magnetic field, declination, and dip angle at only two levels (surface and lateral depth) for analysis of errors in each section. Our investigations indicate that the variable-reference MSA (VR-MSA) can lead to a better determination of errors, specifically in areas of high magnetization. In this methodology, magnetic reference values are estimated at each station using a series of forward and inverse modelling of surface magnetic observations from IFR surveys. The fixed errors in magnetometer components are then calculated by minimizing the variance of the difference between the measured and unique estimated reference values at each station. A Levenberg-Marquardt algorithm is adapted to solve the non-linear optimization problem. Examination of this methodology using MWD data confirms improvements in well path determination by comparing the results with gyro surveys.